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This research conducts a thermodynamic and transient analysis of the hybrid concentrated photovoltaic panel and vapour compression cycle thermal (CPV-VCC/T) system that is designed for combined heat and power purposes. The model includes both the VCC’s thermodynamic model and the evaporator (CPV side) heat exchanger’s convective heat transfer effect. Various commercial refrigerants (R123, R22, R245fa and R134a) are analyzed where their performances are assessed in terms of coefficient of performance (COP) and the CPV’s resulting operating temperature. After the comparative steady state analysis, the CPV-VCC/T system is then tested in a two-day transient simulation involving varying irradiances and hot water demands. Steady state results demonstrate that the COP efficiency relative to Carnot will generally decrease as the condenser temperature increases. Also, although refrigerants R134a and R22 yielded up to 12% higher static COPs than R245fa and R123, the yielded electric energy gain from the two-day transient simulation is different by no more than 1.5%. Positively, the CPV-VCC/T system is found to yield 35% more electric energy than the conventional electric heating system. Although the CPV-VCC/T system yielded 5% less electric energy than direct water cooling, the CPV temperature was successfully maintained at 330 K as opposed to over 380 K.

A novel Stirling-like irreversible electrochemical thermo-capacitive regenerative cycle operating between a heat reservoir and a heat sink for harvesting low-grade heat is proposed, in which the main irreversible losses are considered. Expressions for the efficiency and power output of the cycle are derived. The inherent regenerative heat loss of the cycle is revealed. The influences of several important parameters, such as the capacitance of the supercapacitor, charge ratio, endpoint voltage of the charging process on the power output of the cycle are discussed. The maximum power output and corresponding efficiency are calculated. The results show that when the charging endpoint voltage of the electrochemical thermo-capacitive regenerative cycle is 10 V, the maximum power output is 31.9 W and the efficiency at the maximum power output can reach 12.8%, which is 51.2% of the Carnot efficiency. The optimal values for several parameters at the maximum power output are determined. The parametric optimum selection criteria of the cycle are provided. The results obtained can ensure the cycle to operate in the optimum states.

The heat source and sink temperatures are important external variables that determine the performance of power cycles, such as the ideal Carnot cycle and Rankine cycle. In addition to these external operating variables, variations in the mass flow rate within the system can significantly change the operating characteristics of the power cycle. In this study, the combined effects of these operating variables on the compact Rankine cycle performance were investigated experimentally. A compact ORC with R245fa working fluid and scroll expander was experimentally evaluated at 100–140 °C heat source, 10–30 °C heat sink and 0.042–0.060 kg/s mass flow rate conditions. Comprehensive experiments showed that the external parameters have significant effects on ORC operating temperature and pressure. In particular, the electrical output was well predicted by the parameter of change in the pressure difference rather than the expansion ratio of the expander. It was also found that the mechanical efficiency of expander increased nonlinearly when decreasing the heat source temperature and increasing the pressure difference of expander. The generator efficiency was not affected by the operating parameters as the expander rotational speed was linearly proportional to the electrical power. The maximum system performance showed 0.246 kW electrical output and 1.61% thermal efficiency in contrast to 0.796 kW net fluidic output and 5.72% fluidic thermal efficiency where they were in different conditions.

A Carnot-type engine with a changing phase working fluid is modelled and optimized. The engine is externally heated by a waste heat in order to valorize it. The waste heat is a flue gas at 350 °C, corresponding to a usual temperature of industrial furnace effluents. The objective function is the maximization of the net power output. In the numerical application, the working fluid is water. The optimization variables are the working fluid temperature of vaporization, its temperature of condensation and the allocation of a total thermal transfer area between the boiler and the condenser. This article aims to define an upper bound for waste heat to power conversion, depending on the waste heat temperature and its available mass flow rate, on the working fluid used and on the exchangers’ size. The methodology used can be applied to optimize other externally heated engines. Different control volumes are selected for the analysis: the adiabatic converter, the non-adiabatic converter, the system (the converter and the exchangers) and the system placed in the environment. The impact of the choice of the control volume on optimization results, global efficiency and entropy production is studied. It appears that the optimal heating and cooling temperatures are the same in all sub-systems, but the optimal allocation of the total thermal transfer area differs slightly. However, in all cases the optimal condenser thermal transfer area is larger than the optimal boiler thermal transfer area. This imbalance corresponds to the need to evacuate the mechanical energy degradation into heat. It also appears that the control volume increase implies a decrease of the first law efficiency. The entropy analysis allows an identification and a quantification of the different contributions to entropy production. Then, a sensitivity analysis to the relevant parameters is carried out. The relevant parameters are the heating fluid temperature and mass flow rate, the total thermal transfer area, the compressor efficiency and the turbine efficiency.

The pivotal of organic Rankine cycle (ORC) promotion and optimization is revealing the thermodynamic relationship between cycle configuration, condition and its working fluid properties. Different from the traditional numerical calculation method (TNCM) of ORC, a new thermodynamic cycle separating method (TCSM) is introduced in this paper. Then, efficiency of ORC is conducted expediently by TCSM where Triangle cycle ( η TC ), Carnot cycle ( η CC ) and Brayton cycle ( η BC ) efficiencies are regarded as variables, that is, η SORC = f ( η TC , η CC , η BC ) . When comparing with TNCM, TCSM not only has the acceptable precision for all the investigated 21 working fluids, but also the influence of critical temperature, molecular complexity of the working fluid and superheat degree as well as the reduced operating conditions of ORC can be revealed qualitatively and quantitatively. Finally, three conclusions are revealed: (1) Relationship between ORC limited efficiency (the reduced evaporating temperature of 0.9) and critical temperature of working fluid is revealed; (2) When superheat degree increases, ORC efficiency of dry fluid decreases and wet fluid increases linearly, while the variation of isotropic working fluid remains constant; (3) If the reduced temperatures of two different working fluids are equal, the corresponding efficiencies are equal too. The proposed thermodynamic cycle separating method provides an approach for working fluids selection and performance prediction of ORC.

