Two-stage Organic Rankine Cycle Research Articles

Thermodynamic performance analysis of a novel integrated energy cascade system of liquid air energy storage and two-stage organic Rankine cycles

Liquid air energy storage can enhance the absorptive capacity for renewable energy due to its high energy storage density and extensive application scenarios. This paper proposes an integrated cascade energy system including liquid air energy storage, two-stage organic Rankine cycle, organic Rankine cycle, liquid natural gas regasification and absorption heat pump/chiller to use waste heat and liquid natural gas's cold energy fully and improve the round-trip efficiency. The waste heat of liquid air energy storage and the cold energy of liquid natural gas could increase the temperature and condense the working fluid of organic Rankine cycles. The system's thermodynamic model in design and off-design conditions are established. The charge and discharge minimum loads of liquid air energy storage are 82.5 % and 33.5 %, respectively. The round-trip efficiency of liquid air energy storage obtains a maximum of 49.6 % and a minimum of 29 % in the load ranges. Based on the load changes, the effects on the organic Rankine cycle are discussed, and four control strategies are proposed to absorb the waste heat and cold energy efficiently under off-design conditions. Besides, the organic Rankine cycles are affected by the mass flow of liquid natural gas and split ratio, which are discussed, and its underlying causes are analyzed. The results show that the integrated cascade energy system's round-trip efficiency reaches a maximum of 80.3 % in design conditions and a minimum of 62.7 % in off-design conditions, increasing by 61.9 % and 116.2 %, respectively.

Conventional and advanced exergy-exergoeconomic-exergoenvironmental analyses of an organic Rankine cycle integrated with solar and biomass energy sources

Considering the huge consumption of traditional energy and the rising demand for electricity, the development of renewable energy is very necessary. In this paper, an energy system integrating biomass energy, solar and two-stage organic Rankine cycle (ORC) is proposed, which uses the stable energy output of biomass energy to compensate for the volatility of solar modules. The proposed system comprises a biomass boiler, photovoltaic thermal panels (PV/T), evaporators, condensers, working medium pumps, turbines, a preheater and an air preheater. In addition, conventional and advanced exergy, exergoeconomic and exergoenvironmental (3E) analyses are carried out. Conventional 3E analyses reveal two components that require priority improvement. They are respectively evaporator 1 with the largest exergy destruction (708.2 kW) and exergy destruction environmental impact rate (775.3 mPt/h) and evaporator 2 with the largest exergy destruction cost rate (19.15$/h). The results of advanced 3E analyses show that the largest avoidable endogenous exergy destruction is condenser 1 (136.6 kW), the largest avoidable endogenous exergy destruction cost rate is condenser 2 (3.377$/h), and the largest avoidable endogenous exergy destruction environmental impact rate is condenser 1 (196.1mPt/h). These mean that these components have great potential for improvement in reducing exergy destruction, saving cost and protecting the environment. In addition, the avoidable endogenous exergy destruction/cost/environmental impact rate of evaporator 2 are negative, so evaporator 2 is not suitable as a priority component for improvement, which is contrary to the conclusions of conventional 3E analyses. It is found that conventional 3E analyses can only point out the biggest exergy destruction point, but cannot indicate whether the components with the greatest exergy destruction have the greatest potential for improvement. However, advanced 3E analyses can show the improvement potential of each component by improving its own performance and the external conditions. Therefore, it is necessary to conduct advanced 3E analyses.

