Heat Source Inlet Temperature Research Articles

Thermoeconomic analysis of a novel cogeneration system for cascade recovery of waste heat from exhaust flue gases

In low-grade waste heat recovery scenarios, an absorption refrigeration cycle (ARC) or organic Rankine cycle (ORC) can be used because of its widespread acceptance in combined cooling and power applications. This paper introduces a cogeneration configuration employing a cascade waste-heat recovery architecture (including an ARC, ORC, and intermediate heating loop) to harness waste heat from the flue gas of a gas turbine (GT). The results demonstrated that the first-law efficiency and total power generation were largely influenced by the ARC, whereas the second-law efficiency and total available energy were primarily affected by the ORC. When the ARC generation temperature difference and ORC evaporation temperature were adjusted, the first- and second-law efficiencies exhibited divergent trends, suggesting the absence of a unified strategy to enhance both efficiencies simultaneously in the current system. The system achieved its highest first-law efficiency of 54.84 % and maximum total power generation of 1866.10 kW at an ORC evaporation temperature of 145 °C and a heat-source inlet temperature of 160 °C. Furthermore, at an ORC evaporation temperature of 150 °C and a heat-source inlet temperature of 200 °C, the system reached its highest second-law efficiency of 34.57 % and maximum total available energy of 1233.3 kW. The case study revealed that the GT system (including air compressor, air preheater, combustion chamber, and gas turbine) accounted for the highest level of irreversibility, comprising 74 % of the overall system. Specifically, the combustion chamber in the GT system produced a significant irreversibility due to chemical reactions. Compared with traditional combined cooling and power systems, the proposed system can increase the first-law efficiency and lower the unit energy cost of the output.

Research on geothermal-driven single-effect absorption refrigeration systems with different working fluids: energy and exergy analysis

Abstract Geothermal energy is not only environmentally friendly and has low carbon but also represents a nonintermittent technology. It is anticipated that its utilization in human society will experience rapid growth in the coming decades. Absorption refrigeration can be employed for the recovery of geothermal resources. However, the low performance of absorption refrigeration systems has hindered their development. In order to advance absorption refrigeration technology suitable for geothermal energy recovery, this study developed a mathematical model for an absorption refrigeration system using 55% lithium bromide (LiBr) and 35% lithium chloride (LiCl) solutions as working fluids. The impact of various parameters, including cooling water conditions, chilled water conditions, heat source parameters, and working fluid concentration, on the thermodynamic performance, system efficiency, and exergy efficiency of a single-effect absorption refrigeration system was analyzed. The results reveal that the cooling water inlet temperature and flow, chilled water inlet temperature, heat source inlet temperature, and working fluid concentration are the primary factors affecting the thermodynamic performance and exergy efficiency of the absorption refrigeration system. The coefficient of performance (COP) of the system increases with higher cooling water flow, chilled water inlet temperature, and working fluid concentration, but decreases with higher cooling water inlet temperature. Within a certain temperature range, the COP increases with higher heat source temperature. The effective COP (ECOP) increases with higher cooling water inlet temperature, cooling water flow, chilled water inlet temperature, and working fluid concentration, while it decreases with higher heat source temperature. The highest COP values for the LiBr and LiCl solution single-effect absorption refrigeration systems are 0.795 and 0.785, respectively, while the highest ECOP values are 0.694 and 0.684, respectively.

Performance explorations of a novel high temperature heat pump with multi-adjusted compositions of zeotropic mixture

