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Experimental assessment of the fluid charge distribution in an organic Rankine cycle (ORC) power system
Abstract
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.
... Due to an increasing interest in different applications of these systems, they have been extensively studied in the last decade. This is confirmed by a number of valuable works which examined the effectiveness of ORCs by means of theoretical analyses [3], experimental investigations [4] or numerical simulations [5]. The undoubted advantage of the ORC is the wide choice of possible working fluids, among which the most popular are refrigerants [6], hydrocarbons [7], siloxanes [8] and zeotropic mixtures [9]. ...
... For this application case (Scenario 1) the geothermal water is used as a heat carrier, while the cooling water is utilized as a heat sink of the ORC. In the individual components of the system, the working fluid of the ORC undergoes the following thermodynamic processes: preheating (7)(8), evaporation (8-9) and superheating (9-1) in the vapor generator, expansion (1)(2)(3)(4)(5) in the stage components of the turbine, cooling (5-5 ′′ ) and condensation (5 ′′ -6) in the condenser and pressurizing (6-7) in the pump. By rejecting the heat to the working fluid in the vapor generator, the geothermal water is ...
... As seen, when considering indicators related to size of the heat exchanger, i.e. the total heat transfer area A tot and overall conductance (UA) tot , their sensitivity with respect to n s is very small. Meanwhile, the rotor diameter D 4 and SSI index are affected noticeably (see Fig. 6 and Fig. 7). Despite the fact that the size parameter SP is related to size of the turbine, its sensitivity is very small, similarly as in the case of A tot and (UA) tot . ...
The increasing interest in mobile applications and modular structures of an organic Rankine cycle (ORC) has resulted in growing importance of the optimal sizing of the ORC installation. Most of the existing works were focused on reducing dimensions of the ORC by applying indicators estimating size of a single type of the ORC component. This study aims to define an indicator which comprehensively evaluates the size of the ORC power plant. The proposed indicator, specified as System Size Index SSI, is formulated based on two key ORC design parameters, namely the total heat transfer area of the heat exchangers and the rotor diameter of the turbine. To examine the versatility of SSI, the analysis was conducted considering different ORC applications scenarios, including modular geothermal power plants and mobile waste heat recovery units. The findings of single-objective optimization procedures have shown that by applying indicators evaluating size of the heat exchangers, the turbine rotor diameter may deviate from its smallest possible dimension by up to 70%. Similarly, by utilizing indices evaluating size of the turbine, the heat exchangers area may differ from its optimal size by up to 40%. Meanwhile, by using the SSI indicator, both the total heat transfer area and turbine rotor diameter did not differ from their smallest possible dimensions by more than 1% for each scenario considered. Finally, based on a bi-objective analysis with SSI and net power output as the criteria, it was concluded that by applying such indicators as APR parameter or performance factor PF, the designer is left with a single design solution, while the adoption of the bi-objective approach gives much more flexibility in finding a compromise between the system size and power output of the installation.
... The liquid level variation in the working fluid tank was first observed by Cao et al. (2019), who measured the liquid height using a magnetic float in the outer tube connected to the tank [8]. Dickes et al. (2020) measured the weight of the main components in realtime by implementing the on-line measurement method for the monitoring of the mass distribution, unlike the previous studies that only measured the total charged mass [9]. ...
... The liquid level variation in the working fluid tank was first observed by Cao et al. (2019), who measured the liquid height using a magnetic float in the outer tube connected to the tank [8]. Dickes et al. (2020) measured the weight of the main components in realtime by implementing the on-line measurement method for the monitoring of the mass distribution, unlike the previous studies that only measured the total charged mass [9]. ...
... Thus, the ambient and dissipation heat transfer rate is estimated by curve fitting the customized equations described in Eqs. (8) and (9) to the experimental enthalpy change rate. _ Q pp;amb ¼ 3:47 Â 10 À4 N 0:2837 ...
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.
... They achieved an optimal system thermal efficiency of 7.74 %. Dickes et al. [28] performed an experimental analysis on a 2 kWe ORC system based on working fluid charge quantity and liquid receiver size. The results showed that increasing system charge quantity below the optimal charge did not affect system performance. ...
... From Figs. 9-11, the system performance is not affected to at least − 30 % system charge. In the case of [27,28], these studies recommend a liquid to volume system charge ratio of 32.5 % for ORC systems. But these studies utilized R245fa as the working fluid, which is liquid at atmospheric temperature and pressure. ...
