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Modelling of organic Rankine cycle power systems in off-design conditions: An experimentally-validated comparative study
Abstract
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.
... In literature, three methods are applied for simulating off-design conditions [132]: a constant-efficiency method, a polynomial regression method, and a semi-empirical method. The constant-efficiency method, as its name suggests, is based on an assumption of constant component efficiencies and filling factors independent of the operational design conditions. ...
... In the polynomial regression method, one adapts the component efficiencies and filling factors to the operating conditions by means of a polynomial function. A second-order multivariate polynomial [132] is frequently used. Lastly, in the semiempirical method, one models the components by means of physicallybased equations. ...
... On a component level, the discrepancies with experimental data of a polynomial regression method and a semi-empirical method can give comparable results. The coupling of polynomial regression methods could show unreliability on the system level, while this is not the case for a semi-empirical method [132]. Zeotropic mixtures are also interesting for off-design conditions, even though not a lot of literature covers this topic. ...
Organic Rankine cycle (ORC) systems are a class of distributed power-generation systems that are suitable for the efficient conversion of low-to-medium temperature thermal energy to useful power. These versatile systems have significant potential to contribute in diverse ways to future clean and sustainable energy systems through, e.g., deployment for waste-heat recovery in industrial facilities, but also the utilisation of renewable-heat sources, thereby improving energy access and living standards, while reducing primary energy consumption and the associated emissions. The energetic and economic performance, but also environmental sustainability of ORC systems, all depend strongly on the working fluid employed, and therefore a significant effort has been made in recent years to select, but also to design novel working fluids for ORC systems. In this context, computer-aided molecular design (CAMD) techniques have emerged as highly promising approaches with which to explore the key role of working fluids, and present an opportunity, by focusing on the design of new eco-friendly fluids with low environmental footprints, to identify alternatives to traditional refrigerants with improved characteristics. In this review article, an overview of working-fluid and system optimisation methodologies that can be used for the design and operation of next-generation ORC systems is provided. With reference to wide-ranging applications from waste-heat recovery in industrial and automotive applications, to biomass, geothermal and solar-energy conversion and/or storage, this review represents a comprehensive, forward-looking exposition of the application of CAMD to the design of ORC technology.
... Yet relatively few studies compared the different modelling approaches of ORC systems on the prediction of their design and off-design performances. Among these, in Dickes et al. [39], component modelling approaches including constant efficiency, empirical using polynomial regression, and semiempirical are compared based on their fitting and extrapolation capabilities. The three component model types were coupled in an assumption-based system solver, and it was found that the semiempirical model outperformed the other two for off-design simulations of ORC systems. ...
... Despite its accuracy and internal reliability depending on the number of data points and their range, its assessment is poor. In a comparative modelling study [39], similar accuracy has been reported for the empirical model using polynomial regression and the semi-empirical model showing that each one is relatively superior to the other in certain parameters. Additionally, scarce external reliability for the single-coefficient and empirical models was concluded from [39], with the semi-empirical models having better overall performance [70]. ...
... In a comparative modelling study [39], similar accuracy has been reported for the empirical model using polynomial regression and the semi-empirical model showing that each one is relatively superior to the other in certain parameters. Additionally, scarce external reliability for the single-coefficient and empirical models was concluded from [39], with the semi-empirical models having better overall performance [70]. Nonetheless, these models remain generally valid within the range of experimental data and for the specific component, although their extrapolation should be performed with caution. ...
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.
... With the first approach, the cycle can be modelled following several methods. The constant efficiency models by which constant performance parameters are considered regardless of the operating conditions and are popular because of their simplicity [5][6][7][8][9][10][11]. The polynomial regression model is often used to investigate the effect of variant operation conditions on the system performance using a polynomial regression on the experimental data [11,12]. ...
... The constant efficiency models by which constant performance parameters are considered regardless of the operating conditions and are popular because of their simplicity [5][6][7][8][9][10][11]. The polynomial regression model is often used to investigate the effect of variant operation conditions on the system performance using a polynomial regression on the experimental data [11,12]. The semi-empirical method is also an experimental dependent model relying on numbers of physically meaningful equations in which the individual parameters are to be tuned to fit the reference dataset [8,11,13,14]. ...
... The polynomial regression model is often used to investigate the effect of variant operation conditions on the system performance using a polynomial regression on the experimental data [11,12]. The semi-empirical method is also an experimental dependent model relying on numbers of physically meaningful equations in which the individual parameters are to be tuned to fit the reference dataset [8,11,13,14]. Although all these approaches are practical for system level modeling of the ORCs, they might not be as helpful as the deterministic models when it comes to component-level and individual study of the rotary machines. ...
Expansion devices are a key component that affects the performance of the Organic Rankine Cycles (ORCs), and its improvement has been identified as one of the most important parts of future studies in ORCs. A theoretical model of a modified revolving vane expander was developed in this study. The model was to investigate the inherent physical processes and the effect of losses on the mechanism. The developed model was validated with the experimental results. The experiments were carried out using an improved experimental rig with direct coupling of the dynamometer and expander to eliminate any externally exerted misalignments. The results of the theoretical validation showed that the torque is generally slightly underpredicted, but the trends are very closely predicted. Moreover, a little underprediction was observed in the theoretical study of mass flow rate but still within the uncertainty of the experiments. The experimental results showed the prototype could generate up to almost 0.8 N.m and rotated up to 1700 rpm at a suction pressure of 3 bar(g). Moreover, the prototype demonstrated up to 55% volumetric efficiency and 11% isentropic efficiency at 3 bar(g) of the suction pressure.
... In the analysis, the condenser and evaporator are modeled with the moving-boundary algorithm developed in [65] and [66], and experimentally validated in [67]. The model is equipped with the correlations for calculating the heat transfer coefficients for any phase configuration in plate heat exchangers, as described in [67]. ...
... In the analysis, the condenser and evaporator are modeled with the moving-boundary algorithm developed in [65] and [66], and experimentally validated in [67]. The model is equipped with the correlations for calculating the heat transfer coefficients for any phase configuration in plate heat exchangers, as described in [67]. The heat exchanger model calculates the heat flow rate between the hot and cold streams, given the mass flow rates and inlet temperature and pressure. ...
Solar energy’s growing role in the green energy landscape underscores the importance of effective energy storage solutions, particularly within concentrated solar power (CSP) systems. Latent thermal energy storage (LTES) and leveraging phase change materials (PCMs) offer promise but face challenges due to low thermal conductivity. This work comprehensively investigates LTES integration into solar-thermal systems, emphasizing medium temperature applications. It introduces an innovative LTES tank design with encapsulating tubes modeled through computational fluid dynamics (CFD). The system employs a novel hybrid thermal storage approach, enhancing thermal output through a high-temperature heat pump (HTHP) before storage. This approach aligns with future energy systems, emphasizing energy vector integration. The study offers realistic LTES modeling, accounting for natural convection effects, and integrates LTES within solar thermal systems by taking advantage of time dependent CFD results. Real-world solar irradiance data for an Italian city is integrated into the investigation, providing insights into LTES performance and its role in sustainable solar energy solutions. A multi-objective optimization process follows the year-round simulations to maximize the amount of stored heat and minimize the electric input. This approach facilitates better system sizing and performance evaluation, contributing to the advancement of solar thermal technology.
... The heat exchangers are modeled by means of the moving boundaries method [13]. A corrected version of the Cooper's (Eq. ...
... (13)) and Gnielinski's (Eq. (14)) correlations is used to evaluate, respectively, the evaporating and the condensing convective heat transfer coefficients, as proved to be effective to simulate heat transfer in ORC applications [13]. The correction coefficients are here named as c 1 , c 2 , and c 3 . ...
In the view of reducing the global greenhouse gas emissions it becomes fundamental to exploit the renewable energy sources at their maximum potential by developing effective strategies for their flexible use. Among the available solutions to realize these strategies are the electric energy storages including the innovative Pumped Thermal Energy Storage technology (included in the Carnot battery concept). This can become very interesting in these applications where different energy flows must be handled (both electric and thermal), thanks to the possibility of adding the contribution of a waste heat source, in a thermally integrated energy storage. However, despite the several advantages, the state-of-the-art still lacks experiments and investigation of efficient control strategy for the Carnot battery when inserted into the process. As original contribution to the current literature, this paper presents the off-design model of a reversible Organic Rankine Cycle (ORC)/Heat Pump (HP) Carnot battery configuration with the aim of employing it to simulate the performance of such system and discuss its optimal management when inserted into a generic process. An existing reversible HP/ORC kW-size prototype is considered as reference and its optimal control in both HP and ORC mode under different boundary conditions is assessed.