The Stirling engine has drawn much attention as it can utilize sustainable energy such as solar, waste heat, and biomass. Although the Stirling cycle has the same theoretical efficiency as the Carnot cycle, the thermal efficiency of an actual Stirling engine is lower than that of the ideal Carnot cycle owing to their reversibility. Previous finite-time thermodynamic models of Stirling engines are mainly focused on the temperature difference between the working substance and heat reservoirs. However, there is also a temperature difference between the working substance and the regenerator, which has merely been reported in previous literatures. In this study, we analyzed the regenerative processes of the Stirling engine using finite-time thermodynamics. Using an even distribution temperature assumption, the regenerative effectiveness and its limitations were obtained. For an uneven distribution temperature assumption, the regenerator was modeled by dividing it into n sub-regenerators. Two cases, for either constant or varying temperatures of the sub-regenerators, are discussed in detail, and the same regenerative effectiveness was obtained for the limit n → ∞. Furthermore, the thermal efficiency and output power were obtained, and the effects of the parameters on the performance of the Stirling engine were investigated.

Energy storage is the key to solve the grid connection problem of renewable energy. Carnot Battery is one of the promising energy storage technologies nowadays. In this work, four Carnot Battery systems were constructed using organic Rankine cycle and vapor compression heat pump. Energy, exergy and economic (3E) models of the aforementioned systems were built. Using genetic algorithm as the optimization method, the levelized cost of storage was employed as the objective function. The systems were investigated, analyzed and compared according to the optimization results. The results show that regenerative systems that consist of a regenerative vapor compression heat pump present better thermo-economic performance. With thermal storage temperature increasing from 90 °C to 130 °C, the value of levelized cost of storage gradually decreases from 0.42 $/kWh to 0.29 $/kWh. The power-to-power efficiency decreases continuously with the increase of thermal storage temperature, however, the variation of exergy efficiency with thermal storage temperature is non-monotonic. The maximum value of 19.12% is achieved in system BORC-RVCHP when the thermal storage temperature is 100 °C. Also, it can be found that the value of the levelized cost of storage increases gradually with the lift of power-to-power efficiency. The cost and exergy destruction of each component are also calculated and compared. Two components that take the largest proportion in total investment cost are turbine and compressor for all the considered system configurations in this work.

Heat pump technology can effectively reduce the energy consumption and environmental pollution caused by coal-fired heating. However, commonly used heat pump heating systems have poor performance in low temperature environments, making it challenging to meet the heating demand in cold regions. In this paper, the performance of an electrically driven free-piston Stirling heat pump is studied based on SAGE software. The numerical simulation results show that the 2409 W heating capacity can be obtained with the input electric power of 1 kW, heating temperature of 40 °C and ambient temperature of −20 °C. The overall coefficient of performance reaches 2.41 with corresponding relative Carnot efficiency of 46.2%. In addition, the operation characteristics of this heat pump system under different temperature conditions are investigated in detail. With the constant difference between heating and ambient temperatures, this heat pump can operate efficiently in different temperature ranges. With the increase of temperature difference, the available energy loss in the whole system decreases, leading to the improvement of relative Carnot efficiency. These results show the strong adaptability of the free-piston Stirling heat pump system to different working temperatures.

A growing number of complex thermodynamic cycles, such as different configurations of supercritical carbon dioxide (sCO2) Brayton cycles, have been recently proposed. However, the understanding and analysis of complex cycles are very arduous and time-consuming, due to the lack of a general theory for cycle analysis and numerous equations included in physical models of cycles. To address these challenges, a novel method named the flexible programming of thermodynamic cycles (FPTC) is developed based on the graphical analysis in the T-s diagram. Within the theoretical framework of the FPTC, the complex cycle can be obtained by performing logical operations of several simple cycles in the T-s diagram. Meanwhile, the type of logical operation also determines the relationship of the thermodynamic performance indexes (heat input, heat output, net-work output, and thermal efficiency) between the synthetic complex cycle and simple cycles. There are three basic types of logical operation (addition, subtraction, and interchange) in the FPTC, and the detailed rules in each logical operation are presented by using the Carnot cycle as the basic simple cycle. In the case study, four different configurations of sCO2 Brayton cycles are analyzed using the FPTC, and their evolutions from simple Brayton cycles to complex are shown. Meanwhile, the thermal efficiencies of different configurations can be calculated by efficiency formulas obtained from the FPTC. Results show this case analysis is an efficient approach to analyze complex cycles. The FPTC is not only a graphical analysis method to study the evolution of complex cycles, but also provides a simple method to calculate the performance of synthetic complex cycles. It allows researchers to evaluate the performance of synthetic cycles by physical models of simple cycles, instead of building physical models for complex cycles. Furthermore, the FPTC also provides a new perspective for constructing or designing complex thermodynamic cycles.

An analytical model for predicting the performance of an organic Rankine cycle (ORC) system was reformulated. To do this, a constraint for operating temperature was newly proposed. Typically, based on the Carnot cycle, lots of performance equations for a power system have been derived. Besides, an adequate constraint for the operating temperature has been rarely considered in the previous studies. Those simplicities can increase the chance of error in predicting the performance properly. In this study, the analytical model is comprised of thermodynamic principles and several physical characteristics in the ORC system. Thereby, an analytical expression for the optimal condensation temperature was derived as a function of given parameters. The theoretical model was validated by comparing it with a numerical simulation. Results indicate that the predicted data from the theoretical model are in good agreement with the simulation data. The proposed theoretical model is applicable in the conceptual design of the ORC system or estimating the potential of thermal energy sources.

The Stirling engines are inherently efficient; their thermodynamic cycles reach the Carnot efficiency. These technologies are suitable to operate under any low temperature difference between the hot and the cold sources. For these reasons, these engines can be considered as reliable power conversion systems to promote the conversion of low-grade waste heat generated by industrial plants. The need of a model to predict the behavior of these engines is of primary importance. Nevertheless, a great difficulty is encountered in developing such a model since it is not simple to take into account coupled thermodynamic and dynamic effects. This is the main reason why several models make use of electrical analogies to describe Stirling engines (in particular, free-piston machines): by assuming the pressure equivalent to a voltage and the flow rate to an electrical current, a coupled dynamic-thermodynamic analysis of the engine can be performed. In this paper, an electrical circuit whose behavior is equivalent to that of the engine is derived from the electrical analogy theory. To this aim, we propose an electrical analogy model based on the three conservation laws (mass, momentum and energy). Since limited experimental information is available in the open literature, the results obtained with the proposed model are compared with the experimental data collected at the NASA Lewis Research center for a free-piston Stirling engine i.e., the RE-1000 engine.

