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Charge-sensitive modelling of organic Rankine cycle power systems for off-design performance simulation
... Furthermore, the temperature pinch in the evaporator and condenser was assumed 5 K, and the temperature difference between the evaporation temperature and the thermal storage temperature was assumed 10 K. Ibarra et al. [135] considered the evaporation pressure and the condensation temperature as inputs to their off-design model of an ORC system and assumed a maximum cycle temperature as a limit of the system. Hence, the most common assumptions are an SH degree in the range of 5-10 K and a fixed SC degree [136]. A non-exhaustive list of the assumption-based models of micro and small-scale ORC systems in the literature is reported in Table 6.The main limit of assumption-based off-design models is that they cannot be fully deterministic. ...
... Therefore, choosing a correct VF model is challenging since experimental validation is usually required same as for the HTCs. Looking at Table 7, different VF models were adopted in the literature but only two papers searched for a suitable VF model using experimental data [136,144], while the rest chose the model arbitrarily. ...
... In the three references, the LR was assumed filled with the liquid if the condenser outlet was subcooled. While two papers considered LR filled with inlet saturated liquid, Dickes et al. [136] modelled it differently and assumed the LR partially filled when the inlet fluid is saturated liquid. They also considered the LR emptied if the inlet fluid is two-phase, while the other two modelled the LR partially filled. ...
Organic Rankine cycle (ORC) systems are a technology capable of producing electricity and heat from a wide range of energy sources and are particularly well-suited for medium and low-temperature sources. However, an almost infinite number of technical solutions (cycle configurations, working fluids, components, etc.) can be adopted making the full experimental characterisation of ORC operations for each application unfeasible. To overcome the limitations of extensive experimental investigations, numerical tools are often adopted, thereby supporting the design and operation of these plants. Therefore, over the last two decades, many researchers have put their efforts into developing models to elucidate the design and off-design performances of ORC systems. In this paper, the different modelling approaches for the analysis of ORC systems are discussed and a conclusive review is performed concerning the micro and small-scale ORCs. In total, more than 150 works are reviewed with many of them related to models of volumetric machines and assumption-based system modelling. Semi-empirical models of expanders show good capabilities and accuracy (with errors below 5%) while spatial resolution methods for heat exchangers are used to better capture the dynamics of the system. However, only a limited number of papers (10) deal with assumption-free models of the systems to predict their performance considering the actual boundary conditions. In summary, the present review paper provides a clear overview of the advantages and disadvantages of each modelling approach at both component and system levels to provide insights for interested readers in the advanced simulation of micro and small-scale ORC systems.
... Ibarra et al. [7] considered the evaporation pressure and the condensation temperature as assumed inputs to their offdesign model of an ORC system and also assumed a maximum cycle temperature as a limit of the system. Hence, the most common assumptions are a superheating degree in the range of 5-10 K and a fixed subcooling degree [8]. ...
... In the literature, five out of seven mass-sensitive works had a liquid receiver in their system and the models to calculate the liquid level are presented in different forms. Dickes et al. [8] and Dickes in his doctoral thesis [9] assumed the liquid receiver emptied of liquid if the condenser outlet is two-phase, and with some level of liquid only if the condenser outlet is the saturated liquid. However, Yousefzadeh and Uzgoren [10] and Ziviani et al. [11] modeled the liquid receiver with some level of saturated liquid if the condenser outlet is two-phase flow, and flooded with saturated liquid if the condenser outlet is saturated liquid. ...
... Finally, the total refrigerant mass of the PHE is calculated using Eq (8). ...
Organic Rankine cycle (ORC) systems are one of the most suitable technologies to produce electricity from low-temperature sources. In this paper, the main components of a non-regenerative, micro-scale ORC unit are modeled using the experimental results. These components are then used as functions in the system-level solver developed in MATLAB© to predict the performances of the system at off-design conditions. The proposed system solver is based on a novel approach, in which no assumptions are made about the system’s state, and only the components’ specifications and the real system boundaries that an operator encounters are adopted as inputs. To this end, the conservation of mass is considered in addition to the conservation of energy in the modeling of the system.
Using the assumption-free model, the performances of the ORC system are mapped in the range of the experimental data considering the pump and the expander speeds as variables. The results show that the optimum system net electric performance is achieved at the pump and the expander speeds of 400 rpm and 900 rpm approximately. However, the pump is prone to the risk of cavitation due to low subcooling at the condenser outlet at this condition. Moreover, zero superheating is calculated at the expander suction that is not recommended for its operation.
Hence, the developed assumption-free, object-oriented, mass-sensitive model has led to the full understanding of the system limitations and losses in the case of waste heat recovery applications. The proposed approach could be extended also to other ORC systems thus mapping their performances at off-design conditions without making artificial assumptions.
... However, introducing concerns about safety and environmental impacts, R245fa and R245ca have been found to be the most suitable working fluids [16]. Furthermore, the working fluid charge and liquid receiver size can be optimised to maximise the overall plant performance [17,18]. ...
... Regarding the expander, the power loss is not evaluated through an equivalent approach, but according to the physical relation which expresses the dry friction generated by the rotor and stator surfaces (18), because the expander power loss is more sensitive to operating condition variations in comparison to the pump. ...
... In (18), f represents the friction factor, N v is the point of dry contact between the rotor and the stator (equal to the number of expander vanes), F N is the normal force applied by the rotor on the stator at the contact point, and r v is the distance between the force application point and rotor centre; (18) is generally applicable to all positive-displacement expanders and it can be specialised to a particular type defining a proper F N which strictly depends on the expander technology and its geometric features. For instance, in the case of the SVRE employed in the experimental analysis, the normal force is given by the centrifugal force applied on the blades, pushing them against the stator inner surface. ...
Waste heat recovery (WHR) of internal combustion engine (ICE) exhaust gases through organic Rankine cycle (ORC)-based power units is one of the most effective technological alternatives to increase ICE efficiency, thereby limiting CO2 emissions. Nevertheless, for an optimum design of components and plants, the assessment of intimate ORC-based plant behaviour is a key factor. This is the case of the mass-based permeability of the plant, which represents its attitude to be crossed by the working fluid, defining a specific relation between the mass flow rate and maximum achieved pressure level. Indeed, it was experimentally found that when the pump and expander are volumetric machines, a univocal relation exists between the operating parameters. Thus, the permeability relation defines certain operating paths of the ORC plant, limiting the domain in which the physical quantities can vary and ensuring the prediction of deviations from the ideal design behaviour. The concept of permeability has not been deeply addressed in the literature despite its importance; thus, it was deepened in this study through experimental and theoretical approaches. In particular, the knowledge acquired through a broad experimental campaign allowed us to obtain a physically based relationship, highlighting the main terms which influence this parameter. These terms are then grouped into dimensionless factors that orient the design and outline an easily implementable model-based control of the maximum plant pressure. Theoretical analyses were conducted through a comprehensive mathematical model of the recovery unit validated by an experimental campaign developed on a fully instrumented ORC test bench fed by the exhaust gases of a 3 L turbocharged diesel engine. The results indicate that the maximum values of the recovered power and efficiency are 3 kW and 4.4%, respectively.
... Santos et al. (2018) developed a modular framework to model a completely predictive simulation tool for the micro-combined-heat-andpower (CHP) ORC [13]. The state-of-the-art charge-sensitive ORC model was developed by Dickes et al. (2018), where the sub-models of each component were integrated in order to obtain the system performance without assuming any of the system state [14]. ...
... Santos et al. (2018) developed a modular framework to model a completely predictive simulation tool for the micro-combined-heat-andpower (CHP) ORC [13]. The state-of-the-art charge-sensitive ORC model was developed by Dickes et al. (2018), where the sub-models of each component were integrated in order to obtain the system performance without assuming any of the system state [14]. ...
... The void fraction is defined as the fraction of the cross-sectional area of the channel that is occupied by the vapor phase, which is noted in the common form of Eq. (29) [14]. ...
Experimental and numerical studies on the low-temperature heat recovery organic Rankine cycle operating in off-design conditions are conducted for the in-depth understanding of the system’s underlying mechanisms in terms of mass conservation. Experimental data sets were obtained from a 1 kW lab-scale organic Rankine cycle test bed using R245fa as the working fluid. The effects of several boundary conditions, including charged mass, are thoroughly examined under low-temperature heat source within the range of 65∼95°C. Numerical models of the heat exchangers are developed by applying the discretization method to predict the captured mass inside the phase-changing components and validated within 5% error range. By the integration of experimental and numerical methods, unprecedented and critical results covering the pressure formation process, mass distribution, and liquid receiver modeling are derived from the analysis which could not be discovered through previous approaches. The unconventional thermodynamic state of the working fluid inside the liquid receiver is revealed in detail and a passive design is suggested for the liquid receiver model. An improved solver architecture is proposed for the complete development of a fully deterministic off-design organic Rankine cycle simulation model, where the reality-based logics obtained from the key findings are projected into the novel model.
... Eventually, Ibarra et al. [27] considered the evaporation pressure and the condensation temperature as inputs to their off-design model of an ORC system and also assumed a maximum cycle temperature as a limit of the system. Hence, the most common assumptions are a superheating degree in the range of 5-10 K and a fixed subcooling degree [28]. ...
... The mass sensitive studies are presented in Table 1-1 that include five steady-state models and one dynamic model, two of which did not consider a liquid receiver in their system. In some studies reported in Table 1-1, the liquid receiver was assumed filled with the liquid if the condenser outlet fluid is subcooled, and it was considered emptied of liquid [28,29] or with some level of liquid if the condenser outlet is two-phase flow [30,31]. ...
... The liquid receiver is filled if the condenser outlet is subcooled or saturated liquid and it is partially filled if the fluid is two-phase. The liquid level and outlet enthalpy were modeled as the following: [28] 2 kWel regenerative ORC, R245fa, scroll expander, diaphragm pump, BPHEs, liquid receiver Steady-state model in MATLAB ® using semiempirical models. Four heat transfer coefficients and five void fraction models were used to find the best combination to calculate the BPHE thermal load and system initial mass charge. ...
Organic Rankine Cycle (ORC) systems are one of the most suitable technologies to produce electricity from low-temperature sources. A comprehensive understanding of these systems is necessary for their further deployment especially in micro scales, in which the intrinsic issues such as low isentropic efficiency and unreliability arise more severely.
In this thesis, the components of a non-regenerative, micro-scale ORC unit are modeled empirically using the experimental data. The components models are used as functions in the system-level solver. The system solver uses a novel approach, in which no assumption is made for the modeling and only the components specifications and the real system boundaries that an operator has during the system run are put as inputs. This assumption-free, object-oriented model follows the two fundamental conservation laws of thermodynamics that are the conservation of energy and the conservation of mass. The developed mass-sensitive model is a robust solver that leads to a full understanding of the system limitations and losses using the empirical models of the main system components.
In addition to the system modeling, the thesis theoretically studies the impact of the expander lubricant oil on the system performance using the empirical lubricant-refrigerant miscibility model and a data reconciliation method. The impact of the expander’s lubricant is found significantly effective on the performances of the heat exchangers, the expander, and the overall system especially in micro-scales, despite its influence has been mostly neglected in the literature so far.
Since the use of micro-scale ORC systems for low-temperature waste heat recovery (WHR) applications is investigated, a novel kind of expander for such systems has been here analyzed using computational fluid dynamics (CFD). The regenerative flow turbine (RFT) is found particularly interesting due to its good and reliable performance in very low-pressure ratios making the machine suitable as an alternative expansion.
Finally, a biomass-fed integrated system is studied in detail. The integrated system consists of a dual-fluidized-bed gasifier, a hot gas conditioning unit, a steam-injected-micro gas turbine, and an ORC as the bottom cycle. This system-level study shows the performance of the integrated system when full conservation of energy and mass is applied. The results show that the overall system efficiency improvement by the ORC is limited to 1-2 % using the wet gas turbine compared to 5-8% without the steam injection. Nevertheless, the produced thermal power in the condenser of the ORC unit shows the ability of the integrated system to meet higher users’ thermal demands in small-scale CHP applications.
... While systems at power capacities in the range from hundreds kW el to a few MW el can be considered at a mature stage [4], the case of smallscale waste heat recovery (WHR) applications is still under development [5,6]. Though great potential is expected once a better performance of the control unit for this system is achieved [7,8], the highly fluctuating nature of the heat source (temperature and mass flow) makes WHR applications a challenging task [9], due mainly to heat exchanger dynamics [10,11]. ...
... The first element of the decision variable U obtained from the quadratic programming method in (10), is the control increment that will be applied to the system in the next sampling time (t +1) using (5). In this work the Hildreth QP algorithm to solve (10) is implemented to ensure numerical robustness and stability of the solution, as it is particularly suitable to recover from ill-conditioned problems [61]. ...
