Dual-pressure Organic Rankine Cycle Research Articles

Performance improvement analysis of the regenerative dual-pressure organic flash cycle assisted by ejectors

We proposed a dual-ejector-based regenerative dual–pressure organic flash cycle (DEj–RDPOFC) with higher performance than the RDPOFC. The effects of high and low pressures and the entrainment ratio of the ejector on the DEj–RDPOFC performance were analyzed from the viewpoints of thermodynamics and thermoeconomics. An exergy-flow distribution diagram revealed why the DEj–RDPOFC outperformed the RDPOFC. Next, the performances of the DEj–RDPOFC and dual–pressure organic Rankine cycle (DORC) were compared under the same boundary conditions. Finally, the performance of the DEj–RDPOFC was optimized using the nondominated sorting genetic algorithm (NSGA-II). According to the results, the power output of the high-pressure (HP) turbine of the DEj–RDPOFC can be increased by replacing the HP throttling valve (TV) with an ejector (which increases the working fluid dryness of the HP vapor separator and the mass flow rate of the HP turbine) and the low-pressure (LP) TV with an ejector (which increases the HP turbine expansion ratio). The DEj–RDPOFC achieves a higher maximum net power output (Wnet) and a lower corresponding levelized cost of electricity (LCOE) than the DORC. Under the optimum condition, the Wnet and LCOE of the DEj–RDPOFC are 9.71 % higher and 6.59 % lower, respectively, than those of the RDPOFC.

Multi-objective optimization of the dual-pressure organic Rankine cycle system based on the orthogonal design method under different external conditions

The dual-pressure organic Rankine cycle (DPORC) system can obtain a better temperature match between the heat source and working fluid due to the existence of two evaporation processes, which can significantly improve the utilization rate of the heat source and reduce the irreversible loss. Firstly, single-objective optimization of the DPORC system is conducted based on the orthogonal design method with thermal efficiency, exergy efficiency, electricity production cost (EPC) and annual emission reduction (AER) as the performance indicators, respectively. Then, multi-objective optimization of all performance indicators is carried out based on the comprehensive scoring method. Finally, the contribution rates of the operation parameters and the optimal working conditions under different external conditions are investigated. The results show that condensation temperature, high-pressure evaporation temperature and turbine isentropic efficiency are the most critical parameters for system comprehensive performance. When heat source temperature is 170 °C, thermal efficiency, exergy efficiency, EPC and AER under the optimal working conditions are 11.91%, 48.46%, 2.71 × 10−2$/kWh and 11.17 × 105kg, respectively. With the increase of heat source temperature, condensation temperature and turbine isentropic efficiency always have an important influence on system comprehensive performance. The contribution rates of high-pressure evaporation temperature and low-pressure evaporation temperature increase and decrease, respectively. Better comprehensive performance can be obtained by using HC working fluid.

Assessment of a sustainable power generation system utilizing supercritical carbon dioxide working fluid: Thermodynamic, economic, and environmental analysis

Solar energy stands out as one of the most abundant and cost-effective resources for energy conversion systems, playing a pivotal role in addressing the world's growing energy demands and environmental concerns. This study focuses on a power generation system that integrates multiple technologies for efficient energy conversion, including a parabolic solar trough collector unit, a supercritical carbon dioxide Brayton cycle, and two dual-pressure organic Rankine cycles. The goal is to assess this system from various angles, including energy, exergy, exergoeconomic, and exergoenvironmental aspects. The base mode study results reveal an overall energy efficiency of 16.66 %, exergy efficiency of 17.88 %, a total net power generation of 17.32 MW, a payback period of 4.38 years, and a total exergoenvironmental impact rate of 126.75 Pt/h. Additionally, the study conducts a parametric investigation to analyze how the system performs under different conditions and how changes in design parameters affect its main outputs. Furthermore, the study utilizes a multi-objective particle swarm optimization algorithm and a decision-maker method to identify the most desirable condition for the system's performance. Three optimization scenarios are considered, and various specific costs for electricity are evaluated to determine the payback period and net present value.