... For instance, by referring to performance predicted models for heat exchangers, often a 1-D modelling is used for the determination of a proper overall heat transfer coefficient. This approach, from one hand, increases the reliability and the extrapolation performance of the model adopted, but relatively high computational times are usually demanded [20]. This point could become a crucial issue if a fast response from the ORC model is required or if the simulation of this model is repeated innumerable times. ...
... In particular, it was demonstrated that the main benefits arise from the more accurate prediction of the evaporator effectiveness, which is characterized by the highest variance importance (about 25% and 80% in terms of first order Sobol' indices by referring to net power and the HTF outlet temperature, respectively) and the adoption of correlationbased approach reduces the mean percentage error from 3.6% to 2.6%. The accuracy achieved is in line with other models proposed in literature for small scale ORC units (see [8,12,20,27]). The proposed mathematical model could therefore be suitable for its implementation if a fast response from the ORC model is required or if the simulation of this model is repeated innumerable times. ...
Organic Rankine Cycle is an efficient and reliable technology for the thermal-to-electricity conversion of low-grade heat sources but the variability in boundary conditions often forces these systems to operate at off-design conditions. The development of reliable models for the performance prediction of organic Rankine cycle power systems under off-design conditions is therefore crucial for system-level integration and control implementation. In this paper, a mathematical model for the evaluation of the expected performance of organic Rankine cycle power units in a large range of operating conditions based on experimental data collected in a medium-size solar organic Rankine cycle power plant is presented. Two different empirical approaches for the performance prediction of heat exchangers and machines, namely, constant-efficiency and correlated-based approaches, are proposed and compared. In addition, empirical correlations based on experimental data are proposed for the preliminary assessment of the energy demanded during the start-up phase and the corresponding duration. Results demonstrate that a good achievement in terms of accuracy of the model and reliability of the simulation performance can be obtained by using a constant-efficiency approach, with average errors lower than 5% and 2.5 K for the expected net power and outlet oil temperature respectively. The use of polynomial correlations leads to a more accurate estimation of the performance parameters used for evaporator and the turbine (in particular the evaporator heat effectiveness and the isentropic and electromechanical efficiency for the turbine), which strongly affect the main output variables of the model and, at the same time, are remarkably influenced by the operating conditions. A reduction in the average error in the prediction of the net power and outlet temperature of the heat transfer fluid to about 4% and 1.5 K respectively is therefore achieved by this approach. Average errors of 18.5% and 12.5% are achieved for the start-up time and the corresponding energy absorbed, respectively. Although the results obtained in terms of accuracy could be improved, these correlations can give an initial indication about the duration and energy required during this phase.
... waste heat applications, the organic Rankine cycle has been widely accepted thanks to its viable efficiency, relatively simple structure, and high reliability; therefore, the construction of power facilities based on this technology has shown remarkable growth [2]. However, since the working fluid changes phase, a fraction of the heat is supplied to a constant temperature fluid by a variable temperature heat source (Fig. 1a). ...
... The saturated liquid (3) mixes with the two-phase flow (6) reaching a certain quality (7). The liquid is pumped to the trilateral cycle (7 l -4) and the vapor is expanded by the dry-expander (7 v -8), condensed (8)(9)(10)(11), and heated back to saturated condition (2)(3). Besides eliminating the throttle loss, the ORFC also divides the pumping process in two, avoiding the requirement of highly efficient pumps to handle a high pressure ratio at once. ...
Growing environmental concerns are driving the energy market toward the development of thermodynamic cycles to harness renewable energy and waste heat. This manuscript introduces the novel organic Rankine flash cycle, which combines the organic Rankine cycle with the trilateral cycle, merging their advantages in terms of high specific power output and low heat transfer irreversibility, respectively. By comparing the organic Rankine flash cycle to the organic flash cycle, it was found that the proposed architecture reaches a peak exergy efficiency at a more realistic value of two-phase expansion volume flow ratio, consistently achieves higher energy and exergy efficiencies, presents a lower cost, and is not constrained to operate close to the working fluid saturation temperature, promising easier operability. Considering pentane as working fluid, the exergy efficiency of the organic Rankine flash cycle is 18%p higher for a heat source temperature of 150 °C, 12%p for 175 °C, and 4%p for 200 °C. The attractive thermoeconomic performance of the proposed organic Rankine flash cycle highlights the potential of such a cycle as a new paradigm in the ORC panorama, encouraging further investigation towards practical demonstration.
... However, there is lack of available information regarding real ORC units on industrial level [17]. Moreover, according to Park et al. [28] there is a large gap between research and development for source and sink temperature differences above 150 C, and the majority of published experimental works were performed for micro-and mini- [23,29] and Lecompte et al. [30]. In general there are different approaches to modelling of power generation equipment performance. ...
... The problem is usually tackled using sophisticated strategies, redundant measurements and calculation algorithms tailored for the purpose, as presented in Refs. [29,31,32]. On the other hand it is usually not economically justified in the case of small-scale distributed plants and in the case of existing plant only an existing SCADA system can be used for data acquisition. ...
This paper presents an analysis of operational parameters of the commercial Organic Rankine Cycle (ORC)cogeneration unit integrated with biomass-fired boiler and municipal heating network. The analysis is based on field measurements in real operational conditions using standard industrial sensors installed within the system. Regression based mathematical modelling have been applied to develop robust predictive models of the ORC system for its diagnostics and production planning. Historical data collected within the Supervisory Control and Data Acquisition System of the plant have been used to establish correlations between key thermodynamic parameters. Results reveal off-design performance characteristics of the ORC unit and its individual components such as turbine, evaporator and condenser. There are also demonstrated results of application of the model for technical condition and performance monitoring, which can support decisions on maintenance activities.
... Additionally, the ORC has the unique capability of absorbing heat from a cold source and converting it into usable energy. One practical application of the ORC is its ability to recover heat from machinery and industrial processes [5,6]. In such cases, the temperatures typically fall within a range of average temperatures (above 473 K and below 873.1 K). ...
The technology known as organic rankine cycle (ORC) is a dependable method for transforming heat into electricity, whether it is for use in renewable energy sources such as biomass, geothermal, and solar, or for improving industrial energy efficiency. The range of ORC systems spans from small-scale (a few kW) for home cogeneration to sizable multi-megawatt geothermal power facilities. Since the 1970s, technology has undergone significant progress, largely due to increased economic incentives and rising energy costs, despite a slow start initially. Tracking the evolution of the technology worldwide is challenging due to the wide variety of applications, manufacturers, and countries involved. Hence, the present research scrutinizes the ORC technology to evaluate this system from the energy and economic perspectives. Aspen HYSYS, and Aspen Capital Cost Estimator simulations were used for the process, thermodynamic, and financial evaluations, respectively. In this research, the ORC is evaluated using various organic working fluids, specifically seven different types of fluids. The power and heat flow of the expander in all scenarios are considered at 1200 kW and 1.200 Mw, respectively, to determine the most appropriate organic fluid. Organic fluid toluene, due to its highest boiling point among the investigated fluids, was able to generate the required production power using the lowest molar flow rate for both input and output to the expander, considering these values. The results showed that the organic fluid toluene is technically and economically superior to other fluids. However, cyclopentane performs slightly better in terms of energy consumption and carbon dioxide output. However, toluene is chosen over cyclopentane due to current safety concerns.
... PSO was developed in the 1990s, and its basic idea came from the research on the foraging of birds and fishes in nature. This method has the advantages of simple operation, high calculation accuracy, and good convergence [19]. Therefore, this method has been highly valued by scholars all over the world and has been verified in practice. ...