... The proposed control strategy can be applied to real applications, by implementing the following procedure: (1) identify the temperature and mass flow rate range of the waste heat source (i.e., define a minimum and a maximum), (2) use system identification techniques to model the superheating degree dynamics, for the four extreme waste heat source conditions, paying special attention to the case of low temperature and low mass flow rate as nonlinearity is dominant in these conditions, (3) define other intermediate operating points to model the system in order to have enough models in the bank, a minimum of 12 models should be obtained (i.e., three models at high, intermediate and low superheating degree for each pair of extreme waste heat source conditions), (4) implement the MMPC algorithm and perform initial tests at 'high" superheating degree in order to tune the Bayesian K tuning parameter, (5) perform additional test at lower superheating degrees to verify the closed-loop performance. ...
Waste heat recovery systems are today considered as a valuable solution to increase energy efficiency of industrial applications and heavy-duty vehicles, as it uses a thermodynamic organic Rankine cycle system to recover the heat losses to produce electrical or mechanical power. Optimal performance of such machines is often achieved at conditions where complex time-varying nonlinear dynamics are encountered, making the automatic control strategy a fundamental element to maximise the energy efficiency. In this paper the development of a multiple model predictive controller suitable for industrial implementation is presented, and its effectiveness is experimentally validated for the task of maximising output power of a 11kWel small-scale ORC power unit used in a waste heat recovery application. The main advantage of the proposed controller is the possibility to use different model structures to describe local dynamics without increasing complexity of the optimisation problem. Additionally, experimental results illustrate that the entire operating range of the system might be classified in two regions, a quasi-linear and a highly nonlinear region for ‘high’ and ‘low’ superheating degrees respectively. Closed-loops tests lead to the conclusion that a single linear model predictive controller might only be used under suboptimal operation of low power production (on the quasi-linear region for ‘high’ superheating), otherwise leading to poor performance or even instability. Alternatively, the proposed strategy keeps the cycle stable over the entire range of conditions and allows to increase the net electrical energy produced by at least 6%, even under drastic waste heat source variations, when operating closer to the minimum allowed superheating degree.
... They developed two-phase Nusselt number and friction factor correlations based on the values of vapor Reynolds number and liquid Reynolds number. Dickes et al. [16] developed a simulation model of the ORC and investigated the effect of the total charge of the working fluid on the ORC performance. The PHX was used as the evaporator, and R-245fa was used as the working fluid. ...
... For the analytical study, a correlation developed via the modified Wilson plot method is used for the heat source side. The cor- relations developed by Martin [17] and Amalfi et al. [19] are used for the single and two-phase state of the working fluid, considering the results of Dickes et al. [16] . Table 3 presents a summary of the selected correlations. ...
... However, because both correlations were not validated with the frictional pressure drop data by Dickes et al. [16] , the Darcy friction factor is developed for the single-phase working fluid, as indicated by Eqs. For the two-phase friction factor, the correlation developed by Amalfi et al. [19] is modified with experimental data, as follows: ...
In this study, the experimental investigation of a plate heat exchanger was conducted, which was used as an organic Rankine cycle evaporator. The heat exchanger performance was investigated at the low mass flux and moderate evaporation temperature ranges, as it was designed for a cascade heat utilization in a fourth-generation district heating and cooling system. The experiments were conducted under various operating conditions by changing the R-245fa mass flux, evaporation pressure, R-245fa inlet temperature, heat source inlet temperature, and heat source mass flux. Moreover, the heat transfer and pressure drop mechanisms were thoroughly investigated with an internal process analysis. The two-phase heat transfer coefficient exhibited a strong dependency on the heat flux, indicating that flow boiling heat transfer was mainly dominated by the nucleate boiling mechanism. When both the evaporation pressure and R-245fa mass flux were increased, the single-phase heat transfer accounted for a significantly large portion of the total heat transfer, leading to the rapid decrease in the heat transfer rate. The port and elevation pressure drops together accounted for 27–47% of the total pressure drop due to the low R-245fa mass flux range. With the heat source side variation, the two-phase heat transfer coefficient was more affected by the heat source inlet temperature than the heat source mass flux owing to the increase in the excess temperature. Additionally, the overall heat transfer coefficient strongly depended on the R-245fa side heat transfer coefficient, because the R-245fa had lower heat transfer coefficients than the heat source, even in the two-phase region.
... [7,9]), or the proposed validation was incomplete (e.g. [3,6,8]). At best, the model reliability was assessed by comparing its thermodynamic predictions (i.e. ...
... -the temperature of the pipes (by using a heat gun and hot thermal oil coming from the electrical boiler) 3 . ...
... The same conclusion is true for the expander, but for the opposite reasons: the expander volume is much bigger, but it encloses a vapour phase (low density), resulting too in a negligible charge. 3 The impact of the temperature appeared to be significant for conditions exceeding 120°C, i.e. for the evaporator only. For further details on this aspect, please refer to the original manuscript [25]. ...
This paper reports on experiments aiming to understand and to characterize how the working fluid spreads in an organic Rankine cycle (ORC) power system. To this end, a 2-kWe ORC test rig is constructed and tested over a wide range of conditions. Besides standard thermo-hydraulic sensors, the fluid charge distribution is measured on-line by bending load cells and infrared imaging techniques. Following a complete experimental campaign (which includes more than 330 steady-state points and fully characterizes the ORC off-design behaviour), a dual data reconciliation method is applied to the raw measurements to obtain a reference dataset. The results are then analysed to assess the charge distribution mechanisms occurring in the ORC and how it is correlated with the system performance. Among other aspects, this paper demonstrates (i) how the charge inventory is highly correlated to the temperature profiles in the heat exchangers, (ii) how the evaporator dictates the operating conditions of the low-pressure components, and (iii) how the system charge and the liquid receiver size can be selected to maximize the ORC overall performance.
... The hypothetical methods for calculating porosity can be found in previous studies [25][26][27]. Three instructive classical models including the Tandon model, the Prelimi empirical formula model, and the mass-flux-dependent empirical model (Hughmark model) would be introduced hereafter. ...
... Premoli's calculating model is shown below [27]. ...
... Hughmark 's calculating model is shown below [27]. ...
With regard to concerns about an electric vehicle’s driving range extension in a cold climate, an air-conditioning heat pump (ACHP) shows considerable advantage over thermoelectric heaters for battery energy conservation. The effect of refrigerant charge amount for cooling and heating performance of the ACHP system is significant. The optimal charge, realizing the optimal system performance, is usually determined by experiments of cooling and heating performance. In this paper, the optimal charge determination process based on a newly designed ACHP applied in electric vehicles was introduced. Relationships of characteristics with charge in two modes were investigated by experimental and theoretical methods. Firstly, the performance of the ACHP system was respectively investigated at different charge amounts in cooling and heating operating conditions according to key parameters of system cycles. Secondly, the intersection platforms of subcooling and superheat variation curves with refrigerant charge amount were obtained for determining optimal charge amount of the system further by comprehensive analysis. Finally, the theoretical calculation of charge with three instructive and classical void friction correlation models were applied for better comparisons. It was found that charge amount calculated by the Hughmark model proved to be most consistent with the comprehensive experimental results.
... Yet, very limited work can be found on the impact of working fluid charge on off-design operations. In fact, besides the research conducted by the Authors [6][7][8], only two additional studies could be found in the literature. Liu et al. [9] discussed the effect of different working fluid charge masses on the system performance. ...
... Such detailed models are based upon the extensive literature available for vapor compression cooling and heat pumping systems [16,17]. A recent overview of off-design steady-state performance studies and modeling applied to ORC systems can be found in [8]. ...
... To increase the robustness of the model, a multi-stage solver can be employed to run the simulations. Dickes et al. [8] described these ad-hoc solvers in detail. The thermophysical properties of the working fluid can be retrieved either from CoolProp [20] or REFPROP [21]. ...
It is well known that organic Rankine cycle (ORC) power systems often operate in conditions
differing from the nominal design point due to variations of the heat source and heat
sink conditions. Similar to a vapor compression cycle, the system operation (e.g.,
subcooling level, pump cavitation) and performance (e.g., heat exchanger effectiveness)
of an ORC are affected by the working fluid charge. This chapter presents a discussion of
the effects of the charge inventory in ORC systems. In particular, both numerical and
experimental aspects are presented. The importance of properly predicting the total
amount of working fluid charge for optimizing design and off-design conditions is
highlighted. Furthermore, an overview on state-of-the-art modeling approaches is also
presented.
... Unfortunately, the study was just theoretical and no experimental data was reported. Dickes et al. [38] used a 2 kWe unit as the case study and the simulations of various correlations for plate and fan coil heat exchangers were compared with the experimental values. In addition, all previously existing heat transfer correlations are obtained from steadystate experimental measurements. ...
... The applicability of different void fraction models in refrigerant systems has been well studied by comparing the calculation results with the experi-mental measurements. However, the investigations related to ORC systems have rarely been reported in literature [37,38]. ...
... Their investigations were performed at both the component-and the cycle-level. Later on, they [38] developed a steady-state charge-sensitive model, which used the system's boundary conditions rather than any pre-set constraint to predict the off-design performance of a 2kWe recuperative ORC system. ...
Organic Rankine Cycles (ORCs) are an effective way to produce electricity from low-grade heat sources, which cannot be effectively obtained using conventional high-temperature Rankine cycles. Due to the lack of available information regarding the real Organic Rankine Cycle units on industrial level, off-design simulation under diversified operating conditions plays a significant role for both the system performance prediction and control strategy design. This paper summarizes the theoretical basis, modeling approaches and tools for ORC off-design simulations. Firstly, a review was conducted on the individual state-of-the-art convective heat transfer correlations and void fraction models. Secondly, different kinds of modeling approaches and simulation tools were proposed, highlighting their relevant characteristics, and were categorized for their specific applications. Moreover, an in-depth analysis of technical challenges related to various applications and focusing on the model accuracy and complexity, computational efficiency, as well as the model compatibility were extensively described and discussed. Finally, the current research trends in this field and the development for further investigations were presented.
... The mass charge of a system is particularly important since not only it has economic and environmental impacts, but it also affects the superheating and subcooling degrees in the evaporator and the condenser and consequently, the system's overall performance [26]. In the literature, the mass-sensitive modeling of ORC systems was performed in several studies considering the system initial mass charge as the system boundary and the mass distribution in the system as the result [27,28]. ...
... Then, the average cell density is calculated using the cell enthalpy and pressure, and the cell mass is calculated using Eq. (27). ...
Organic Rankine Cycles (ORCs) are recognized as suitable systems to convert the thermal energy from low-grade heat sources to electricity. However, their performance and reliability are subjected to several deficiencies especially at micro-to-small scales. In this work, the impact of the expander lubricant oil on the performance of the heat exchangers and the scroll expander of a non-regenerative, micro-scale ORC unit is investigated. In particular, the oil circulation rate (OCR) is theoretically assessed for each experimental data set based on the thermal balance between the hot and cold streams of the condenser. Then, the performance of the other components is assessed using the lubricant-R134a mixture properties, assuming the same OCR.
Results have shown that the presence of the lubricant oil leads to 5-15% capacity loss of the evaporator and the condenser of the studied ORC system. The calculated mass charge of the evaporator can also be underestimated up to 6.5% approximately if the oil is neglected. In addition, neglecting the lubricant oil may lead to over-estimation of the expander mechanical efficiency and under-estimation of its volumetric efficiency up to about 50% in very low shaft speeds. Hence, despite it is usually neglected in the literature, the results of the present analysis show that the impact of expander oil is relevant in micro-scale ORC systems.
... The model validation showed an excellent accuracy considering the variations of refrigerants and the different conditions. The total refrigerant charge could be further improved by implementing detailed heat transfer coefficients as conducted by Dickes (2018). ...
... Meanwhile, the thermodynamic properties of water were determined using the Xsteam water software (https://xsteam.sourceforge.net/). The isentropic efficiency of the turbines and pumps was 85%, the equipment was operated adiabatically, and changes in the kinetic and potential energy were disregarded [22][23][24]. Thus, the design parameters were as follows: ...