Optimizing solar-driven multi-generation systems: A cascade heat recovery approach for power, cooling, and freshwater production

The pursuit of an optimal solution for performance is essential in ensuring that a solar-driven multi-generation system functions efficiently. Due to the vast amount of solar power received by such systems, the production of significant waste heat ensues. Therefore, this study aims to propose and optimize a new cascade heat recovery method for a solar system that generates power, cooling, and freshwater. The designed configuration encompasses a parabolic trough solar collector to supply the required input energy of integration of a dual-pressure organic Rankine cycle with an ejector refrigeration cycle and an integration of a steam power cycle with a thermal vapor compression- multi-effect desalination. Employing both thermodynamic and economic assessments leads to attaining the performance indexes, which are involved in the optimization procedure. The outcomes of the simulation reveal that the net power, cooling, and freshwater productions are attainted at about 12.56 MW, 2.01 MW, and 138.3 kg/s. Also, the mass fraction 0.5–0.5 is an ideal composition for the highest values of net power and exergy efficiency, and these indexes are highly sensitive to changes in the temperature of vapor generator 3. The optimal operation mode provides an exergy efficiency of 14.76 % and a payback period of 5.49 years. In conclusion, the proposed cascade heat recovery method not only enhances the overall efficiency of the solar-driven multi-generation system but also demonstrates economic viability with a reasonable payback period.

Green and Efficient Recovery and Optimization of Waste Heat and LNG Cold Energy in LNG-Powered Ship Engines

This study focuses on the Wartsila 9L34DF engine and proposes an integrated system for low-temperature carbon capture using the coupling of cold and hot energy recovery with membrane separation in LNG-powered ships. By utilizing a series dual-pressure organic Rankine cycle (SDPORC) system to recover waste heat from the engine exhaust gases and generate electricity, the system provides power support for the low-temperature carbon capture compression process without consuming additional ship power. To validate the accuracy and reliability of the mathematical model, the simulation results are compared with the literature’s data. Once the model’s accuracy is ensured, the operational parameters of the integrated system are analyzed. Subsequently, working fluid optimization and genetic algorithm sensitive parameter optimization are conducted. Finally, under the optimal operating conditions, the thermodynamic performance and economic evaluation of the integrated system are assessed. The results demonstrate that the net power output of the integrated system is 100.95 kW, with an exergy efficiency of 45.19%. The unit carbon capture cost (UCC) is 14.24 $/ton, and for each unit of consumed LNG, 1.97 kg of liquid CO2 with a concentration of 99.5% can be captured. This integrated system significantly improves the energy utilization efficiency of ships and reduces CO2 emissions.

Multi-aspect evaluation of a novel double-flash geothermally-powered integrated multigeneration system for generating power, cooling, and liquefied Hydrogen

An integrated renewable geothermal-driven multigeneration system is demonstrated in the present evaluation, consisting of a double-flash geothermal unit, a modified dual-pressure organic Rankine cycle, a Proton exchange membrane electrolyzer, and a Claude Hydrogen liquefication subsystem. The proposed system is examined from various standpoints, and the performance of the system is quantified through the output parameters. Besides, a case study is performed to evaluate the operation of the system in a specified state. Moreover, a sensitivity analysis is conducted to follow the behavior of the system under different conditions. Furthermore, a multi-objective particle swarm optimization algorithm is employed to define the optimum solutions. The results of the study bring out a total generated cooling load of 2.27 MW, a net output power of 10.48 MW, and a liquefied Hydrogen rate of 37.83 kg/h. Also, exergetic efficiency and exergy destruction rate values of 55.89 % and 9.39 MW are obtained, respectively. The exergoeconomic and exergoenvironmental outputs are discussed through a total cost rate of 482.62 $/h, a payback period of 3.27 years, and a product exergy-related environmental impact rate of 9.84 Pts/h.

