Variable-temperature Heat Reservoirs Research Articles

Exergetic efficiency and exergy-based ecological function performance optimizations for two irreversible simple Brayton refrigeration cycle models

Air (Brayton) refrigerator has some application potentials nowadays. Based on two irreversible models of constant- and variable-temperature heat reservoir simple Brayton refrigerators established in previous references, this paper further derives exergetic efficiencies and ecological functions, and provide exergy-based comprehensive comparative analyses and optimizations for the two cycle models. Comparative studies among coefficient of performance (COP), cooling load, exergetic efficiency and ecological function characteristics are performed, including general performance analyses for fixed effectivenesses of heat exchangers and performance optimization by optimizing heat conductance distribution and optimizing heat capacity rate marching between heat-reservoir and working substance. For constant-temperature heat reservoir cycle, when select optimal pressure ratio, exergetic efficiency indicator is more reasonable than cooling load and ecological function indicators. When optimizing heat conductance distribution, cooling load indicator is almost the same as exergetic efficiency indictor. For variable-temperature heat reservoir cycle, the reasonable range of pressure ratio is that slightly larger than optimal one at maximum COP. For fixed pressure ratio, the difference between two optimal heat conductance distributions corresponding maximum cooling load and maximum exergetic efficiency is small. Various fixed design parameters have their influence features on the general performances and optimal performances of the cycle models, they should be considered specifically and pointedly. The major difference comparing with classical thermodynamic analysis is the consideration of heat-transfer loss in heat exchangers herein.

Effects of variable-temperature heat reservoirs on performance of irreversible Carnot refrigerator with heat recovery

The outlet temperature of the heat recovery reservoir is an important parameter in the design of refrigeration with heat recovery systems. In this paper the second law of thermodynamics has been applied to an irreversible Carnot refrigerator with heat recovery (CRHR) coupled to variable-temperature heat reservoirs. The refrigerating rate, input power, refrigeration coefficient, heat recovery coefficient, comprehensive coefficient of performance and exergy efficiency are chosen as the objective functions. The design rule chosen for this study is that the heat transfer area should be constrained. The mathematical expressions for assessing performance parameters with respect to area ratio, were derived for this study. These expressions are transcendental equations. The numerical solution method was employed to calculate the approximate solutions of the optimum performance parameters in a numerical example. The results indicate that the increase in the outlet temperature of heat recovery reservoir could lead to a rise in the maximum value of refrigerating rate and minimum value of input power; also it will lead to the decline in the maximum value of refrigeration coefficient, heat recovery coefficient, comprehensive coefficient and the exergy efficiency. When the ratio of heat recovery heat exchanger area to the summation of high temperature heat exchanger area and the heat recovery heat exchanger area is 1.0, the performance coefficients would attain their limit values and all of the condensing heat could be recycled. Our findings are helpful to the design and optimization to inform preparation of standard relating to the development of refrigerator with heat recovery.

Optimizing power of a variable-temperature heat reservoir Brayton cycle for space nuclear power plant

Optimizing power of a variable-temperature heat reservoir Brayton cycle for space nuclear power plant

Performance analysis of an ejector-assisted two-stage evaporation single-stage vapor-compression cycle

• An ejector-assisted 2-stage evaporation 1-stage vapor-compression cycle is proposed. • New cycle works for different refrigerants, especially for CO 2 transcritical cycle. • Energy savings depend on reasonable design and variable operating conditions. For actual vapor-compression refrigeration or heat pump systems with variable-temperature heat reservoirs, two-stage evaporation cycle is an efficient way to improve the coefficient of performance (COP). However, its application is limited because the two-stage evaporation cycle requires two individual suction pressures carried out by two cylinders or two compressors. To solve this problem, a two-stage evaporation cycle with a common single-stage compressor is proposed. An ejector is applied to lift the lower evaporation pressure up to the higher evaporation pressure or the compressor suction pressure. A thermodynamic model is developed to analyze its performance. Numerical results indicate that the COP of the novel cycle is 12.1% higher than the ejector cycle, 6.9% higher than the two-stage evaporation cycle and 22.0% higher than the basic refrigeration cycle under rated operating conditions. The exergy analysis indicates that TSEC-E can reduce 49.6% exergy loss of expansion process and 21.5% exergy loss of heat transfer process in evaporators in comparison with the basic refrigeration cycle. Although TSEC-E is applicable with various refrigerants, particularly it is more effective for CO 2 transcritical cycle. Moreover, performance improvement of the novel cycle under variable operating conditions is numerically investigated.