Direct steam generation (DSG) is a promising technology for introducing solar energy into industrial applications, yet it still faces significant challenges. This work analyzes two critical issues associated with DSG: temperature gradients on the receiver tube wall caused by direct and concentrated radiation and flow instability resulting from the phase transition of the working fluid from liquid–vapor to vapor. These phenomena can reduce the mechanical strength of the receiver tube and lead to sudden pressure increases, deformation, or rupture, which hinder the implementation of DSG in solar thermal plants. To address these challenges, the behavior of a receiver tube composed of copper on the inside and an Al2O3 envelope is studied. A 50 MWe hybrid solar thermal plant is proposed for Mulegé, Baja California Sur, Mexico, including a solar field designed to analyze the production of superheated steam during peak solar irradiance hours. The effect of the Cu-Al2O3 ratio on the receiver tube is evaluated, with Al2O3 serving as a thermal regulator to reduce temperature gradients and mitigate flow instability. This combination of materials improves the receiver tube’s performance, ensuring mechanical stability and enhancing the viability of DSG systems. By reducing temperature gradients and flow instability, DSG-based plants can double thermal efficiency and significantly lower environmental impact by eliminating the need for thermal oils, which require frequent replacement. These findings demonstrate the potential for hybrid solar thermal plants to provide sustainable and efficient solutions for industrial energy needs.
... However, because of its characteristics, water is not suitable for all applications, especially when the power of the heat source is low (unlike in coal, gas, or nuclear power stations). In a study by Dickes et al. [36] various case studies for ORC applications were presented, covering different power levels. The study discussed several working fluids, considering their impact on the environment (zero Ozone Depletion Potential [ODP] and lower Global Warming Potential [GWP]), as well as their inflammability and toxicity. ...
The Organic Rankine Cycle (ORC) power cycle is a well-established solution for harnessing heat sources to generate energy. Presently, ORC systems predominantly employ hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs) as working fluids. However, these substances possess significant greenhouse gas effects and are slated for future bans. To address this, it is imperative to establish rational selection criteria and corresponding techniques for evaluating working fluids suitable for industrial ORC applications. This chapter presents the working fluid selection criteria and screening methods for environmentally friendly working fluids. The chapter is organized as follows: (1) The fundamentals of working fluids section provide a broad introduction to the core principles of working fluids; (2) the working fluid section outlines reasonable selection criteria for identifying potential alternatives; (3) the screening of ORC working fluids section discusses possible working fluid candidates, simulation approach, and thermodynamic models in detail, which is very important to access the thermodynamic performance of ORC cycle; and (4) an example of the simulation of an ORC for working fluid selection section demonstrates the strategy for the selection of a working fluid considering a defined ORC architecture.
... The heat exchangers (evaporator, recuperator and condenser) are modelled according to a moving boundary approach [31]. In the most general case, the heat transfer surface is divided into three zones, corresponding to the subcooled liquid, two-phase mixture and superheated vapour [32]. The extent of each zone is not fixed but it depends on the heat transfer area needed by the fluid to complete the process related to each specific zone (i.e., in the case of the evaporator, the processes are the heating of the subcooled liquid, the vaporization, and the superheating). ...
In the effort to enhance the recovery of waste energy, data centers are drawing attention because of the huge amount of heat discharged from the computer racks. Organic Rankine cycle (ORC) power systems are a viable solution to exploit servers’ waste heat, as it is available at very low temperatures. The purpose of this study is to assess the feasibility of integrating a micro-ORC system into data centers cooling systems and its potential energy saving. An experimental analysis is carried out on a kW-scale ORC test bench, with R134a as working fluid. Heat is supplied at temperatures and flow rates in the range respectively 40-55 °C and 1.8-5 m ³ /h, consistently with typical values observed in data centers application, showing the second law efficiency varying between 5% and 13%. Furthermore, a steady-state model of the micro-ORC has been recalibrated and validated against experimental data; the built-in volume ratio of the reciprocating piston expander has been optimized to improve the filling performance of the machine. A parametric analysis, varying the boundary conditions within their range of interest for this application, and the working fluid (R1234yf and R1234ze(E)), shows that a maximum second law efficiency of 30% is achievable with R1234ze(E).
... At present, the application of data mining based on association rules to power systems has achieved some research results [5][6]. Some researchers proposed an improved April algorithm based on association rule mining, analyzed the data characteristics of the three grid states, created a test database, improved the traditional April algorithm, and now has a support number, accept and improve the current support number [7][8]. Some researchers also apply the association rules of data mining technology to the relevant analysis of 2 power transmission and transformation device failures and weather factors, and use the Aprilo algorithm to obtain a large amount of relevant historical data, obtain one-dimensional extreme climate events, and establish multi-dimensional association rules [9][10]. ...
With the continuous development and progress of society, electricity has been integrated into people’s lives. However, power transmission is a very complex process that requires power transmission and power system conversion. A large amount of data will be generated during the operation of the power system. Through these data, we can use electricity better and more efficiently. This paper aims to study the power system data application based on data association rules. Based on the analysis of data association rules related algorithms and the application of data association algorithms in the power system, a power failure prediction system is designed and the performance of the system is analyzed. The test results show that the system has a very high transaction success rate, the longest response time does not exceed 20 seconds, and the CPU is operating normally, reaching the expected expectations.
... The rising interest in small-scale organic Rankine cycle (ORC) plants for waste heat recovery (WHR) purposes, in applications characterized by time variable conditions associated with both the heat source and heat sink, raises the issue of off-design plant operation [1][2][3]. Whereas extensive literature deals with proving ORC thermodynamic feasibility in various WHR scenarios [4][5][6][7], the optimum plant design is relatively unexplored, based on the analysis of the plant hydraulics [8,9], i.e., the performance sensitivity to the (i) actual fluid charge in the plant [10]; (ii) revolution speed of the pump and expander [11,12]; (iii) lower and upper cycle pressures [13]; and (iv) heat exchangers behavior at part load conditions [14]. ...
The applicability of organic Rankine cycle (ORC) technology to waste heat recovery (WHR) is currently experiencing growing interest and accelerated technological development. The utilization of low-to-medium grade thermal energy sources, especially in the presence of heat source intermittency in applications where the thermal source is characterized by highly variable thermodynamic conditions, requires a control strategy for off-design operation to achieve optimal ORC power-unit performance. This paper presents a validated comprehensive model for off-design analysis of an ORC power-unit, with R236fa as the working fluid, a gear pump, and a 1.5 kW sliding vane rotary expander (SVRE) for WHR from the exhaust gases of a light-duty internal combustion engine. Model validation is performed using data from an extensive experimental campaign on both the rotary equipment (pump, expander) and the remainder components of the plant, namely the heat recovery vapor generator (HRVH), condenser, reservoirs, and piping. Based on the validated computational platform, the benefits on the ORC plant net power output and efficiency of either a variable permeability expander or of sliding vane rotary pump optimization are assessed. The novelty introduced by this optimization strategy is that the evaluations are conducted by a numerical model, which reproduces the real features of the ORC plant. This approach ensures an analysis of the whole system both from a plant and cycle point of view, catching some real aspects that are otherwise undetectable. These optimization strategies are considered as a baseline ORC plant that suffers low expander efficiency (30%) and a large parasitic pumping power, with a backwork ratio (BWR) of up to 60%. It is found that the benefits on the expander power arising from a lower permeability combined with a lower energy demand by the pump (20% of BWR) for circulation of the working fluid allows a better recovery performance for the ORC plant with respect to the baseline case. Adopting the optimization strategies, the average efficiency and maximum generated power increase from 1.5% to 3.5% and from 400 to 1100 W, respectively. These performances are in accordance with the plant efficiencies found in the experimental works in the literature, which vary between 1.6% and 6.5% for similar applications. Nonetheless, there is still room for improvement regarding a proper design of rotary machines, which can be redesigned considering the indications resulting from the developed optimization analysis.
... However, unlike the conventional centralized power generation system, microgrids face a challenge producing energy from low-to-medium temperature heat sources scattered around. In an attempt to utilize renewable energy sources and industrial waste heat, an organic Rankine cycle (ORC) has lured much interest and is considered a promising solution [5][6][7][8]. Being the core component of the ORC, scroll expanders are considered suitable for a small-scale power generation system due to its advantages of lightweight, high efficiency, low rotational speed, and two-phase tolerance [9]. On the contrary, velocity type expansion machines, such as radial turbines, are of limited use due to their high cost, low efficiency, and vulnerability of two-phase flow, where the system is in low mass flow rate and high pressure ratio conditions. ...
A new numerical model of a scroll expander is developed applying the deterministic method to analyze its thermal-hydraulic characteristics for the application to the organic Rankine cycle. Several geometrical models are refined as a subsection function of the orbiting angle and area models regarding suction and tangential gas force are newly developed. The geometrical models match well with the actual values extracted from the 3D computer-aided design model. In addition, mathematical models that reflect the effects of mass leakage and heat transfer are integrated in a closed-loop iteration process. Two critical unknown parameters of overall leakage clearance and wall-to-gas temperature ratio are identified via the coupling of the simulation model and experimental data. Experiments were conducted on a 1 kW scale organic Rankine cycle test rig using R245fa as the working fluid. The identified parameters are expressed as functions of the operating conditions and the simulation model is validated within 5% error range. The best isentropic efficiency appeared to be 62.8% according to the test results of the scroll expander. The simulation model allows the analysis of various aspects of the internal expansion process that could not be examined through empirical or semi-empirical models. Mechanisms regarding under and over-expansion and heat transfer are thoroughly investigated for profound understanding of the scroll expander transient behavior and energy level degradation.
... As a matter of fact, the spread of transcritical configurations may be impeded by the challenge connected with the design of the turbine, whose expansion process occurs 40 in the close proximity of the thermodynamic critical point, thereby featuring larger departures from the ideal-gas behaviour than the one found in the subcritical layout. A well-know parameter to quantify the volumetric deviation from the ideal-gas law 45 is the compressibility factor Z = P v/RT , where R is the specific gas constant. Besides, the evolution of the speed of sound along isentropic expansion can be fundamentally different for molecularly complex fluids when approaching the critical point 50 from that observed in ideal flows. ...
In high-temperature transcritical organic Rankine cycles (ORCs) the expansion process may take place in the neighbourhood of the thermodynamic critical point. In this region, many organic fluids feature a value of the fundamental derivative of gas dynamics Γ that is less than unity. As a consequence, severe non-ideal gas-dynamic effects may be possibly observed. Examples of these non-ideal effects are the non-monotonic variation of the Mach number along an isentropic expansion, oblique shocks featuring an increase of the Mach number and a significant dependence of the flow field on the upstream total state. To tackle this latter non-ideal effect, an uncertainty-quantification strategy combined with Reynolds-averaged flow simulations is devised to evaluate the turbine performance in presence of operational uncertainty. Results clearly indicate that a highly non-ideal expansion process leads to an amplification of the operational uncertainty. Specifically, given an uncertainty of ≈ 1% in cycle nominal conditions, the mass flow rate and cascade losses vary ±4% and ±0.75 percentage points, respectively. These variations are four and six times larger than those prompted by an ideal-like expansion process. The flow delivered to the first rotating cascade is severely altered as well, leading to local variations in the rotor incidence angle up to 10°. A decomposition of variance contributions reveals that the uncertainty in the upstream total temperature is mainly responsible for these variations. Finally, the understanding of the physical mechanism behind these changes allows us to generalise the present findings to other organic-fluid flows.
... To this end, each correlation is simply scaled by a constant factor c j as proposed in [8], i.e. ...
Although crucial for simulations, a proper identification of the convective heat transfer coefficients in multi-zone heat exchangers is a challenging task. While well in-strumented thermal systems permit to accurately record the energy balance in such components, the sole knowledge of the global heat transfer rate is not enough to reliably assess these coefficients. In this work, it is proposed to use the zones spatial distribution (i.e. the spatial fraction occupied by the liquid phase, the vapour phase and/or the two-phase regions) as a second identification criteria. An air-cooled condenser into which flows R245fa is considered as case study and a dedicated in-frared imaging method is presented to assess the spatial distribution of its different phases. These new data, combined with standard heat transfer rate measurements, are exploited to identify the best heat transfer correlations pre-selected from the scientific literature. In order to further improve the model predictions, the original heat transfer correlations are ultimately adjusted so as to best fit both the global heat transfer rate and the zones distribution data.
... Regarding advances in simulations, Haervig et al. [21] screened with a genetic optimization algorithm 26 common working fluids for a heat source in the temperature range of 50-280 • C. Critical conditions and the slope of the vapor line were found to be important criteria of optimization parameters for a given heat source temperature. Dickes et al. [22] developed a charge-sensitive model of a 2 kWe recuperative ORC to characterize and optimize the performances of ORC for off-design operation. Regarding the available experimental studies screened thanks to the Landelle et al. [23] database, two working fluids have been principally investigated in the corresponding ORC literature: R-245fa [24,25] and R-123 [26,27]. ...
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This work shows the interest in new low Global Warming Potential replacement fluids for Organic Rankine Cycles and suggests the use of zeotropic mixtures to improve Organic Rankine Cycle systems.