Simulation and multi-aspect investigation of a geothermal-based power, hydrogen, oxygen, and fresh water production integrated into a flue gas-driven combined power plant

This paper introduces and evaluates a novel multi-generation structure that employs geothermal energy and utilizes waste heat from flue gas. This innovative structure includes a combined flash and binary geothermal system, a multi-effect desalination subsystem, a proton electrolyte membrane electrolyzer, a steam Rankine cycle, and a dual-pressure organic Rankine cycle. The proposed plant is evaluated from energy, exergy, economic, and environmental viewpoints. Moreover, a detailed parametric analysis is performed to study the effect of different parameters on the system performance. Results obtained demonstrate that this proposed process can generate 21.93 kg/s of fresh water, 346.1 kg/h of green hydrogen, 2739 kg/h of green oxygen, and 19,090 kW of power. Furthermore, the overall energy, exergy, electrical efficiencies, and total unit cost of the product for this new process are determined to be 16.44%, 53%, 8.7%, and 4.56 $/GJ, respectively. Additionally, through exergy analysis, it is established that the Proton Electrolyte Membrane Electrolyzer boasts the highest efficiency within the proposed process, with an exergy efficiency of 88.84%. Its contribution to the total irreversibility of the system is calculated at 1%. Moreover, the analysis reveals that the combined flash and binary geothermal system is the most significant contributor to irreversibility, accounting for 58% of the total, while turbine 2 represents 35.53% of the system's overall irreversibility. From an environmental perspective, employing this proposed process for power generation leads to an annual saving of 44,482,754 liters of petroleum. From an economic standpoint, the total investment cost of the proposed process amounts to $20,743,728. Among the subsystems, the combined flash-binary geothermal system incurs the highest investment cost, representing 41% of the total, while the proton electrolyte membrane electrolyzer has the lowest cost share at 6%.

An energy-saving hydrogen liquefaction process with efficient utilization of liquefied natural gas cold energy

Storage and transportation of hydrogen in liquid form offers considerable advantages, but the high energy consumption of hydrogen liquefaction processes needs to be addressed. In this study, a novel hydrogen liquefaction process integrated with a dual-pressure organic Rankine cycle (DORC) is proposed. It offers significant energy-saving by efficiently utilizing the cold energy of liquefied natural gas (LNG). The key parameters in the proposed process are investigated, and the results are combined with an improved genetic algorithm for efficient optimization of the process. The optimal exergy efficiency of the proposed process is 48.7%, and the minimal energy consumption is 6.29 kWh/kgLH2. Thanks to the power output (381.3 kW) of the DORC and the energy saved by the cryogenic compression of nitrogen (586.6 kW), energy consumption of the proposed process is reduced by approximately 3.1% compared to that of the process without a DORC. Moreover, an improved helium Brayton refrigeration cycle for hydrogen cryo-cooling is simpler, but less the refrigerant usage and energy consumption compared to the previously proposed cycles. The proposed process demonstrates excellent performance in terms of energy saving and efficiency improvement, providing a low-cost scheme to produce liquid hydrogen.

Achieving reasonable waste heat utilization in all truck operating conditions via a dual-pressure organic rankine cycle and its operating strategy

The engine of truck contains multiple waste heat sources. Waste heat recovery (WHR) in truck is a necessary approach to achieve energy saving and cleaner production. These heat sources vary transiently and have thermal management requirements. Therefore, effective waste heat utilization should fulfill both waste heat recovery and thermal management requirements. The dual-pressure organic Rankine cycle (DPORC) is suitable for complex and fluctuating heat sources. Furthermore, a suitable operation strategy is essential for the reasonable waste heat utilization under all engine working conditions and complex road conditions. Therefore, this study designs a DPORC-WHR system and its operation strategy. The essence of this operating strategy is that the evaporation pressure of system can be regulated at different engine loads, thus regulating the heat exchange of each waste heat source. The goal of this operating strategy is to control the waste heat utilization of exhaust gas, coolant and charge air to near 100% at all operating conditions. The dynamic model is established to validate the effectiveness of the proposed DPORC-WHR system and its operation strategy. The results show that the utilization rate of coolant, charge air and exhaust gas is greater than 87.47%, 88.34% and 95.45% respectively for all engine operating conditions. Under the road conditions tested, the DPORC-WHR system can improve engine BTE by 5.98%. The temperature of the coolant and charge air after heat exchange, superheat degree and pressure can be maintained within the target range. This study provides a basis for the operation and control of ORC-WHR system after coupling with the vehicle in the future.