Optimizing Power and Thermal Efficiency of an Irreversible Variable-Temperature Heat Reservoir Lenoir Cycle

Applying finite-time thermodynamics theory, an irreversible steady flow Lenoir cycle model with variable-temperature heat reservoirs is established, the expressions of power (P) and efficiency (η) are derived. By numerical calculations, the characteristic relationships among P and η and the heat conductance distribution (uL) of the heat exchangers, as well as the thermal capacity rate matching (Cwf1/CH) between working fluid and heat source are studied. The results show that when the heat conductances of the hot- and cold-side heat exchangers (UH, UL) are constants, P-η is a certain “point”, with the increase of heat reservoir inlet temperature ratio (τ), UH, UL, and the irreversible expansion efficiency (ηe), P and η increase. When uL can be optimized, P and η versus uL characteristics are parabolic-like ones, there are optimal values of heat conductance distributions (uLP(opt), uLη(opt)) to make the cycle reach the maximum power and efficiency points (Pmax, ηmax). As Cwf1/CH increases, Pmax-Cwf1/CH shows a parabolic-like curve, that is, there is an optimal value of Cwf1/CH ((Cwf1/CH)opt) to make the cycle reach double-maximum power point ((Pmax)max); as CL/CH, UT, and ηe increase, (Pmax)max and (Cwf1/CH)opt increase; with the increase in τ, (Pmax)max increases, and (Cwf1/CH)opt is unchanged.

Power, Efficiency, Power Density and Ecological Function Optimization for an Irreversible Modified Closed Variable-Temperature Reservoir Regenerative Brayton Cycle with One Isothermal Heating Process

One or more isothermal heating process was introduced to modify single and regenerative Brayton cycles by some scholars, which effectively improved the thermal efficiency and significantly reduced the emissions. To analyze and optimize the performance of this type of Brayton cycle, a regenerative modified Brayton cycle with an isothermal heating process is established in this paper based on finite time thermodynamics. The isothermal pressure drop ratio is variable. The irreversibilities of the compressor, turbine and all heat exchangers are considered in the cycle, and the heat reservoirs are variable-temperature ones. The function expressions of four performance indexes; that is, dimensionless power output, thermal efficiency, dimensionless power density and dimensionless ecological function are obtained. With the dimensionless power density as the optimization objective, the heat conductance distributions among all heat exchangers and the thermal capacitance rate matching among the working fluid and heat reservoir are optimized. Based on the NSGA-II algorithm, the cycle’s double-, triple- and quadruple-objective optimization are conducted with the total pressure ratio and the heat conductance distributions among heat exchangers as design variables. The optimal value is chosen from the Pareto frontier by applying the LINMAP, TOPSIS and Shannon entropy methods. The results show that when the pressure ratio in the compressor is less than 12.0, it is beneficial to add the regenerator to improve the cycle performance; when the pressure ratio is greater than 12.0, adding the regenerator will reduce the cycle performance. For single-objective optimization, the four performance indexes could be maximized under the optimal pressure ratios, respectively. When the pressure ratio is greater than 9.2, the cycle is simplified to a closed irreversible simple modified Brayton cycle with one isothermal heating process and coupled to variable-temperature heat reservoirs. Therefore, when the regenerator is used, the range of pressure ratio is limited, and a suitable pressure ratio should be selected. The triple objective (dimensionless power output, dimensionless power density and dimensionless ecological function) optimization’ deviation index gained by LINMAP or TOPSIS method is the smallest. The optimization results gained in this paper could offer some new pointers for the regenerative Brayton cycles’ optimal designs.