Recent developments in thermoacoustic technologies have demonstrated that multi-stage looped thermoacoustic Stirling engine would be a promising option for harvesting waste heat. Previous studies on multi-stage looped thermoacoustic systems were mainly focused on heat-driven refrigeration or heat pumping, while much fewer work were done on power generations, especially those for recovering low temperature heat. In this work, a four-stage looped thermoacoustic Stirling power generator for generating electricity from low temperature waste heat at 300 °C is systematically studied. A numerical model is built and then validated on an experimental four-stage looped thermoacoustic Stirling engine. On the basis of the validated model, the effects of the coupling position for the linear alternators and the regenerator position on the acoustic characteristics and performances of the power generation system are numerically investigated. The distributions of the acoustic fields along the loop, including the pressure amplitude, volume flow rate, phase angle, specific acoustic impedance and acoustic power, are presented and analysed for three representative coupling modes. Superior efficiency is achieved when the linear alternators are coupled near the cold ends of the thermoacoustic cores on the resonators, while more electric power is generated at the hot ends. The worst performance is expected when the linear alternators are connected at the middle of the resonators. The underling mechanisms are further explained detailedly by analysing the characteristics of the acoustic fields and output acoustic impedances for different coupling modes. Furthermore, the regenerator position in the thermoacoustic cores is found to have a remarkable influence on the output electric power, while it is less important for the efficiency. The optimized four-stage looped thermoacoustic Stirling power generator is able to provide a maximum electric power of 1223 W with a highest relative Carnot efficiency of around 0.20. This work provides in-depth insights into the operation principle of the looped thermoacoustic Stirling power generator and will be helpful for future developments of similar waste heat recovery systems. Accepted version

Multi-stage thermoacoustic engines with a looped structure have become a promising technology to use heat at a low temperature level. This study forwards a single-stage engine with comparable capabilities. A stub is used to suppress acoustic impedance disturbances. This conception shows to be a powerful tool to accurately tune acoustic conditions in the regenerator. Furthermore, a systematic parametric study is carried out numerically with DeltaEC to examine their relations. This could help in a system-wide optimization according to various application requirements. The study comprises geometrical parameters of regenerator, heat exchangers, and feedback loop as well as process parameters like operational temperatures, mean pressure and working gas. Discrepancies between ideal conditions on component level and on system level have been observed and an approach to interpret them is given. Recommendations for parameter settings depending on an efficiency or power driven operation are supplied. Since a proper coupling between acoustic load and acoustic field in the loop is essential for a sufficient overall-performance, joining mechanisms have been examined in terms of acoustic impedance and position of the load. Eventually it is shown that the proposed engine type is able to power an acoustic load with a relative Carnot efficiency greater than 50% when heat is supplied at 150 °C and rejected at 15 °C.

A new model of the electrochemical system consisting of multiple thermally regenerative electrochemical cycles (TRECs) working in different states is proposed to solve the problem of discontinuous power output. This system can efficiently harvest low-temperature waste heat and realize the continuous power output. Based on the performance of a single TREC, the power output and efficiency of the system are analytically derived. The effects of the electric current on the power output and the upper and lower boundaries of the efficiency are discussed. The general performance characteristics of the system at the maximum power output are revealed. Moreover, it is expounded that when the system is operated at the state of the maximum efficiency-power product, the systemic performance relative to the state of the maximum power output may have an obvious improvement. For example, when the Carnot efficiency of the system is smaller than 0.2, the improvement ratio of the systemic performance is larger than 16%. The results obtained here show that the system should be operated between the maximum power output and the maximum efficiency-power product. Consequently, the optimal criteria of the parametric design are provided and the rationally operating region of the system is determined.

This paper focuses on a looped travelling-wave thermoacoustic refrigerator powered by thermal energy. Based on a simplified model for the regenerator, key issues for a highly efficient thermoacoustic conversion, including both thermal-to-acoustic and heat-pumping processes, are summarized. A looped travelling-wave thermoacoustic refrigerator with one engine stage and one refrigerator stage is proposed, with emphasis on high normalized acoustic impedance, sufficient volumetric velocity and appropriate phase relation close to travelling wave in the regenerators of both engine and refrigerator. Simulation results indicate that for the ambient temperature of 30 °C, the looped travelling-wave thermoacoustic refrigerator can be powered by the heat at 210–250 °C to achieve the refrigeration at −3 °C with the overall coefficient of performance above 0.4 and the relative Carnot coefficient of performance over 13%. The characteristics of the acoustic field inside the loop configuration are analyzed in detail to reveal the operation mechanism of the looped travelling-wave thermoacoustic refrigerator. Additional analyses are conducted on the impact of the cooling and the heating temperatures, which are of great concern to the refrigeration applications and the utilization of low-grade thermal energy.

A vast amount of heat energy is available at low cost within the range of medium and low temperatures. Existing thermal cycles cannot make efficient use of such available low grade heat because they are mainly based on conventional organic Rankine cycles which are limited by Carnot constraints. However, recent developments related to the performance of thermal cycles composed of closed processes have led to the exceeding of the Carnot factor. Consequently, once the viability of closed process based thermal cycles that surpass the Carnot factor operating at low and medium temperatures is globally accepted, research work will aim at looking into the consequences that lead from surpassing the Carnot factor while fulfilling the 2nd law, its impact on the 2nd law efficiency definition as well as the impact on the exergy transfer from thermal power sources to any heat consumer, including thermal cycles. The methodology used to meet the proposed objectives involves the analysis of energy and entropy on trilateral closed process based thermal cycles. Thus, such energy and entropy analysis is carried out upon non-condensing mode trilateral thermal cycles (TCs) characterised by the conversion of low grade heat into mechanical work undergoing closed adiabatic path functions: isochoric heat absorption, adiabatic heat to mechanical work conversion and isobaric heat rejection. Firstly, cycle energy analysis is performed to determine the range of some relevant cycle parameters, such as the operating temperatures and their associated pressures, entropies, internal energies and specific volumes. In this way, the ranges of temperatures within which the Carnot factor is exceeded are determined, where carbon dioxide, nitrogen, helium and hydrogen are considered as real working fluids, followed by an entropic analysis in order to verify 2nd law fulfilment. The results of the analysis show that within a range of relatively low operating temperatures, high thermal efficiency is achieved, reaching 44.9% for helium when the Carnot factor is 33.3% under a ratio of temperatures of 450/300 K. With respect to entropy analysis, it is verified that the results of the latter demonstrate compliance with the second principle, while violating Carnot constraints, since the Carnot factor is constrained only by the Carnot, Stirling and Ericsson cycles and its associated Carnot engine characteristics. However, the most relevant findings through the performed analysis concern the detection of some inconsistencies regarding the conventional 2nd law efficiency definition and the exergy transfer definition from thermal power sources to thermal cycles. In summary, a TC undergoing isochoric heat absorption, adiabatic expansion and isobaric heat rejection under closed transformations can yield improved performance over traditional thermal cycles, even exceeding the Carnot factor under relatively low top temperatures, for which Carnot efficiency is lower. Furthermore, the concept of 2nd law efficiency, defined as the ratio of the thermal to the Carnot efficiency, has been reconsidered in agreement with the results achieved. That is, the definition of 2nd law efficiency lacks both theoretical and practical sense. In the same way, as a result of discarding the Carnot factor as limiting the thermal efficiency, the definition of the exergy transfer to a thermal cycle (the maximum available energy) must be defined as the product of the transferred heat from a heat source and the thermal efficiency.