Exergoeconomic and exergoenvironmental analysis and optimization of an integrated double-flash-binary geothermal system and dual-pressure ORC using zeotropic mixtures; multi-objective optimization

The current study focuses on proposing a double-flash geothermal system with a dual-pressure organic Rankine cycle that utilizes zeotropic mixtures. This system aims to harness the geothermal energy efficiently and sustainably. The performance of the system is evaluated using the exergy, exergoeconomic, and exergoenvironmental approaches, taking into account the utilization of different mass fractions of the R123/R142b mixture. The results of the analysis indicate that the designed system is capable of producing a net power output of 6336.04 kW with an exergetic efficiency of 66.70%. The payback period of the system is estimated to be 3.48 years, indicating its economic viability. Moreover, the electricity sale price has a substantial impact on the system's payback period and revenue, surpassing the influence of the geofluid purchase cost. Through the application of triple-objective optimization scenarios, the best optimum state of the system is determined to achieve a 66.96% exergetic efficiency, 3017.82 mPts/h exergoenvironmental impacts, and a payback period of 3.38 years. Furthermore, this state yields a revenue of 11.1 M$. Overall, the study highlights the potential of the proposed double-flash geothermal system with a dual-pressure organic Rankine cycle utilizing zeotropic mixtures for sustainable power generation.

Integrated hydrogen liquefaction process with a dual-pressure organic Rankine cycle-assisted LNG regasification system: Design, comparison, and analysis

Improving the efficiency and reducing the cost of hydrogen liquefaction systems have become a subject of importance nowadays. Through the comparison and analysis of existing processes, a hydrogen liquefaction system that can enhance the utilization efficiency of liquefied natural gas (LNG) cold energy is developed. The system includes a dual-pressure organic Rankine cycle (DORC)-assisted LNG regasification process and improved cascade Joule-Brayton refrigeration cycles. To assess the performance of the proposed process, two forms of ORC and three Brayton refrigeration cycles are compared, respectively. The genetic algorithm is applied to optimize the proposed process. The coefficient of performance, exergy efficiency, and specific energy consumption of the proposed process is 0.20, 46.9%, and 6.61 kWh/kgLH2, respectively. Owing to the efficient utilization of LNG cold energy, the power output of the DORC is 16.2% higher than that of a regular ORC. Furthermore, the improved cryo-cooling process exhibits a significant performance enhancement. Compared to two existing processes, the UA value of the heat exchangers in the proposed cryo-cooling process is reduced by 47.6% and 8.3%, respectively, and the required mass flow rate of helium is reduced by 45.3% and 8.6%, respectively. Moreover, the exergy analysis performed on the system indicates that the exergy utilization rate of the hydrogen pre-cooling process is 80.2%, while that of the cryo-cooling process is only 48.7%.

Multi-objective optimization and exergoeconomic analysis of solar and geothermal-based power and cooling system using zeotropic mixtures as the working fluid

The current paper proposes a double-flash geothermal system with a dual-pressure organic Rankine cycle binary system. To improve the system’s performance, the inlet geofluid is preheated via an indirect mode of a solar system, and an Ejector Refrigeration Cycle system is installed to the geofluid’s brine line to produce cooling capacity. Comprehensive investigations are performed to select the Dual-pressure Organic Rankine Cycle s working fluid among various pure substances and zeotropic mixtures that refer to choosing the R245ca/R142b. The system presents 12.12 MW net power at the base mode with a 1.87 MW cooling load and 4.88 years payback period. Some main parameters’ change effect on the system is studied. The low-pressure steam turbine’s exit pressure influences the cooling load and energetic efficiency. The high-pressure turbine’s input pressure affects the total exergy destruction and net power higher than the other. The optimum state of the system is obtained through two energy-economic and exergy-economic assessments. The energy-economic assessment provides a higher cooling load of about 5.7 MW and a lower net power of about 10.8 MW with 4.59 years payback period. The exergy-economic assessment represents 12.22 MW net power and a cooling load of about 1.97 MW with 4.77 years payback period.