With the progress of society, the supply of electricity has become an inevitable choice for the development of all walks of life. The stability, safety, and economic dispatch of the electric energy system directly impact the entire country’s economic development, from national defense, military, and even people’s daily life, all closely associated with the smooth running of the network. In the aspect of power grid planning and operation, the economic security dispatching problem of power system is a very typical optimization problem. The problem is how to maximize the operating cost of the system under the premise of satisfying the system’s load limit and safety and stability. Due to the rapid development of power technology, the research on power system security and economic planning is essential. As an important means to promote China’s energy structure adjustment and promote the supply side structural reform, the regional spot electricity market is playing an increasingly important role in building a resource optimization allocation mechanism, improving China’s energy resource allocation efficiency, and promoting social and economic development. However, the development of China’s regional spot power market is still in its infancy, and the transaction price of the regional spot power market is quite different from that of other countries. It was given these problems, this article discusses the use of particle swarm algorithms for secure and economic scheduling of power systems in the context of securely reading blockchains and distributed data. The research results showed that: The introduction of environmental pollution penalty programs improved the priority dispatch of wind and photovoltaic power when considering environmental penalty programs and backup penalties for wind and photovoltaic power. The setting of the coefficient of the environmental penalty term depended on the designer’s emphasis on the priority scheduling of wind power and photovoltaic (PV) power generation (PG), and the standby capacity penalty clause achieved reasonable dispatch and utilization of wind power and PV PG, which reduced the dramatic fluctuations and intermittent volume of the system, and facilitated safe and consistent operation of the network. The research of this paper shows the positive relationship between the blockchain and distributed data security reading algorithm and the clearing method of regional spot power market, and points out a new method for its development.
... Experimental validation of the developed off-design models by means of experimental data is a main issue in the context of a reliable prediction of the ORC's performance. Hence, this topic is tackled in different recent publications (Petrollese et al., 2020;Dickes et al., 2017;Mazzi et al., 2015). According to Lecompte et al. (2018), the models can be divided into black box, gray box and white box models. ...
Organic Rankine Cycle (ORC) systems are a widespread technology for recovering the waste heat of low to medium temperature energy sources. Despite the fact that they are commercially used in a wide range of applications, the data situation concerning the thermodynamic performance of the different components of those plants is rather poor, especially regarding part-load operation. In the scope of the present paper, comprehensive experimental results of an ORC test rig with hexamethyldisiloxane (MM) as working fluid and a Quasi-Impulse Cantilever turbine as expansion machine are presented. For preheating, evaporating and superheating the working fluid, a Plate & Shell heat exchanger was applied. Exhaust gas from a propane gas burner served the test rig as heat source. At design point, MM was evaporated at a pressure level of 6 bar. By expanding the working fluid to 0.32 bar, an electrical power output of approx. 12 kW was generated. All main components of the cycle, i.e. heat exchangers, feed pump and turbine, were analyzed concerning their thermodynamic operational behavior. Part-load operating points down to 50% of the ORC design mass flow rate were considered. Due to the fixed swallowing capacity of the used turbine, a decrease in mass flow rate was always associated with a drop of the evaporation pressure of the cycle. Experimental results for each component of the ORC system were analyzed. The corresponding off-design characteristics were implemented in a commercial cycle modelling tool for quasi-stationary simulations. Hence, a digital twin of the experimental test rig was provided. In this simulation model, it could be shown that WHR efficiency at 50% design mass flow rate could be improved from 4.6% to 7.2% by substituting the fixed geometry turbine by one with adjustable swallowing capacity.
... A semi-empirical model of the whole system has been implemented and validated with experimental data, collected operating with the conventional fluid R134a. Semi-empirical model type is chosen as this demonstrated to be the most suitable modelling approach for similar applications, with robust prediction in both fitting and extrapolation more than usually adopted constant efficiency models [21]. Indeed, this kind of models allows to account for actual operating conditions of the volumetric machines and not only for the only thermodynamic process. ...
This study proposes a comprehensive evaluation of the actual greenhouse effect related to the operation of ORC systems in the kW scale. The method is derived from the TEWI (total equivalent warming impact) concept for refrigeration systems, since it includes both direct and indirect contributions to the greenhouse gas emission related to the ORC system. A comparison between traditional HFC-134a (R134a) and some of its low-GWP replacements has been performed, accounting for the effect of the operating fluid leakage during system operation, but also for the indirect contribution associated to the lower performance that can be achieved using more sustainable working fluids, such as hydrofluoroolefins (HFO). Alternative fluids that have been tested are two pure compounds (R1234yf and R1234ze(E)), and four mixtures (R134a-R1234yf; R-134a-R1234ze(E); R515A; R430A). A semi-empirical lumped-parameters model has been employed for simulating the behavior of the ORC system. For the model validation, the experimental data collected on a reference 2-kW ORC test bench with R-134a have been used. The model was then applied to investigate the performance of the system working with alternative fluids. The results show that the indirect emissions associated to HFOs may lead to higher values of total equivalent CO2 emissions, with respect to the employment of R134a as working fluid. The main factors affecting the environmental evaluation, such as emission factors, fluid leak rate and R134a concentration in the mixture, can be decisive and are discussed in this work.
... In contrast to the interpolation capability, the extrapolation capability for such empirical models is typically poor. This has been investigated i.e. by Dickes et al. [273]. They compared three different approaches for ORC part-load modeling, containing a constant efficiency method as well as an empirical and a semi-empirical modeling approach. ...
The Organic Rankine Cycle (ORC) can be applied to generate power from low-temperature heat sources and thus supports a sustainable energy system. In order to make this technology more competitive, this thesis contributes to their development, particularly for geothermal applications. To this end, environmentally friendly working fluids and new plant architectures are being investigated.
Recently, a new generation of working fluids with significantly lower Global Warming Potential (GWP) has been developed. However, operating experience with this novel fluids is rare, especially concerning existing systems designed for older generation fluids. Therefore, the applicability of two modern fluids as drop-in replacements for the currently widespread fluid R245fa is experimentally investigated. It can be concluded that both novel fluids R1233zd(E) and R1224yd(Z) are generally suitable as a drop-in replacement for R245fa in ORC systems. The highest thermal efficiency is reached with R1233zd(E), while the highest power output is still obtained with the high-GWP fluid R245fa.
The concept of regenerative preheating is investigated in literature by numerical studies and
its thermodynamic and economic performance is predominantly evaluated positively. However, regenerative preheating has not yet been realized in experimental ORC test rigs or commercial products. For this purpose, a novel ORC test rig is designed, constructed and commissioned in the course of this thesis. For evaluation purposes, the regenerative preheating concept is compared to a standard ORC configuration. As a result, a 9.9% higher net thermal efficiency is achieved with regenerative preheating, while the net power output is equal to the one achieved with the standard configuration. This result is particularly important for combined heat and power (CHP) generation due to the reduced cooling of the heat source. In order to evaluate this concept for CHP applications, a novel ORC-CHP architecture based on regenerative preheating is experimentally compared to three state of the art ORC-CHP concepts. Moreover, three different supply and return temperatures of the district heating system (DHS) are considered. The results reveal that the novel architecture, in combination with low- and medium-temperature DHS, leads to an increased part-load performance and a wider operating range. This enables an up to 9.4% higher annual net electricity production, which accounts for additional revenues of 4.56 million e in the case of a typical geothermal project. Thus, it can be concluded that this novel ORC-CHP architecture is beneficial from a technical perspective and promising in terms of economics.
... The respond performance of ORCs using fourteen working mediums was studied. Dickes et al. [15] evaluated different modeling methods for the off-design operation performance of ORC systems. Their results indicated that semiempirical-efficiency models were more accurate and reliable than constant-efficiency and polynomial-efficiency models. ...
The organic Rankine cycle (ORC) has been regarded as one of the most potential technologies for converting low-grade heat into power. In the past decades, the design and operation optimization of ORC has undergone investigation, and most approaches use a sequential or step-by-step approach by separating the off-design operation from the system design and typically lose solution optimality. The current study proposes a simultaneous optimization of the design and off-design operation for a waste heat-driven ORC. The detailed component configuration models for the heat exchangers are developed. A multi-period optimization model is developed to synchronously obtain the configurations of components and scheme of off-design operation that minimizes the annual average electricity production cost. A case study validates the superiority of the proposed multi-period optimization method over a two-step optimization. The ORC obtained using multi-period simultaneous optimization method features 5.7%–16.6% lower average electricity production cost than those achieved with the conventional two-step method.
... As a matter of fact, a significant breakthrough in ORC plant design and optimization should be sought in a less constrained off-design analysis, with a deeper investigation of the effects of the system's inertia in transient operation [22], optimum selection of the rotational speed for the expander and pump [11], and an optimum working fluid mass flow rate for each operating condition, which, in turn, reflects on the limited operating pressures of the plant. Another fundamental aspect is the working fluid selection, and to this aim, molecular-based models were developed to predict the thermodynamic properties of organic working fluids [23]. ...
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.
... An optimization of the ORC design with the objective of maximizing the power output was applied and the operating control strategy was implemented to achieve the maximum power output also with ambient temperatures other than the nominal one. Dickes et al. [14] compared three modelling methods for the ORC off-design simulation, where experimental measurements gathered on two ORC facilities were used as reference for the models' calibration and evaluation. ...