The aim of the paper is to demonstrate that a regenerative Otto cycle with adiabatic or polytropic expansion can achieve improved performance over traditional Otto engines, even exceeding the Carnot factor. Thus, the work deals with a novel regenerative Otto based internal combustion engine which differs from the conventional Otto thermal cycles in that the process of heat conversion into mechanical work is performed obeying a polytropic path function instead of the conventional adiabatic expansion without regeneration. Design characteristics concern the fact that combustion at constant volume is carried out undergoing large air excess so that the top combustion temperature is significantly lower than in conventional Otto cycles and consequently NOx emissions are neglected. Furthermore, during the polytropic expansion based path function, heat is absorbed by being submitted to a controlled heat flow rate, to achieve the desired polytropic expansion. The analysis of the regenerative Otto based on polytropic expansion is presented and results are compared with a regenerative Otto based on the adiabatic expansion and CF. The results show that a relevant advantage of the proposed regenerative Otto with polytropic expansion over the regenerative Otto cycle with adiabatic expansion involves performance enhancement within a wide range of combustion pressures, temperatures and regeneration capacities. Thus, thermal efficiency and specific work as function of the top combustion pressure ranges are of 71.95–58.43% and 143.5–173.6 kJ/kg respectively, when combustion pressures vary between 105 kPa and 200 kPa and CF is 60.8% (lower than the thermal efficiency). The successful results involving a compact engine structure, technically and economically viable, promises a new generation of efficient reciprocating internal combustion engines operating at relatively low temperatures.

A three-stage looped traveling-wave thermoacoustic system with high efficiency thermoacoustic conversion and acoustic power transmission potential is studied through computational fluid dynamics simulation. ωτ, an important dimensionless parameter representing the heat exchange efficiency of the system, is related to the efficiency of sound production and depends on the frequency, hydraulic radius and temperature of the regenerator. The influence of resonance tube radius on the system performance is analyzed, while the acoustic impedance and exergy loss of main components are given. The proposed unequal-radius resonance tubes help with the realization of acoustic power amplification and impedance matching. After parameter evaluation, the proportion of traveling wave in the whole system increases, and the impedance amplitude of some resonance tubes also becomes higher. The mechanism and effect of ωτ on the system performance are expounded. The simulation results show that when the ωτ is 5.7, acoustic power of 8.9 kW and relative Carnot efficiency of 53.1% are obtained, which are higher than the performance of the baseline model.

This paper presents a novel reverse Carnot cycle-organic Rankine cycle (RCC-ORC) dual-function system (DFS) for electricity and heat output using a two-screw machine which consists of heat pump (HP) mode and power generation (PG) mode. Compared with single PG and HP system, the proposed system can recover low-grade waste heat to generate electricity or high-grade thermal energy in accordance with consumer demand. A mathematical model has been established to analyze the variation of the energy, exergy, economic and environmental performance with different temperature. Simulation results show that the DFS has good feasibility and stability in two modes. In addition, there is a contradiction between the exergy performance and economic performance. When the DFS has shorter payback period (PBP) of 1.39 year, the irreversible destruction is high at 225.58 kW, therefore, the optimized DFS should be determined according to the exergy performance and economic performance. TOPSIS method is chosen as the decision method to select the final optimal value of the system. The optimization results show that if the optimized payback period project is chosen, PBP is 70% longer than its smallest possible value at the condensing temperature of 120 °C; whereas, if exergy destruction based optimized design is chosen, exergy destruction is 6.5% more than the smallest possible value. At the optimal condition, the heat output and electricity are 945.7 kW 81.9 kW, respectively. Payback period and exergy destruction are 4.34 year and 201.21 kW, respectively.

In this work a numerical model was developed to optimize a kinematic Gamma Stirling engine. In this model thermal and mechanical losses are included to approximate experimental results. The influences of three operation parameters (filling pressure (3, 5, 10 bar), hot end temperature (300, 400, 500 °C) and rotation speed (160, 360, 600 rpm)) as well as heat exchangers efficiencies on the Stirling engine performances are studied. The studied parameters are dependent and their interactions are significant for the Stirling engine brake power. For high initial filling pressure, the brake power became more sensitive to the rotation speed and the hot end temperature. The set of operation parameters giving the optimal brake power will be presented and discussed. It was found that the performances of the engine are more sensitive to its regenerator efficiency. For e r = 0.5 the output brake power and the Carnot efficiency were dropped respectively of 16% and 5% relatively to e r = 1.

The authors of the above mentioned paper claim that “recent developments related to the performance of thermal cycles composed of closed processes have led to the exceeding of the Carnot factor” (Garcia et al., 2016). However, this unusual result is based on an erroneous analysis of the proposed trilateral cycles. The authors forget to take into account the compression work of the working fluid. By considering both the expansion and the compression work of the cycle, the correct thermal efficiency is far lower than the Carnot efficiency.

Stirling engine is an external combustion engine that uses any external heat source to generate mechanical power which operates at closed cycles. These engines are good choices for using in power generation systems; because these engines present a reasonable theoretical efficiency which can be closer to the Carnot efficiency, comparing with other reciprocating thermal engines. Hence, many studies have been conducted on Stirling engines and the third order thermodynamic analysis is one of them. In this study, multi-objective optimization with four decision variables including the temperature of heat source, stroke, mean effective pressure, and the engine frequency were applied in order to increase the efficiency and output power and reduce the pressure drop. Three decision-making procedures were applied to optimize the answers from the results. At last, the applied methods were compared with the results obtained of one experimental work and a good agreement was observed.