Cryogenic carbon-neutral fuels are potential alternatives as future marine fuels, releasing waste cryogenic energy during regasification and waste thermal energy during combustion. Organic Rankine Cycles (ORCs), using flammable hydrocarbon working fluids, are the preferred waste energy reutilization technology, prioritized over Brayton and Kaline cycles due to their compact system configuration. However, hydrocarbon flammability and explosiveness poses a huge safety risk. Therein lies the novelty of this study which presents an advanced dynamic model of a cryogenic enhanced ORC utilizing low flammability hydrofluorocarbons as working fluids for simultaneous reutilization of waste thermal and cryogenic energy from carbon-neutral cryogenic fuels. The evaporation temperature exhibits a direct correlation with energy and an inverse correlation with the exergy performance. System overcharging leads to a drastic performance decline, while undercharging can be tolerated to a certain liquid-to-volume ratio until critical failure. Marine classification societies' recommendations-based scenarios were employed to gauge the emission reduction potential of low flammability working fluids for cryogenic ORCs, pitted against traditional combustion technologies. A maximum specific network , thermal efficiency, exergy efficiency, and cryogenic energy efficiency of 45.64 kJ/kg, 10.43 %, 12.75 %, and 11.8 % was achieved, respectively, with 85 % reduction in GHG emissions, using R452B as the working fluid.
... The investigation of phase change heat exchangers is a bigger challenge. Dickes et al. [3] studied the fluid distribution in ORC heat exchangers by means of an infra-red camera and gravimetric methods. The current work extends this research to a reversible apparatus used in ORC mode as well as in HP mode. ...
... Different temperature and pressure levels in HP and ORC mode affect the density of the working fluid and thus its total mass in the fixed volume of piping and instrumentation changes. While the global discrepancies can be countered with a collector vessel (described in [3][4][5]), there are still local differences in the apparatuses' filling. The following procedure shows an analytical method of determining the filling of the heat exchanger, which operates as an HP-condenser and ORC-evaporator. ...
The combined heat pump–organic Rankine cycle is a thermal–electrical storage concept which allows the reversible use of components in both operation modes (loading and unloading the storage). This saves in terms of investment costs but also creates challenges during design and operation. A heat exchanger is an expensive component destined to be used for the reversible purposes of a heat pump condenser and an organic Rankine cycle evaporator. In this study, the operation of such an apparatus was evaluated based on an analytical model, experimental data and thermal imaging. This study shows that the model can predict the filling of the apparatus distinguished by liquid, vapour and the two-phase region. The thermal imaging supports the model and gives the location of the regions. Connecting both methods, a valid statement about the current condition of the heat exchanger is possible. Due to very small pinch points, the apparatus is not efficiently used in the investigated modes. Extending the pinch to 2 K can already save up to 46.1% of the heat exchange area. The quality of the heat transfer in the evaporator (q˙ORC = 10.9 kW/m2) is clearly higher than in the condenser (q˙HP = 6.1 kW/m2).
... The mass of PHEs depends on their thermal performance and the length of the heat transfer zones. Instead, if the liquid receiver is flooded due to the high initial system charge, the remaining refrigerant is located in the PHEs and the liquid receiver dictates the mass of the PHEs and subsequently affects their thermal performance [13]. ...
... respectively [13]. The total mass charge of an ORC system is calculated as the sum of the mass of the working fluid in all the different components of the system. ...
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.
... Therefore, today, active research on Organic Rankine Cycle (ORC) system using stack waste heat energy in addition to fuel cell systems is being done [14]. Dickes' research team experimentally examined the temperature distribution of the working fluid for power Energies 2020, 13, 6163 3 of 18 generation in the evaporator heat exchanger in the ORC power system, and Jang's research team conducted a study on the performance of the compact ORC system at the 1 kW-level using a heat source in the range of 100 to 140 • C [15,16]. In addition, Jeong's research team conducted a study on the heat exchange performance and characteristics of the plate heat exchanger for each working fluid operating condition applied to the ORC system [17]. ...
... The values of the regressions used are shown in Table 1. It was assumed that all cell unit performances of the HT-PEMFC were the same, and the electric power (W FC ) and thermal power (Q FC ) of HT-PEMFC were calculated in proportion to the number of cells (N cell ) and single cell active area (A cell ) as in Equations (14) and (15). Moreover, the power efficiency (η FC ) of HT-PEMFC can be obtained as in Equation (16) based on the lower heating value (LHV) of hydrogen. ...