A novel optimization method for transcritical-subcritical parallel organic Rankine cycle under variable engine loads and cooling water temperature

Organic Rankine cycle (ORC) is an effective technology for engine waste heat recovery. However, when engine loads and cooling water temperature are variable, the optimization of ORC system will be difficult and time-consuming, especially for complex system like transcritical two-stage ORC. In this study, a novel optimization method is proposed for optimizing the transcritical-subcritical parallel ORC (TSPORC) system, and the system operates under variable engine loads and cooling water temperature. The key idea of the optimization method is a combination of artificial neural network (ANN) and non-dominated sorting genetic algorithm (NSGA-II), the ANN is used to establish a fast and accurate prediction model of the system performance to reduce computing time significantly, and the NSGA-II is furtherly used to optimize system performance fast and accurately based on the ANN prediction model. To improve the accuracy of ANN model, three different artificial neural network models are optimized and compared. To satisfy different objective optimization for application, both single-objective and multi-objective optimization are conducted. The results demonstrate that the optimization method can achieve real time optimization for TSPORC system operating under variable engine loads and cooling water temperature fast and accurately.

Tri-objective optimization of a waste-to-energy plant with super critical carbon dioxide and multi-effect water desalination for building application based on biomass fuels

In the present research, an innovative biomass-based energy system for the production of electricity and desalinated water for building application is proposed. The main subsystems of this power plant include gasification cycle, gas turbine (GT), supercritical carbon dioxide cycle (s-CO2), two-stage organic Rankine cycle (ORC) and MED water desalination unit with thermal ejector. A comprehensive thermodynamic and thermoeconomic evaluation is performed on the proposed system. For the analysis, first the system is modeled and analyzed from the energy point of view, then it is examined similarly from the exergy point of view, and then an economic analysis (exergy-economic analysis) is performed on the system. Then, we repeat the mentioned cases for several types of biomasses and compare them with each other. Grossman diagram will be presented to better understand the exergy of each point and its destruction in each component of the system. After energy, exergy and economic modeling and analysis, the system is analyzed and modeled using artificial intelligence to help the system optimization process, and the model obtained with genetic algorithm (GA) to maximize the output power of the system, minimize the cost system and maximizing the rate of water desalination is optimized. The basic analysis of the system is analyzed inside the EES software, then it is transferred to the MATLAB software to optimize and check the effect of operational parameters on the thermodynamic performance and the total cost rate (TCR). It is analyzed and modeled artificially and this model is used for optimization. The obtained result will be three-dimensional Pareto front for single-objective and double-objective optimization, for work-output-cost functions and sweetening-cost rate with the specified value of the design parameters. In the single-objective optimization, the maximum work output, the maximum rate of water desalination, and the minimum TCR will be 55,306.89 kW, 17216.86 m3/day, and $0.3760/s, respectively.

Thermo-economic performance comparison and advanced analysis using orthogonal experiment for organic Rankine cycle

To improve the thermo-economic performance of organic Rankine cycle (ORC) systems, a multi-level analysis method is proposed in this study. In the first level, the optimal configuration is determined based on the thermo-economic optimization. The exergy efficiency and electricity production cost are selected as the objective functions of optimization for three ORC configurations, including simple ORC, TORC (two-stage ORC), and DORC (dual-loop ORC). In the second level, the real potential of performance improvement for the optimal configuration is revealed through advanced exergy analysis method. Finally, the order of operating parameters to different performance indicators of the optimal configuration is determined by orthogonal experiment method. The results show that the TORC system exhibits better thermo-economic performance. Although the low-temperature condenser in the TORC system has the highest exergy destruction, the LTT (low-temperature turbine) contributes the largest avoidable-endogenous exergy destruction. Thus, the LTT has the greatest potential for technical improvement. The condensation temperature and pinch point temperature difference in the low-temperature evaporator are the primary factors for the thermodynamic and economic performance of the TORC system, respectively. While the evaporation temperature of the high-temperature evaporator has the greatest effect on the potential of technical improvement for the LTT.