High temperature heat pump has received its invigorated interests in recovering industrial waste heat due to its abilities of improving energy efficiency and reducing carbon emissions. But it is subjected to performance degradation at the varied operating conditions. To tackle this challenge, a novel vapor injected heat pump that ingeniously integrates with a liquid-separation condenser as the separating device and an ejector as the combining device (LEHP) is proposed, and it is featured by actively adjusting of the composition of zeotropic mixture. The mathematical model is established and verified. Six environmentally friendly zeotropic mixtures with different temperature glides are selected as working fluids. Comparison results show that the coefficient of performance of the proposed system is 21.86% higher than that of the conventional heat pumps. Parametrical studies indicate that a decrease in the vapor quality for liquid separation or an increase in the flash tank pressure is beneficial to the system performance and temperature matching in heat transfer processes. Its potentials under various operating conditions are further explored and mapped by using Genetic Algorithm. Propane/Cyclobutene is suitable at the heat source inlet temperature lower than 30 °C, and Cyclopropane/Pentane or Cyclopropane/Hexane is favored as ranging from 40 °C to 50 °C, when it is higher than 50 °C, Butene/Cyclohexane should be selected. Under unique scenarios of equivalent inlet temperatures for heat sink and heat source, Butene/Cyclohexane and Cyclopropane/Hexane demonstrate broad applicability across various operating conditions. The maximum coefficient of performance of 4.72 and exergy efficiency of 68.29% are found when Butene/Cyclohexane is employed.

Optimization of exergetic performance and payback period of organic rankine cycle integrated vapor compression refrigeration system based on exergy, economic, and environmental criteria

In this paper, a multi-objective optimization study is carried out to determine the optimal exergetic performance (𝜂𝑒𝑥) and the payback period (PB) of the organic Rankine cycle (ORC) integrated vapor compression refrigeration (VCR) system based on exergy, economic, and environmental criteria using non-dominated sort genetic algorithm-II (NSGA-II). Moreover, a sensitivity analysis is carried out to improve the exergetic performance of the system. The working fluid, the pinch-point temperature difference of the ORC evaporator (ΔToep ), con-denser (ΔTpcond), and the VCR evaporator (ΔTpve), cooling water inlet temperature (Tcw1), chilled fluid temperature (Tcf2), and heat-source inlet temperature (Th1) are taken as the decision vari-ables. The results of this study indicate that butane is the most suitable working fluid for op-timal exergetic efficiency, 𝜂𝑒𝑥 (33.7%) and payback period, PB (4.9 years) of the system. The optimal values of other decision variables such as Th1, Tcw1, Tcf2, ΔToep , ΔTpve , and ΔTpcondare 378 K, 313 K, 276 K, 10 K, 5 K, and 5 K respectively.

Parametric analysis and multi-objective optimization of biomass-fired organic Rankine cycle system combined heat and power under three operation strategies

Biomass-fired organic Rankine cycle (ORC) systems for combined heat and power (CHP) generation are a feasible technology to provide carbon-neutral power production. In this study, the parametric analysis and multi-objective optimization of a biomass-fired ORC-CHP system under three operation strategies are conducted. The thermodynamic, exergoeconomic, environmental and sustainability models are established, while the effects of seven operation parameters on exergy efficiency (ηex), unit cost of exergy (UCE), equivalent carbon emission (ECE) and ecological life cycle cost (ELCC) are examined. A further four-objective optimization problem is addressed using NSGA-II and the Pareto-optimal solutions for the three operation strategies are derived and compared. The results indicate that evaporation temperature, condensation temperature and heat source inlet temperature present positive effects on ηex and UCE, whereas opposite effects are imposed by degree of superheat and pinch point temperature difference. System B yields a 0.21% higher ηex and a 0.41 tons higher CO2,eq than system C. The Pareto-optimal solutions of ηex, UCE, ECE and ELCC for system A using TOPSIS decision-making are 12.380%, 0.183 $/kWh, 10.250 tons CO2,eq, and 13.677 × 10−3 $/kWh, respectively. System A may be regarded as the preferred operation strategy for biomass-fired ORC-CHP systems due to its moderate performance.

Experimental study on high temperature heat pump system with a double heat source cascade

In order to improve the thermal performance of a high temperature heat pump system, to realize the efficient heat recovery of industrial waste heat, and to make the combined utilization of low and medium temperature heat sources, a high temperature heat pump system with a double heat source cascade is proposed, which can absorb heat from industrial waste heat and the system is capable of heating water up to the required high temperature. The performance of the system is tested and analyzed. It is found that the inlet temperature of the medium temperature heat source has a significant impact on the performance of the heat pump system under working mode of the medium and low temperature heat sources. When the inlet temperature of the intermediate temperature heat source rises from 42?C to 53?C, the heating capacity of the cascade heat pump increases by 18.6%, and its performance coefficient increases by 8.9%. This paper provides a reference for the high efficiency of the cascade high temperature heat pump technology.