Power density analysis and multi-objective optimization for a modified endoreversible simple closed Brayton cycle with one isothermal heating process

By using finite time thermodynamics, a modified endoreversible simple closed Brayton cycle with one isothermal heating process and variable isothermal pressure drop ratio is established in this paper. The cycle is composed of a compressor, a regular combustion chamber, a converging combustion chamber, a turbine and a precooler. Three variable temperature heat reservoirs are considered. The dimensionless power density is selected as the optimization objective, and the effects of inlet temperature ratios of combustion chambers on the optimal performances are studied. The results show that adding a convergent combustion chamber to the cycle can increase the dimensionless power density by 18.57%. There exit the optimal heat conductance distributions among three heat exchangers and the optimal pressure ratio leading to maximum dimensionless power density. The Pareto front based on dimensionless power output, thermal efficiency and dimensionless power density is further obtained by applying NSGA-II algorithm, and three decision methods are employed to choose the appropriate schemes from Pareto front. The different optimization schemes gained in this paper can satisfy different design demands for practical Brayton cycle power plants.

Power output, thermal efficiency and exergy-based ecological performance optimizations of an irreversible KCS-34 coupled to variable temperature heat reservoirs

An irreversible Kalina cycle system 34 (KCS-34) model with variable temperature heat reservoirs is studied by using finite-time thermodynamics. The power output, thermal efficiency and exergy-based ecological function of the KCS-34 are derived, and the effects of evaporator outlet dryness and mass flow rate of the working fluid on these performances are analyzed. With constant total heat transfer area, the heat transfer area distributions are optimized, and the optimal performances of the KCS-34 are obtained. The results show that the power output, thermal efficiency and ecological function have their triple maximum by optimizing the heat transfer area distributions, and the optimal distributions are different from each other. The ecological function shows a trade-off between power output and thermal efficiency to some extent. Compared with the initial designs, the power output, thermal efficiency and ecological function after triple optimizations of the heat transfer area distributions are increased by 17.27%, 5.79% and 1.05%, respectively. It shows that parameter optimization is important to improve the performances of the KCS-34, and the corresponding optimization work is significant.

Thermodynamic modeling and performance analysis of the variable-temperature heat reservoir absorption heat pump cycle

For practical absorption heat pump (AHP) plants, not all external heat reservoir heat capacities are infinite. External heat reservoir heat capacity should be an effect factor in modeling and performance analysis of AHP cycles. A variable-temperature heat reservoir AHP cycle is modeled, in which internal working substance is working in four temperature levels and all irreversibility factors are considered. The irreversibility includes heat transfer irreversibility, internal dissipation irreversibility and heat leakage irreversibility. The general equations among coefficient of performance (COP), heating load and some key characteristic parameters are obtained. The general and optimal characteristics are obtained by using numerical calculations. Besides, the influences of heat capacities of heat reservoirs, internal dissipation irreversibility, and heat leakage irreversibility on cycle performance are analyzed. The conclusions can offer some guidelines for design and operation of AHP plants.

Exergetic performance optimization of an endoreversible variable-temperature heat reservoirs intercooled regenerated Brayton cogeneration plant

An endoreversible intercooled regenerative Brayton cogeneration plant model coupled to variable-temperature heat reservoirs is established. On the basis of the exergetic analysis, the performance of the plant is investigated using finite time thermodynamics. Analytical formulae about dimensionless exergy output rate and exergy efficiency are deduced. By taking the maximum exergy output rate and exergy efficiency as the objectives, the heat conductance allocations among the hot-, cold- and consumer-side heat exchangers, the intercooler and the regenerator, the choice of intercooling pressure ratio are optimized by detailed numerical examples, and the corresponding exergy efficiency and exergy output rate are obtained. When the optimization is performed further with respect to the total pressure ratio of the cogeneration cycle, double-maximum exergy output rate and double-exergy efficiency are obtained. The effects of the total heat conductance, the consumer-side temperature and the thermal capacitance rate matching between the working fluid and the heat reservoirs on the double-maximum dimensionless exergy output rate and the double-maximum exergy efficiency are analyzed, it is found that there exists an optimal consumer-side temperature which leads to a thrice-maximum dimensionless exergy output rate, and there exist optimal thermal capacity rate matchings between the heat reservoirs and the working fluid, respectively, which lead to a thrice-maximum dimensionless exergy output rate and a thrice-maximum exergy efficiency.