Energy and exergy analyses of a two-stage organic rankine cycle with low pressure stage regeneration for IC engine waste heat recovery

A two-stage Organic Rankine Cycle (ORC) with Low Pressure Stage Regeneration (LPSR) proposed in this article is intended to utilize the engine coolant energy completely for vaporization of organic fluid in a Low Pressure stage Heat Exchanger (LPHE) and the engine exhaust energy for sensible heating, vaporization and super heating of organic fluid in a High Pressure stage Heat Exchanger (HPHE) besides utilizing the superheated vapor energy of exhaust from Low Pressure stage Turbine (LPT) in a regenerator. Since regeneration is used only at low pressure stage, the energy associated with the engine exhaust gases can be utilized to the maximum extent by lowering its temperature nearer to the temperature of liquid phase working fluid after High Pressure stage Pump (HPP), thereby maximizing the Waste Heat Recovery (WHR) potential of the bottoming two stage ORC. The WHR efficiency of two-stage ORC with and without LPSR is analyzed at a typical operating condition of the engine using a nearly dry fluid R123 and a nearly isentropic fluid R134a as the working substances. It is observed that the thermal efficiency of the two-stage ORC with R123 is higher than that with R134a. The LP stage regeneration has been found to be effective in increasing the thermal efficiency and, in turn, the WHR efficiency of the two-stage ORC with both R123 and R134a. The increase in the fuel efficiency of the IC engine due to the bottoming two-stage ORC is found to be 7.22% with R123 and 6.21% with R134a with LPSR and 6.58% with R123 and 5.51% with R134a without LPSR. The optimum pressure in HPHE is found to be about 2.5 MPa and 3.5 MPa with R123 and R134a respectively.

Techno-economic optimization of a fuel cell with nanomaterial structure membrane for electricity and heating production

In this research study, a biofuel cell system for simultaneous electricity and heat production is modeled and investigated from the perspectives of energy, exergy, and economics. The polymer-based fuel cell in this study is employed with nanomaterial Pt for enhanced performance. The system includes the primary components of a gasifier, a polymer membrane fuel cell, and a two-stage organic Rankine cycle. Additionally, the impacts of employing the Rankine cycle to recover the waste heat generated by the fuel cell on system efficiency have been explored. Five series of binary combinations for the first and second cycles of the organic Rankine system were explored during this investigation. Among them, the first and second cycles using propane and ethane, respectively, have the maximum energy effectiveness and exergy.The fundamental and effective variables were found. Eventually, the system was optimized using the objective functions of exergy cost and efficiency after conducting a thermodynamic and economic analysis of the cycle using parametric assessment. Ultimately, the most acceptable point of system design, taking both cost and exergy efficiency into account, results in exergy efficiency and cost of 39.86 percent and 32.038 $/h, respectively.

Thermo-economic optimization of a hydrogen storage structure using liquid natural gas regasification and molten carbonate fuel cell

Liquid natural gas (LNG) regasification and fuel cells can be employed to supply the required precooling and electrical power for hydrogen liquefaction, respectively. Such an approach helps reduce the process complexity and the number of equipment, total costs, and carbon footprints, and enhance the controllability of the process. This paper proposes an integrated structure for hydrogen liquefaction and storage by employing the LNG regasification process for precooling and four multi-component refrigerant cycles for liquefaction. Molten carbonate fuel cells, gas turbines power cycles, the two-stage organic Rankine cycle, and the carbon dioxide power cycle are used for supplying the electrical power. A liquid hydrogen production capacity of 1766 kmol/h and 1576.7 MW electrical power generation are attained in this integrated process. The coefficient of performance, specific energy consumption, and specific power consumption of the integrated structure are calculated to be 0.175, 4.772 kWh/kgLH2, and 3.872 kWh/kgLH2, respectively. The exergy efficiency of 83.03% and the exergy destruction of 47.87 MW are obtained based on the exergy analysis. The maximum shares of exergy destruction are attributed to the heat exchangers, gas turbines, and fuel cells with the magnitudes of 45.43%, 24.75%, and 10.93%, respectively. The prime cost of product and rate of return are calculated to be 0.0840 US$/kWh and 24.02%, respectively. According to the sensitivity analysis, it is found that increasing the molar fraction of helium in the multi-component refrigerant cycle from 81.31% to 93.61% leads to a reduction in the net power production (151.2 MW) and total exergy efficiency (81.39%) of the proposed process. Using the multi-objective genetic algorithm method, the total exergy efficiency and the rate of return are optimized to be 86.32% and 29.66%, respectively.