Off-design performance of an organic Rankine-vapor compression cooling cycle using R1234ze(E)

In this study, a 264 kWth organic Rankine-vapor compression cycle (ORVC) was experimentally tested over a range of conditions to quantify the individual impact of off-design external conditions. The ORVC was designed to recover waste heat at 91°C, reject heat to a liquid condenser stream at 30°C, and produce chilled water at 7°C. The condenser heat rejection temperature was varied from 16.6°C to 32.6°C, the chilled water delivery temperature was varied between 2.1°C and 13.1°C, and the heat input temperature was varied from 91°C to 120°C. As the condenser heat rejection temperature decreased, the coefficient of performance (COP) of the ORVC improved from 0.558 at 30°C to 0.682 at 16.6°C, despite a reduction in compressor isentropic efficiency. Although the chilled water temperature variation had almost no impact on the organic Rankine cycle performance, the compressor efficiency decreased when the delivery temperature was below 7°C. At the highest chilled water temperature, 13.1°C, the COP of the ORVC was 0.643. Compressor stall was noted when the condenser heat rejection temperature was greater than 32.6°C and the chilled water delivery temperature was below 2.1°C. Increasing the driving heat source inlet temperature improved the COP of the ORVC and the thermal efficiency of the organic Rankine power cycle, while decreasing the efficiency of the compressor and the COP of vapor compression cycle. In addition, the integrated part load value of the system was determined through experimentation to be 0.682, which provides a realistic estimate of real-world performance.

Analysis and assessment of a novel organic flash Rankine cycle (OFRC) system for low‐temperature heat recovery

AbstractIn this paper, a novel organic flash Rankine cycle (OFRC) system for low‐temperature heat recovery is analyzed and optimized using the 3E (energy, exergy, and economic) analysis method and particle swarm optimization algorithm, respectively. Five environmentally friendly organic fluids and three typical heat source conditions are simultaneously discussed during the parametric analysis and optimization process, to obtain a comprehensive understanding of the thermo‐economic characteristics of this novel system. The main analysis results show that the biggest exergy loss of the OFRC system is caused by the condenser, followed by the evaporator. About 70% of the specific power cost is caused by the capital investment of the system, and more than 60% of the capital investment is spent on purchasing the high‐pressure and low‐pressure expanders. Under the same operating conditions, the working fluids with high critical temperatures can achieve higher cycle thermal efficiency and lower specific power costs than those with low critical temperatures. The optimization results show that the OFRC system is able to achieve a better thermodynamic performance than the organic Rankine cycle (ORC) and organic flash cycle (OFC) systems, and when the heat source's inlet temperature is set to 100°C, 120°C, and 140°C, R245fa, R290, and R152a are, respectively, recommended as the best working fluid for the OFRC system. Besides this, it is found that the OFC system has the worst thermo‐economic performance, and the ORC system can achieve the lowest specific power cost. A new finding is that, for all three systems, the higher the critical temperature of the working fluid, the lower the specific power cost of the system.

Integrated Vapor Compression Chiller with Bottoming Organic Rankine Cycle and Onsite Low-Grade Renewable Energy

The present study offers a scheme to improve the performance of existing large-scale chillers. The system involves raising the temperature of the chiller’s cooling water stream using renewable energy sources by incorporating an organic Rankine cycle (ORC). The thermal analysis was conducted by raising the temperature of one-third of the approximately 200 ton chiller’s cooling water. The investigation was considered for ORC evaporator inlet temperature of 90~120 °C by the step of 10 °C. Various working fluids for the different ORC evaporator inlet temperatures were examined. Sensitivity analyses conducted on the degree of superheating, degree of subcooling, condenser saturation temperature, pinch point temperature differences of the ORC evaporator and condenser, and the mass flowrates of the heating and cooling streams were also reported. Genetic algorithm was employed to carry out the optimization. The best options for the ORC working fluid at the heating source ORC evaporator inlet temperatures of 90 °C was found to be DME, presenting an improvement of 48.72% in comparison with the rated coefficient of performance (COP) value of the VCC, with a renewable energy input requirement of 710 kW. At the heat source temperatures of 100 °C and 110 °C, butene, which presented an improvement in the COP equal to 48.76% and 68.85%, respectively, with the corresponding renewable energy requirements of 789.6 kW and 852 kW, was found to be the ideal candidate. Meanwhile, at the heat source inlet temperature of 120 °C, R1233zd (E), representing an improvement of 140.88% with the renewable energy input of around 1061 kW, was determined to be the most favorable ORC working fluid candidate.