Abstract
The Organic Rankine Cycle (ORC) is widely used in industry to recover low-grade heat. Recently, some research on the ORC has focused on micro power production with new low global warming potential (GWP) replacement working fluids. However, few experimental tests have investigated the real performance level of this system in comparison with the ORC using classical fluids. This study concerns the experimental analysis and comparison of a compact (0.25 m³) Organic Rankine Cycle installation using as working fluids the NovecTM649 pure fluid and a zeotropic mixture composed of 80% NovecTM649 and 20% HFE7000 (mass composition) for low-grade waste heat conversion to produce low power. The purpose of this experimental test bench is to study replacement fluids and characterize them as possible replacement fluid candidates for an existing ORC system. The ORC performance with the pure fluid, which is the media specifically designed for this conversion system, shows good results as a replacement fluid in comparison with the ORC literature. The use of the mixture leads to a 10% increase in the global performance of the installation. Concerning the expansion component, an axial micro-turbine, its performance is only slightly affected by the use of the mixture. These results show that zeotropic mixtures can be used as an adjustment parameter for a given ORC installation and thus allow for the best use of the heat source available to produce electricity.
... The authors concluded that the overall cycle efficiency can be estimated within the maximum relative error of ±20%, while the accuracy of the degree of subcooling was within ±1.5 K. The second work was completed by Dickes et al. [44]. The main contribution of the current work was the demonstration of how state-of-the-art correlations may be used to identify the convective heat transfer coefficients and how the modelling of charge helps in assessing their reliability. ...
... Lecompte et al. [23] also took into account the part load operation of a CHP system using an ORC when optimizing the size of the heat exchanger-staking. Dickens et al. [24] focused on the complexity of heat exchangers modelling, analyzing the influence of heat transfer coefficients and of void fraction on the off-design prediction accuracy of the model for both the heat exchangers and the ORC system. They concluded that a proper estimation of the mass enclosed in the heat exchanger is more important than a slight improvement of the heat transfer predictions. ...
The objective of this work was to simulate the behavior of an Organic Rankine Cycle (ORC) system with two expanders in series at off-design working conditions. The influence of both the intermediate pressure and the volumetric expansion ratio of the expanders on the off-design performance of the ORC was studied and the irreversibilities of the components were analyzed. The performance of the ORC with two expanders for two different designs was also discussed. The thermal efficiency reached using two expanders was higher than the obtained using only one. However, this increase conveyed an increase in the complexity of the design and control of the expanders. As an additional conclusion, it was found that the influence of the intermediate pressure is higher than that of the volume expansion ratio of each expander. The irreversibility of the first expander was mainly due to leaks. However, the performance of the second expander was particularly affected by the difference between the discharged pressure and the condensation pressure. The off-design analysis allowed the definition of a methodology to achieve the desired power with the maximum thermal efficiency, and the identification of the best actuation for the part load operation.
... Developed, sized, built and operated by engineering students of the University of Liège, this solar system aims to offer a practical case study for training the students during their formation. While extensive investigations on the ORC unit have been presented in previous works [6]- [8], this paper presents the first experimental feedback gathered on the Sun2Power solar field. More specifically, this contribution aims to analysis the recorded performance, to identify the main sources of losses, and to investigate the potential means for improving the solar field efficiency. ...
The present work investigates the solar field performance of a pilot system installed at the University of Liège in Belgium. The system includes eight parabolic trough collectors connected in series (66m² in total) and is aimed for small-scale power generation (2kWe) at low-temperature operation (150°C max.). The paper describes the test facility and presents experimental data gathered under various operating conditions. After posttreatment, the measurements are used to calibrate a deterministic model of the solar collectors. This model is used to better estimate the various sources of losses during the solar energy conversion and to investigate potential means of performance improvements. While the collection efficiency recorded experimentally does not exceed 30%, it is shown that an improvement of the tracking could raise this value to 65% and thus meet the design expectations.
Mass-based analysis of thermodynamic cycle can be a link to realize the matching between the fluid properties, components performance, and operation parameters. The present paper proposes a mass-based analysis for ORC systems using zeotropic fluid mixture, focusing on evaporator and condenser as the phase-change heat transfer components. This proves to be an effective way to investigate the impact of composition shift and the initial working fluid charge on system design performance. Results indicated that liquid-phase zones in both heat exchangers account for about 60 % of total mass. Since the largest composition shift occurs in condensation zone, this relation affects the masses and composition shifts in other zones as well as the overall system. Meanwhile, the relationship between the initial charge and composition shift are analyzed. It is found that the increase of total mass leads to a decrease of mass in two-phase zones and an increase of mass in single-phase zones. It therefore causes a reduction of the average composition shift. Moreover, analysis shows the highest net output work of 2.34 kW, which occurs when the initial working fluid charge rises to 130 % of the design value. However, further increases in mass will have a negative impact on system performance.
Charge-based studies, in particular investigations of mass distribution, are still almost absent, although the efficiency of the organic Rankine cycle (ORC) has attracted a great deal of scholarly attention. This paper aims to provide a new perspective on the intrinsic relationship among the mass distribution, phase-zone distribution in the heat exchanger (HEX), charge of working fluid (WF), rotation speed of the pump (RSP), and system performance. A comprehensive ORC simulation model is presented by linking each component’s sub-models, including the independent models for HEX, pump, and expander in an object-oriented fashion. The visualization study of mass distribution of the WF in the system is investigated under different working conditions. Furthermore, the volume and mass of the gas phase, two-phase and liquid phase of WF in the HEX and their variation rules are analyzed in-depth. Finally, the strategies of charge reduction considering HEX areas and pipe sizes are investigated. The results show that the model based on the interior-point method provides high levels of accuracy and robustness. The mass ratio of the WF is concentrated in the liquid receiver, especially in the regenerator, which is 32.9% and 21.9% of the total mass, respectively. Furthermore, 2.4 kg (6.9%) WF in the system gradually migrates to the high-temperature side as the RSP increases while 6.1 kg (17.4%) WF migrates to the low-temperature side, especially to the condenser, as the charge in the system increases. Output power and efficiency both decrease gradually after the peak due to changes in RSP and charge. Last, reducing heat transfer areas of the condenser and regenerator is the most effective way to reduce WF charge.
Hydrocarbons are a promising working fluid for organic Rankine cycles and refrigeration systems that have gained increasing attention in recent decades as a replacement for the currently used higher global warming potential fluids. A novel standalone cascade refrigeration system (CRS) has been developed in this study for simultaneous ultra-low temperature cooling and heating production to medical applications most suitable for rural and power outages. Using the energy utilization factor (EUF) metrics, neopentane was chosen as the working fluid for the organic Rankine cycle (ORC) system among the selected hydrocarbon (HC) refrigerants. The standalone ORC-CRS system performance was investigated in terms of cooling and heating capacity, EUF, exergy, and environmental metrics by varying both ORC and CRS key temperature parameters. The exergy destruction of individual components was evaluated, and it was found that the ORC evaporator has the highest exergy destruction, followed by the expander. The highest EUF and exergy efficiency for a combined ORC-CRS system was about 0.64 and 38%. The maximum cooling and heating capacity of novel standalone CRS system was achieved by 70 kW and 225 kW respectively at a typical operating condition. At a higher ORC evaporation temperature, the exergy efficiency of the ORC-CRS system is decreased, leading to reduction in SI. The standalone ORC-CRS system at ORC evaporator temperature of 150 °C achieved the maximum CO2 emission tax savings of /h for the variable operating condition. Non-dominated sorting genetic algorithm II (NSGA-II) method was used to conduct the multi-objective optimization analysis, and the technique for order of preference by similarity to ideal solution (TOPSIS) method used to found the optimal point, which had the following values of TAC 42.22 $/h and exergy efficiency was around 46.10%.
The purpose of this study is to estimate the electricity production obtainable by coupling an existing kW-size recuperated Organic Rankine Cycle (ORC) prototype with a commercial solar thermal collector to reduce the yearly electricity purchased by a single-family user. A detailed semi-empirical steady-state model, validated against experimental data, is employed for the power plant simulation. The optimal sizes of both the collector surface and the storage tanks were assessed considering that a solar collector surface larger than 32.25 m² would lead the micro-ORC working in off-design conditions; while storage volumes higher than 6000 l become too large to be completely exploited.
Then, different low global warming potential fluids and blends were simulated for comparison with HFC-134a, the reference fluid for low-temperature ORC. Results show that the integrated system working with R134a can cover approximately 39% of the yearly electricity demand, corresponding to more than 1150 kWh. The replacement of R134a with the alternative fluids results in a penalization in the output electric power, related to thermodynamic properties such as density, liquid viscosity, and latent heat. Indeed, with R1234yf barely 16% (466 kWh) of the yearly electricity demand is covered; whilst the blend R513A allows to reach only 17.5% (525 kWh).
Due to the rapid development of artificial intelligence and big data technology, State Grid has entered the era of full intelligence. The amount of data has increased exponentially. How to manage these data is a major challenge encountered in the development of smart national grid. The rapid development of big data analysis and visualization technology has opened up new development opportunities for power grid data analysis and research. This paper aims to study the data visualization technology in power grid planning and government affairs under the association rule algorithm. Based on the analysis of the application of data mining technology in the power system, the association rule algorithm and data visualization technology, it adopts the electricity consumption of an enterprise in S city. The data construction model is used for data mining and analysis to verify the effectiveness of the association rule algorithm in data analysis in power grid planning and government affairs. The experimental results show that through the analysis of association rules, the business efficiency of the power grid is continuously improved, and the best allocation of resources is finally realized.KeywordsData miningAssociation rule algorithmE-planning government affairsData visualization
A fully deterministic simulation model that requires only external boundary conditions as input parameters is newly developed for predicting the off-design performance of an organic Rankine cycle. Accurate prediction of the evaporation and condensation pressures without using any assumptions has been of major concern. In this context, the actual pressure formation characteristics are reflected into the model to identify the high- and low-pressures. The mass balance of the system is realized by applying the proposed passive design of the liquid receiver. The sub-models of each component are integrated into a three-stage solver which iterates under the law of mass and energy conservation. 77 sets of experimental data were collected from a 1 kW scale testbed using R245fa as the working fluid. The developed model is verified by examining the thermodynamic states of the working fluid and the energy balance error of the simulation results is less than 0.3%. The simulation results are compared to the experimental results and are validated within 8% error range. Also, the computational time reduced from 412 h to 93 s after applying the meta-models with only 0.03% difference in thermal efficiency. The effects of various modelling methods are compared to each other which emphasizes the importance of the newly proposed reality-based logics. The simulation model can detect several operational failure scenarios and accurately predict the off-design performance of the system. The developed model is fully predictive without imposing internal assumptions and has the potential to be utilized in various applications without conducting excessive experiments.
An organic Rankine cycle (ORC) system with R123 working fluid has been utilised for generating electricity from low-temperature geothermal resources. The degree of superheated vapour warrants attention to be studied further. This is because the degree of superheated vapour is the last point to absorb heat energy from geothermal heat sources and influence the amount of expansion power produced by the expander. Therefore, achieving high ORC system efficiency requires a parameter of superheated vapour degree. This paper presents an experimental study on a binary cycle, applying R123 as the working fluid, to investigate the effect of variation in superheated vapour degree on the ORC efficiency. Geothermal heat sources were simulated with conduction oil as an external heat source to provide input heat to the ORC system. The temperature high inlet (TH in) evaporator was designed to remain at 120 °C during the experiment, while mass flow rate was adjusted to make superheated vapour variations, namely set at 278, 280, 282, 284, and 286 K. Furthermore, the effect was observed on heat transfer inlet, pinch, heat transfer coefficient, expander work output, isentropic efficiency, expander shaft power, power generation, thermal efficiency, and ORC efficiency. The experimental results showed that the mass flow rate nearly remained unchanged at different degrees of superheated vapour. The ranges of heat transfer inlet, pinch temperature, and heat transfer coefficient were 25.34–27.89 kJ/kg, 9.35–4.08 °C, 200.62–232.54 W/m²·K, respectively. In conclusion, ORC system efficiency can be triggered by various parameters, including the temperature on the exit side of the evaporator. The superheated vapour of R123 working fluid to higher temperatures has caused a decrease in ORC system efficiency due to the decrease in heat transfer inlets, although theoretically, the work total increased. Further investigation has found that the magnitude of the mass flow rate affects the behaviour of the components of the ORC system.
Refrigeration systems can experience operating faults that negatively impact their performance, such as reducing their ability to maintain the temperature setpoint, increasing energy consumption, or reducing the life of the equipment. To better understand the impacts on the physical operating characteristics of a system, a numerical model can be used. However, fully physics-based models require significant computational effort and detailed system information that is typically unavailable; whereas data-driven models require a prohibitively large amount of test data. In this paper a gray-box model is described, using a semi-empirical approach from the literature. It combines into a complete cycle the models for each component: compressor, heat exchangers, pipelines, expansion valve, and liquid line receiver. A set of 35 test data from laboratory experiments on a commercial walk-in freezer was used for training and validating the model. The liquid line receiver was found to provide particular challenges for charge estimation, but the overall model performs quite well and is able to predict the effects of faults on most operating variables with reasonable accuracy.