A Two-Stage ORC Integration to an Existing Fluidized Bed Sewage Sludge Incineration Plant for Power Production in the Scope of Waste-to-Energy

This paper presents the design, evaluation, and optimization of an electricity generation system based on the two-stage organic Rankine cycle (TS-ORC), which utilizes the waste heat of an existing fluidized bed sewage sludge incineration (FBSSI) facility. The facility incinerates an average of 300 tons per day of sewage sludge with a dry matter content of 22%. After the drying process, the sewage sludge is burned in a fluidized bed combustor, and exhaust gas at a temperature of about 850-900ºC is released due to the combustion. The system provides the energy required to dry the sludge from this exhaust gas. In this study, a TS-ORC is designed to be coupled to the exhaust gas flowlines discharged to the atmosphere at two different points in the FBSSI plant. The exergy efficiency of the FBSSI facility is found to be 70.5%. Three different working fluids are selected to examine the variations of thermodynamic and thermoeconomic performance parameters of the designed TS-ORC system. The highest power generation in the TS-ORC system (183.40 kW) is achieved using R1234yf as working fluid. R1234yf is also the most expensive fluid for electricity generation among the other working fluids (10.57 $/h). The least electricity generation in the TS-ORC (142.70 kW) occurs at the thermoeconomically most affordable cost with R245fa (9.35 $/h).

Research on synchronous optimization effect of working fluid components and node parameters of organic Rankine cycle (ORC)

In this paper, a new numerical optimization algorithm which can simultaneously optimize the components and operating parameters of organic Rankine cycle is proposed and applied to single-stage organic Rankine cycle and two-stage organic Rankine cycle. Compare with the Nested method, the optimization effect of the numerical method proposed can reach more than 99.99% of the optimal effect if the working medium had not been considered in the two-stage organic Rankine cycle. The optimization time is only about 0.02% of the Nested method. 99.52% of the best performance after the working medium being considered. The results indicate that when the complexity of the system and the number of independent variables increase, the optimization effect of the system decreases slowly. Compared with the previous publication, the overall performance is finally improved 1.68% by the new numerical optimization algorithm.

A novel transcritical-recuperative two-stage Organic Rankine Cycle for dual/multi-source heat recovery applications

The Transcritical Regenerative Series Two-stage Organic Rankine Cycle (TR-STORC) has improved performance over STORC and single-stage ORC. However, the TR design requires vapor extraction between turbine stages, which prevents the use of compact two-stage ORC turboexpanders. An alternative Transcritical-Recuperative (TREC) design is proposed which eliminates vapor extraction allowing direct integration of compact two-stage turbines. The novel TREC-STORC operates with a recuperator acting as an additional evaporator at the low-pressure turbine exit (LP mode). Another TREC design requiring vapor extraction from the high-pressure turbine stage (HP mode), similar to the TR design, is also presented for comparison purposes. TREC-STORC in LP mode presents a marginally improved performance of 1–3% over HP mode for various IC engine heat source conditions. With cyclopentane as working fluid, at engine design point conditions, LP mode delivers 4% higher power outputs over HP mode and TR-STORC. Compared to STORC and single-stage ORC, the LP mode of TREC-STORC delivers 21% and 28% higher power outputs, respectively. For other working fluids, TREC-STORC (LP mode) delivers between 2–5% additional power output over TR-STORC, with 18–38% and 20–56% increased power outputs over STORC and single-stage ORC, respectively. TREC-STORC (LP mode) presents a superior and robust ORC architecture for dual/multi-source heat recovery applications.