The application of Organic Rankine cycle (ORC) units in concentrating solar power systems is a very promising solution. However, fluctuations in the available solar energy often force solar-based ORC systems to operate at part-load conditions. An innovative methodology for finding robust design solutions of such ORC systems, based on the minimization of the expected Levelized Cost of Energy (LCOE), is therefore proposed. The expected variations in the ORC heat source and heat sink are considered during the design stage by adopting a multi-scenario approach. The proposed methodology has been tested by referring to a medium-scale ORC unit and by considering different working fluids. As cases study, the direct coupling of the ORC unit with a solar field and the integration of a Thermal Energy Storage system have been investigated. The comparison of the results obtained by using a multi-scenario and a single-scenario approach highlights a reduction of the actual LCOE. The ORC configuration obtained by adopting a multi-scenario approach is characterized by lower performance under design conditions, but it is less sensitive to variations in the main inputs during off-design operating periods. This fact is particularly noteworthy for the case with the direct coupling of the solar field.
... The data-driven models are based on the knowledge of the system coming from measurements or previous simulations, and make use of computational methods such as machine learning [30] or transfer function identification to develop models of high computational efficiency [31]. The drawback is that the accuracy of the model is highly dependent on the quality of the data set [32]. Extrapolation out of the operating range of the data set can lead to poor accuracy and estimation errors. ...
Organic Rankine cycle systems are suitable technologies for utilization of low/medium-temperature heat sources, especially for small-scale systems. Waste heat from engines in the transportation sector, solar energy, and intermittent industrial waste heat are by nature transient heat sources, making it a challenging task to design and operate the organic Rankine cycle system safely and efficiently for these heat sources. Therefore, it is of crucial importance to investigate the dynamic behavior of the organic Rankine cycle system and develop suitable control strategies. This paper provides a comprehensive review of the previous studies in the area of dynamic modeling and control of the organic Rankine cycle system. The most common dynamic modeling approaches, typical issues during dynamic simulations, and different control strategies are discussed in detail. The most suitable dynamic modeling approaches of each component, solutions to common problems, and optimal control approaches are identified. Directions for future research are provided. The review indicates that the dynamics of the organic Rankine cycle system is mainly governed by the heat exchangers. Depending on the level of accuracy and computational effort, a moving boundary approach, a finite volume method or a two-volume simplification can be used for the modeling of the heat exchangers. From the control perspective, the model predictive controllers, especially improved model predictive controllers (e.g. the multiple model predictive control, switching model predictive control, and non-linear model predictive control approach), provide excellent control performance compared to conventional control strategies (e.g. proportional–integral controller, proportional–derivative controller, and proportional–integral–derivative controllers). We recommend that future research focuses on the integrated design and optimization, especially considering the design of the heat exchangers, the dynamic response of the system and its controllability.
... where f i are the fitted interpolatant, e i are the load cells output voltage 2 Although the pump encloses a high-density fluid (i.e. a liquid phase), its inner volume is limited (<20 cc). Combining both effects, its charge enclosure is negligible (<30 g) in comparison to the heat exchangers or the liquid receiver (enclosing several kilograms of fluid). ...
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.
... A screening campaign on ORC working fluids is presented in [7][8], for the cases of flat plate and vacuum tube solar thermal collectors (90°C and 125 °C heat source fluid temperature, respectively) and 1 kW electric power output [7]. The energy performance of small-scale ORC units in both steady-state and transient operation is investigated for exploitation of low-grade heat from solar thermal collectors in [9][10][11]. In addition to the efforts oriented to the optimum design of the thermodynamic cycle and control strategy [12,13] and the proper working fluid [14,15] and rotary equipment selection [16,17], the more efficient exploitation of the thermal power associated to the heat source is crucial to fill the gap between the ORC -particularly, subcritical cycles -and other recovery and micro-cogeneration layouts. ...
The paper presents an extensive modeling activity for optimum design of the Heat Recovery Vapor Generator, HRVG, in an ORC-based unit, bottoming a solar thermal collector for domestic hot water production. Sliding vanes rotary machines-equipped ORC units are widely acknowledged among the most suitable options for low grade (100-120°C source temperature) heat recovery, due to their technological advantages (low cost, low maintenance) and higher energy merit (flexibility and applicability to a variety of upper thermal sources). In such plants, the energy performance boost calls for the exergy losses reduction at the heat recovery vapor generator, via the improved matching of the heat exchange curves. The paper investigates the potential gain on exergy and energy efficiency of two different techniques, i.e. the evaporation split over two pressure levels for pure R245fa and the use of R245fa/R227ea zeotropic mixtures, as the working fluid. Optimum evaporation pressures, pinch point temperature difference, mass flowrates and superheating degree on both loops are selected for each case study and the system energy performance assessed, hence providing a reliable criterion for optimum design of small-scale plants.
... Considering the boundaries imposed by availing low-grade heat, the organic Rankine cycle (ORC) is being widely accepted due to a simple structure, an easy maintenance, and a high reliability; other options such as the trilateral and Kalina cycles have not found significant market penetration due to challenges in two-phase expansion and complex layout, respectively [3]. Thus, the development of ORC projects has shown remarkable growth, reaching 2 GWe of worldwide installed capacity in 2017 [4]. Both organic and steam Rankine cycles are quite similar, being the basic design comprised of four processes: (1), heat addition to vaporize the fluid; (2), expansion to extract work; (3), heat rejection to condensate the fluid; and (4), compression to add work. ...
In an increasingly decentralized energy market, micro gas turbines are seen with great potential due to their low emissions and fuel flexibility, which aligns with growing environmental concerns. Although presenting a relatively low efficiency, these machines could be improved by coupling it with an organic Rankine cycle. This manuscript covers the thermoeconomic design and optimization of such bottoming cycle for a 100 kWe micro gas turbine. The tool employed for such calculations is extensively described and was developed using solely open resources. The results shown that the saturation temperature at ambient pressure was an important variable when the minimum pressure is constrained above ambient and that a high degree of superheating was favored when the recuperated cycle is heated directly by the microturbine flue gases. Pentane was flagged as the best working fluid, generating 14.1 kWe of additional power and increasing the overall electric efficiency from 30 to 34.2%. The Authors show that at the current state of the art an efficiency of around 35% is the upper practical limit for such microturbine organic Rankine cycle combination.
... Dickes et al. experimentally compared three approximations (constant-efficiency method, a polynomial-based method and a semi-empirical method) in order to obtain the best simulation method for ORC. As a result, the semi-empirical method presented the most reliable and precise results [32]. Peris et al. experimentally characterised the performance of an ORC to recover low grade waste heat in cement industry. ...
The wastes in wood industries (waste chips) are commonly used as fuel for burners to produce steam and to use the remaining heat in the drying process. However, in spite of that, there is a considerable amount of heat evaluated from the burn of waste chips still released to the atmosphere without use. Therefore, in the present study, a cogeneration cycle design by used of ORC was designed and parametrically optimised for six organic working fluids (acetone, ethanol, R11, RE245fa2, R365mfc and R601a). During the ORC optimisation, the ORC turbine inlet temperature was changed from the saturated steam temperature of the fluid to the maximum temperature of the fluid. The ORC turbine inlet pressure was increased from 7.5 bar to the critical pressure of the fluid. As a result of the study, the maximum net power, net thermal efficiency and exergy efficiency of the ORC were found as 453.91 kW, 30.01% and 67.56% at 340 °C and 62.5 bar from the ORC with ethanol. This means that almost 30% of the waste heat could be recovered by use of the ORC with ethanol. By using the designed cogeneration system, it was calculated that the thermal efficiency of the system can be increased up to 74.01%.
... The ORC development halted in the 1980s due to high interest rates and cheap fossil fuels, shutting down most experiments since the technology was no longer economically attractive [15]. From there on, the development of ORC projects slightly increased until the 21 st century when the technology showed remarkable growth, accounting for 2 GWe of worldwide installed capacity in 2017 [16]. ...
Current energy conversion machines such as the micro gas turbine can be improved by harvesting the low-grade energy of the exhaust. A prominent option for such is the organic Rankine cycle due to its relatively efficient and reliable design. This manuscript presents a review on the subject and is the first step toward the design of an organic Rankine cycle bottoming a 100 kWe recuperated gas turbine. After introducing and covering the historical development of the technology, appropriate guidelines for defining the cycle arrangement and selecting the fluid are presented. At last, the viability of the cycle is assessed by assuming an appropriate efficiency value and general cost functions. The organic Rankine is expected to generate an additional 16.6 kWe of power, increasing the electrical efficiency from 30 to 35%. However, the capital cost increase was estimated in 48%.