The Organic Rankine Cycle (ORC) technology is applicable for conversion of low-grade waste heat to electric power. Off-design performance of ORC system is one of the research hotspot. In the present paper, a small-scale ORC experimental bench was constructed with a scroll expander whose nominal output power was 3 kW. Experimental data including thermodynamic parameters at inlet and outlet of the main components from 15 steady-state operation conditions were collected. Special attention has been paid to analyzing the amplitude of data fluctuation. Standard deviation has been introduced to quantify the fluctuation amplitude. The comparison between the accuracy of measuring sensors and the amplitude of data fluctuation was conducted. Moreover, critical parameters affecting the expander output power calculation were investigated. Results indicated that the largest amplitude of the measured pressure and temperature fluctuation were at the pump outlet and evaporator outlet respectively. Besides, the fluctuation amplitude of measured volume flow rate in gas phase was greater than the liquid phase due to strong turbulence of gas phase. Furthermore, the measured volume flow rate of working fluid had a vital effect on the calculated expander output power compared to the measured temperatures and pressures.
Liquid charge ratio is defined as the total liquid volume charged in an organic Rankine cycle (ORC) system divided by the total internal volume of the system, which influences the two-phase (vapor-liquid) phase distribution to dominate the component operation. In this paper, the effect of liquid charge ratio on ORC performance is explored for a ~10 kWe ORC system. It is shown that ORC can operate with liquid charge ratios in the range of 35-50%. At an optimal liquid charge ratio of 42.5%, the system attains the maximum thermal efficiency, power efficiency and net efficiency of 7.74%, 7.02% and 5.62%, respectively. When the system operates below the optimal liquid charge ratio of 42.5%, the liquid flushing may occur to induce pump cavitation and unstable flow due to insufficient liquid suction during pumping process. Alternatively, when the system operates above the optimal liquid charge ratio of 42.5%, more liquid is occupied in the condenser, decreasing the effective heat transfer area to elevate the condensation pressure, thus the system efficiency is worsened. This paper presents the important criterion for optimal liquid charge ratio for ORC operation.
The diaphragm pump is commonly utilized in the small-scale ammonia-water power cycle for pumping the liquid from absorber to evaporator. The electricity consumption and possible leakage of such a pump influence the system efficiency and reliability significantly. In order to find an alternative “pump” with high reliability and low cost, a gravity assisted thermal driven “pump” (GTP), which is consisted of three top-down organized units connecting absorber and evaporator separately, is designed. With the charging and discharging phases, the pressure in each unit fluctuates, and the level of the liquid increases and decreases alternately by the function of gravity. The results of the system show that the net work and thermal efficiency are 10.68 kW and 9.9%, respectively, when the evaporator and absorber are at 140 °C/4000 kPa and 25 °C/800 kPa separately. The optimal net work, thermal efficiency and exergy efficiency are improved by 4.87%, 3.62% and 10.06% respectively compared with the conventional cycle. An application of the GTP power cycle with the capacity of 10 kW driven by the biomass boiler is analyzed, and the results show that the electricity produced by 645 kg biomass pellets can support more than 12 households per day.
The organic Rankine cycle (ORC) is among the most suited technologies to convert low-grade and low-capacity heat sources into useful work. For many reasons - including predictive control, optimal sizing or performance forecast - a proper understanding and characterization of the ORC behaviour under off-design conditions is of significant interest. In order to avoid any intrinsic state assumption, predictive models must account for both fundamental conversation laws of hermodynamics, namely the conservation of energy and the conservation of mass. Besides of modelling the energy transfers, a true off-design model must account for the constant amount of working fluid in the system and simulate its distribution among the different components. Although well-known for HVAC systems, such charge-sensitive considerations are quasi absent for the ORC technology and existing models have never been completely validated. The goal intended by this PhD thesis is to fill this gap. To begin this work, experiments are conducted to assess the fluid distribution and the impact of the charge on a real system operation. To this end, a 2 kWe ORC test rig is constructed and tested over wide range of conditions. Besides of standard thermohydraulic sensors, the fluid charge distribution is measured on-line by bending load cells and infrared imaging techniques. Following a complete experimental campaign (which includes more than 300 steady-state points), a dual reconciliation method is applied on the raw measurements to obtain a reference dataset. An extensive study of the experimental data is then conducted. Among many results, the important contribution of the heat exchangers in the charge inventory is highlighted, so as the impact of oil circulation on the ORC performance rating. Following this experimental study, a complete modelling library is developed to replicate and extrapolate the system off-design behaviour. The intended goal is to create a true performance simulator, i.e. a predictive tool able to estimate the ORC behaviour based solely on its boundary environment (without state assumption, i.e. accounting for the charge distribution in the system). In a first step, a miscibility model of R245fa/POE oil is developed to account for the presence of the lubricant in the ORC operation. Afterwards, the modelling of each system component is conducted, with a particular focus on the heat exchangers and their charge estimation. Then a global ORC model is constructed by coupling the various components sub-models accordingly to a robust resolution scheme. The ORC model predictions are ultimately confronted to the experimental measurements, both in terms of thermodynamics and charge inventory predictions, and a good fit is demonstrated for all the model outputs. Finally, the utility of such a charge- and lubricant-sensitive ORC model is highlighted for different tasks. Considering the experimental test rig as case study, the off-design modelling tool is employed (i) to fully characterize the ORC response under off-design conditions, (ii) to prevent operating conditions where pump cavitation is likely to occur, (iii) to build optimal performance mappings for full- and part-load operations, and, finally, (iv) to optimally select the charge of working fluid and to accordingly size the liquid receiver.
This study focuses on the design phase of ORC systems recovering the heat wasted from two of the sources available on a Heavy-Duty Truck (HDT): the exhaust and recirculated gases. From these heat sources and their combinations, 5 possible architectures are considered. The main components (i.e. the heat exchangers, the pump and the expander) of the WHR systems are investigated and modeled. Plate type heat exchangers are considered for both the hot and cold sides of the system. Regarding the expansion devices, 5 positive displacement machine technologies, the scroll, screw, piston, vane and roots expanders, are considered and modeled while, among the turbo-expanders, the radial-inflow turbine is taken into consideration. A semi-empirical model is proposed to simulate a volumetric pump. The models of components are first confronted with experimental data. The validated models are then used as references for the design of the new components, which is achieved following similitude rules. This ultimately leads to 30 typologies that will be used with 6 of the various investigated working fluids. In order to identify the most promising system(s), a 3-step optimization tool is developed. First, the most suitable conditions are identified for the design of the ORC systems using a simplified model of an expansion machine. In the second step, the design phase, using more detailed models for the expanders and a proposed economic model for the overall system, a thermo-economic optimization is performed. In the third step, the output power for each of the obtained system models is maximized, optimizing the evaporating pressure and the superheating degree for various off-design conditions. The average power weighted using the frequency distribution of the gas operating conditions is computed and used to compare the 180 systems. Finally, because power is not the only criterion to select the most suitable system topology, additional criteria are taken into consideration.
Automotive air‐conditioning heat pump systems are particular interest worldwide in energy conservation and emission reduction for electric vehicles, hybrid electric vehicles, and fuel cell electric vehicles. Refrigerant charge amount is a key factor for the air‐conditioning heat pump system optimization affecting the condensing pressure and subcooling in both heating and cooling modes. In this paper, the influence of the refrigerant charge on system performances was investigated using the experiment method on a secondary loop air‐conditioning heat pump system. The typical heat transfer and flow parameters were recorded, and both cooling and heating performances of the system were investigated and illustrated by pressure‐enthalpy diagrams. The critical refrigerant charges were determined in both heating and cooling modes. Three typical void fraction correlation models were also applied for the refrigerant charge determination modeling as a system off‐design method. Results show that the Hughmark void fraction correlation method has the best prediction of the critical refrigerant charge in both cooling and heating modes.
Organic Rankine cycle (ORC) has been demonstrated to be an effective and promising technology to recover low-grade heat source. Constant turbine efficiency is generally assumed in most of ORC studies, which is not necessarily accurate. To address this issue, a one-dimensional analysis model of radial-inflow turbine is presented. A comparative analysis of the thermodynamic and economic performances of ORC system with constant turbine efficiency and dynamic turbine efficiency is conducted for eight working fluids. The multi-objective grey wolf optimizer (MOGWO) is employed to conduct the multi-objective optimization of the ORC system with constant turbine efficiency and dynamic turbine efficiency. The optimization results are analyzed and compared to investigate the effects of the turbine efficiency on the working fluid selection and system parameters determination. A comparative analysis of the off-design performance of the ORC system with both types of turbine efficiency is also carried out, and the influence of the heat source inlet temperature on the multi-objective optimization results of the ORC system is also studied. The results show that the turbine efficiency increases with the decrement of evaporation temperature or the increment of condensation temperature, and there are significant differences of turbine efficiency among different working fluids. The optimal working fluid and operating parameters are different between the ORC system with constant and dynamic turbine efficiency. Isopentane and pentane are considered to be the optimal working fluids for the constant turbine efficiency ORC system, while only pentane is considered to be the optimal working fluid for the dynamic turbine efficiency ORC system. As the heat source inlet temperature increases, the difference in the optimal evaporation temperature and the net power output between the two ORC systems increases and the error caused by adopting constant turbine efficiency increase accordingly
This paper presents experimental results from a prototype of an 11 kWe organic Rankine cycle (ORC). Special attention has been paid to detecting reliable steady state data points. This has been achieved by application of a novel steady state detection algorithm. This method also allows evaluating the performance of the sensors in relation to remaining steady state fluctuations. The reliability of the experimental data is evaluated by investigating the heat balances over the heat exchangers and through error propagation of the measurement uncertainties. In addition, the experimental data from the setup is used for validating an off-design ORC model. The proposed simulation model solely requires the pump and expander speed as inputs besides the given heat source and sink conditions while subcooling is assumed to be constant. The important dependent model outputs are the evaporation pressure, the condensation pressure and the working fluid mass flow rate. All three predicted outputs show a maximum deviation of less than ±1% from the measured values. The modelled net power output deviates less than ±2% from the measured values. In general, this is a satisfactory result that gives confidence in using these models in future off-design simulations.
A novel sodium hydroxide-water solution absorption refrigeration, heating and organic Rankine cycle power system is proposed for low-temperature heat source utilization. The sodium hydroxide-water solution absorption refrigeration is employed as top cycle which directly absorbs heat from low-temperature heat source. While R218 organic Rankine cycle and heating subsystem are adopted to produce power and heating as designed in bottom cycles. Under the considered condition, model results indicate that refrigeration, heating and electricity efficiency are 0.8244, 0.4019 and 0.03437 with the capacity of 945.2, 460.8 and 39.41 kW, respectively. Energy and exergy efficiency of refrigeration, heating and electricity are also theoretically analyzed with various condensation temperature, evaporation temperature, turbine inlet pressure, split ratio and mass fraction of rich sodium hydroxide solution. Sensitivity of parameters to system performance is also analyzed and results indicate that energy and exergy efficiency are remarkably influenced by operating parameters. A comparison between the proposed system and three independent electrical refrigeration, heating and power systems shows that the proposed system has superior performance and the maximum primary energy saving ratio can reach approximately 0.4889. In conclusion, with the multi-productions of refrigeration, heating and electricity, the proposed polygeneration system provides a more rational and effective energy utilization from a single low temperature heat source at suitable level.
The design of efficient ORC systems involves selection of thermodynamic cycles and working fluids, optimization of operating variables and selection of components for available heat sources. These optimization processes involve determining heat exchanger performance and selection calculations suitable for various cycles and dealing with multiple heat sources.
The current article presents a steady state ORC system simulation tool in which the main emphasis is given to braze plate heat exchanger (BPHE) modeling and selection. A unique 1D pressure-enthalpy based discretized method is developed for the simulation of heat exchanger components. The developed method is generic and can be adopted for simulating single phase, super-critical, evaporation and condensation processes with full or partial phase change. To improve the stability and speed of fluid property retrieval, a bilinear finite grid interpolation method based on property maps obtained from standard libraries such as NIST REFPROP and COOLPROP is developed.
Heat exchangers of any types are fully necessary for sourcing heat energy to, as well as disposing the low temperature waste energy from, the ORC system. Depending on the temperatures and the number of heat sources; the working fluids, choice of thermodynamic cycle and internal cycle, operating conditions can be varied to determine the optimum fit of the system design for the available heat source and sink.