Cost optimization of heat exchanger inventory in power cycles with one open feedwater heater is undertaken. In this regard, thermoeconomic analysis for an endoreversible power cycle with an open feedwater heater is shown. The scenarios of constant heat rejection and addition rates, power as well as rate of heat transfer in the open feedwater heater are studied. All cost functions displayed minima with respect to the high-side absolute temperature ratio (θ1). In this case, the effect of the Carnot temperature ratio (Φ1), absolute temperature ratio (ξ) and the phase-change absolute temperature ratio for the feedwater heater (Φ2) are qualitatively the same. Furthermore, the constant heat addition scenario resulted in the lowest value of the cost function. For variation of all cost functions, the smaller the value of the phase-change absolute temperature ratio for the feedwater heater (Φ2), lower the cost at the minima. As feedwater heater to hot end unit cost ratio decreases, the minimum total conductance required increases.

A new energy-efficient refrigeration system subcooled by liquid desiccant dehumidification and evaporation was proposed in this paper. In the system, liquid desiccant system could produce very dry air for an indirect evaporative cooler, which would subcool the vapor compression refrigeration system to get higher COP than conventional refrigeration system. The desiccant cooling system can use the condensation heat for the desiccant regeneration. Thermodynamic analysis is made to discuss the effects of operation parameters (condensing temperature, liquid desiccant concentration, ambient air temperature and relative humidity) on the system performance. Results show that the proposed hybrid vapor compression refrigeration system achieves significantly higher COP than conventional vapor compression refrigeration system, and even higher than the reverse Carnot cycle at the same operation conditions. The maximum COPs of the hybrid systems using hot air and ambient air are 18.8% and 16.3% higher than that of the conventional vapor compression refrigeration system under varied conditions, respectively.

Thermoelectric generators (TEG) are cost effective solid-state devices with low thermal efficiencies. The limitations, due to the operational temperature of the thermoelectric materials, suppress the Carnot efficiency increase of the device. Although thermoelectric generators have considerable advantages over the other renewable energy devices, low convergence efficiency of the device retains thermoelectric devices behind its competitors. In order to keep up with the competition, improvement of device efficiency becomes crucial for the practical applications. The design configuration of the device pin legs, lowering the overall thermal conductance, can improve the device efficiency. Therefore, in the present study, the influence of pin leg geometry on thermal performance of the device is formulated thermodynamically. In this case, the exponential area variation of pin legs is considered and dimensionless geometric parameter ‘a’ is introduced in analysis. The influence of dimensionless geometric parameter on efficiency and power output is demonstrated for different temperature ratios and external load resistance ratios. It is found that increasing dimensionless geometric parameter improves the thermal efficiency of the device; however, the point of maximum efficiency does not coincide with the point of the maximum device output power.

A comparative performance analysis for various optimization criterion functions has been carried out for an irreversible Brayton cycle applicable to turbojet engines; the cycle includes the engine core and nozzle and diffuser. Newly defined parameters are introduced as power loss parameter (PLOS), effective power loss parameter (EPLOS) and Carnot-Brayton shape factor (CBSF) for a better assessment of the performance and power losses throughout the operation of the engine cycle. In addition, optimization functions, such as maximum power (MP), maximum power density (MPD), ecological coefficient of performance (ECOP) and ecological function (ECOL) are considered and their optimal operation conditions are compared with respect to each other. Some important evaluations are made for the optimal operation conditions with respect to the ideal Carnot cycle case and also with respect to the ideal Brayton cycle case. Power losses due to internal irreversibilities, as a result of the second law of thermodynamics and due to shape of the cycle model are classified and evaluated. In addition, this paper introduces an assessment of the performance of the turbojet engines in terms of a mass flow rate, gas generator speed and shaft force models as well as considerations for variations of heat leakage. Various parameters affecting the performance analysis are also considered such as compressor pressure ratio θ c , compressor and turbine efficiencies ( η c and η t respectively), altitude and flight Mach number M ∞ . The comparisons show that the maximum power conditions have the lowest EPLOS but the highest PLOS values with respect to the other optimal function criterions, indicating the effect of the internal irreversibilities at the lowest but exergy destruction for the environment at the highest.

A novel dual functional heat pump and power generation integration system (DFHPPGIS) using a scroll machine as compressor and expander is proposed in this paper. It can be operated in both reverse Carnot cycle (RCC) heat pump mode and organic Rankine cycle (ORC) power generation mode. Compared with single system, this system can improve the utilization efficiency of geothermal water and generate more economic benefits. Three kinds of ORC and RCC working fluids are compared to select refrigerant R245fa as the most suitable fluid for the system. Then the model of DFHPPGIS is built to analyze its energy, exergy and economic performance as the variation of geothermal water inlet temperature. Results of theoretical calculation show that there is a conflict between total cost and payback period of the system. The system with lower total cost shows high payback period and vice versa. Thus, a multi-objective optimization for the system is conducted to determine the optimal geothermal water inlet temperature based on the ideal point decision making method. Exergy loss of the system under the optimal condition is analyzed to reveal which component has the largest exergy loss. The results indicate that in this proposed system, if the total cost based optimized design is chosen, payback period is 136.4% higher than its minimum value. If the method of payback period based optimized design is selected, total cost is 15.3% higher than its minimum value. The system has a heat capacity of 250.19 kW and a power output of 17.83 kW at the optimal geothermal water inlet temperature of 80℃ as well as the total cost and payback period of the system are 37.31 k$ and 4.87 years. In addition, the scroll machine has the largest exergy loss in both RCC and ORC mode, which is the component that needs to be mostly optimized in the further development.

This work reports the analyses of some thermodynamic systems with the concepts of entropy generation, entropy generation numbers and revised entropy generation number, as well as exergy destruction number and exergy-work conversion efficiency that are proposed in this paper. The applicability of entropy generation minimization (EGM) is conditional if the optimization objective is the output power. The EGM leads to the maximum output power when the net exergy flow rate into the system is fixed, but it may not be appropriate if the net exergy flow rate into the system is not fixed. On the other hand, smaller exergy destruction number always corresponds to larger exergy-work conversion efficiency. The refrigeration cycle with the reverse Carnot engine is also analyzed in which mechanical work is input. The result shows that the EGM leads to the largest COP if the temperature of the high temperature heat source is fixed.