... Refs. [34,35] investigated the performance of ORC engines with scroll expanders and plate/helical coil HEXs. Single-stage single screw expanders in off-design operation were also considered by Ziviani et al. [36] for smallscale, low-temperature ORC systems, with particular focus on the control strategy of the expander to maximise power output or efficiency. ...
Organic Rankine cycle (ORC) engines in real applications experience variable heat-source conditions. In this paper, the off-design performance of small- to medium-scale ORC engines recovering heat from stationary internal combustion engines (ICEs) is investigated. Of particular interest are the employment of screw vs. piston expanders, and two heat exchanger (HEX) architectures. Unlike previous studies where the performance of the expander and HEX are assumed fixed during off-design operation, here we consider explicitly their varying and interacting characteristics within the overall system. Nominal sizing results reveal indicated isentropic efficiencies > 80% for twin-screw and > 85% for piston expanders. Following nominal design, the ORC engine operation is optimised for ICE part-load (PL) operation. Although the heat transfer coefficients in the evaporator decrease by up to 30% at PL, the effectiveness in this HEX increases by 20% due to the larger temperature differences across the component. The screw expander efficiency reduces by up to 3% at off-design operation, whilst that of the piston expander increases by up to 16%. Optimised off-design maps indicate that the ORC engine power output reduces to 77% (piston) or 68% (screw) of its full-load value when the ICE operates at 60% PL, and that ORC engines with plate HEXs generate 5–11% more power than those with double-pipe HEX designs. Under variable ICE operation, smaller ORC engines with piston expanders generate more power than larger engines with screw expanders, highlighting the resilient off-design operation of piston machines. The modelling tool developed here can predict ORC performance over a wide operating envelope and provides performance maps that can be used by operators to optimise ORC engine operation in variable conditions and by ORC vendors to inform component design decisions.
... The proposed solar ORC model was validated using the data reported in the literature [14]. The parameters compared are the working fluid flow, the power generated by the expander, the pump mechanical power, the air temperature at the condenser outlet and the working fluid temperature at the condenser inlet. ...
This study presents an experimental and numerical investigation into the performance and control optimization of an Organic Rankine Cycle (ORC)-based micro-combined heat and power (micro-CHP) system. A steady-state, off-design, charge-sensitive model is developed to design a control strategy for an ORC micro-CHP combi-boiler, aiming to efficiently meet real-time domestic hot water demands (up to 40 °C and 35 kW) while generating up to 2 kW of electricity. The system utilizes a natural gas burner to evaporate the working fluid (R245fa), with combustion heat power, volumetric pump speed, and expander speed as control variables. Experimental and numerical evaluations generate steady-state control maps to identify optimal operating regions. A PID-based dynamic control strategy is then developed to stabilize operation during start-ups and user demand variations. The results confirm that the strategy delivers hot water within 1.5 min in simple boiler mode and 3 min in cogeneration mode while improving electricity generation stability and outperforming manual control. The findings demonstrate that integrating steady-state modeling with optimized control enhances the performance, responsiveness, and efficiency of ORC-based micro-CHP systems, making them a viable alternative for residential energy solutions.
In the following years, the building sector will have to considerably reduce its final energy demand and emissions by adopting new technologies.
This paper introduces two innovative technologies capable of reducing the final energy demand of residential buildings: A Heat Pump/Organic Rankine Cycle system coupled to solar thermal collectors (HP/ORC-COL), and a Heat Pump coupled with Photovoltaic panels (HP-PV). The HP/ORC-COL system alternates between generating either heat or electricity, whereas the HP-PV system produces them separately. The goal is to compare both systems through their Seasonal Performance Factor (SPF), equivalent emissions, operational cost, and Net Zero Energy Building (NZEB) potential. Additionally, the impact on the heating demand of retrofitting existing buildings with the Passivhaus Standard is studied.
The heating, domestic hot water, and electricity demands are determined by considering the climate conditions of three polluted Chilean cities: Santiago, Concepción, and Temuco. The proposed systems are evaluated through numerical models developed in Python and validated with data from manufacturer catalogues. The validation of the models allows to enhance the result accuracy of the integrated overall model by closely aligning outputs with real conditions, and therefore increases the reliability of the prediction on the potential of the building to reach net zero energy.
Results show that applying the Passivhaus standard allows to reduce heating requirements by up to 30% in all cities. The SPF and electricity production of the HP-PV system is better than the HP/ORC-COL, reaching a NZEB potential of 76.3% in Santiago, 66.6% in Concepción and 60.4% in Temuco. The average cost reduction of the HP/ORC-COL system compared to oil, natural gas, and wood pellet boilers is 82.7%, 90.4%, and 72.3%, whereas for the HP-PV system is 93.3%, 96.3%, and 89.2% respectively. On the CO2 emissions, the HP/ORC-COL system presents an average reduction of 63.6% and 59.7% compared to an oil and natural gas boiler, while the HP-PV system has an average reduction of 72.5% and 69.5% respectively.
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.
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).
The efficient utilization of renewable energy, e.g., solar energy, has become a major requirement to build a clean and efficient energy system and achieve the goal of carbon neutrality. Organic Rankine cycle (ORC) is one of the most promising technologies for converting solar energy into power. However, life-span low operation performance is still a barrier to the wide-spread application of ORC. Design and off-design operation optimization is significant in improving the life-span performance of ORC. In the routine methods, the ORC system is usually designed under a specific rated condition. However, the ORC mostly operates under off-design conditions. The conventional method separates the off-design operation process from the design process and typically loses solution optimality. In the present study, the off-design operation conditions are incorporated into the design process. A time series aggregation-based method for design and operation optimization of a solar driven ORC is proposed. K-means algorithm is applied to achieve the optimal aggregation operating points for the environmental temperature and solar radiation. The design and operation optimization model incorporating the off-design component performance is developed. The optimal design and operating schemes are achieved based on the aggregated operating points, and the effectiveness of the proposed method is validated. The results indicate that the ORC obtained by the proposed method features a 37.37% lower electricity production cost compared with the conventional method.
This study focuses on a thermal power plant design working according to an Organic Rankine Cycle using solar energy. A study will be conducted on the potential of the solar deposit in southern Tunisia. The results proved that Kebili is a suitable site to install the thermal power plant. A thermodynamic analysis of a solar power plant is conducted to simulate the cycle performances during the 15th day of January. The maximum net power was obtained at noon because the increase in solar radiation improved the net power. A calculation code was performed using equation engineering solver (EES) software to simulate the developed model.
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.
The lack of an open-source dynamic simulation framework integrated with well-established thermodynamic package represents a major constraint facing open-source software users in process industry. To overcome this limitation, many approaches can be adopted to create a unified dynamic co-simulation framework. The purpose of this work is to demonstrate one of them which aims at simulating a cyclopentane based ORC cycle powered by solar thermal energy using SAM, coolprop, and Python. All unit operations (UO) involved in the ORC cycle are coded in Python language, while thermodynamic properties are estimated using Coolprop Python wrapper. The required thermal power produced by the solar field (SF) has been simulated through PySAM wrapper and then transmitted to the power cycle. Once the co-simulation architecture is established, the overall system is dynamically simulated using 10 minutes time step weather data file measured in Benguerir city. Subsequently, the tool is validated against literature data for the aforementioned site. The estimated mean absolute error between the simulated and the literature results was found in the range 0.7 % to 10.0 % in terms of the SF produced thermal power, Qsf, and less than 2.25 % in terms of the ORC produced electrical power, W˙net. Thus, the proposed simulation approach allows properly simulating the entire system dynamics response to the solar irradiation evolution overtime with good accuracy. The simulation tool is also used to compare the dynamic performance of the ORC cycle using the weather data of seven sites in Morocco. The obtained dynamic profiles ofQsf, the working fluid (WF) temperatureTwf, and W˙net, for the studied sites are in good agreement with their respective DNI profiles. The most significant ORC time averaged efficiency have been achieved in Tata and Benguerir, its value found to be arround 18 %.