Recently, the need for energy-saving and eco-friendly energy systems is increasing as problems such as rapid climate change and air pollution are getting more serious. While research on a power generation system using hydrogen energy-based fuel cells, which rarely generates harmful substances unlike fossil fuels, is being done, a power generation system that combines fuel cells and Organic Rankine Cycle (ORC) is being recognized. In the case of High Temperature Proton Exchange Membrane Fuel Cell (HT-PEMFC) with an operating temperature of approximately 150 to 200 °C, the importance of a thermal management system increases. It also produces the waste heat energy at a relatively high temperature, so it can be used as a heat source for ORC system. In order to achieve this outcome, waste heat must be used on a limited scale within a certain range of the temperature of the stack coolant. Therefore, it is necessary to utilize the waste heat of ORC system reflecting the stack thermal management and to establish and predict an appropriate operating range. By constructing an analytical model of a combined power generation system of HT-PEMFC and ORC systems, this study compares the stack load and power generation performance and efficiency of the system by operating temperature. In the integrated lumped thermal capacity model, the effects of stack operating temperature and current density, which are important factors affecting the performance change of HT-PEMFC and ORC combined cycle power generation, were compared according to operating conditions. In the comparison of the change in power and waste heat generation of the HT-PEMFC stack, it was shown that the rate of change in power and waste heat generation by the stack operating temperature was clearly changed according to the current density. In the case of the ORC system, changes in the thermal efficiency of the ORC system according to the operating temperature of the stack and the environmental temperature (cooling temperature) of the object to which this system is applied were characteristic. This study is expected to contribute to the establishment of an optimal operation strategy and efficient system configuration according to the subjects of the HT-PEMFC and ORC combined power generation system in the future.
... A challenging aspect in the design of reversible HP-ORC systems is an adequate fluid management in order to supply both processes with the required amount of working fluid at all operating conditions. Excess fluid typically accumulates in the condenser, increases the condensation pressure and reduces the power output in ORCs while too low fluid charges lead to pump cavitation (Dickes et al., 2020). A recent experimental study varied the fluid charge of a 1 kWel reversible HP-ORC test rig and concluded that the fluid charge has a significant impact on overall efficiency, reversible machine performance, system stability and obtainable operation points (Tassenoy et al., 2022). ...
The reversible operation of the heat pump process and the organic Rankine cycle as a Carnot battery has gained attraction of the scientific community in recent years. Despite numerous publications investigating the potential of such systems as a new energy storage technology, experimental results are scarce. The present work presents the outcomes on process level of a first experimental characterization of a reversible heat pump – organic Rankine cycle (HP-ORC) pilot plant designed for a maximum charging capacity of 15 kWel in heat pump mode and discharging capacity of 9 kWel in ORC mode. The experimental campaign comprised 150 stationary operating points. The plant yielded a maximum COP of 9.5 and a maximum ORC net efficiency of 4.4% for the defined boundary conditions of this experimental campaign. While the heat pump favors low temperature lifts for a high COP, high temperature gradients are favorable for ORC operation. Moreover, limitations on component and system level were determined. The challenging aspect of a proper fluid management of such reversible systems was identified as a crucial factor. Finally, the impact of fluid charge on ORC operation was experimentally evaluated and revealed significant optimization potential by means of an adequate fluid management.
... According to Imran et al. [2] dynamic system models suffer from different numerical problems that lead to long simulation times or to simulation failures that might make the model inappropriate for some operating ranges. For example, semi-empirical charge-sensitive models are considered to investigate the off-design operation characteristics of ORCs within the range for power outputs up to 2 kW [3], [4] and [5]. For such approaches both the experimental and the simulation effort are large. ...
For teaching and research issues, a micro-scale low-temperature organic Rankine cycle (MORC) has been investigated by using the method of Design of Experiments (DoE). A central composite plan was constructed, and multiple quadratic regression analysis has been applied to derive an approximation of the MORC’s thermal efficiency based on measured data. The application of the statistical approach allows for the empirical determination of the accuracy. The regression model predicts a maximum efficiency of 1.88 %. In terms of quality of fit, the model has a coefficient of determination of 92 %. The largest discrepancies between model and measured data are being noted for lowest efficiencies. Nevertheless, a good fit can be observed for efficiencies across a large range of operating parameters, i.e. the method provides an efficient and accurate search for the best operating point with low experimental effort. Both experimental setup and the applied method have good potential to convey basic concepts of energy systems in education at moderate costs in terms of hardware, maintenance and testing time.
... The investigation of the influence of the working fluid charge on the performance of small-and micro-scale ORC systems is also a topic that has gathered substantial interest, including pure-fluid and zeotropic ORCs [154][155][156]. In the case of the latter, the working fluid charge affects the working fluid composition in the different system components and thus the overall performance. ...