Brazed Plate Heat Exchangers (BPHEs) are used for a wide variety of applications and are very suitable for ORC applications due to their compactness and high efficiency. In this article, we will present a tool which is useful for early phase decision making, such as system optimization based on brainstorming of the concept and choice of thermodynamic cycle, during the design phase involving system design and heat exchanger selections, and for the late post-launch phase where an existing system design might be adopted for use with different heat sources.
The principle strength of the developed freeware tool is that it combines system efficiency calculations for various thermodynamic cycles and comes with a large fluid database as well as a heat exchanger price indicator. The tool communicates with a commercial heat exchanger selection software (SWEP SSPG7), in which detailed calculations at component level can be performed followed by possibility of configuration of heat exchangers, check stock availability, compare price and generate 3D drawings.
Cases studies within low and medium temperature applications, based on different installations of the Viking CraftEngine, are presented in a separate performance verification section. Several variations of the same basic concepts are obtained by varying temperatures, internal recuperation, working fluid and secondary fluids, which all in all shows some of the possibilities which can be explored virtually with good reliability.
While the conventional design of thermal power plants is mainly focused on high process efficiency, market requirements increasingly target operating flexibility due to the continuing shift towards renewables. Dynamic simulation is a cost-efficient tool for improving the flexibility of dispatchable power generation in transient operation such as load changes and start-up procedures. Specific applications include the optimisation of control structures, stress assessment for critical components and plant safety analysis in malfunction cases. This work is a comprehensive review of dynamic simulation, its development and application to various thermal power plants. The required mathematical models and various components for description the basic process, automation and electrical systems of thermal power plants are explained with the support of practical example models. The underlying flow models and their fundamental assumptions are discussed, complemented by an overview of commonly used simulation codes. Relevant studies are summarised and placed in context for different thermal power plant technologies: combined-cycle power, coal-fired power, nuclear power, concentrated solar power, geothermal power, municipal waste incineration and thermal desalination. Particular attention is given to those studies that include measurement validation in order to analyse the influence of model simplifications on simulation results. In conclusion, the study highlights current research efforts and future development potential of dynamic simulation in the field of thermal power generation.
Despite the increasing interest in organic Rankine cycle (ORC) systems and the large number of cycle models proposed in the literature, charge-based ORC models are still almost absent. In this paper, a detailed overall ORC simulation model is presented based on two solution strategies: condenser subcooling and total working fluid charge of the system. The latter allows the subcooling level to be predicted rather than specified as an input. The overall cycle model is composed of independent models for pump, expander, line sets, liquid receiver and heat exchangers. Empirical and semi-empirical models are adopted for the pump and expander, respectively. A generalized steady-state moving boundary method is used to model the heat exchangers. The line sets and liquid receiver are used to better estimate the total charge of the system and pressure drops. Finally, the individual components are connected to form a cycle model in an object-oriented fashion. The solution algorithm includes a preconditioner to guess reasonable values for the evaporating and condensing temperatures and a main cycle solver loop which drives to zero a set of residuals to ensure the convergence of the solution. The model has been developed in the Python programming language. A thorough validation is then carried out against experimental data obtained from two test setups having different nominal size, working fluids and individual components: (i) a regenerative ORC with a 5 kW scroll expander and an oil flooding loop; (ii) a regenerative ORC with a 11 kW single-screw expander. The computer code is made available through open-source dissemination.
When modeling low capacity energy systems, such as a small size (5–150 kWel) organic Rankine cycle unit, the governing dynamics are mainly concentrated in the heat exchangers. As a consequence, the accuracy and simulation speed of the higher level system model mainly depend on the heat exchanger model formulation. In particular, the modeling of thermo-flow systems characterized by evaporation or condensation requires heat exchanger models capable of handling phase transitions. To this aim, the finite volume (FV) and the moving boundary (MB) approaches are the most widely used. The two models are developed and included in the open-source ThermoCycle Modelica library. In this contribution, a comparison between the two approaches is presented. An integrity and accuracy test is designed to evaluate the performance of the FV and MB models during transient conditions. In order to analyze how the two modeling approaches perform when integrated at a system level, two organic Rankine cycle (ORC) system models are built using the FV and the MB evaporator model, and their responses are compared against experimental data collected on an 11 kWel ORC power unit. Additionally, the effect of the void fraction value in the MB evaporator model and of the number of control volumes (CVs) in the FV one is investigated. The results allow drawing general guidelines for the development of heat exchanger dynamic models involving two-phase flows.
A small-scale solar organic Rankine cycle (ORC) is a promising renewable energy-driven power generation technology that can be used in the rural areas of developing countries. A prototype was developed and tested for its performance characteristics under a range of solar source temperatures. The solar ORC system power output was calculated based on the thermal and solar collector efficiency. The maximum solar power output was observed in April. The solar ORC unit power output ranged from 0.4 kW to 1.38 kW during the year. The highest power output was obtained when the expander inlet pressure was 13 bar and the solar source temperature was 120 °C. The area of the collector for the investigation was calculated based on the meteorological conditions of Busan City (South Korea). In the second part, economic and thermoeconomic analyses were carried out to determine the cost of energy per kWh from the solar ORC. The selling price of electricity generation was found to be 0.39/kWh for the prototype and low cost solar ORC, respectively. The sensitivity analysis was carried out in order to find the influencing economic parameters for the change in NPV. Finally, the sustainability index was calculated to assess the sustainable development of the solar ORC system.
Because of the depletion of fossil fuels and global warming issues, the world energy sector is undergoing various changes towards increased sustainability. Among the different technologies being developed, solar energy, and more specifically CSP (Concentrated Solar Power) systems are expected to play a key role to supply centralized loads and off-grid areas in the medium-term. Major performance improvements can be achieved by implementing advanced control strategies accounting for the transient and random nature of the solar heat source. In this context, a lab-scale solar power plant has been designed and is under construction for experimental purposes and dynamic analysis. The test rig includes an Organic Rankine Cycle (ORC) unit, a field of parabolic trough collectors and a thermal energy storage system. This paper presents the results of an experimental campaign conducted on the ORC module alone. This power unit, designed for a 2.8 kW net electrical output, consists of two scroll expanders in series, an air-cooled condenser, a recuperator, a volumetric pump and an oil-heated evaporator. The ORC engine is constructed using standard mass manufactured components from the HVAC industry, this practice reducing considerably the system cost. The overall unit performance and components effectiveness are presented in different operating conditions and relevant empirical correlations are derived to be implemented in a steady state model of the ORC unit.
Under the economic and political pressure due to the depletion of fossil fuels and global warming concerns, it is necessary
to develop more sustainable techniques to provide electrical power. In this context, the present study aims at designing,
building and testing a small-scale organic Rankine cycle (ORC) solar power plant (∼3 kWe) in order to define and optimize
control strategies that could be applied to larger systems. This paper presents a first step of the design of the solar power
plant and focuses more specifically on the ORC engine. This design is defined on the basis of simulation models of the ORC
engine and takes into account some technical limitations such as the allowed operating ranges and the technical maturity of
the components. The final configuration includes a diaphragm pump, two plate heat exchangers for the regenerator and the evaporator,
an air-cooled condenser, two hermetic scroll expanders in series and R245fa as the working fluid. Simulations indicate that
an efficiency close to 12% for the ORC engine can be reached for evaporating and condensing temperatures of 140 and 35°C,
respectively.
[en] This thesis contributes to the knowledge and the characterization of small-scale Organic Rankine Cycles (ORC). It is based on experimental data, thermodynamic models and case studies.\The experimental studies include:\1. A prototype of small-scale waste heat recovery ORC using an open-drive oil-free scroll expander, declined in two successive versions with major improvements.\2. A prototype of hermetic scroll expander tested on vapor test rig designed for that purpose.\The achieved performance are promising, with expander overall isentropic effectivenesses higher than 70% and cycle efficiencies comparable or higher than the typical efficiencies reported in the scientific literature for the considered temperature range.\New steady-state semi-empirical models of each component are developed and validated with the experimental data. The global model of the ORC prototype allows predicting its performance with a good accuracy and can be exploited to simulate possible improvements or alternative cycle configurations.\Dynamic models of the cycle are also developed for the purpose of evaluating the system's reaction to transient conditions. These models are used to define and compare different control strategies.\The issues of cycle optimization and fluid selection are treated using the steady-state semi-empirical models. The thermodynamic optimization of such cycles is first demonstrated by practical examples. Furthermore, three different methods for fluid selection are proposed, investigated and compared. Their respective advantages and fields of application are described.\Finally, two prospective studies of small-scale ORC systems are proposed. The first one is a solar ORC designed for the rural electrification of remote regions in Africa. This prototype aims at competing with the photovoltaic technology, with the advantage of generating hot water as by-product. \The second prospective study deals with the recovery of highly transient heat sources. Advanced regulation strategies are proposed to address the practical issues inherent to such systems. These strategies are compared with the state-of-the-art strategies and show a non-negligible potential of performance improvement.
Over the last few decades, researchers have developed a number of empirical and theoretical models for the correlation and prediction of the thermophysical properties of pure fluids and mixtures treated as pseudo-pure fluids. In this paper, a survey of all the state-of-the-art formulations of thermophysical properties is presented. The most-accurate thermodynamic properties are obtained from multiparameter Helmholtz-energy-explicit-type formulations. For the transport properties, a wider range of methods has been employed, including the extended corresponding states method. All of the thermophysical property correlations described here have been implemented into CoolProp, an open-source thermophysical property library. This library is written in C++, with wrappers available for the majority of programming languages and platforms of technical interest. As of publication, 110 pure and pseudo-pure fluids are included in the library, as well as properties of 40 incompressible fluids and humid air. The source code for the CoolProp library is included as an electronic annex.
New heat conversion technologies need to be developed and improved to take advantage of the necessary increase in the supply of renewable energy. The Organic Rankine Cycle is well suited for these applications, mainly because of its ability to recover low-grade heat and the possibility to be implemented in decentralized lower-capacity power plants. In this paper, an overview of the different ORC applications is presented. A market review is proposed including cost figures for several commercial ORC modules and manufacturers. An in-depth analysis of the technical challenges related to the technology, such as working fluid selection and expansion machine issues is then reported. Technological constraints and optimization methods are extensively described and discussed. Finally, the current trends in research and development for the next generation of Organic Rankine Cycles are presented.
Ten void fraction correlations and four heat flux assumptions are evaluated for their effect on refrigerant charge inventory predictions. Comparisons between mass inventory predictions are made for condensers and evaporators over representative heat pump operating ranges of saturation temperature, mass quality, and mass flux. The choice of void fraction model is found to have a major effect on refrigerant inventory prediction. The maximum variation of predictions ranges from a factor of 10 for low-ambient, heating-mode evaporators to 4.2 for cooling-mode evaporators and 1.7 for high-ambient cooling-mode condensers assuming no subcooling. The correlations of Hughmark, Premoli, Tandon, and Baroczy are found to give the highest predictions and closest agreement to measured total system charge. The choice of heat flux assumption is shown to be insignificant for forced-flow evaporators and of secondary to possibly equal importance to choice of void fraction model for condensers. Implications for charge balancing, off-design and transient performance prediction, and unit reliability are discussed.
A mathematical model describing the transient interactions in one-dimensional two-phase flows with heat transfer is presented. A moving-boundary refrigerant model is used to predict the position of the two-phase/vapor interface. A boundary immobilization technique is used to predict the temperature profile along the heat-exchanger wall. Typical results of an evaporator model, in terms of interface position and discharge superheat, are presented for inlet flow disturbances. The model is then used in an overall heat-pump simulation to predict cyclic performance. The results compare favorably to those obtained with a high-fidelity spatially dependent heat-pump model, but require significantly less computational effort.
An interior-point method for nonlinear programming is presented. It enjoys the flexibility of switching between a line search
method that computes steps by factoring the primal-dual equations and a trust region method that uses a conjugate gradient
iteration. Steps computed by direct factorization are always tried first, but if they are deemed ineffective, a trust region
iteration that guarantees progress toward stationarity is invoked. To demonstrate its effectiveness, the algorithm is implemented
in the Knitro [6,28] software package and is extensively tested on a wide selection of test problems.
Plate heat exchangers are used regularly in the heating, ventilating,
air conditioning, and refrigeration industry. There is an urgent
need for detailed and systematic research regarding heat transfer
and the fluid flow characteristics of these types of exchangers.
As an initiative in this respect, a literature search is presented
on plate heat exchangers. New correlations for evaporation heat transfer
coefficient and friction factor are introduced, which are applicable
to various system pressure conditions and plate chevron angles. The
correlations are based on actual field data collected during several
years of installation and operation of chillers, and they are intended
to serve as design tools and perhaps as a starting point for future
research.