Modeling and analysis of a two-stage ORC for recovering waste heat of single flash geothermal cycle

Reusing heat dissipation in thermodynamic cycles is an exciting proposal to increase efficiency. In this paper, a two-stage ORC (Organic Rankine Cycle) is proposed to recover and reuse wasted energy from an SFGC (Single Flash Geothermal Cycle). The working fluids studied for the recovery system include R227ea and R116 and R124 and R125. The effect of the main elements of system performance is investigated using sensitivity analyses. Exergy degradation of various components is also calculated. For working fluids R227ea and R116, the thermal efficiency improved by 7.66%, from 0.2023 to 0.2178. The system's thermal efficiency is improved from 0.2023 to 0.2177 by 7.61% using R124 and R125. The exergy efficiency of the initial working fluid improves by 15.04%, from 0.5044 to 0.5803. Further, the second pair of working fluids from 0.5044 to 0.5852, which indicates a 16.01% system efficiency improvement. 85% of the system exergy is eliminated through the expansion valve, turbine 3, heat exchanger 2, and mixer. Including the recovery phase in the base, SFGC will positively affect the power plant's performance.

Investigation and optimization of a two-stage cascade ORC system for medium and low-grade waste heat recovery using liquefied natural gas cold energy

This study proposed a two-stage cascade organic Rankine cycle (ORC) to harvest the medium and low-temperature discharged heat from internal combustion engine (ICE), and a liquefied natural gas (LNG) cold energy system is integrated into the proposed cycle in order to realize the cascade utilization of ICE multi-grade waste heat more efficiently. The thermodynamic and economic models are developed to investigate the proposed system performance under steady-state conditions. The influence of the important thermodynamic parameters, e.g. the ORC turbine inlet temperature, the high pressure of first-stage ORC and the outlet temperature of condenser are investigated. The calculated results indicated that the thermal efficiency and the exergy efficiency achieved 27.31% and 33.73%, respectively. In addition, it was found that the total sum unit cost of system product was decreased by increasing turbine Ⅰ inlet pressure. Furthermore, to reach the conflicting compromise between the two aspects of thermodynamic and economic, the Non-dominated Sorting Genetic Algorithm (NSGA-II) is employed to carry out the multi-objective optimization study by which the external investment cost and the thermal efficiency are selected as the objective function. At the same time, the influence region of decision variables related to the optimization is determined according to the scatter distribution of decision variables. Moreover, the exergy destruction and the investment cost ratio of each component under the optimization conditions are analyzed to obtain the preferred performance of the key components. The findings could provide references for the ORC system design and operation driven by multi-grade waste heat.

An ejector based Transcritical Regenerative Series Two-Stage Organic Rankine Cycle for dual/multi-source heat recovery applications

The Transcritical Regenerative Series Two-stage Organic Rankine Cycle (TR-STORC) was shown to deliver improved performance than Series Two-stage ORC (STORC) and single-stage ORC for dual-source heat recovery applications. However, TR-STORC utilizes partial evaporation for regeneration which requires precise control of two-phase flows. In real-time operations involving dual/multiple heat sources, this is difficult to achieve due to fluctuating heat inputs, leading to liquid carryover and subsequent corrosion of turbine blades. This study explores a novel Transcritical Ejector Regenerative STORC (TER-STORC), which replaces the need for two phase flows with a avapor-vapor regeneration via an ejector operating entirely with fully evaporated vapor (FE mode). Partial evaporation (PE) mode of TER-STORC requiring two-phase flows is also analyzed for comparison. Results indicate that the FE mode of TER-STORC can achieve performance comparable to PE mode and TR-STORC. FE mode of ejector operation is less sensitive to variations in ejector pressure drop than PE mode while delivering 0.2–4% lower power outputs with lower heat exchanger requirements than TR-STORC by up to 18%.At the engine design point, only a 2% drop in power output is seen for FE mode compared to TR-STORC. TER-STORC presents a robust system with reduced complexity for multisource heat recovery.