The idea is to find out whether 2nd law efficiency optimization may be a suitable trade-off between maximum work output and maximum 1st law efficiency designs for a regenerative gas turbine engine operating on the basis of an open Brayton cycle. The primary emphasis is placed on analyzing the ideal cycle to determine the upper limit of the engine. Explicit relationships are established for work and entropy production of the ideal cycle. To examine whether a Brayton cycle may operate at the regime of fully reversible characterized by zero entropy generation condition, the cycle net work is computed. It is shown that an ideal Brayton-type engine with or without a regenerator cannot operate at fully reversible limit. Subsequently, the analysis is expanded to an irreversible cycle and the relevant relationships are obtained for net work, thermal efficiency, total entropy production, and second law efficiency defined as the thermal efficiency of the irreversible cycle divided by the thermal efficiency of the ideal cycle. The effects of the compressor and turbine efficiencies, regenerator effectiveness, pressure drop in the cycle and the ratio of maximum-to-minimum cycle temperature on optimum pressure ratios obtained by maximization of 1st and 2nd law efficiencies and work output are examined. The results indicate that for the regenerator effectiveness greater than 0.82, the 2nd law efficiency optimization may be considered as a trade-off between the maximum work output and the maximum 1st law efficiency. (C) 2013 Elsevier Ltd. All rights reserved.

The performance of a heat engine is analyzed subject to two types of irreversibility: a non-zero heat capacity, together with the more common finite heat transfer rate between the engine and the external heat reservoirs. The heat capacity represents an engine body that undergoes significant temperature variations during the engine cycle. An option to cut off the heat exchange between the engine and the external surrounding for part of the engine cycle is also explored. A variational approach was taken to find the engine’s internal temperature profile (which defines the internal thermodynamic cycle) that would produce maximum power. The maximum power is shown to be lower than the case of zero heat capacity, due to a loss of heat that is stored in the engine body and then lost, bypassing the thermodynamic cycle. The maximum efficiency and the efficiency at maximum power are also lower than the zero heat capacity case. Similar to the Curzon–Ahlborn analysis, power can be traded for increased efficiency, but for high heat capacity, the range of efficiency that is available for such a trade is diminished. Isolating the engine during part of the cycle reduces maximum power, but the efficiency at maximum power and the maximum efficiency are improved, due to better exploitation of heat stored in the engine body. This might be useful for real engines that are limited by the internal energy change during a single engine cycle or by the operating frequency, leading to a broader power–efficiency curve.

A micro heat engine, based on a cavity filled with a stationary working fluid under liquid–vapor saturation conditions and encapsulated by two membranes, is described and analyzed. This engine design is easy to produce using MEMS technologies and is operated with external heating and cooling. The motion of the membranes is controlled such that the internal pressure and temperature are constant during the heat addition and removal processes, and thus the fluid executes a true internal Carnot cycle. A model of this Saturation Phase-change Internal Carnot Engine (SPICE) was developed including thermodynamic, mechanical and heat transfer aspects. The efficiency and maximum power of the engine are derived. The maximum power point is fixed in a three-parameter space, and operation at this point leads to maximum power density that scales with the inverse square of the engine dimension. Inclusion of the finite heat capacity of the engine wall leads to a strong dependence of performance on engine frequency, and the existence of an optimal frequency. Effects of transient reverse heat flow, and ‘parasitic heat’ that does not participate in the thermodynamic cycle are observed.

Thermodynamic cycle transferring heat and work was executed in thermoacoustic engines, when the acoustic resonators substituted the moving mechanical components of the traditional heat engines. Based on the traveling-wave phasing and reversible heat transfer, thermoacoustic Stirling engines could achieve 70% of the Carnot efficiency theoretically, if the inevitable viscous dissipation in resonators was also counted as exported power. It should be pointed out an error on this efficiency evaluation in the previous literatures. More than 70% of the acoustic power production was often consumed by the side-branch resonator that was the essential configuration to build up a thermoacoustic Stirling engine. According to the simulation results and some experimental data, the actual available acoustic power consumed by the acoustic loads was restricted by the operating peak-to-mean pressure ratio, i.e. | p 1 / p m |. When the peak-to-mean pressure ratio operated on 4–6.5%, the thermal efficiency and power density of the available acoustic power reached higher levels. But the available acoustic power would approach zero when | p 1 / p m | attained 10%. It was approved that the turbulence oscillation occurred on the higher | p 1 / p m | (usually >4%) was the main reason of the excess dissipation in the side-branch resonator. This character of the available power limited the wide application of thermoacoustic Stirling engines. The evaluation of thermal efficiency and energy conversion also indicated the improving direction of thermoacoustic Stirling engines. Generators driven by the thermoacoustic Stirling engines were an effective way, due to the elimination of the side-branch resonator. To achieve a high power density and a high pressure ratio on the higher available power efficiency level, the standing-wave thermoacoustic engines might outvie the traveling-wave thermoacoustic engines. To enjoy the best features of standing-wave engines and traveling-wave engines simultaneously, exploiting multi-stage thermoacoustic engines, such as cascade engines, etc., would be an important research direction.

The Diesel refrigeration cycle using an ideal quantum gas as the working substance is called quantum Diesel refrigeration cycle, which is different from Carnot, Ericsson, Brayton, Otto and Stirling refrigeration cycles. For ideal quantum gases, a corrected equation of state, which considers the quantum behavior of gas particles, is used instead of the classical one. The purpose of this paper is to investigate the effect of quantum gas as the working substance on the performance of a quantum Diesel refrigeration cycle. It is found that coefficients of performance of the cycle are not affected by the quantum degeneracy of the working substance, which is the same as that of the classical Diesel refrigeration cycle. However, the refrigeration load is different from those of the classical Diesel refrigeration cycle. Lastly, the influence of the quantum degeneracy on the performance characteristics of the quantum Diesel refrigeration cycle operated in different temperature regions is discussed.