Optimization of a heat pipe-radiator assembly coupled to a recuperated closed Brayton cycle for compact space power plants

• Analysis of recuperated closed Brayton cycle conditions is carried out. • Brayton cycle and heat pipe-radiator models are numerically coupled. • Heat source and cold heat exchanger inlet temperatures are optimized. • The specific mass minimization is used as the objective variable. • Optimization provides cycles with specific masses ranging from 40 to 50 kg/kW. Compact and efficient energy conversion systems for space applications enable the appearance of new mission opportunities and technological discoveries resulted from space exploration. Besides energy availability, another crucial factor of any energy conversion system for space purposes is its total mass and size. Focusing on a recuperated closed Brayton cycle (CBC), thermodynamic modeling of a CBC is proposed. Moreover, a thermal model is carried out to predict the overall properties of the cold side of the system (i.e., heat pipes and radiator) for different CBC conditions. Both models are coupled and their conjunct solution provides operational data for the design of the heat rejection system, such as the number of heat pipes (HP), total assembly mass, length, and second-law efficiency. Furthermore, by means of this coupling, the heat source temperature and the cold heat exchanger (CHE) inlet temperature are defined, using an optimization procedure where the specific mass (i.e., radiator mass to cycle power ratio) is minimized. Based on this objective variable, the optimized heat source temperature of 1200 K is achieved, while the CHE inlet temperature of 513.2 K is obtained. Such temperature conditions ensure the future design of a space energy conversion system that aligns good efficiency and compactness.

Transient behavior investigation of a regenerative dual-evaporator organic Rankine cycle with different forms of disturbances: Towards coordinated feedback control realization

This study utilizes an innovative regenerative dual-evaporator organic Rankine cycle (RDORC) with R1234ze to recover solid bulk waste heat by using air as heat transfer medium. A comprehensive transient model for RDORC is constructed based on finite volume method. Furthermore, a novel coordinated feedback control system for RDORC including two PI controllers is proposed to remain turbine inlet main vapor and resupplied vapor superheat degrees constant. The transient behaviors of RDORC and simple ORC (SORC) are compared under external disturbances of heat source inlet temperature. The results indicate that under 10 °C step decrease disturbance, the RDORC presents a 0.5 °C larger undershoot and a 164s longer setting time of superheat degree than that of SORC. Compared with SORC, the exergy efficiency enhancement of RDORC increases from initial value 4.13% to final value 4.51% after the step disturbance. Under sinusoidal disturbance with amplitude of 10 °C in RDORC, the peak amplitude of turbine main vapor inlet pressure (3.6%) is far higher than that of turbine resupplied vapor inlet pressure (0.82%). Besides, under random disturbance with variance of 1, the system exergy efficiency and thermal efficiency fluctuate around the design values of 35.67% and 7.65% with maximum deviations of 2.2% and 0.37%, respectively.