... Similar parametric studies were presented by Gao et al. [2], Kosmadakis et al. [3], and Jang and Lee [4]. Dickes et al. [5] carried out extensive experiments including 330 steady-state points to assess the R245fa distribution in an ORC unit. Li et al. [6,7] investigated the performance of a small-scale ORC with R245fa at different heat source temperatures and pump speeds. ...
In this paper, a performance prediction model of the cryogenic ORC was presented based on the back propagation neural network optimized by the genetic algorithm (BPNN-GA). Firstly, an experimental setup was established to obtain the database for BPNN-GA model training and test. Then, the expander output power, working fluid mass flow rate, and the cold energy efficiency were selected as the BPNN-GA model outputs and the model structure was determined as 9-12-3. Finally, the accuracy of the BPNN-GA model was verified, and the parametric study was further conducted. The mean absolute relative errors (MARE) are 1.1876%, 0.9037%, and 2.6464%, the root mean square errors (RMSE) are 5.3789 W, 1.0260 kgh⁻¹, and 0.3151%, and the correlation coefficients (R) are 0.9974, 0.9977, and 0.9665 for the expansion work, the working fluid mass flow rate, and the cold energy efficiency, respectively. The BPNN-GA is proved as a promising methodology, which could provide direct guidance for the determination of relevant parameters in experimental analysis and control strategy optimization.
... The relative thermal capacity losses of the evaporator and the condenser are presented in Figure 5 The presence of the lubricant also affects the calculations of the system mass charge due to the changes in the calculated mass density. 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 [113]. 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 [28,31]. ...
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.
... The presence of the lubricant also affects the calculations of the system mass charge due to the changes in the calculated mass density. 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]. ...
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.
... Consequently, the dependence between the volumetric efficiency, leakages, and rotational speed is worth investigating, particularly in small-scale ORC units for WHR from unsteady thermal sources. It has been already observed [9,31,32] that real performance can be quite different from the idealized one evaluated by means of thermodynamic analysis without considering typical real plant features such as hydraulic permeability and the effect of expander leakages on the whole performance. From it, in fact, having fixed a specific cycle design for optimal operating conditions, the main relevant variables can be calculated in order to fulfill the desired goal: maximum efficiency, maximum thermal energy recovered, and maximum specific work, after which the optimum pressure or superheating degree can be easily determined. ...
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.
Power-to-Gas technologies are high-capacity systems, which can be used to optimise the use of renewable energy to meet energy demands. Among them, Power-to-Methane (PtM) stands out due to the ease of storage and use of the produced methane. However, its total storing efficiency (Power-to-Methane-to-Power) is likely still low as there are various losses. This paper presents an overview of PtM improvement through the utilisation of low temperature waste heat produced upon the two steps by converting power to gas, namely electrolysis and methanation. An organic Rankine cycle, Stirling engine, and thermoelectric generator were chosen as promising technologies that could be implemented to recover the waste heat and improve the PtM system. Several experimental studies of each technology were reviewed. Moreover, some technological and economic surveys are described and discussed. At the end of the article, the study discussed the direction of further research and development for waste heat recovery in the system to increase the competitiveness of the PtM system and its efficiency.
Organic Rankine cycle (ORC) plays an important role in addressing the issue of energy shortage and climate change. The condenser should release heat to the environment. Due to the seasonal and daily variations of ambient conditions, the condensation temperature, power consumption of the cooling system, the operation conditions of the working medium pump and expander would suffer drastic changes in different condensing ambient conditions. Therefore, the main aim of this study is to investigate the cross-seasonal performance of the organic Rankine cycle under variable condensation conditions. Simultaneously, the single screw expander with quasi two-stage expansion was used to improve the performance of ORC in different condensing ambient conditions. A model was developed to depict the variations of operation parameters of ORC system. The results show that the performance of ORC was adversely correlated with the wet-bulb temperature throughout the year. The net power and net efficiency can be reached at 8.04 kW and 11.31 % when the wet-bulb temperature is 1.1 ℃, respectively. Additionally, the variation in cooling water flow rate has little effect on the volumetric expansion process of the new single screw expander, but has a significant influence on the velocity expansion process, which increases by nearly 56.4 %. Meanwhile, decreasing the pipeline impedance coefficients of cooling system can optimize ORC system performance effectively. It was also discovered that the isentropic efficiency of single screw expander decreases by only 8.68 percentage points during the expansion ratio increase from 3.31 to 11.54.