This article proposes a new analytical model that calculates the thermal efficiency of subcritical Organic Rankine Cycles (ORCs) and integrates the main cycle components and parameters in a single analytical solution accounting the effect of turbine and pump isentropic efficiencies, heat recuperation, superheating, fluid type, and hot and cold reservoir temperatures. Previous analytical models for ORCs covered neither heat recuperation, nor pump isentropic efficiency, which are important features for improving the cycle thermal efficiency. The analytical solution was compared with numerical solutions of ORCs for 22 fluids, and the model showed highly accurate for wet and isentropic fluids. The highest percentage divergences were verified for both dry fluids of heavier molecular weight and evaporation temperatures close to the fluid́s critical temperature. The model can be applied to most fluids and for the main operation conditions of ORC applications.
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.
The worrying effects of climate change have led, in the last decades, to the improvement of innovative solutions for low greenhouse emission energy conversion, among which, is the use of micro-ORC (Organic Rankine Cycle) systems for distributed generation, in the framework of combined heat and power applications and renewables exploitation. However, micro-ORCs environmental impact, due to high GWP (global working potential) working fluid leak rate, is an issue still to overcome. Neverthless the interest in using new low GWP refrigerants and their blends is increasing, new fluids have not yet been properly tested into ORC. Numerical studies reveal that low GWP fluids do not always guarantee the same performance of typically used fluids, leading to indirect emissions related to the use of fossil fuels to compensate the lower power production. This study proposes to investigate performance and impact of an innovative micro-ORC test bench when working with HFCs (HydroFluoroCarbons), low GWP fluids and mixtures, with the main aim of comprehensively evaluating its impact due to both direct and indirect greenhouse gas emissions produced in a typical annual operation.
In this study, low-GWP fluids (R1234yf and R1234ze(E)) have been compared with R134a when used in a kW-size reciprocating piston expander. Semi-empirical models of the pump and the expander are employed to analyze how the different fluids thermodynamic characteristics could influence machines behavior into real operation of a micro-ORC. Parameters related to thermo-fluid-dynamic fluids properties are updated compared to the original values calibrated over R134a. Results show that the use of HFOs alternative fluids leads to a loss of electric power and expander efficiency, whose detriment depends on fluids properties and on operation strategy. At a given pressure ratio the decrease of power output is close to 21% and 42%, while the loss on expander efficiency is more limited, being around 6% and 11%, for R1234yf and for R1234ze(E), respectively. Main factors of influence such as saturation pressure, viscosity, heat transfer coefficients and vapour density are discussed. The expander model has also been used to perform the optimization of the built-in volume ratio for each fluid, revealing that a significant enhancement of the expander overall performance could be obtained modifying the intake valve timing, thus reducing under-expansion losses and improving its volumetric efficiency.
Solar-driven organic Rankine cycle (s-ORC) power generation is a promising technology with thermal storage for flexible operation to meet domestic variable electricity demand. A satisfactory efficiency of this technology can be obtained only at medium-to-high temperature, for which conventional flat plate and evacuated tube solar collectors are not suitable while solar concentrators cannot efficiently utilize diffuse solar radiation. Evacuated flat plate (EFP) collectors have recently been developed for efficient solar heat collection in the temperature range from 100 to 200°C, suitable for the ORC system. At present, the cost of EFP collectors is relatively high and will lead to a long payback period of the s-ORC system. To increase the annual power yield and reduce the payback time, inexpensive amorphous silicon (a-Si) solar cells are proposed to be integrated into the EFP collectors. It is the first time to put forward such photovoltaics/thermal (PV/T) design combining a-Si cells and EFP collectors. Compared with polycrystalline silicon cells (poly-Si), a-Si cells may have a higher electrical efficiency at a higher operating temperature due to the thermal annealing effect and are expected to have a long lifetime without encapsulation in the vacuum environment provided by the EFP collectors. In this study, the a-Si PV/T-ORC system using EFP collectors is investigated. Transient performance analysis of a-Si PV/T-ORC is given for the weather data of two selected days. A comparison is also made with a stand-alone poly-Si PV system, poly-Si PV/T-ORC system and s-ORC system with EFP collectors alone, respectively. The results indicate that for a typical day in July, the a-Si PV/T-ORC system has the highest daily power output of 0.822 kWh/m², 102.3% more than the s-ORC system, 23.8% more than the stand-alone poly-Si PV system and 12% more than the poly-Si PV/T-ORC system, respectively.
This study presents a scroll expander modelling methodology for small scale power generation systems by combining scroll geometry and semi-empirical model. Although the semi-empirical model is quite popular, its dependence on several experimentally-determined scroll geometrical and operational parameters makes this approach inflexible for different capacities and operating conditions. Some studies have sought to improve its flexibility in terms of using different working fluids and more accurate empirical parameters, however, those improved models still depend on a considerable number of experimentally-obtained scroll parameters. Therefore, in this study, a practical methodology for a simpler semi-empirical model combined with the operational flexibility of the scroll geometry is presented. Firstly, the flow rates of mainstream and leakage flows are analysed, where a correlation between scroll clearance and pressure ratio is determined. Secondly, a simpler approach to the semi-empirical model of scroll expander is proposed, whereby dependent parameters have been reduced to two parameters by using scroll geometrical calculations. The model is further improved to predict the rotational speed and electricity output by considering the overall friction coefficient of the coupled expander-generator unit. The findings are then compared with the results of an experimental study. The results show that the effective clearance values between scrolls vary according to pressure ratios, increasing from 20μm to 34μm. Mass flow rate can be predicted within 10% deviation from the experimental results for the same inlet conditions and rotational speed at a transient state. Additionally, considering steady state conditions, modelling results show that the rotational speed and electricity output can also be predicted within 8% and 7.5% of deviation, respectively.
This dissertation is devoted to the analysis of selected operating parameters of a prototype micro-cogeneration system with a biomass-fired boiler. The work presents the relevant steps in installation development and the results of research, along with an analysis of the obtained results.
Chapters 1 and 2 present an introduction to the thesis (justification for the selection of research topics) and a list of main symbols used in the dissertation.
Chapter 3 contains an introduction to the issues of using straw in microscale energy systems, including biomass characteristics as a fuel, presentation of biomass conversion possibilities, and comparison of selected micro-cogeneration technologies.
Chapter 4 presents the objectives and thesis. The main goal was the analysis of the possibility of extending the basic functionality of the biomass-fired batch boiler (i.e. heat generation for heating purposes) with electricity generation. This analysis includes (i) the selection of technology, design, construction, and launching the research version of the micro-cogeneration system, (ii) conducting measurement campaigns with implementation of the necessary improvements and (iii) analysis of results in terms of creating a prototype system design, and mathematical models for future analysis of the system. The main thesis was the statement that it is possible and expedient to develop a micro-cogeneration system based on a dedicated biomass-fired batch boiler, which will have high implementation potential with support to manufacturers at the stage of technology development and implementation.
Chapter 5 discusses the theoretical foundations and current state of research in the area of microscale biomass cogeneration systems. The theoretical foundations of the Clausius-Rankine cycle with the use of saturated steam and superheated steam and the possibilities of increasing the efficiency of this type of cycle are discussed. Also, the efficiency of power devices is discussed, and the Clausius-Rankine cycle is compared with the organic Rankine cycle. Then, the current state of research on micro-cogeneration systems is presented, taking into account the work on various micro-scale technologies of combined heat and electricity generation based on biomass and biogas combustion.
Chapter 6 discusses the construction of the experimental rig, taking into account the two heat sources tested (batch boilers), the oil circuit, the steam-condensate circuit, the water circuit, the power generation system, and the control and measurement system with data acquisition.
Chapter 7 presents information on measuring and computational methodology as well as measurement results and the discussion thereof. The experimental section presents the operating characteristics of the KW1 boiler in the oil circuit in the base system, the KW2 boiler in the prototype system, the oil circuit, the steam-condensate circuit, the water circuit, and the power generator. Based on the results obtained, an analysis of data is carried out covering the working characteristics of the KW2 boiler, working characteristics of the evaporator and superheater, the characteristics of changes in steam pressure as a function of temperature as well as issues regarding the pressure drop and temperature of steam in the steam pipeline together with the parameters of the steam engine and generator. Based on the results of the measurements and the analyses, a mobile container installation project and the first version of the computational model for the needs of dynamic simulations in TRNSYS software were developed.
Chapter 8 presents the summary and conclusions resulting from the implementation of this dissertation. This chapter sets out to justify achieving the assumed goals of work and confirms the truthfulness of the theses.
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.
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.