Performance analysis and multi-objective optimization of the high-temperature cascade heat pump system

This paper presents a high-temperature cascade heat pump system with a high compression ratio for recovering the waste heat of the chemical plant and replacing the steam. To analyze the economic and the thermodynamic performance of the high-temperature cascade heat pump, a mathematical model, considering energy, exergy, economy and environment, has been developed under different heat source inlet temperature. The proposed system here indicated good stability and feasibility under various operating conditions according to the simulation results. However, as the heating capacity and the annual net profit were found to be conflicting with each other, a multi-objective optimization method was developed to solve this problem. In addition, the technique for Order of Preference by Similarity to Ideal Solution method was selected as the decision-making method to search for the final optimal result of the system. The optimization results showed that if the optimized heating capacity project was selected, the average heating capacity and the average annual net profit were 2.8% and 6.7% lower than their maximum possible values. At the optimal heat source inlet temperature of 71 °C and condensing temperature of 145 °C, the heating capacity of the system was 498.9 kW and the payback period was 3.9 year.

Dynamic Modeling and Operation Characteristics Analysis of LiBr Absorption Heat Pump

In order to fully exploit the dynamic operation characteristics of absorption heat pump, a dynamic simulation model of LiBr absorption heat pump considering mass transfer and distribution parameters is established. The thermal inertia characteristics of the system are studied, and the thermal performance and crystallization characteristics of the system are analyzed under the conditions of heat source temperature change, cooling water temperature change and refrigerant water temperature change. The results show that: the model can accurately simulate the steady-state and dynamic characteristics of the absorption heat pump; the thermal inertia of the unit is mainly related to the mass of the solution stored in the unit; the upper limit of the heat source inlet temperature is affected by the COP of the system and the crystallization risk, the decrease of the inlet temperature of cooling water can increase the COP of the system, and the lower limit is limited by the crystallization risk, and the cooling water inlet temperature is affected by the crystallization risk , and the lower limit of the inlet temperature of cooling water is limited by the crystallization characteristics. The upper limit of the inlet temperature of refrigerant water is not limited by the crystallization characteristics, but is mainly limited by the user’s energy demand. This provides a reference for the dynamic operation of absorption heat pump.

Performance analysis of compressor-assisted two-stage triple effect absorption refrigeration cycle for power and cooling

Ammonia-water absorption and power cycles offer a viable option for the effective use of low and medium temperature heat sources for sub-zero cooling and power generation. The proposed system is a compressor assisted two-stage triple effect absorption cycle for power and cooling. This system comprises of power sub-system and absorption-compressor sub-system and both systems utilize ammonia-water mixture as the working fluid. The power generated by the cycle is used for the compression required to enhance the performance of the cycle. The mathematical model of the proposed system is analyzed using first and second law of thermodynamics. The engineering equation solver (EES) software is employed to analyze the model. Thermodynamic performance of the combined cycle was evaluated based on the compression ratio and the heat source inlet temperature. The pressure ratio (Prcom) of the integrated compressor is varied between 1.0 and 1.6 to observe the effect on the operation, output, and performance of the system. The simulation result shows that the heat source inlet temperature can be reduced by 50 °C compared to the traditional two-stage triple effect absorption system. The effective energy and exergy efficiencies are 17% and 76% respectively while the cooling load was 165.4 kW. The results also showed that asorber1 and absorber 2 have the highest rate of exergy destruction of 30.91% and 37.32% followed by condenser 1 with 16.3% and solution heat exchanger 2 with 7.89%. It is observed that the proposed system performs better in comparison to its counterparts operated at medium temperature heat sources.

Thermodynamic analysis and optimization of organic Rankine cycles based on radial-inflow turbine design

In recent years, high-temperature organic Rankine cycle (ORC) with a heat source temperature over 300 °C has attracted interests, which has wide range of applications from industrial point of view. However, few researchers have focused on the coupled design and optimization of the high-temperature ORC system and radial-inflow turbine (RIT). In this work, a coupled design of the ORC system and RIT with the heat source inlet temperatures of 300, 350 and 400 °C is developed using non-dominated sorting genetic algorithm. The ORC thermal efficiency and turbine performance factor (PF) are introduced as optimization objectives. Two mixtures of siloxanes, seven pure siloxanes and nine pure hydrocarbons are considered as working fluid candidates. In addition, the thermodynamic comparison between mixtures of siloxanes and pure working fluids is presented. Moreover, the influences of the turbine inlet total temperature, total-to-static expansion ratio, and specific speed on the ORC thermal efficiency, turbine performance, and turbine size are investigated. The results show that the turbine power output and cycle thermal efficiency are more sensitive to the expansion ratio while turbine size is more sensitive to the turbine inlet total temperature and specific speed. In addition, PF increases with the rise of the turbine inlet total temperature, expansion ratio and specific speed. Moreover, compared with pure working fluids, mixtures of siloxanes have higher efficiencies, but larger turbine and heat exchanger size. Cyclohexane, toluene, m-xylene and hexane with higher PF and smaller size of turbine and heat exchanger are promising working fluids in applications where the compact turbine and heat exchanger is preferred, such as vehicle waste heat recovery.