Die Carnot-Batterie ist eine Energiespeichertechnologie, die elektrische Energie in Form von Wärme speichert. Die Kombination aus Wärmepumpenkreislauf (HPC), Organic Rankine Cycle (ORC) und einem sensiblen Wärmespeicher ist eine Variante dieser Technologie, die insbesondere für kleinere bis mittlere Skalen attraktiv ist. Zur Beladung des Speichers erhöht der linksläufige Wärmepumpen-Kreisprozess unter Verwendung von Abwärme und elektrischer Energie die Temperatur des Speichermediums. Der rechtsläufige ORC-Kreisprozess entlädt den Speicher bei Bedarf, indem thermische wieder in elektrische Energie gewandelt wird. Die beiden Kreisprozesse besitzen eine deutliche Übereinstimmung hinsichtlich der verwendeten Komponenten. Dies ermöglicht theoretisch eine Kombination der Prozesse in einer einzigen Anlage. Durch die reversible Verwendung von Komponenten können die Investitionskosten der Anlage gesenkt werden, es treten aber gleichzeitig gewisse Herausforderungen bei der Auswahl der Komponenten und dem Betrieb der Anlage auf. Um den Planungsprozess zu studieren und erste praktische Betriebserfahrungen zu sammeln, wird im Rahmen dieser Arbeit eine solche reversible HPC-ORC-Pilotanlage aufgebaut. Ziel dieser Forschung ist es, herauszufinden, inwieweit sich die beiden Prozesse als Carnot-Batterie kombinieren lassen und was dabei zu beachten ist. Im vorliegenden Werk werden die wichtigsten Vorüberlegungen zur Planung vorgestellt: Die Kriterien für die Auswahl des Arbeitsfluids, eine Methode zur Festlegung der Betriebsbedingungen sowie eine Diskussion zur Auswahl der Hauptkomponenten. Die Anlage wird mit einer ölgeschmierten reversiblen Schraubenmaschine und Plattenwärmeübertragern als reversible Kondensatoren und Verdampfer ausgerüstet. Zahlreiche Messreihen liefern erste Erfahrungen mit dem Betrieb einer solchen Anlage. Im Hinblick auf das Gesamtsystem erweist sich der Unterschied der Arbeitsfluidmasse in den verschiedenen Betriebszuständen als ein relevanter Punkt. Darüber hinaus werden die wichtigsten reversiblen Komponenten (Schraubenmaschine und Wärmeübertrager) genauer untersucht: Ein analytisches Modell wird mit thermographischen Messungen kombiniert, um den Füllstand eines Wärmeübertragers zu bestimmen und Aussagen über die Auslastung dieser Komponente im reversiblen Betrieb zu treffen. Die Geometrie der reversiblen Schraubenmaschine wird im Hinblick auf die unterschiedlichen Druckverhältnisse in den beiden Betriebszuständen detailliert betrachtet. Es wird eine minimalinvasive Optimierung der Öffnungsgeometrie erarbeitet, die ihre positiven Auswirkungen insbesondere auf den Betrieb als ORC-Expander in experimentellen Untersuchungen bestätigen kann.
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.
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.
The exhaust gases of an ICE leave the equipment still with enough temperature to generate more electric power if a proper system is used for that purpose. Therefore, due to this scenario, in this work a study of an ORC system to recovery energy from the flow gases of a stationary diesel engine. The analyses are in three parts: 1) parametric analysis of an ORC system in on-design to determine the cycle's optimal operating point; 2) parametric analysis in off-design condition, to verify the behavior of power production and cycle efficiency when the heat source's flow and temperature are varied; 3) An economic analysis, a CEPCI based model was used. The system design was determined for a working fluid flow rate of 0.09 kg/s, evaporation and condensation pressure of 3,870 kPa and 25 kPa, respectively, and with exhaust gas at 420 °C and 0.1697 kg/s. For the off-design simulations, the evaporation pressure, the working fluid flow and the heat source inlet conditions were varied. A minimum, average, and maximum net power production of 8.56 kW, 15.59 kW, and 26.29 kW, respectively, was verified, while in the design condition it was 14.72 kW, there was also an average increase in exergy of 9.35%. The initial investment for the system's implementation is US$ 23,257.02 and the financial return and rate of return reach an average of 1.5 years and 90%, respectively. The study shows that the system increases the power plant's thermal efficiency and decreases fuel consumption as well as the emission of pollutant gases.
Sliding vane rotary expanders (SVREs) are widely used in organic Rankine cycle (ORC)-based power units for low-grade heat recovery because of their capability to deal with severe off-design working conditions. In particular, the speed of SVREs is a very effective operating parameter, together with the speed of the pump, to regulate the recovery unit and to lead the involved components in an acceptable operating behaviour when they are far from the design conditions.