Energy and exergy and economic (3E) analysis of a two-stage organic Rankine cycle for single flash geothermal power plant exhaust exergy recovery

A two-stage ORC (Organic Rankine Cycle) is being proposed for the recovery and utilization of low-grade heat from single flash geothermal power plant exhaust flue fluids. The working fluids for two-stage the recovery system are R227ea and R116. The impact of essential design parameters such as inlet temperature of the cycle, ambient temperature, geothermal turbine inlet pressure, geothermal condensation temperature on system performance are investigated using thermodynamic mathematical models regarding energy and exergy efficiencies as the objective function. The addition of the recovery section to the base cycle improves the thermal efficiency from 0.2023 to 0.2178, a 7.66% improvement in this critical metric. Furthermore, the exergy efficiency rises from 0.55044 to 0.5803, representing a 15.04% boost in exergy efficiency. Furthermore, the system's overall output rises from 7690 kW to 9898 kW, representing a substantial gain of 28.71%. The suggested recovery system's cycle has a lower LEC than a single flash geothermal cycle from an economic standpoint. LEC Reduced from $ 0.125 per kilowatt to $ 0.108 per kw, a cost savings. As a result, including a recovery section in the basic single flash geothermal cycle will significantly improve system performance.

The Effect of Using Different Refrigerants on System Performance in Two-Stage Organic Rankine Cycle

In this study, energy analysis of a double-stage Organic Rankine Cycle (ORC) was performed using the Engineering Equation Solver program (EES). In the first and second stages, eight different organic working fluids (R134a, R410a, RC318, R407c, R22, R23, R116, and R218) were used in pairs for certain temperature limits. Choosing a suitable refrigerant or refrigerant pair is crucial for the ORC’s high thermal efficiency and net work output value. This study aims to determine the effect of working fluid on system efficiency and find out the most suitable working fluid pair. The results showed that the highest net power output and the highest thermal efficiency were obtained from the R23+R23 pair. It was also observed that the lowest net power output from the RC318+RC318 pair and the lowest thermal efficiency was obtained from the R218+R218 pair.

A geospatial analysis approach for the operational assessment of solar ORC systems. Case study: Performance evaluation of a two-stage solar ORC engine in Greece

Renewable energy potential depends on many variables that vary both in space and time. Therefore, geospatial analysis approaches may effectively support proper planning and technoeconomic analysis. In this study, a simulation model of a two-stage Organic Rankine Cycle (ORC) engine combined with evacuated tube solar collectors was developed and applied throughout Greece. The simulation model uses daily meteorological data for a thirty-four years reference period. These data are downscaled in hourly time step and used to predict the system's performance in spatially distributed form based on experimental data. The obtained results are statistically analysed and a set of ranking maps presenting various aspects of the integrated system performance is created. The results indicate that there is a profound spatial variability. The interannual average power production ranges spatially between 17.4 and 27.7 kWh/d, the total efficiency between 0.022 and 0.028 and the corresponding energy cost between 0.41 and 0.7 €/kWh. Finally, the effect of climate variability on energy production performance and the presence of trends are investigated. The proposed approach was proven to be an effective tool for the assessment of the feasibility and potential of a solar heat-to-power engine for Greece and to be expanded in other regions as well.

Energy, exergy and pinch analyses of a novel energy storage structure using post-combustion CO2 separation unit, dual pressure Linde-Hampson liquefaction system, two-stage organic Rankine cycle and geothermal energy

The cost of electricity production and its purchase price markedly differ at different hours of the day; therefore, the idea of storing electricity during off-peak hours (low price) and its use during peak hours (high price) is discussed. In this study, a novel integrated structure for separating CO2 from the output smoke of a power plant using a chemical absorption unit via the post-combustion and CO2 liquefaction method by the dual pressure Linde-Hampson liquefaction system during off-peak hours is developed. The low-temperature organic Rankine cycle and the gas turbine power production unit are used to produce power during peak hours. The post-combustion CO2 separation unit produces 9307 kmol/h CO2 from the power plant's output smoke by absorbing 249.6 MW heat flow from the geothermal energy and 24.42 MW power. At hours when the energy demand is low, the produced CO2 is converted into condensed liquid form by receiving 18.31 MW power. When the demand for electric energy consumption is increased, this integrated structure produces 11.61 MW power. The multi-functional LCES system round trip and baseline LCES system exergy efficiencies are 36.75% and 58.48%, respectively. The exergy analysis results illustrate that the highest exergy destruction contribution in the equipment belongs to the fractionating columns (48.52%), heat exchangers (43.62%), and compressors (2.86%). The heat exchanger network related to the HE12, HE15, HE16, and HE18 multi-stream heat exchangers are extracted by pinch analysis. The sensitivity analysis illustrates that the exergy efficiency of the power generation cycle, baseline LCES plant, and multi-functional LCES structure increase up to 33.19%, 60.87%, and 21.14%, respectively when the pumped pressure of liquid carbon dioxide at peak consumption increases from 80 bar to 120 bar.