This paper discusses the behavior of methanol through an ejector operating in a refrigeration system with a medium temperature thermal source. For detailed calculation of the proposed system, a method has been developed, which employs analytical functions describing the thermodynamic properties of methanol. The proposed cycle has been compared with a Carnot cycle working at the same temperature levels. The influences of three major parameters, generator, condenser and evaporator temperatures, on ejector efficiency and coefficient of performance are discussed. Also, the maximum value of COP was estimated by correlation of the above three temperatures for constant superheated temperature 150 °C, and it was 0.139–0.467. The design conditions were generator temperature 117.7–132.5 °C, condenser temperature 42–50 °C and evaporator temperature −10–5 °C.

In this study, a performance analysis based on alternative performance criteria has been performed for a reversible Carnot cycle, modified for cogeneration, with external irreversibilities. The optimum values of the design parameters of the cogeneration cycle were determined for different criteria. In this context, the effects of design parameters on the exergetic performance are investigated, and the results are discussed. The obtained results may guide towards a more realistic criterion for actual cogeneration plants.

We present the results of efficiency and power output for irreversible Carnot-like heat engines with nonlinear inverse, Dulong–Petit and Stefan–Boltzmann heat transfer laws when optimized with a recent criterion. A unified working regime is found intermediate between those predicted by the maximum efficiency and maximum power for all realistic values of the parameters accounting for the irreversibilities: finite rate heat transfer between the working fluid and the external heat sources, internal dissipation in the working fluid, and heat leak between reservoirs.

There appear to be some fundamental disagreements between those scientists and engineers who adhere to the principles of macroscopic classical thermodynamics and those who adhere to the principles of finite time thermodynamics. In many instances, the disagreements can be traced to differing assumptions concerning fundamental principles, definitions, etc., as well as differing interpretations of fundamental principles, definitions, etc. This paper is an attempt to clarify the issues by consideration, in somewhat laborious detail, of the basic definitions and their interpretations, considering the fundamental principles of classical thermodynamics and nonequilibrium thermodynamics. It appears to the author that nonequilibrium thermodynamics might offer a reconciliation of the disagreements and provide an approach acceptable to both sides. The work is purposely limited to considerations of importance in thermal cycles, mostly isothermal heat addition and rejection, and no attempt is made to treat even this broad field comprehensively. It is hoped that such discussions will lead to resolution of the disagreements, at least in part. Given the tendency of thermodynamicists to argue, in all probability, the discussions will lead to additional disagreements.

By employing the energetic optimization technique, the optimal performance of a focusing collector-driven, irreversible Carnot cogeneration heat pump system which consists of an irreversible Carnot heat engine, an irreversible Carnot vapor compression heat pump and an irreversible Carnot absorption heat pump, is investigated. The optimum temperature and minimum operating parameter of the solar collector are obtained. A minimum value for the total solar insolation needed to overcome internal irreversibilities for start-up of the system is defined, and the effect of the collector design parameters on this value is investigated.

It is acknowledged that the Curzon–Ahlborn efficiency ηCA determines the efficiency at maximum power production of heat engines only affected by the irreversibility of finite rate heat transfer (endoreversible engines), but ηCA is not the upper bound of the efficiencies of heat engines. This is conceptually different from the role of the Carnot efficiency ηC which is indeed the upper bound of the efficiencies of all heat engines. Some authors have erroneously criticized ηCA as if it were the upper bound of the efficiencies of endoreversible heat engines. Although the efficiencies of real heat engines cannot attain the Carnot efficiency, it is possible, and often desirable, for their efficiencies to be larger than their respective maximum power efficiencies. In fact, the maximum power efficiency is the allowable lower bound of the efficiency for a given class of heat engines. These important conclusions may be expounded clearly by the theory of finite time thermodynamics.

We construct thermodynamic criteria for optimization of sequential work-assisted heating and drying operations which run jointly with ‘endoreversible’ thermal machines. The total power input is minimized with constraints which take into account the dynamics of heat and mass transport and rate of work consumption. Finite-rate, endoreversible models include the irreducible losses of classical exergy potential caused by the thermal resistances. Extremum performance functions for optimal work, which incorporate residual entropy production, are formulated in terms of end states, duration and (in discrete processes) number of stages. Formal analogies between entropy production expressions for work-assisted and conventional drying operations help formulate optimization models of the former. An optimization procedure for a two-stage drying operation with an endoreversible heat pump at each stage is outlined.

In this paper, a comparison between the Entropy Generation Minimization method and the Power Maximization technique is presented. The assessment is performed by analyzing, as a typical example of direct conversion heat engines, the thermoelectric generator. The effects of heat-leak and finite-rate heat transfer on the performance of the generator, which is modeled as a Carnot-like engine with internal irreversibilities, are studied. Even though both methods lead to the same conclusions, the entropy generation minimization method, when applied for such an inherently irreversible device, is shown to be less straight forward than the power maximization technique requiring careful accounting of the different sources of irreversibility. Moreover, the entropy generation rate vs. efficiency behavior of the generator reveals that the efficiency at minimum entropy generation rate and the maximum efficiency are distinct.

The technique of energetic optimization is employed to investigate the optimal performance of focusing collector driven, irreversible Carnot ejector-vapor compression cascaded cooling and heating systems. The optimal operating temperature of the solar collector and the maximum overall coefficient of performance of the cooling and heating modes of the combined system are derived. The results obtained here may serve as a good guide for the evaluation of existing real solar assisted hybrid systems.

This paper reports the finite time thermodynamic optimization based on the maximum power density criterion for an irreversible Carnot heat engine model which includes three types of irreversibilities: finite rate heat transfer, heat leakage and internal irreversibility. The obtained results are compared with those results obtained by using the maximum power criterion. The design parameters under the optimal conditions have been derived analytically and the effects of the irreversibilities on the general and optimal performances are investigated. The results showed that the design parameters at maximum power density lead to smaller and more efficient heat engines. It is also seen that the irreversibilities have a greater influence on the performances at maximum power density conditions with respect to the ones at maximum power conditions. Also in this study, the optimal conductance allocation parameter is investigated at both maximum power density and maximum power conditions by assuming a constrained total thermal conductance in the case when there is no heat leakage. The relation between the conductance allocation parameter and the thermal efficiencies at maximum power density and maximum power is also investigated. The obtained results generalize the result of previous studies on this subject and provide guidance to optimal design in terms of power, thermal efficiency and engine sizes for real heat engines.