In this study, a control strategy based on the variation in revolution speed of a SVRE was developed, where the inlet pressure of the expander is the main controlled property, which must be verified when the flow rate of the working fluid is changed to match the thermal power recovery at the hot source. In fact, pressure level control is a key point of the recovery unit for thermodynamic reasons and for the safety and reliability of the expander and, more generally, of the whole recovery unit.
The proposed control strategy is based on an original theoretical procedure that relates the expander speed, inlet pressure, volumetric efficiency, and working fluid mass flow rate in an analytical form. This analytical formulation is widely nonlinear and is simplified for use as a tool for the model-based control of the inlet expander pressure. An experimental activity performed on a SVRE operating in an ORC-based power unit, fed by the exhaust gases of a supercharged diesel engine, was the base of the analytical formulation. This provided the possibility of deriving a simplified model-based control of the expander inlet pressure and assessing its effectiveness and limits during off-design conditions. Higher expander global efficiencies were obtained (up to 45%), allowing a greater mechanical energy recovery (up to 2 kW).
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.
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.
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.
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.
This paper deals with the evolution of infrared (IR) thermography into a powerful optical tool that can be used in complex
fluid flows to either evaluate wall convective heat fluxes or investigate the surface flow field behavior. Measurement of
convective heat fluxes must be performed by means of a thermal sensor, where temperatures have to be measured with proper
transducers. By correctly choosing the thermal sensor, IR thermography can be successfully exploited to resolve convective
heat flux distributions with both steady and transient techniques. When comparing it to standard transducers, the IR camera
appears very valuable because it is non-intrusive, it has a high sensitivity (down to 20mK), it has a low response time (down
to 20μs), it is fully two dimensional (from 80k up to 1M pixels, at 50Hz) and, therefore, it allows for better evaluation
of errors due to tangential conduction within the sensor. This paper analyses the capability of IR thermography to perform
convective heat transfer measurements and surface visualizations in complex fluid flows. In particular, it includes the following:
the necessary radiation theory background, a review of the main IR camera features, a description of the pertinent heat flux
sensors, an analysis of the IR image processing methods and a report on some applications to complex fluid flows, ranging
from natural convection to hypersonic regime.
Air Conditioning and Refrigeration Project 141
A modular framework to model the steady off-design behavior of micro-CHP natural gas boilers based on Rankine technology is presented. The system charge integration into the model eliminates the use of any assumptions (i.e. subcooling, superheating, condensing pressure, etc.) which makes the presented model completely predictive. It is illustrated in the modelling a micro-CHP that satisfies the hot waters and central heating domestic needs (35 kWt) and produces electricity (≤1.5 kWe). A library of sub models of components with empirical (rotary vane pump and vapor scroll expander), semi-empirical (compact plates condenser) and spatially detailed physical (gas burner and evaporator) models is used to construct a model, using R245fa as thermal fluid.
The model is calibrated and validated in tests in which 0.1 kg/s of water was heated from 20 °C to 30–36 °C, and 80–500 W mechanical power was delivered at the expander shaft, sweeping restrict ranges of three control variables: burner thermal power of combustion (10–14.5 kWt), pump (500–740 rpm) and expander (2500–2750 rpm) rotation speeds. The model predicts most output variables with acceptable errors, e.g., less than ±10% for the expander outlet pressure (190–220 kPa, abs) or the temperatures at the outlet of the evaporator (80–150 °C) or the expander (60–120 °C).
Distribution of working fluid mass will significantly impact off-design performance of the Organic Rankine Cycle. In this paper, both the evaporator and the condenser are divided into three different zones, and the impact of heat transfer correlations and void fraction models selection on the working fluid mass calculation results have been discussed. Afterwards, impact of evaporation and condensation temperatures on working fluid mass distribution in each zone calculated by proposed correlations and models have been evaluated. Four common working fluids which feature different physical properties are taken into account. Results indicate that the selection of heat transfer correlations and void fraction models shows great importance for the calculation of working fluid mass in condenser but rarely affects the calculation results in evaporator. Accordingly, condensation correlation of Shah (1979) and void fraction model of Premoli (1971) is proposed for the discussed system. Furthermore, in condenser the working fluid mass monotonically decreases as the condensation temperature increases, while in evaporator the working fluid mass firstly rises and then falls when the evaporation temperature increases. It can be found that the relevant evaporation temperature at which the largest working fluid mass occurs is related to critical temperatures of the employed working fluids.