Waste heat recovery from a flame-assisted fuel cell utilizing recompression supercritical CO2 Brayton and dual-pressure organic Rankine cycles

A Flame-assisted Fuel Cell is the combination of the Solid Oxide Fuel Cell with fuel-rich combustion. In this work, two different bottoming power cycles i.e., the RSCBC and the DORC are employed to the waste heat recovery of a methane-fueled Flame-assisted Fuel Cell (FFC). Energy and exergy analyses of the FFC-RSCBC combined cycle show that this combination improves the energy and exergy efficiencies from 18.62% to 22.91% and 18.05%–22.21%, respectively. The cathode inlet air is preheated by the exhaust gases from fuel-lean combustion in the FFC-DORC combined cycle, which increases the efficiency. The improvements in energy and exergy efficiencies are 20.54–20.94% and 19.91–20.3% respectively. Comparison of the performance of the proposed cycles show that the FFC-RSCBC generates more power compared to the FFC-DORC (W˙FFC−RSCBC=184.4kW,W˙FFC−DORC=168.6kW) and the exergy loss of FFC-RSCBC and FFC-DORC is 22.02 kW and 3.46 kW, respectively.

Exergoeconomic and exergoenvironmental assessment of a geothermal-driven cogeneration system utilizing dual-pressure organic Rankine cycle and zeotropic mixtures

In the current study, a detailed assessment and multi-objective optimization of a geothermal-driven cogeneration system for producing hydrogen and electricity have been carried out. The system includes two parts; the primary cycle is the geothermal cycle, which extracts geothermal energy. The secondary cycle consists of an organic Rankine cycle (ORC) and a proton exchange membrane (PEM) electrolyzer. In order to enhance ORC performance, zeotropic mixes have been used as working fluids. The concentration on the destructive environmental impacts of the system, which has not been considered in prior studies, is the research’s main novelty. This study mainly focuses on environmental evaluation using Life Cycle Assessment (LCA) to decrease destructive environmental impacts. From the perspectives of energy, exergy, exergoeconomic, and exergoenvironmental (4E), the pentane (0.3)-butane (0.7) working fluid performed better than other working fluids. Energy and exergy efficiency for the cogeneration system increased by 19.34% and 58.67%, respectively. According to the results of the exergoenvironmental analysis, the PEM showed significant product environmental consequences at a speed of 261 mpts/h. The multi-objective optimization findings illustrated that energy efficiency and destruction cost in optimum conditions reach 20.06% and 22.64 $/h. Furthermore, exergy efficiency and destruction environmental impacts will reach 59.81% and 1762.11 mpts/h, respectively.

Exergoeconomic and Exergetic Sustainability Analysis of a Combined Dual-Pressure Organic Rankine Cycle and Vapor Compression Refrigeration Cycle

In this paper, an exergoeconomic and exergetic sustainability analysis of a dual-pressure organic Rankine cycle (ORC) and vapor compression refrigeration cycle (VCRC) driven by waste heat is performed for power generation and cooling production. In addition, the most suitable fluid couple among the thirty-five different fluid pairs was investigated for the proposed combined system. The results indicate that the highest energy utilization factor, exergy efficiency, the system coefficient of performance, and net power are calculated for the R123-R141b fluid pair. In terms of exergetic sustainability indicators, the best performance results are obtained for the R123-R141b fluid combination. The minimum unit electricity generation cost and the shortest payback period are calculated as 0.0664 $/kWh and 2.5 years, respectively, for the R123-R290 fluid pair. The system component with the highest exergy destruction is the boiler, with 21.67%. The result of the parametric analysis showed that the thermodynamic performance parameters increase with the increment of the ORC’s boiling temperature. In addition, with the increasing boiling temperature, the environmental effect factor of the system decreases, while the exergetic sustainability index increases. Additionally, as the boiling temperature increases, the total system cost increases, while the unit electricity production cost and payback period decrease. It is suggested to use a R123-R141b fluid couple among fluid pairs created as a result of thermodynamic, exergoeconomic and sustainability analysis.