An endoreversible Carnot cycle is used to describe heat engine operation. This provides upper limits for real performance. The output power is maximized. Meteorological and actinometric data provided by the Viking Lander 1 are used as inputs. Four strategies of collecting solar energy are considered. Results concerning the following three parameters are briefly reported: (1) optimum solar collector surface area, (2) optimum solar collector temperature and (3) maximum output power.

In this work an irreversible extension of the Carnot problem of maximum mechanical work delivered from a system of two fluids with different temperatures is analysed. We consider the active heat exchange between the fluid of a limited thermal capacity and an ambient or environmental fluid whose thermal capacity is so large that its intensive parameters do not change. In the classical problem the rates vanish due to the reversibility requirement, in the extended problem irreducible irreversibilities occurring in boundary layers are admitted. Our thermodynamic modelling is linked with ideas and methods of the optimal control. A variational theory treats an infinite sequence of infinitesimal Curzon–Ahlborn–Novikov processes (CAN processes) as the pertinent theoretical model leading to a finite time exergy of the system with a finite exchange area or a finite contact time. This dissipative exergy is discussed in terms of the process intensity and finite duration. An analytical formalism, analogous to that in analytical mechanics is effective. A novel approach is worked out based on the Hamilton–Jacobi–Bellman equation for the dissipative exergy and related work functionals (HJB theory). The HJB formulation is helpful to deal with work potentials by numerical methods which use Bellman’s recurrence equation as practically sole method of extremum seeking for functionals with constrained rates and states and for complex boundary conditions. It is pointed out that only an irreversible process can be optimal for a finite duration. A link is shown between the process duration, dissipation and optimal intensity measured in terms of a Hamiltonian. Hysteretic properties are effective which cause the difference between the work supplied and delivered, for inverted end states of the process. A decrease of maximal work received from an engine system and an increase of minimal work added to a heat pump system is shown in the high-rate regimes and for short durations. The results prove that bounds of the classical availability should be replaced by stronger bounds obtained for finite time processes.

An extended exergy allowing some inevitable dissipative properties (e.g. those occurring in boundary layers) can be formulated for finite rate transitions, which lead a system not in equilibrium with the environment (or “dead state”) to equilibrium with it in a finite time. This exergy is a function of the end states and the process duration or a process intensity index. This exergy simplifies to classical thermal exergy in the limiting case of infinite duration, when the property of reversibility is recovered due to the vanishing rates. The functional of the extended exergy can be derived either as a finite time extension of the classical thermodynamic work extracted from a system and its environment or by evaluating a functional of the dissipated classical exergy. With finite time exergy, one can effectively optimize various continuous and multistage processes encountered in the theory of energy conversion and energy exchange in systems with a finite exchange area or with a finite contact time. However, the most important application of the dissipative exergy is the enhanced bounds predicted for the work delivery (consumption) in finite rate processes. These bounds are stronger than classical thermostatic bounds. In our formulation, nonlinear thermodynamic models are linked with ideas and methods of variational calculus. The variational formalism, with certain energy like and momentum like quantities strongly analogous to those known from analytical mechanics and optimal control theory, is an effective tool in the optimization of work. In this theoretical framework, one can easily discuss the role of finite process intensity and finite duration. The optimality of a specific irreversible process for the finite time transition of a fluid from one thermodynamic state to another is pointed out as well as a connection between the process duration, optimal dissipation and the optimal process intensity measured in terms of a Hamiltonian function. Our analysis proves that finite time exergy is different for processes approaching equilibrium with the environment, in which work is released (engine mode) and for processes departing equilibrium in which work is supplied (heat pump mode). However, the coincidence between the dissipative exergies and classical exergy is shown for the quasistatic case. The hysteretic properties of the finite time exergy cause a decrease in the maximum work received from a system in engine mode and an increase of work added to a system in heat pump mode, features which are particularly important in high rate regimes or for short duration thermodynamic processes. These results prove that the limits implied by the classical exergy theory should be replaced by the more realistic, stronger limits which are obtained for finite time processes.

The purpose of this paper is to study the optimal performance for an endoreversible Carnot heat pump with a working fluid system consisting of many spin-1/2 fermi. The relationship between the optimal heating load and the coefficient of performance (COP) for the quantum Carnot heat pump is derived. The optimum parameters are obtained at a maximum heating load point and at a maximum ecological function point, respectively.

The operation of a Carnot heat pump is viewed as a production process with exergy as its output. The short-run economic optimization of the endoreversible heat pump is performed in this paper. The profit of the heat pump is taken as the optimization objective function. Using the method of finite-time exergoeconomic analysis, which emphasizes the compromise optimization between economics (profit) and the utilization factor of energy (Coefficient of Performance, COP) for finite-time (endoreversible) thermodynamic cycles, this paper derives the relation between optimal profit and COP of an endoreversible Carnot heat pump based on a relatively general heat transfer law: q∞Δ(Tn). The COP bound at the maximum profit is also obtained. The results obtained involve those for three common heat transfer laws: Newton's Law (n = 1), the linear phenomenological law in irreversible thermodynamics (n = − 1), and the radiative heat law (n = 4).

In this paper, finite-time thermodynamics has been applied to ecologically optimize the power output of closed Brayton cycles for infinite thermal capacitance rates of the heat reservoirs. The optimum values of power, thermal efficiency and second-law efficiency of Brayton cycles are presented. The ratio of ecologically optimum power to maximum power is independent of the numbers of transfer units of the hot-side and the cold-side heat exchangers, and this ratio is much higher than the ratio of the entropy generation rate at maximum ecological function to that at maximum power. With the introduction of the ecological function, the improvement in second-law efficiency and in thermal efficiency is evident, especially for low hot-cold temperature ratios. Moreover, the thermal efficiency at maximum ecological function is about the average of the maximum-power efficiency and the reversible Carnot efficiency.

The optimization problem of a combined heat pump cycle made of two endoreversible Carnot heat pump cycles in series without intermediate reservoirs is investigated using finite-time thermodynamics. The fundamental relation between heating quantity and coefficient of performance (COP) of the combined cycle is derived. Relations are also derived between the SRHP (specific rate of heat pumping), the average rate of heat pumping per unit of total heat-transfer surface area and the COP for reciprocating and stable flow models of combined cycles. The results obtained from this paper may be extended to an endoreversible, combined heat pump cycle formed by more than two endoreversible Carnot heat pump cycles.