This paper presents experimental investigations on the transient refrigerant migration during the start-up and shut-down of a R410A split air conditioner. The refrigerant distribution inside the main components of the air conditioner is measured by using “quick-closing valves” technique in which the components are isolated and the refrigerant is removed to be weighed. Results indicate that the refrigerant distribution rapidly changes within 300 s during start-up and becomes gently after 300 s. On the other hand, the refrigerant distribution rapidly changes within 60 s during shut-down and becomes gently after 60 s. In addition, the dynamic characteristics of the air conditioner after startup are also investigated at the standard cooling capacity rating condition. Several active control strategies for refrigerant mass migration during on–off cycling are proposed to improve quick cooling performance during startup. And it is verified that increasing the refrigerant mass in the condenser can improve cooling performance during startup.
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.
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.
This study presents a mass-conserving dynamic numerical model to capture dynamic response of ORCs (organic Rankine cycles) when the system experiences a change in expander's rotational speed, pump's capacity factor, and conditions of hot and cold heat transfer fluids. ORC's dynamic response is tracked specifically considering evaporator pressure, condenser pressure, degree of superheating at the evaporator exit, degree of sub-cooling at the condenser exit, and the mass distribution along evaporator, condenser and liquid receiver tank. The developed model is novel due to the way subcomponent models are integrated together. Specifically, this integration includes fully coupled tank and condenser models. The model is validated against an experimental benchmark study for various steady state conditions and further verified considering mass and energy conservation principles. A parametric study is carried out to identify parameters which can be used for devising a new autonomous control strategy for organic Rankine cycles. It is illustrated through simulations that the mass distribution over ORC sub-components are linked to the system's overall state; and both the liquid level in the tank and the degree of sub-cooling at the tank exit can be used to devise a control system by changing pump's capacity factor and expander's rotational speed.
The relationship between cyclic refrigerant migration and cyclic loss for a residential, split-system air conditioner has been investigated. The unit tested was found to have a high initial capacity as migrated refrigerant was removed from the evaporator and then a low, slowly increasing capacity as trapped refrigerant was returned to the system from the accumulator. The unit performance was also compared to single and double time constant regressive approximations and to the time constant calculated from the evaporator mass and heat transfer coefficient.
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.
R-290 (propane) has been chosen as one of the most potential next generation working fluids of RACs because of its favourable environmental and thermo-physical properties. However, its use is hindered by its flammability and resulting concerns on safety. In addition, the charge mass is limited strictly by the standards which would impact RAC’s heating performance. In this paper, refrigerant mass distributions within a R290 split type air conditioner were experimentally investigated at both static and dynamic state, in which the liquid nitrogen method (LNM) was used to determine the refrigerant mass inside the components of the circuit. The distribution of refrigerant that changes with temperatures and compressor speed were also measured and discussed. The results can assist with improving the design of the products (performance improvement, safety measures, etc.) and providing data for further theoretical study and simulation analyses.
This paper presents HFC32 average boiling heat transfer coefficients and pressure drops measured inside a small Brazed Plate Heat Exchanger (BPHE): the effects of heat flux, saturation temperature (pressure), and outlet conditions are investigated. The experimental tests were carried out at four different saturation temperatures (5, 10, 15, and 20°C) and four different evaporator outlet conditions (vapour quality around 0.80 and 1.00, vapour super-heating around 5 and 10 °C). The average heat transfer coefficients show great sensitivity to heat flux and outlet conditions and weak sensitivity to saturation temperature (pressure). The saturated boiling heat transfer coefficients were compared with a new model for refrigerant vaporisation inside BPHE (Longo et al., 2015): the mean absolute percentage deviation between calculated and experimental data is 4.7%. The heat transfer and pressure drop measurements are complemented with a IR thermography analysis for a better understanding of the vaporisation process inside a BPHE.
This paper presents the experimental heat transfer coefficients and pressure drop measured during vaporisation of the new low Global Warming Potential (GWP) refrigerant HFO1234yf inside a Brazed Plate Heat Exchanger (BPHE): the effects of heat flux, mass flux, saturation temperature (pressure) and outlet conditions are investigated. The heat transfer coefficients show great sensitivity to heat flux and outlet conditions and weak sensitivity to saturation temperature (pressure). The frictional pressure drop shows a linear dependence on the kinetic energy per unit volume of the refrigerant flow and therefore a quadratic dependence on refrigerant mass flux. The saturated boiling experimental heat transfer coefficients are reproduced by two well-known equations for nucleate boiling, Cooper (1984) and Gorenflo (1993), with reasonable agreement. The heat transfer and pressure drop measurements are complemented with IR thermography analysis in order to quantify the portion of the heat transfer surface affected by vapour super-heating.