Ecological performance analysis of an endoreversible modified Brayton cycle

An endoreversible closed modified simple Brayton cycle model with isothermal heat addition coupled to variable-temperature heat reservoirs is established using finite-time thermodynamics. Analytical expressions of dimensionless power output, thermal efficiency, dimensionless entropy generation rate and dimensionless ecological function are derived. Influences of cycle thermodynamic parameters on ecological performance and optimal compressor pressure ratio, optimal power output, optimal cycle thermal efficiency and optimal entropy generation rate corresponding to maximum ecological function are obtained and compared with those corresponding to maximum power output. The results show that cycle thermal efficiency improvement and entropy generation rate reduction are obtained at the expense of higher compressor pressure ratio and a little sacrifice of power output at maximum ecological function. The compromises between power output and entropy generation rate and between power output and cycle thermal efficiency, respectively, are achieved.

Optimal Allocation of Heat Exchanger Area for Vapor-Compression Heat Pump Plants

In order to optimize coefficient of heat pump, the finite time thermodynamic optimization theory is applied in this paper. By introducing area ratio of the condenser and evaporator, the analytical expressions for the optimal allocation of heat exchanger area for the irreversible Carnot heat pump with variable-temperature heat reservoirs are derived. For illustration, a real vapor compression heat pump example is utilized. Empirical results show that the optimal values for real vapor-compression heat pump cycles are obtained when area ratio is equal to 0.916. The results can guide the evaluation of existing heat pump plants or influence design practice for future plants.The results can guide the evaluation of existing heat pump plants or influence design practice for future plants.

Efficiency optimization of a closed indirectly fired gas turbine cycle working under two variable-temperature heat reservoirs

Efficiency optimization of a closed indirectly fired gas turbine cycle working under two variable-temperature heat reservoirsIndirectly or externally fired gas turbines (IFGT or EFGT) are interesting technologies under development for small and medium scale combined heat and power (CHP) supplies in combination with micro gas turbine technologies. The emphasis is primarily on the utilization of the waste heat from the turbine in a recuperative process and the possibility of burning biomass even "dirty" fuel by employing a high temperature heat exchanger (HTHE) to avoid the combustion gases passing through the turbine. In this paper, finite time thermodynamics is employed in the performance analysis of a class of irreversible closed IFGT cycles coupled to variable temperature heat reservoirs. Based on the derived analytical formulae for the dimensionless power output and efficiency, the efficiency optimization is performed in two aspects. The first is to search the optimum heat conductance distribution corresponding to the efficiency optimization among the hot- and cold-side of the heat reservoirs and the high temperature heat exchangers for a fixed total heat exchanger inventory. The second is to search the optimum thermal capacitance rate matching corresponding to the maximum efficiency between the working fluid and the high-temperature heat reservoir for a fixed ratio of the thermal capacitance rates of the two heat reservoirs. The influences of some design parameters on the optimum heat conductance distribution, the optimum thermal capacitance rate matching and the maximum power output, which include the inlet temperature ratio of the two heat reservoirs, the efficiencies of the compressor and the gas turbine, and the total pressure recovery coefficient, are provided by numerical examples. The power plant configuration under optimized operation condition leads to a smaller size, including the compressor, turbine, two heat reservoirs and the HTHE.