Selection maps of dual-pressure organic Rankine cycle configurations for engine waste heat recovery applications

Dual-pressure organic Rankine cycles are widely used for engine waste heat recovery. Moreover, several dual-pressure organic Rankine cycle configurations exist. The configuration design and parameter optimisation of an optimal dual-pressure organic Rankine cycle system under different engine waste heat source conditions is difficult. Thus, an efficient and accurate selection method is required. To select system parameters for different waste heat sources quickly and accurately, this study proposes a method for constructing system selection maps. To use this method and verify its correctness, we applied it to a practical example of a dual-pressure organic Rankine cycle system selection process. The selection maps demonstrate that the best dual-pressure organic Rankine cycle system can increase the net output power and thermal efficiency by 8.34 %–28.52 % and 8.43 %–32.38 %, respectively, compared to the dual-pressure organic Rankine cycle system with the lowest net output power and thermal efficiency. After obtaining selection maps, the key parameters affecting the distribution of each selection map were analysed, as well as the map itself. The selection maps considered in this study provide a reference for appropriately selecting dual-pressure organic Rankine cycle systems for engine waste heat recovery. This research method can be extended to other areas (geothermal and solar) for waste heat recovery and other cycle configurations for optimum selection and design. In engineering applications, the selection map allows for quick and accurate determination of the best matching system for the target waste heat source, which reduces the capital and time costs of technical advice.

Comparison Study of Four Typical ORC Configurations for Different Waste Heat Characteristics of Engine

The organic Rankine cycle (ORC), as the most promising technology for engine waste heat recovery, has many configurations available to match the complicated engine waste heat conditions. This paper is aimed at revealing the characteristics of the engine waste heat condition for which different ORC configurations are matched so as to give selection guidance. The engine waste heat sources are characterized by the exhaust gas temperature (300°C-650°C) and the heat ratio of the high temperature waste heat to low temperature waste heat (0.5-1.5). Four mostly investigated configurations including the single-loop ORC, the cascade dual-loop ORC, the dual-pressure ORC, and the CO2 transcritical Rankine cycle (CTRC) are selected and modified to enable efficient utilization of all waste heat sources. Their thermodynamic performance is optimized for different waste heat characteristics. The results show improvements in net power for all configurations when exhaust gas temperature and heat ratio rise from 300°C, 0.5, to 650°C, 1.5, respectively, of which the high-temperature split CTRC (HTS-CTRC) recorded the largest increase of 183.58%, while the split regenerative ORC (SR-ORC), dual-pressure ORC (D-ORC), and split cascade ORC (SC-ORC) increased by 17.41%, 23.71%, and 35.45%, respectively. Two selection maps are established to provide general references for quickly selecting the configuration according to different waste heat characteristics whatever the engine type and operating conditions.

Thermodynamic Optimization of Advanced Organic Rankine Cycle Configurations for Geothermal Energy Applications