The purpose of this paper is to present methods for measuring refrigerant mass distribution inside a refrigeration system conveniently and accurately. The quasi on-line measurement method (QOMM) was presented for measuring refrigerant mass inside heat exchangers. Compared with the existing liquid nitrogen method (LNM), QOMM can avoid the refrigerant waste and accelerate the measurement process. For measuring refrigerant mass inside the compressor, QOMM was used together with the oil level observation method. The liquid level method (LLM) was used to measure the refrigerant mass inside the accumulator and the receiver. In order to verify the accuracy of the measurement methods, not only the deviation of the measurement method for refrigerant in single component was analyzed, but also the prediction of the total refrigerant charge in an air conditioner was verified. The results showed that the maximal prediction deviation of the refrigerant charge in the whole refrigeration system is 1.7%.
Experimental results are presented of the refrigerant mass charge distribution in a steadily cycling domestic refrigerator. In detail it is shown how the charge is displaced at compressor start-up and shut-down. At start-up it was found that the charge was temporarily displaced towards the condenser before returning to a steady state distribution in the latter part of the on-period. As a result, initially the evaporator was starved with a lowered evaporation temperature and a peak 10 °C superheat. The superheat disappeared within 3 min as the evaporator was gradually refilled with refrigerant. At shut-down the pressure equalised within 3 min as refrigerant was pushed into the evaporator from the condenser. The losses due to charge displacements were estimated to 11% (capacity) and 9% (efficiency). Possible ways to reduce the losses are discussed.
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.
The off-cycle refrigerant mass migration has a direct influence on the on-cycle performance since compressor energy is necessary to redistribute the refrigerant mass. No studies, as of today, are available in the open literature which experimentally measured the lubricant migration within a refrigeration system during cycling or stop/start transients. Therefore, experimental procedures measuring the refrigerant and lubricant migration through the major components of a refrigeration system during stop/start transients were developed and implemented. Results identifying the underlying physics are presented. The refrigerant and lubricant migration of an R134a automotive A/C system-utilizing a fixed orifice tube, minichannel condenser, plate and fin evaporator, U-tube type accumulator and fixed displacement compressor-was measured across five sections divided by ball valves. Using the Quick-Closing Valve Technique (QCVT) combined with the Remove and Weigh Technique (RWT) using liquid nitrogen as the condensing agent resulted in a measurement uncertainty of 0.4 percent regarding the total refrigerant mass in the system. The determination of the lubricant mass distribution was achieved by employing three different techniques-Remove and Weigh, Mix and Sample, and Flushing. To employ the Mix and Sample Technique a device-called the Mix and Sample Device-was built. A method to separate the refrigerant and lubricant was developed with an accuracy-after separation-of 0.04 grams of refrigerant left in the lubricant. When applying the three techniques, the total amount of lubricant mass in the system was determined to within two percent. The combination of measurement results-infrared photography and high speed and real time videography-provide unprecedented insight into the mechanisms of refrigerant and lubricant migration during stop-start operation. During the compressor stop period, the primary refrigerant mass migration is caused by, and follows, the diminishing pressure difference across the expansion device. The secondary refrigerant migration is caused by a pressure gradient as a result of thermal nonequilibrium within the system and causes only vapor phase refrigerant migration. Lubricant migration is proportional to the refrigerant mass during the primary refrigerant mass migration. During the secondary refrigerant mass migration lubricant is not migrating. The start-up refrigerant mass migration is caused by an imbalance of the refrigerant mass flow rates across the compressor and expansion device. The higher compressor refrigerant mass flow rate was a result of the entrainment of foam into the U-tube of the accumulator. The lubricant mass migration during the start-up was not proportional to the refrigerant mass migration. The presence of water condensate on the evaporator affected the refrigerant mass migration during the compressor stop period. Caused by an evaporative cooling effect the evaporator held 56 percent of the total refrigerant mass in the system after three minutes of compressor stop time-compared to 25 percent when no water condensate was present on the evaporator coil. Foam entrainment led to a faster lubricant and refrigerant mass migration out of the accumulator than liquid entrainment through the hole at the bottom of the U-tube. The latter was observed for when water condensate was present on the evaporator coil because-as a result of the higher amount of refrigerant mass in the evaporator before start-up-the entrainment of foam into the U-tube of the accumulator ceased before the steady state refrigerant mass distribution was reached.
Dynamic Modeling of Organic Rankine Cycle Power Systems
- A Desideri
A. Desideri, Dynamic Modeling of Organic Rankine Cycle Power Systems, Phd thesis
University of Liège, 2016.
Transient mass flow rate of a residential air-to-Fig. B.23. Example of ID card for one operating point
- M I Belth
- T E Grzymala
- D R Tree
M.I. Belth, T.E. Grzymala, D.R. Tree, Transient mass flow rate of a residential air-to-Fig. B.23. Example of ID card for one operating point (point #3).
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