Performance comparison of an endoreversible closed variable-temperature heat reservoir intercooled regenerated Brayton cycle under maximum power and maximum power density conditions

In this paper, the power density, defined as the ratio of power output to the maximum specific volume in the cycle, is taken as the objective for performance analysis of an endoreversible closed intercooled regenerated Brayton cycle coupled to variable-temperature heat reservoirs from the viewpoint of finite-time thermodynamics (FTT) or entropy generation minimization. The analytical formulae for the relations between power density and pressure ratio are derived with the heat resistance losses in the hot- and cold-side heat exchangers, the intercooler, and the regenerator. Some results in the recent FTT literature are replicated. The intercooling pressure ratio is optimized for dimensionless power density. The effects of component (the intercooler, the regenerator, and the hot- and cold-side heat exchangers) effectivenesses, the thermal capacity rate of the working fluid, the heat reservoir inlet temperature ratio, and the inlet temperature ratio of the cooling fluid in the intercooler and the cold-side heat reservoir on maximum power density and its corresponding efficiency and intercooling pressure ratio are analysed using detailed numerical examples. The obtained results are compared with those obtained by using the maximum power criterion, including the cycle efficiencies, the normalized maximum specific volumes, and the normalized maximum specific volume differences at two design objectives. The advantages and disadvantages of maximum power density design are obtained via comparison of the main parameters of the cycle at maximum power and maximum power density conditions. The regeneration makes the maximum dimensionless power density decrease, and the intercooling makes the maximum dimensionless power density increase. The finite thermal capacity rates of the heat reservoirs do affect the performance of the variable-temperature heat reservoir cycle even for reversible cycles free of heat transfer irreversibility in general. The maximum power density design has the advantage of smaller size, but it requires a higher pressure ratio and has a lower efficiency than the maximum power design.

Heating load, COP, exergy loss rate, exergy output rate and ecological analyses and optimisations for irreversible universal steady flow variable temperature heat reservoir heat pump cycle model

AbstractThe performance of an irreversible universal steady flow heat pump cycle model with variable temperature heat reservoirs is analysed and optimised by using finite time thermodynamics. The universal heat pump cycle consists of two heat absorbing branches, two heat releasing branches and two irreversible adiabatic branches, with the losses of heat resistance and internal irreversibility. Expressions of heating load, coefficient of performance (COP), exergy output rate, exergy loss rate and ecological function of Brayton, Otto, Diesel, Atkinson, Dual, Miller and Carnot heat pump cycles are derived. There exists an optimal heat conductance distribution and an optimal thermal capacity rate matching between the working fluid and heat reservoir which leads to maximum heating load, maximum exergy output rate and maximum ecological function respectively. Performance comparisons among heating load, COP, exergy output rate, exergy loss rate and ecological function objectives are carried out. The effects of t...

Exergoeconomic performance analyses of an endoreversible intercooled regenerative Brayton cogeneration-type model

The exergoeconomic performance of an endoreversible intercooled regenerative Brayton cogeneration plant coupled to variable-temperature heat reservoirs is investigated using finite-time thermodynamics. The analytical formulae for the dimensionless profit rate and exergy efficiency of the cogeneration plant are deduced. The two cases with fixed and variable total pressure ratios are discussed. When the total pressure ratio is variable, by means of numerical calculations, the optimal intercooling pressure ratios corresponding to the optimal profit rate and exergy efficiency are obtained, respectively. The maximum profit rate, the maximum exergy efficiency and the corresponding optimal total pressure ratios are obtained. The effects of the design parameters on the exergoeconomic performance of the cogeneration plant are analysed. The results show that there exist two optimal consumer-side temperatures which lead to a double-maximum profit rate and a double-maximum exergy efficiency.