Experimental investigation of performance of plate heat exchanger as organic Rankine cycle evaporator

In this study, the experimental investigation of a plate heat exchanger was conducted, which was used as an organic Rankine cycle evaporator. The heat exchanger performance was investigated at the low mass flux and moderate evaporation temperature ranges, as it was designed for a cascade heat utilization in a fourth-generation district heating and cooling system. The experiments were conducted under various operating conditions by changing the R-245fa mass flux, evaporation pressure, R-245fa inlet temperature, heat source inlet temperature, and heat source mass flux. Moreover, the heat transfer and pressure drop mechanisms were thoroughly investigated with an internal process analysis. The two-phase heat transfer coefficient exhibited a strong dependency on the heat flux, indicating that flow boiling heat transfer was mainly dominated by the nucleate boiling mechanism. When both the evaporation pressure and R-245fa mass flux were increased, the single-phase heat transfer accounted for a significantly large portion of the total heat transfer, leading to the rapid decrease in the heat transfer rate. The port and elevation pressure drops together accounted for 27–47% of the total pressure drop due to the low R-245fa mass flux range. With the heat source side variation, the two-phase heat transfer coefficient was more affected by the heat source inlet temperature than the heat source mass flux owing to the increase in the excess temperature. Additionally, the overall heat transfer coefficient strongly depended on the R-245fa side heat transfer coefficient, because the R-245fa had lower heat transfer coefficients than the heat source, even in the two-phase region.

Interactive heat transfer characteristics of 5 kW class shell-and-tube methane steam reformer with intermediate temperature heat source

A 5 kW class shell and tube methane steam reformer (MSR) with intermediate temperature heat source was evaluated to find a correlation between the methane conversion and heat transfer performance. First, performance evaluation of MSR1 was conducted by varying experimental conditions such as reformer reactant flow rate, steam to carbon ratio (SCR), inlet temperature of reforming zone, and inlet temperature of heat source. Sensitivity study of overall heat transfer coefficient was also carried out to find the major parameter affecting the heat transfer. Next, the heat transfer performance and methane conversion rate of MSR1 and 2 were compared. Both reformers have the same gas hourly space velocity (GHSV) in the form of shell and tube heat exchangers, but are designed to compare the heat transfer characteristics of the reformer by designing with the different heat transfer areas. The results show that the main factors affecting the performance of the reformer are load, heat source inlet temperature, and heat transfer area.

Thermodynamic and thermoeconomic analysis of innovative integration of Kalina and absorption refrigeration cycles for simultaneously cooling and power generation

The energy consumption and the emission of greenhouse gases have been motivated researchers toward using renewable energy sources for the development of a sustainable society. Low-temperature heat sources like geothermal sources can be a desirable option to produce the demanded power, cooling, heating, and other byproducts. In the present literature, a novel combined cooling and power generation system is presented which is an internal integration of the Kalina Cycle and the absorption refrigeration cycle. The system operates using the low-temperature heat source. The thermodynamic and thermoeconomics analysis is carried out using Engineering Equation Solver software. Besides, considering the mathematical modeling, the parametric investigation is implemented to evaluate the effect of the key parameters on the sum unit cost of the products. Based on the obtained results, the cooling and power generation system can provide the net output power of 158.3kW and the cooling capacity of 1084kW. The results indicate that the system is most appropriate for cooling provision rather than for power generation aims. The energy and the exergy efficiencies and the net power of the combined cycle obtained 41.33%, 27.47%, and 158.3kW respectively. The sum unit cost of the product cooling, power, and total system are respectively evaluated 148.5$/GJ, 97.16$/GJ, and 19.44$/GJ. The minimum and maximum values for the rate of exergy destruction cost belong to the pump and absorber, respectively. The parametric analysis delivered some valuable results such as a drop in the sum unit cost of the net power, cooling output, and overall system by increasing the generator hot pinch point temperature difference, an increase in the overall system sum unit cost of the products with increment in the low-temperature heat source inlet temperature, and rising in the sum unit cost of the net power, cooling output, and the overall system with an increase in the evaporator temperature.