Experimental data are subject to different sources of disturbance and errors, whose importance should be assessed. The level of noise, the presence of outliers or a measure of the “explainability” of the key variables with respect to the externally-imposed operating condition are important indicators, but are not straightforward to obtain, especially if the data are sparse and multivariate. This paper proposes a methodology and a suite of tools implementing Gaussian processes for quality assessment of steady-state experimental data. The aim of the proposed tool is to: (1) provide a smooth (de-noised) multivariate operating map of the measured variable with respect to the inputs; (2) determine which inputs are relevant to predict a selected output; (3) provide a sensitivity analysis of the measured variables with respect to the inputs; (4) provide a measure of the accuracy (confidence intervals) for the prediction of the data; (5) detect the observations that are likely to be outliers. We show that Gaussian processes regression provides insightful numerical indicators for these purposes and that the obtained performance is higher or comparable to alternative modeling techniques. Finally, the datasets and tools developed in this work are provided within the GPExp open-source package.
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.
Experimental data is often the result of long and costly experimentations. Many times, measurements are used directly without (or with few) analysis and treatment. This paper, therefore, presents a detailed methodology to use steady-state measurements efficiently in the analysis of a thermodynamic cycle. The reconciliation method allows to correct each measurement as little as possible, taking its accuracy into account, to satisfy all constraints and to evaluate the most probable physical state. The reconciliation method should be used for multiple reasons. First, this method allows to close energy and mass balances exactly, which is needed for predictive models. Also, it allows determining some unknowns that are not measured or that cannot be measured precisely. Furthermore, it fully exploits the collected measurements with redundancy and it allows to know which sensor should be checked or replaced if necessary. An application of this method is presented in the case of a reversible HP/ORC unit. This unit is a modified heat pump which is able to work as an organic Rankine cycle by reversing its cycle. Combined with a passive house comprising a solar roof and a ground heat exchanger, it allows to get a positive energy building. In this study case, the oil mass fraction is not measured despite its strong influence on the results. The reconciliation method allows to evaluate it. The efficiency of this method is proven by comparing the error on the outputs of steady-state models of compressor and exchangers. An example is given with the prediction of the pinch-point of an evaporator. In this case, the normalized root mean square deviation (NRMSD) is decreased from 14.3 to 4.1 % when using the reconciliation method. This paper proves that the efficiency of the method and also that the method should be considered more often when dealing with experimentation.
Organic Rankine Cycle systems onboard heavy- duty vehicles require effective control to ensure safety and attain satisfactory performance over a broad range of operating conditions. Publications on this subject, however, are surprisingly scarce. After an overview of the different ORC architectures proposed so far, this paper presents the main features required for supervision and control and the most promising solutions in the literature.
In this paper the optimal operation of an Organic Rankine Cycle (ORC) unit is investigated both in terms
of energy production and safety conditions. Simulations on a validated dynamic model of a real regenerative
ORC unit, are used to illustrate the existence of an optimal evaporating temperature which
maximizes energy production for some given heat source conditions. This idea is further extended using
a perturbation based Extremum Seeking (ES) algorithm to find online the optimal evaporating temperature.
Regarding safety conditions we propose the use of the Extended Prediction Self-Adaptive Control
(EPSAC) approach to constrained Model Predictive Control (MPC). Since it uses input/output models
for prediction, it avoids the need of state estimators, making of it a suitable tool for industrial applications.
The performance of the proposed control strategy is compared to PID-like schemes. Results show
that EPSAC-MPC is a more effective control strategy as it allows a safer and more efficient operation of
the ORC unit, as it can handle constraints in a natural way, operating close to the boundary conditions
where power generation is maximized.
An organic Rankine cycle system comprised of a preheater, evaporator, condenser, turbine, generator, and pump was used to study its off-design performance and the operational control strategy. R245fa was used as the working fluid. Under the design conditions, the net power output is 243 kW and the system thermal efficiency is 9.5%. For an off-design heat source flow rate (mW), the operating pressure was controlled to meet the condition that the R245fa reached the liquid and vapor saturation states at the outlet of the preheater and the evaporator, respectively. The analytical results demonstrated that the operating pressure increased with increasing mW; a higher mW yielded better heat transfer performance of the preheater and required a smaller evaporator heat capacity, and the net power output and system thermal efficiency increased with increasing mW. For the range of mW studied here, the net power output increased by 64.0% while the total heat transfer rate increased by only 9.2%. In summary, off-design operation of the system was examined for a heat source flow rate which varied by –39.0% to +78.0% from the designed rate, resulting in –29.2% to +16.0% and –25.3% to +12.6% variations in the net power output and system thermal efficiency, respectively.
This paper shows monitoring results of the operation of a biomass fuelled combined heat and power (CHP) plant using the Organic-Rankine-Cycle (ORC) technology and a district heating network as a heat sink. Furthermore a decentralised thermal cooling system, connected to the district heating is being described. Annual analyses focus on the performance of the facilities and the optimisation potential. Monitoring data from the control system and various sensors in the cycle are assessed over several annual periods. The results give an overview of the economic and ecologic performance of the facility. Auxiliary energies are discussed in detail. Economical and ecological aspects of the system are discussed.
Improving access to energy in developing countries without exacerbating climate change requires novel technical strategies. Micro-CSP power plants using organic Rankine cycles (ORCs) are one example of a promising ap-proach for meeting this challenge. Specifically, expander development has been identified as a critical component for enhancing the performances ofsmall ORC units. The goal of this project is to develop an optimized two-stage scroll expander to be integrated in a micro-CSP power plant designed to supply remote, off-grid areas.
The expansion requirements are firstly defined so as to meet production specifications. Then, employing a deterministic model and a selection process, optimal scroll geometries are chosen for both expansion stages. The deterministic model is executed with Matlab and accounts for the relevant physical phenomena i.e. radial and flank leakages, throttling losses at the in-take and exhaust processes, friction between the two scrolls and mechanical losses in the different bearings. A performance enhancement of about 6% of the isentropic effciency is predicted in comparison with baseline practice. A CAD model of a single-stage prototype is developed in Solidworks following the main architectural features of a compliant HVAC scroll compressor. This CAD is updated and improved with the optimal scroll geometries and some parts are 3D printed to verify their mechanical assembly. Toolpaths to control CNC machines are generated using the CAM program HSMXpress and the parts constituting the expander prototype are manufactured in a machine shop.
In conclusion, the properties of the enhanced scroll expander are evaluated in the context of a typical Micro-CSP plant. A simple steady state model in EES is developed to quantify how three parameters (T_ev, T_cond and eta_exp) influence the power plant overall sunlight to electricity conversion effciency (a gain of 0.73% is predicted). This project underscores the importance of optimizing the expansion process and identifies further research pathways to progress towards this goal.
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.
ORCs (Organic Rankine Cycles) represent an effective option to exploit low grade heat fluxes, the characteristics of which not only affect design, but also performance and stability during operation.
This paper presents a detailed design and off-design dynamic model of a superheated regenerative ORC system using the exhaust gases of an industrial process. The point of view is that of a designer who has to predict the system behavior both at steady-state and transient operation to get a reliable and efficient operation. Real physical and operating characteristics of all components are considered, with particular attention to the geometries of shell-and-tube commercial heat exchangers to properly simulate mass and thermal inertias. A suitable control system is chosen to govern the off-design operation taking into account all real operating constraints.
Results show a slight decrease in gross system efficiency (less than 1% point) either varying the oil mass flow rate (in the range 80–110%) at constant temperature of the cold sink or this temperature (of 10 °C) at constant oil mass flow rate. Simulation of the transient behavior demonstrates the effectiveness of the control system on ORC stability under variation of the hot source mass flow rate and cold sink temperature.
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.
The RCS (Rankine cycle system) used to recover the WHE (waste heat energy) from engines has been regarded as one of the most potential ways of achieving higher efficiency. However, it is of great challenge to keep the RCS still in good performance under driving cycle. This paper tries to reveal and explain its on-road inefficiency. The operating process of the RCS under driving cycle was analyzed in advance. Afterwards, four basic operating modes were defined, including startup mode, turbine turning mode, power mode and protection mode. Then, a RCS model was established and operating performances of the RCS under an actual driving cycle were discussed based on this model. The results indicate that the on-road RCS-E (Rankine cycle system efficiency) is as low as 3.63%, which is less than half of the design RCS-E (7.77%) at the rated operating point. Despite the inevitable vapor state fluctuation, it is the operating mode switching during the driving cycle that leads to the on-road inefficiency. Further investigations indicate that the expander safety temperature and its safety margin affected by the working fluids, designed superheat degree and evaporating pressure are the main factors determining the operating mode switching. Finally, the effects of the working fluids, designed superheat degree and evaporating pressure on the operating mode switching and RC (Rankine cycle) efficiencies were profoundly investigated. The study shows that the dry and isentropic fluids are superior to the wet ones due to their less probabilities of droplets formation as a consequence of their saturated vapor characteristics. The effects of the vapor parameters on the RCT-E (Rankine cycle thermal efficiency) and operating mode switching are opposite. Therefore, in order to optimize the RCS, it would be better to take full consideration in reducing the operating mode switching, while pursuing the maximum RCT-E.