Analysis and optimization of a fuel cell integrated with series two-stage organic Rankine cycle with zeotropic mixtures

In this article, thermodynamic modeling of a cogeneration system consisting of a series two-stage organic Rankine cycle (STORC) and a proton exchange membrane (PEM) fuel cell is conducted. The fuel cell dissipated heat is utilized as STORC plant input. In order to gain a higher efficiency for the proposed cogeneration system, the condenser of the organic Rankin cycle is replaced by a thermoelectric generator (TEG) to minimize heat loss. Moreover, zeotropic mixtures have been employed due to their lower irreversibility compared to single working fluid. Simulation code is developed in MATLAB software linked with the REFPROP software to extract the thermodynamic properties. This simulation code calculates the exergy efficiency and system's total cost rate. Since the performance of the system is affected by the working fluid, three zeotropic mixtures are compared with R123. The parametric study shows that high pressure (HP) and low pressure (LP) evaporator temperature, current density, and PEM operating pressure significantly affect the total cost rate and the second law efficiency. The results indicate that Ipentane-cis Butane has better efficiency among the selected zeotropic mixtures. Furthermore, the genetic algorithm multi-objective optimization is applied to determine the optimal design parameters of the system in a scatter distribution schematic. Finally, the normalized Pareto frontier of Ipentane-cis Butane is given and the related best point of working as a higher exergy efficiency and lower cost rate are specified. Eventually, it is concluded that the integration of STORC with primary PEM fuel cell improves overall exergy efficiency by 1.9%. The total cost rate for optimum point can be in a range of 1.36–14.94 ($/h), depending on the hydrogen production process.

Exergy Analysis of Two-Stage Organic Rankine Cycle Power Generation System.

Organic Rankine cycle (ORC) power generation is an effective way to convert medium and low temperature heat into high-grade electricity. In this paper, the subcritical saturated organic Rankine cycle system with a heat source temperature of 100~150 °C is studied with four different organic working fluids. The variations of the exergy efficiencies for the single-stage/two-stage systems, heaters, and condensers with the heat source temperature are analyzed. Based on the condition when the exergy efficiency is maximized for the two-stage system, the effects of the mass split ratio of the geothermal fluid flowing into the preheaters and the exergy efficiency of the heater are studied. The main conclusions include: The exergy efficiency of the two-stage system is affected by the evaporation temperatures of the organic working fluid in both the high temperature and low temperature cycles and has a maximum value. Under the same heat sink and heat source parameters, the exergy efficiency of the two-stage system is larger than that of the single-stage system. For example, when the heat source temperature is 130 °C, the exergy efficiency of the two-stage system is increased by 9.4% compared with the single-stage system. For the two-stage system, analysis of the four organic working fluids shows that R600a has the highest exergy efficiency, although R600a is only suitable for heat source temperature below 140 °C, while other working fluids can be used in systems with higher heat source temperatures. The mass split ratio of the fluid in the preheaters of the two-stage system depends on the working fluid and the heat source temperature. As the heat source temperature increases, the range of the split ratio becomes narrower, and the curves are in the shape of an isosceles triangle. Therefore, different working fluids are suitable for different heat source temperatures, and appropriate working fluid and split ratio should be determined based on the heat source parameters.