Organic Rankine Cycle system is one of the most widely used technique for low-grade waste heat recovery. Developing of dynamic Organic Rankine Cycle models played an increasingly important part in system performance prediction. The present paper developed a working fluid charge oriented model for an small scale Organic Rankine Cycle to calculate the theoretical value of working fluid charge level for the system under rated condition. The two heat exchangers are divided into three different zones and related heat transfer correlations are employed to estimate the length variation of each zones. Steady state models have been applied to describe the performance of pump and expander. Afterwards, an overall solution algorithm based on the established model has been proposed in order to exact simulate the system’s off-design performance. Additionally, the impact of different working fluid charge volumes has also been discussed. Simulation results clearly shows the variation trend of different zones in both heat exchangers, as well as the variation trend of system operating parameters under various expander output work. Furthermore, the highest thermal efficiency can be reached 6.37% under rated conditions with a working fluid charge volume of 34.6 kg.
Organic Rankine Cycle (ORC) Power Systems: Technologies and Applications provides a systematic and detailed description of organic Rankine cycle technologies and the way they are increasingly of interest for cost-effective sustainable energy generation. Popular applications include cogeneration from biomass and electricity generation from geothermal reservoirs and concentrating solar power installations, as well as waste heat recovery from gas turbines, internal combustion engines and medium- and low-temperature industrial processes. With hundreds of ORC power systems already in operation and the market growing at a fast pace, this is an active and engaging area of scientific research and technical development. The book is structured in three main parts: (i) Introduction to ORC Power Systems, Design and Optimization, (ii) ORC Plant Components, and (iii) Fields of Application. Provides a thorough introduction to ORC power systems Contains detailed chapters on ORC plant components Includes a section focusing on ORC design and optimization Reviews key applications of ORC technologies, including cogeneration from biomass, electricity generation from geothermal reservoirs and concentrating solar power installations, waste heat recovery from gas turbines, internal combustion engines and medium- and low-temperature industrial processes Various chapters are authored by well-known specialists from Academia and ORC manufacturers.
Because of environmental issues and the depletion of fossil fuels, the world energy sector is undergoing many changes toward increased sustainability. Among the many fields of research and development, power generation from low-grade heat sources is gaining interest and the organic Rankine cycle (ORC) is seen as one of the most promising technologies for such applications. In this paper, it is proposed to perform an experimentally-validated comparison of different modelling methods for the off-design simulation of ORC-based power systems. To this end, three types of modelling paradigms (namely a constant-efficiency method, a polynomial-based method and a semi-empirical method) are compared both in terms of their fitting and extrapolation capabilities. Post-processed measurements gathered on two experimental ORC facilities are used as reference for the models calibration and evaluation. The study is first applied at a component level (i.e. each component is analysed individually) and then extended to the characterization of the entire organic Rankine cycle power systems. Benefits and limitations of each modelling method are discussed. The results show that semi-empirical models are the most reliable for simulating the off-design working conditions of ORC systems, while constant-efficiency and polynomial-based models are both demonstrating lack of accuracy and/or robustness.
Organic Rankine cycles (ORC) are used to convert lowgrade heat sources into power. Current research and development investigate small scale and variable heat sources application such as waste heat recovery. Many experimental data on ORC are available. Feed-pump performances achieved are lower than expected and some authors reported cavitation issue. Pump performance has a non-negligible impact over the ORC performance, especially for transcritical cycles. Operations of diaphragm pumps in three different test benches with different fluid and pump size are analyzed. A semi-empirical model of the pump power chain is proposed and validated. Energetic analysis show highlevel of losses in the variable speed drive and electric motor, mainly due to design oversizing. Then a model and analysis of reciprocating pump volumetric efficiency is proposed, taking into account fluid properties. Finally, cavitation limits in different conditions are calculated. Required Net Positive Suction Head (NPSHr) calculated for R134a are found to be in accordance with manufacturer limits for water. Pump vibration sensor could be used for cavitation monitoring. This work gives information for ORC feed-pump simulation, design and operation.
Low temperature geothermal sources with brilliant prospects have attracted more and more people’s attention. Kalina cycle system using ammonia water as working fluid could exploit geothermal energy effectively. In this paper, the quantitative analysis of off-design performance of Kalina cycle for the low temperature geothermal source is conducted. The off-design models including turbine, pump and heat exchangers are established preliminarily. Genetic algorithm is used to maximize the net power output and determine the thermodynamic parameters in the design phase. The sliding pressure control strategy applied widely in existing Rankine cycle power plants is adopted to response to the variations of geothermal source mass flow rate ratio (70%-120%), geothermal source temperature (116 °C-128°C) and heat sink temperature (0 °C-35°C). In the off-design research scopes, the guidance for pump rotational speed adjustment is listed to provide some reference for off-design operation of geothermal power plants. The required adjustment rate of pump rotational speed is more sensitive to per unit geothermal source temperature than per unit heat sink temperature. Influence of the heat sink variation is greater than that of the geothermal source variation on the ranges of net power output and thermal efficiency.
A one-dimensional analysis method has been proposed for the organic Rankine cycle (ORC) system in this paper. The method contains two main parts: a one-dimensional aerodynamic analysis model of the radial-inflow turbine and a performance prediction model of the heat exchanger. Based on the present method, an ORC system for the industrial waste heat recovery is designed and analyzed. The net power output of the ORC system is 534 kW, and the thermal efficiency reaches 13.5%. System performance under off-design conditions is simulated and considered. The results show that the inlet temperatures of the heat source and the cooling water have a significant influence on the system. With the increment of the heat source inlet temperature, the mass flow rate of the working fluid, the net power output and the heat utilization ratio of the ORC system increase. While, the system thermal efficiency decreases with increasing cooling water inlet temperature. In order to maintain the condensation pressure at a moderate value, the heat source inlet temperature considered in this analysis should be kept within the range of 443.15–468.15 K, while the optimal temperature range of the cooling water is between 283.15 K and 303.15 K.
Plate Heat Exchangers (PHEs) are used in a wide variety of applications including heating, ventilation, air-conditioning, and refrigeration. PHEs are characterized by compactness, flexible thermal sizing, close approach temperature, and enhanced heat transfer performance. Due to their desirable characteristics, they are increasingly utilized in two-phase flow applications. Detailed research on heat transfer and fluid flow characteristics in these types of exchangers is required to design and use plate heat exchangers in an optimal manner. This paper reviews the available literature on the correlations for heat transfer and pressure drop calculations for two-phase flow in PHEs as an initial process step in order to understand the current research status. Comparative evaluations for some of the existing correlations are presented in the light of their applicability to different refrigerants. Overall, there is a significant gap in the literature regarding two-phase heat transfer and fluid flow characteristics of these types of exchangers. © 2016 Elsevier Ltd and International Institute of Refrigeration. All rights reserved.
In the second part of this study a sensitivity analysis on the prediction methods is performed to consider the effect of plate geometry on thermal–hydraulic performance and an extensive comparison of all the two-phase pressure drop and flow boiling heat transfer prediction methods available in the open literature are also provided versus the large diversified database presented in Part 1. The experimental databank, from numerous independent research studies, is then utilized to develop the new prediction methods to evaluate local heat transfer coefficients and pressure drops. These new methods were developed from 1903 heat transfer and 1513 frictional pressure drop data points (3416 total), respectively, and were proved to work better over a very wide range of operating conditions, plate designs and fluids (including ammonia). The prediction for flow boiling heat transfer coefficients was broken down into separate macro- and micro-scale methods.
This paper presents the off-design performance analysis of an organic Rankine cycle system in the view of control strategies. Variable inlet guide vanes and evaporating pressure are considered as control variables to adapt the system to the variable geothermal fluid mass flow rate and temperature. The optimal control strategy is studied to maximize the net power under the given geothermal source conditions. The constant pressure operation, the sliding pressure operation and the optimal control strategy are compared in order to analyze their differences. The results indicate that the constant pressure operation with variable inlet guide vanes generates more net power than the sliding pressure operation when the geothermal fluid mass flow rate is relative low. The optimal control strategy is determined by the off-design performance of evaporator and turbine. With fixed geothermal fluid temperature and variable geothermal fluid mass flow rate, the potential increase of the net power under the optimal operation can reach 4.7% and 11.0% for the constant and sliding pressure operation, respectively. When the geothermal fluid temperature decreases, the curve of net power tends to shift to the direction of larger geothermal fluid mass flow rate in all control strategies.
This paper presents a new computational procedure for refrigerant condensation inside herringbone-type Brazed Plate Heat Exchanger (BPHE). A transition point between gravity controlled and forced convection condensation was found for an equivalent Reynolds number around 1600. At low equivalent Reynolds number (<1600) the heat transfer coefficients are not dependent on mass flux and are well predicted by a simple model based on the Nusselt (1916) equation for vertical surface. For higher equivalent Reynolds number (>1600) the heat transfer coefficients depend on mass flux and condensate drainage is controlled by the combined actions of gravity and vapour shear. A new model was developed for predicting the heat transfer coefficients in the forced convection condensation region. This new model was also applied to super-heated vapour condensation by using the equation of Webb (1998) to account for super-heating effects. The new computational procedure was compared against data from the literature: the mean absolute percentage deviation between experimental and calculated heat transfer coefficients was lower than 16%.
The Solar ORC is an approach to off-grid generation that utilizes parabolic concentrating solar collectors and a heat engine - an organic Rankine cycle (ORC) - constructed from mass-manufactured parts from the HVAC and automotive industries. This technology can supply both electricity and thermal products to rural institutions such as primary schools or health clinics, which face levelized costs of electricity (LCOE) of over 0.30/kWh with hot water included (approximately 500 L per day at 50° C) at no extra cost. Various components of the technology, including the solar collectors and prototype ORC engines, were field-tested in Lesotho beginning in 2006 as part of a World Bank supported technology transfer project. A 3 kW precommercial pilot of the system is currently being installed at a health clinic in the Berea district, in partnership with the Government of Lesotho's Appropriate Technology Services (ATS) division. With electricity and hot water from a Solar ORC, the quality of service provided to rural communities can be greatly improved for a fraction of the cost of competing technologies.
In this work, a novel and robust solution approach is presented that can be used to predict the steady-state thermal heat transfer rate for counterflow heat exchangers with any combination of single-phase and two-phase conditions within the heat exchanger. This methodology allows for multiple internal pinching points, as well as all permutations of subcooled liquid, two-phase and superheated vapor sections for the hot and cold fluids. A residual function based on the matching of the required and available thermal conductances in each section is derived, and Brent's method is then used to drive the residual to zero. Examples are presented for the application of this methodology to a water-heated n-Propane evaporator. The computational time required to execute the model for a simple case is on the order of one millisecond when the tabular interpolation methods of CoolProp are applied. Source code for the algorithm is provided in the Python programming language as an appendix.
An analysis of steam-void fraction in two-phase flow is carried out, utilizing the principle that in a steady-state thermodynamic process the rate of entropy production is minimum. The two-phase flow is idealized in the analysis to be a truly steady-state process. The effects of liquid entrainment and wall friction on the void fraction and slip ratio are evaluated. It is found that the slip-ratio in an idealized two-phase flow with zero wall friction and zero entrainment equals (ρf /ρg )1/3 . Data from a number of experiments are found to be bracketed between this result and the result obtained by assuming complete entrainment (slip ratio = 1).
Power generation from low enthalpy geothermal resources using Organic Rankine Cycle systems is markedly influenced by the temperature level of the heat source and heat sink. During plant operation the actual temperature of the geofluid may be different from the value assumed in the design phase. In addition, the seasonal and daily variations of the ambient temperature greatly affect the power output especially when a dry condensation system is used. This paper presents a detailed off-design model of an Organic Rankine Cycle that includes the performance curves of the main plant components. Two capacitive components in the model have the key function of damping the temporary disequilibrium of mass and energy inside the system. Isobutane and R134a are considered as working fluids, mainly operating in subcritical and supercritical cycles, respectively. The off-design model is used to find the optimal operating parameters that maximize the electricity production in response to changes of the ambient temperatures between 0 and 30°C and geofluid temperatures between 130 and 180°C. This optimal operation strategy can be conveniently applied both to already existing plants and in the choice of new design plant configurations.
This paper analyzes the performance of an Organic Rankine Cycle (ORC) system at part load operation. The objective is to understand its behavior from a thermodynamic perspective, identifying which elements are the most critical and which are the best operating points for each level of demanded power. This paper also compares two working fluids: R245fa and Solkatherm ES36 (SES36) for the same cycle specifications. The results have shown that the scroll isentropic efficiency has a great influence on the cycle performance and its thermal efficiency and that SES36 arises as a potential better fluid than R245fa. At the given maximum and minimum temperatures, the best operation point was determined. This allows reaching a maximum efficiency for each demanded level of power; depending on the required amount of power, the expander speed and the working pressure are adjusted.