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 presents the performance of an organic Rankine cycle (ORC) powered by medium-temperature heat sources. A simulation model, developed by the authors, has been improved to this scope. The model is based on zero-dimensional energy and mass balances for all the components of the system. It is also strictly related to the geometrical and design parameters of its components, especially in case of heat exchangers. The model evaluates the energetic and economic performance of the system, for different operating conditions and design criteria. In particular, the model allows one to set the geometrical parameters of heat exchanger and evaluate the off-design performance of the system. Hence, it could be an useful tool in the preliminary design of the plant. The n-butane has been used as working fluid according to results of the previous authors’ work.
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.
This literature review focuses on the applications of Computational Fluid Dynamics (CFD) in the field of heat exchangers. It has been found that CFD has been employed for the following areas of study in various types of heat exchangers: fluid flow maldistribution, fouling, pressure drop and thermal analysis in the design and optimization phase. Different turbulence models available in general purpose commercial CFD tools i.e. standard, realizable and RNG k − ε RSM, and SST k − ε in conjunction with velocity-pressure coupling schemes such as SIMPLE, SIMPLEC, PISO and etc. have been adopted to carry out the simulations. The quality of the solutions obtained from these simulations are largely within the acceptable range proving that CFD is an effective tool for predicting the behavior and performance of a wide variety of heat exchangers.
In this paper, the dynamics of organic Rankine cycles (ORCs) in waste heat utilizing processes is investigated, and the physical model of a 100 kW waste heat utilizing process is established. In order to achieve both transient performance and steady-state energy saving, a multivariable control strategy for the waste heat recovery system is proposed by incorporating a linear quadratic regulator (LQR) with a PI controller. Simulations demonstrate that the proposed strategy can obtain satisfactory performance.
The present paper focuses on the experimental characterization of an open-drive scroll expander integrated into an Organic Rankine cycle using R245fa as working fluid. The expander is a commercially available air compressor that was modified to operate in expander mode. The ORC (Organic Rankine Cycle) system is designed for a nominal heat input of 20 kW and a nominal net power output of 1.8 kW. A total of 74 steady-state operating points are measured to evaluate the expander performance over a wide range of conditions. The operating parameters that are varied include the inlet pressure (from 9 to 12 bar), outlet pressure (from 1.5 to 4 bar) and rotational speed (from 2000 to 3500 rpm). The maximum isentropic efficiency and shaft power are, respectively, 75.7% and 2.1 kW. A maximum cycle efficiency of 8.5% is reached for evaporating and condensing temperatures of 97.5 °C and 26.6 °C respectively. For most of the tests, hot water is produced in the condenser and the system therefore behaves as a CHP (combined heat and power). Depending on the water temperature requirement, a power to heat ratio varying between 1.9% and 11.8% is obtained. Water over 50 °C can be produced with a power to heat ratio higher than 8%.The experimental data points are then used to generate a performance map of the expander. This performance map allows for simulation of the use of such an expander in other ORC system.
This paper develops a strategy to maintain steady operation of an organic Rankine cycle (ORC) by adjusting evaporator flow rates in relation to the available thermal energy. ORC unit under investigation uses R245fa as the working fluid, for which a regression based approach was implemented to evaluate its state properties. Steady and transient models for unit’s subcomponents (pump, evaporator, expander and condenser) were developed. Heat source is considered as solar heated water between 80 °C and 95 °C at mass flow rates between 2 kg/s and 12 kg/s, while the flow rate of R245fa is ranging between 0.5 kg/s and 1.5 kg/s. Due to possible changes in the available thermal energy, unit’s evaporator was identified as the critical component of the ORC. Evaporator’s effectiveness was characterized as a function of inlet temperatures and mass flow rates to map steady operation scenarios for changing conditions. Steady state analysis shows that the selected ORC system is capable of producing 13–39 kW power for heat inputs varying between 125 kW and 367 kW with maximum efficiency in the defined operating range. Subsequently, the developed steady state map is used to construct a control strategy. This strategy aims to adjust evaporator flow rates in order to achieve maximum and steady energy recovery for any given level of heat input. The unit is simulated to study its dynamic response when available thermal energy gradually or abruptly changes with and without the control strategy. It is demonstrated that adjusting flow rates not only improves the thermal efficiency but also helps maintaining the steady state operation.
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.
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.
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.
This paper presents a detailed analysis of an organic rankine cycle (ORC) heat recovery power plant using R134a as working fluid. Mathematical models for the expander, evaporator, air cooled condenser and pump are developed to evaluate and optimize the plant performance. Computer programs are developed based on proposed models and algorithms. The effects of controlled variables, including working fluid mass flow rate, air cooled condenser fan air mass flow rate, and expander inlet pressure, on the system thermal efficiency and system net power generation have been investigated. ROSENB optimization algorithm combining with penalty function method is proposed to search the optimal set of operating variables to maximize either the system net power generation or the system thermal efficiency. The optimization results reveal that the relationships between controlled variables (optimal relative working fluid mass flow rate, the optimal relative condenser fan air mass flow rate) and uncontrolled variables (the heat source temperature and the ambient dry bulb temperature) are near liner function for maximizing system net power generation and quadratic function for maximizing the system thermal efficiency.
Organic Rankine Cycles (ORCs) are particularly suitable for recovering energy from low-grade heat sources. This paper describes the behavior of a small-scale ORC used to recover energy from a variable flow rate and temperature waste heat source. A traditional static model is unable to predict transient behavior in a cycle with a varying thermal source, whereas this capability is essential for simulating an appropriate cycle control strategy during part-load operation and start and stop procedures. A dynamic model of the ORC is therefore proposed focusing specifically on the time-varying performance of the heat exchangers, the dynamics of the other components being of minor importance. Three different control strategies are proposed and compared. The simulation results show that a model predictive control strategy based on the steady-state optimization of the cycle under various conditions is the one showing the best results.
The paper proposes two alternative approaches for the design of a dynamic model for an Organic Rankine Cycle (ORC) to be used for the design of control and diagnostics systems. The model has been developed in Modelica language and simulated with Dymola. The two modeling approaches, based on moving boundary and discretization techniques, are compared in terms of accuracy, complexity and simulation speed. Compared to experimental data simulations evidence that while both models have good accuracy. And the moving boundary model is faster, therefore more suitable for control design applications.
Short review of the long history of ORC power systems. In: Keynote lecture of the 2nd International seminar on ORC power systems -ASME-ORC
- L Y Bronicki
Bronicki LY. Short review of the long history of ORC power systems. In:
Keynote lecture of the 2nd International seminar on ORC power systems -ASME-ORC 2013, Rotterdam (NL); 2013.
Organic rankine cycle power systems: from the concept to current technology, applications and an outlook to the future
- P Colonna
- E Casati
- C Trapp
- T Mathijssen
- J Larjola
- T Turunen-Saaresti
Colonna P, Casati E, Trapp C, Mathijssen T, Larjola J, Turunen-Saaresti T, et al.
Organic rankine cycle power systems: from the concept to current technology,
applications and an outlook to the future. J Eng Gas Turbines Power
2015;137(October):1e19.
http://dx.doi.org/10.1115/1.4029884.
http://
gasturbinespower.asmedigitalcollection.asme.org/article.aspx?doi¼10.1115/
1.4029884.
ORCmKit : an open-source library for organic Rankine cycle modelling and analysis
- R Dickes
- D Ziviani
- M De Paepe
- M Van Den Broek
- S Quoilin
- V Lemort
Dickes R, Ziviani D, de Paepe M, van den Broek M, Quoilin S, Lemort V.
ORCmKit : an open-source library for organic Rankine cycle modelling and
analysis. In: Proceedings of ECOS 2016, Portoroz (Solvenia); 2016.
Microsol -a 10 kW solar power plant for rural electrification
- V Rieu
Rieu V. Microsol -a 10 kW solar power plant for rural electrification. In:
Presentation at the SolarPACES 2012 conference, Marrakech; 2012.
Heat transfer calculations for extended surfaces
- T Schmidt
Schmidt T. Heat transfer calculations for extended surfaces. ASHRAE J 1949:
351e7.