The Organic Rankine Cycle (ORC) is commonly accepted as a viable technology to convert from low to medium temperature geothermal energy into electrical energy. In practice, the reference technology for converting geothermal energy to electricity is the subcritical simple ORC system. Over time, geothermal ORC plants with more complex configurations (architectures) have been developed. In the open literature, a large number of advanced architectures or configurations have been introduced. An analysis of the scientific literature indicates that there is some confusion regarding the terminology of certain advanced ORC system architectures. A new categorization of advanced configurations has been proposed, with a special emphasis on the application of geothermal energy. The basic division of advanced plant configurations is into dual-pressure and dual-stage ORC systems. In this study, the real potential of advanced ORC architectures or configurations to improve performance as compared with the simple ORC configuration was explored. The research was conducted for a wide range of geothermal heat source temperatures (from 120 °C to 180 °C) and working fluids. Net power output improvements as compared with the basic subcritical simple ORC (SORC) configuration were examined. The ability to produce net power with different ORC configurations depends on the magnitude of the geothermal fluid temperature and the type of working fluid. At a lower value of geothermal fluid temperature (120 °C), the most net power of 18.71 (kW/(kg/s)) was realized by the dual-pressure ORC (DP ORC configuration) with working fluid R1234yf, while the double stage serial-parallel ORC configuration with a low-temperature preheater in a high-temperature stage ORC (DS parHTS LTPH ORC) generated 18.51 (kW/(kg/s)) with the working fluid combination R1234yf/R1234yf. At 140 °C, three ORC configurations achieved similar net power values, namely the simple ORC configuration (SORC), the DP ORC configuration, and the DS parHTS LTPH ORC configuration, which generated 31.03 (kW/(kg/s)) with R1234yf, 31.07 (kW/(kg/s)) with R1234ze(E), and 30.96 (kW/(kg/s)) with R1234ze(E)/R1234yf, respectively. At higher values of geothermal fluid temperatures (160 °C and 180 °C) both the SORC and DP ORC configurations produced the highest net power values, namely 48.58 (kW/(kg/s)) with R1234ze(E), 67.23 (kW/(kg/s)) with isobutene for the SORC configuration, and 50.0 (kW/(kg/s)) with isobutane and 69.67 (kW/(kg/s)) with n-butane for the the DP ORC configuration.

Process synthesis for the valorisation of low-grade heat: Geothermal brines and industrial waste streams

The increasing demand of power and the need to reduce our dependency on fossil resources represent an opportunity to valorise low-to-medium grade heat streams such as mild hot streams from industry and natural brines into electricity. A systematic approach is required for the simultaneous selection of thermodynamic cycle which includes its configuration, the thermal fluid, and the optimal operating conditions. A methodology that integrates heuristics, for pre-screening, machine learning, to include rigorous thermodynamics, and mathematical optimization, for process flowsheet design is proposed. The pre-screening yields three fluids, benzene, toluene and 1,1,1,2,3,3,3-heptafluoropropane (R227ea) and two promising cycles, dual pressure organic Rankine cycle (ORC) and organic flash Rankine cycle (OFRC). The mathematical optimization shows that for temperatures over 120 °C, the OFRC using Benzene is the configuration of choice in terms of thermodynamic performance, but the ORC provides the most economical electricity. For hot resources below 120 °C, the efficiency of both cycles converges, but the best fluid turns out to be R227ea alongside the dual ORC cycle showing better performance and lower cost. The cooling costs present a minimum at ΔTmin equal to 8 °C. The results on process design are used to evaluate a exploitation of geothermal resources across Spain.

Exploring low-grade heat in exhaust gases with moisture via power generation cycles

Low-grade exhaust gases from industrial processes can be reutilised for electricity generation by ways of various thermodynamic cycles. Previous studies assumed that only sensible heat is available in the heat source. However, the presence of moisture is common, and as the moisture condenses, a significant amount of additional heat will be released for reutilisation. To gain further understandings to the influence of moisture in heat recovery, comparative analyses between a simple organic Rankine cycle and alternative cycles, including organic flash cycle, Kalina cycle, transcritical ORC and dual-pressure organic Rankine cycle has been carried out. The results show that with the presence of moisture, the net power output per kg/s of heat source flow increases significantly for all cycles examined. It is due to the large amount of latent heat release during moisture condensation process, that is not seen for dry gases. Among all cycles considered, the dual-pressure organic Rankine cycle shows the most convincing performance enhancement compared to a simple organic Rankine cycle, in terms of the cycle net power output. This is attributed to its flexibility to cope with both high and low-temperature heat sources. With a moisture content of 0.1 (mass fraction of moisture in the heat source), a dual-pressure organic Rankine cycle can achieve a performance enhancement up to 58%. As the moisture content further increases or decreases, its performance enhancement tends to reduce, although still noticeably higher than the simple organic Rankine cycle throughout the range of moisture content examined.