Performance evaluation of a thermodynamic system under off-design conditions is very important for reliable and cost-effective operation. In this study, an off-design model of an organic Rankine cycle driven by solar energy is established with compound parabolic collector (CPC) to collect the solar radiation and thermal storage unit to achieve the continuous operation of the overall system. The system off-design behavior is examined under the change in environment temperature, as well as thermal oil mass flow rates of vapor generator and CPC. In addition, the off-design performance of the system is analyzed over a whole day and in different months. The results indicate that a decrease in environment temperature, or the increases in thermal oil mass flow rates of vapor generator and CPC could improve the off-design performance. The system obtains the maximum average exergy efficiency in December and the maximum net power output in June or in September. Both the net power output and the average exergy efficiency reach minimum values in August.
Experiments to measure the condensation heat transfer coe�cient and the pressure drop in brazed
plate heat exchangers (BPHEs) were performed with the refrigerants R410A and R22. Brazed
plate heat exchangers with di�erent chevron angles of 45�, 35�, and 20� were used. Varying the
mass
ux, the condensation temperature, and the vapor quality of the refrigerant, we measured the
condensation heat transfer coe�cients and the pressure drops. Both the heat transfer coe�cient and
the pressure drop increased proportionally with the mass
ux and the vapor quality and inversely
with the condensation temperature and the chevron angle. Correlations of the Nusselt number and
the friction factor with the geometric parameters are suggested for the tested BPHEs.
One variable required for modelling the heat transfer and pressure drop of refrigerant inside air conditioner evaporators and condensers is the void fraction. Eight void fraction models were used in a refrigeration system model to evaluate their impact on the estimation of important system variables in an air conditioner as a function of charging. The void fraction models included: (i) homogeneous, (ii) Lockhart and Martinelli, (iii) Thom, (iv) Zivi, (v) Baroczy, (vi) Hughmark, (vii) Premoli and (viii) Tandon. The system variables considered included: power, capacity, refrigerant flow, subcooling and superheat. Comparisons were made with a 10.6 kW capacity air conditioner with capillary tube expansion. Results indicate that the Hughmark void fraction model appeared to provide the best comparison to measured data over the range of charging conditions considered.
Organic Rankine Cycles (ORC’s) are particularly suitable for recovering energy from low-grade heat sources. This paper first presents the results of an experimental study carried out on a prototype of an open-drive oil-free scroll expander integrated into an ORC working with refrigerant HCFC-123. By exploiting the overall expander performance measurements, the eight parameters of a scroll expander semi-empirical model are then identified. The model is able to compute variables of first importance such as the mass flow rate, the delivered shaft power and the discharge temperature, and secondary variables such as the supply heating-up, the exhaust cooling-down, the ambient losses, the internal leakage and the mechanical losses. The maximum deviation between the predictions by the model and the measurements is 2% for the mass flow rate, 5% for the shaft power and 3K for the discharge temperature. The validated model of the expander is finally used to quantify the different losses and to indicate how the design of the expander might be altered to achieve better performances. This analysis pointed out that the internal leakages and, to a lesser extent, the supply pressure drop and the mechanical losses are the main losses affecting the performance of the expander.
Experiments on the evaporative heat transfer and pressure drop in the brazed plate heat exchangers were performed with refrigerants R410A and R22. The plate heat exchangers with different 45°, 35°, and 20° chevron angles are used. Varying the mass flux of refrigerant (13–34 kg/m2s), the evaporating temperature (5, 10 and 15 °C), the vapor quality (0.9–0.15) and heat flux (2.5, 5.5 and 8.5 kW/m2), the evaporation heat transfer coefficients and pressure drops were measured. The heat transfer coefficient increases with increasing vapor quality and decreasing evaporating temperature at a given mass flux in all plate heat exchangers. The pressure drop increases with increasing mass flux and quality and with decreasing evaporating temperature and chevron angle. It is found that the heat transfer coefficients of R410A are larger than those of R22 and the pressure drops of R410A are less than those of R22. The empirical correlations of Nusselt number and friction factor are suggested for the tested PHEs. The deviations between correlations and experimental data are within ±25% for Nusselt number and ±15% for friction factor.
Low-grade heat can be converted to electricity using power plants based on conventional Rankine cycles but with an organic Rankine fluid. Design and construction of such plants have been known for a long time and they are now a commericial reality. Applications include industrial waste heat recovery systems, solar thermal systems, low-temperature geothermal power plants, stand-alone electricity generators like those used for cathodic protection of pipelines, etc. In the past, simulation studies of such systems have usually suffered from the lack of an efficient, reliable and fast algorithm to predict system performance under part-load and off-design conditions. In this study, an efficient algorithm is introduced to simulate ORC Plant performance and the part-load and off-design efficiencies of ORC Plants.
A simple dimensionless correlation for predicting heat-transfer coefficients during film condensation inside pipes is presented. It has been verified by comparison with a wide variety of experimental data. These include fluids water, R-11, R-12, R-22, R-113, methanol, ethanol, benzene, toluene, and trichloroethylene condensing in horizontal, vertical, and inclined pipes of diameters ranging from 7 to 40mm. One data set for condensation inside an annulus has also been analyzed. The range of parameters covered includes reduced pressures from 0.002 to 0.44, saturation temperatures from 21 to 310°C, vapor velocities from 3 to 300m/s, vapor qualities from 0 to 100%, mass flux 39000-758 000kg/m2 h, heat flux from 158 to 1 893000W/m2, all liquid Reynolds numbers from 100 to 63 000, and liquid Prandtl numbers from 1 to 13. The mean deviation for the 474 data points analyzed was found to be 15.4%.
Dynamic models of vapor compression systems are useful tools in developing feedback controllers and are gaining increasing attention in recent years. The dominant dynamics are typically those of the evaporator and condenser, which are difficult to model. Accuracy and execution speed of system models therefore are highly dependent on the modeling approach of the heat exchangers. The two common approaches are the finite-volume (FV) and the moving-boundary (MB) methods. Both have been used successfully and reported in the literature, but there is little discussion presented for either choice. This paper presents the development and comparative study of shell-and-tube heat exchanger dynamic models using both the FV and the MB approaches. Detailed model formulations are provided and stability is demonstrated as components and within a complete centrifugal chiller system model. The system models are validated using data from a 300kW R134a centrifugal chiller test stand. The FV formulation is found to be more robust through start-up and all load-change transients, but executes slower. The moving-boundary method can handle all load-change transients but start-up stability is more sensitive to compressor and expansion valve formulations. The moving-boundary formulation also executes about three times faster than the finite-volume while maintaining nearly identical accuracy. With the homogenous two-phase assumption, charge prediction is seen to be less accurate in the moving-boundary approach.
An organic Rankine cycle (ORC) machine is similar to a conventional steam cycle energy conversion system, but uses an organic fluid such as refrigerants and hydrocarbons instead of water. In recent years, research was intensified on this device as it is being progressively adopted as premier technology to convert low-temperature heat resources into power. Available heat resources are: solar energy, geothermal energy, biomass products, surface seawater, and waste heat from various thermal processes. This paper presents existing applications and analyzes their maturity. Binary geothermal and binary biomass CHP are already mature. Provided the interest to recover waste heat rejected by thermal devices and industrial processes continue to grow, and favorable legislative conditions are adopted, waste heat recovery organic Rankine cycle systems in the near future will experience a rapid growth. Solar modular power plants are being intensely investigated at smaller scale for cogeneration applications in buildings but larger plants are also expected in tropical or Sahel regions with constant and low solar radiation intensity. OTEC power plants operating mainly on offshore installations at very low temperature have been advertised as total resource systems and interest on this technology is growing in large isolated islands.
A comparison of the performance of 68 void fraction correlations based on unbiased data set (2845 data points) covering wide range of parameters than previous assessments was made. A comprehensive literature search was undertaken for the available void fraction correlations and experimental void fraction data. After systematically refining the data, the performance of the correlations in correctly predicting the diverse data sets was evaluated. Comparisons between the correlations were made and appropriate recommendations drawn. The analysis showed that most of the correlations developed are very restricted in terms of handling a wide variety of data sets. Based on the observations made, an improved void fraction correlation which could acceptably handle all data sets regardless of flow patterns and inclination angles was suggested. It was shown that this correlation has the best predictive capability than all the correlations considered in this study.
This paper studies refrigeration cycles in which plate heat exchangers are used as either evaporators or condensers. The performance of the cycle is studied by means of a method introduced in previous papers which consists of assessing the goodness of a calculation method by looking at representative variables such as the evaporation or the condensation temperature depending on the case evaluated. This procedure is also used to compare several heat transfer coefficients in the refrigerant side. As in previous works the models of all the cycle components are considered together with the heat exchanger models in such a way that the system of equations they provide is solved by means of a Newton–Raphson algorithm. Calculated and measured values of the evaporation and the condensation temperatures are also compared. The experimental results correspond to the same air-to-water heat pump studied in other papers and they have been obtained by using refrigerants R-22 and R-290.
Manufacturers of plate and frame heat exchangers nowadays mainly offer plates with chevron (or herringbone) corrugation patterns. The inclination angleof the crests and furrows of that sinusoidal pattern relative to the main flow direction has been shown to be the most important design parameter with respect to fluid friction and heat transfer. Two kinds of flow may exist in the gap between two plates (pressed together with the chevron pattern of the second plate turned into the opposite direction): the crossing flow of small substreams following the furrows of the first and the second plate, respectively, over the whole width of the corrugation pattern, dominating at lower inclination angles (lower pressure drop); and the wavy longitudinal flow between two vertical rows of contact points, prevailing at highangles (high pressure drop). The combined effects of the longer flow paths along the furrows, the crossing of the substreams, flow reversal at the edges of the chevron pattern, and the competition between crossing and longitudinal flow are taken into account to derive a relatively simple but physically reasonable equation for the friction factor ξ as a function of the angleand the Reynolds number Re. Heat-transfer coefficients are then obtained from a theoretical equation for developing thermal boundary layers in fully developed laminar or turbulent channel flow — the generalized Lévêque equation — predicting heat-transfer coefficients as being proportional to (ξ·Re2)1/3. It is shown, by comparison, that this prediction is in good agreement with experimental observations quoted in the literature.
A correlation for fin-and-tube heat exchanger having plain fin geometry is proposed in this study. A total of 74 samples were used to develop the correlation. For practical considerations, the proposed heat transfer correlation had absorbed the contact conductance in the development of correlation. The proposed heat transfer correlation can describe 88.6% of the database within ±15%, while the proposed friction correlation can correlate 85.1% of the database within ±15%. The mean deviation of the heat transfer correlation is 7.51%, while that for the proposed friction correlation is 8.31%.
This paper presents the performance analysis of a power plant with the Organic Rankine Cycle (ORC). The power plant is supplied by thermal energy utilized from a solar energy collector. R245fa was the working fluid in the thermodynamic cycle. The organic cycle with heat regeneration was built and tested experimentally. The ORC with a heat regenerator obtained the maximum thermodynamic efficiency of approximately 9%. IMPLICATIONS The ORC engine can cooperate with any low-temperature waste heat source. Hence, the waste heat can be used for electricity or mechanical work production.
This thesis develops techniques for automated detection, diagnostics, and evaluation of faults in vapor compression equipment. Fault evaluation was added to the more common steps of fault detection and diagnostics to consider the special aspects of performance degradation faults over abrupt faults. A model for testing these techniques in a simulation environment was developed. The model is described and experimental validation results are presented in this thesis. The model has a modular design that enables easy adaptation to different equipment configurations. It can be easily tuned with a few simple measurements and can simulate faults in the cycle. The fault detection technique described in this thesis evaluates the impact of measurement errors on the confidence that current measurements are different from the predictions of a normal performance model. If the statistical confidence exceeds a predetermined threshold, then a fault is indicated. Diagnostics are performed by statistically evaluating a generic set of rules indicating the direction change of each measurement. This diagnostic technique does not require a learning phase for each piece of equipment, is capable of detecting a 5% refrigerant leak, and can distinguish between refrigerant leaks, condenser fouling, evaporator fouling, liquid line restrictions, and compressor valve leakage. Four fault impact evaluation criteria were developed to determine if the fault is severe enough to justify the service cost. These criteria are: comfort, economics, safety, and environmental hazard. Evaluating these criteria evolved into a constrained optimization problem to minimize lifetime service and energy costs while maintaining the other criteria as constraints. This problem was solved exactly using dynamic programming to create a minimum cost baseline for comparison with a simplified near-optimal scheduler, regular interval maintenance, and comfort constrained only maintenance. It was found that optimal service scheduling reduced lifetime operating costs by as much as a factor of two over regular service intervals and 50% when compared to constrained only service. The near-optimal algorithm gave operating costs that were within 1% of the optimal results.