Performance investigation of transcritical and dual-pressure Organic Rankine Cycles from the aspect of thermal match

Dual-pressure and transcritical Organic Rankine Cycles (DORC and TORC) all have advantages of increasing the thermal match between the working fluid and heat source. However, their temperature ranges of applicable heat source are different. This study focuses on the investigation and comparison of subcritical ORC (SORC), TORC and DORC using various working fluids for recovering energy of hot water which temperature is between 120 °C and 200 °C. Effects of evaporation pressure of SORC, two-stages pressures of DORC, and turbine inlet parameters (temperature and pressure) of TORC on the thermal match and the net power output have been studied. Then the thermal match and net power output comparisons of SORC, DORC and TORC under the optimal conditions for various temperatures of heat source (120–200 °C) have been presented. Furthermore, the maximum net powers of threes cycles using various working fluids are also optimized and compared when the heat source temperatures are given. The results indicate that for the DORC system, the thermal match improves as high stage pressure rises. And compared with the low stage pressure, the high stage pressure has a greater effect on the system performance. For the TORC system, the slop of temperature rising curve of supercritical working fluid is affected by the factors such as heat source inlet temperature, turbine inlet temperature and turbine inlet pressure of turbine. And it influences the thermal match in the vapor generator. There is a boundary temperature of heat source for the selection of DORC and TORC using various working fluids. If heat source temperature is less than the boundary temperature, DORC shows better system performance, otherwise, TORC possesses better performance. The boundary temperatures of R1234ze(E), R227ea, Isobutane, R245fa are 151.4 °C, 125.5 °C, 170.8 °C and 195.8 °C respectively. And the analyzed results also present that there exists suitable cycle layout and appropriate working fluid to achieve best thermal match and produce maximum net power when the heat source temperature is given.

Rigorous modelling and deterministic multi-objective optimization of a super-critical CO2 power system based on equation of state and non-linear programming

CO2 power cycle is one of the promising sustainable technologies that generate power from fossil fuel, nuclear, solar, and various waste heat energy. The heat transfer process plays a key role in influencing the thermodynamic or thermo-economic performance of CO2 power cycle. The drastic variation of CO2 thermo-physical properties results in great difficulty in modeling and optimizing the heat exchangers and CO2 cycle configurations. In the present study, a mathematical programming method is proposed for the simulation and optimization of simple, regenerative, and recompression super-critical CO2 power systems. An accurate equation of state is applied to calculate CO2 properties, thereby enabling the performance indicator a continuous function of system parameters. A multi-objective non-linear programming model is formulated in GAMS for the automatic heat exchange pinch locating and system optimization. The simulation and optimization models are validated by comparison with REFPROP-based method. The maximum and average simulation errors are 0.064% and 0.015%. The thermal efficiency values achieved using the developed optimization method are improved by 6.14–10.85% compared with previous solutions. Then, a case study of a 573.15 K waste heat driven super-critical CO2 power system optimization is elaborated. Results show that pinch locating of all heat exchangers and system optimization can be simultaneously conducted within a short time. The solution optimality is validated by comparing the traditional pinch assignment concept and optimization method. Sensitivity analyses of heat source inlet temperature and heat sink temperature rise are also conducted. For both objectives of maximizing thermal efficiency and net power output, the recompression super-critical CO2 cycle features highest in thermal efficiency and the regenerative super-critical CO2 cycle features highest in net power output. Finally, a multi-objective optimization is conducted to achieve the Pareto Front for the studied three super-critical CO2 power systems.