Finite time thermodynamic modelling and analysis for irreversible IFGT cycles

<title/>Indirectly or externally fired gas turbines (IFGTs or EFGTs) are interesting technologies under development for small and medium scale combined heat and power supplies in combination with micro gas turbine technologies. The emphasis is primarily on the utilisation of the waste heat from the turbine in a recuperative process. The possibility also exists for burning biomass even ‘dirty’ fuel by employing a high temperature heat exchanger (HTHE) to avoid the combustion gases passing through the turbine. In this paper, the theory of finite time thermodynamics is used in the performance analysis of a class of irreversible closed IFGT cycles coupled to variable temperature heat reservoirs. The analytical formulae for the dimensionless power output and efficiency, as functions of the total pressure ratio, component (HTHE, hot and cold side heat exchangers) effectiveness, compressor and turbine efficiencies and the thermal capacity rates of the working fluid and the heat reservoirs, the pressure recovery coefficient, the heat reservoir inlet temperature ratio, are derived. Analyses are carried out based on the formulation. Indirectly fired gas turbine cycles are most efficient under low compression ratio ranges (2·0-5·0) and fit for low power output circumstances for integration with micro gas turbine technology. Optimal total pressure ratio π c under maximum power output is always higher than that under maximum cycle thermal efficiency. When either of the heat transfer effectiveness of the hot or the cold side heat exchanger, the pressure recovery coefficient, isentropic efficiencies of the gas turbine and the compressor and the heat reservoir inlet temperature ratio increase, the dimensionless power output, cycle efficiency and their corresponding optimal total pressure ratios increase. It must be noted that the optimal total pressure ratio π c under the maximum cycle thermal efficiency decreases with increase in heat transfer effectiveness of the HTHE. The model derived can be further used to optimise the operational parameters and forecast performance of practical IFGT configurations and choices.

Heating load and COP optimizations for a class of generalized irreversible universal steady-flow variable-temperature heat reservoir heat pump cycle model

The heating load and coefficient of performance (COP) of a class of generalized irreversible universal steady-flow heat pump cycle model with variable-temperature heat reservoirs and the losses of heat transfer, heat leakage and internal irreversibility are investigated by using the theory of finite-time thermodynamics. The universal heat pump cycle model consists of two heat-absorbing branches, two heat-releasing branches and two adiabatic branches. Expressions of heating load and COP of the universal heat pump cycle model are deduced, respectively. By means of numerical calculations, the heat conductance distributions between hot- and cold-side heat exchangers are optimized by taking the maximum heating load as objective. There exist both optimal heat conductance distributions and optimal thermal capacity rate matching between the working fluid and heat reservoirs which lead to the double maximum heating load. The effects of heat leakage, internal irreversibility, total heat exchanger inventory and thermal capacity rate of the working fluid on the optimal performance of the cycle are discussed in detail. The results obtained herein include the optimal performance of endoreversible and irreversible, constant- and variable-temperature heat reservoir Brayton, Otto, Diesel, Atkinson, Dual, Miller and Carnot heat pump cycles. The results obtained can provide some theoretical guidelines for the designs and operations of practical heat pumps.

A Parametric Study of an Irreversible Closed Intercooled Regenerative Brayton Cycle

Entropy generation minimization technique is used in the analysis of an irreversible closed intercooled regenerative Brayton cycle coupled to variable-temperature heat reservoirs. Mathematical models are developed for dimensionless power and efficiency for a multi-stage Brayton cycle. The dimensionless power and efficiency equations are used to analyze the effects of total pressure ratio, intercooling pressure ratio, thermal capacity rates of the working fluid and heat reservoirs, and the component (regenerator, intercooler, hot- and cold-side heat exchangers) effectiveness. Using detailed numerical examples, the optimal power and efficiency corresponding to variable component effectiveness, compressor and turbine efficiencies, intercooling pressure ratio, total pressure ratio, pressure recovery coefficients, heat reservoir inlet temperature ratio, and the cooling fluid in the intercooler and the cold-side heat reservoir inlet temperature ratio are analyzed.

Thermodynamic modelling and performance of variable-temperature heat reservoir absorption refrigeration cycle

An irreversible variable-temperature heat reservoir four-temperature-level absorption refrigeration cycle model is established in this paper. The considered losses include heat resistances between heat reservoirs and working fluid, internal irreversibilities due to internal dissipation of the working fluid, and the heat leakages between the heat reservoirs and the surroundings. The general relationships between the cooling load and the COP are derived. Besides, the optimal performance characteristics between the cooling load and the COP are obtained using numerical examples. The effects of heat resistances, internal irreversibilities, heat leakages, and the thermal capacitance rates of heat reservoirs on the cycle performance are analysed.