Heat Exchanger Inventory Research Articles

Power density analysis and optimization of an irreversible closed intercooled regenerated Brayton cycle

In this paper, power density, defined as the ratio of power output to the maximum specific volume in the cycle, is optimized for an irreversible closed intercooled regenerated Brayton cycle coupled to constant-temperature heat reservoirs in the viewpoint of the theory of thermodynamic optimization. The analytical formulae for dimensionless power density and efficiency, as functions of the total pressure ratio, the intercooling pressure ratio, the components (the regenerator, the intercooler, the hot- and cold-side heat exchangers) effectiveness, the compressor and turbine efficiencies, the heat reservoir temperature ratio, and the temperature ratio of the cooling fluid in the intercooler and the cold-side heat reservoir, are derived. The optimum dimensionless power density is obtained by optimizing the intercooling pressure ratio. The maximum dimensionless power density is obtained by searching the optimum heat conductance distributions between the hot- and cold-side heat exchangers for a fixed total heat exchanger inventory and fixed heat conductance distributions of the regenerator and the intercooler, and by searching the optimum intercooling pressure ratio. When the optimization is performed with respect to the total pressure ratio of the cycle, the maximum dimensionless power density can be maximized again, and a double-maximum power density and the corresponding optimum total pressure ratios are obtained. The effects of the heat reservoir temperature ratio, the temperature ratio of the cooling fluid in the intercooler and the cold-side heat reservoir, the efficiencies of the compressors and the turbine, and the total heat exchanger inventory on the optimum power density, the maximum power density, and the double-maximum power density and the corresponding optimal total pressure ratio are analyzed by numerical examples. In the analysis, the heat resistance losses in the four heat exchangers, and the irreversible compression and expansion losses in the compressors and the turbine are taken into account.

Assessment of Bejan’s heat exchanger allocation model under the influence of generalized thermal resistance, relaxation effect, bypass heat leak and internal irreversibility

This article reports an analytical investigation of the optimal heat exchanger allocation and the corresponding efficiency for maximum power output of a Carnot-like heat engine. To mimic a real engine, the generalized power law for the resistance in heat transfer external to the engine, relaxation effect in heat transfer, bypass heat leak and finally internal irreversibility of the power producing compartment of the engine is taken into consideration. From the engineering perspective the temperature ratio of the heat source and sink as well as to that of hot end and cold side of the working fluid is considered not to be the controllable parameters. A parametric study is presented for the other possible controllable variables. Selection of a power law over a linear model has a significant effect on the optimal heat exchanger allocation for maximum power output and the corresponding efficiency. For a higher degree of relaxation effect the drop in the maximum power efficiency is prominent along with the shift of equipartitioned allocation of heat exchanger inventory. Bypass heat leak and internal irreversibility exhibits relatively less pronounced effects on the maximum power efficiency and on the optimal heat exchanger allocation. Thus the endoreversible formulation of thermodynamic model is physically realistic. Strikingly when the optimal allocation of the heat exchanger inventory obeys the principle of equipartition in macroscopic organization for the linear law of the external heat resistance, the thermal efficiency appears to assume the representative documented value. Hence the linear model due to Bejan is also capable of capturing the essential features of a real power plant.

Performance optimization of a diabatic distillation-column by allocating a sequential heat-exchanger inventory

A diabatic distillation model is improved and the effects of the allocation of a heat-exchanger inventory on the diabatic-distillation performance are taken into account. Minimizing the entropy-production rate of the diabatic distillation column is taken as the optimization objective and the allocations of the heat-exchanger inventory on each tray in the diabatic distillation column are optimally redistributed under the condition of the fixed total heat-exchanger inventory. The optimal performance of the diabatic distillation column is obtained. The diabatic distillation model is meaningful for the design and optimization of diabatic distillation plant.

Power optimization of a regenerated closed variable-temperature heat reservoir Brayton cycle

In this paper, the power output of the cycle is taken as an objective for performance analysis and optimization of an irreversible regenerated closed Brayton cycle coupled to variable-temperature heat reservoirs in the viewpoint of finite time thermodynamics or entropy generation minimization. The analytical formulae about the relations between power output and pressure ratio are derived with the heat resistance losses in the hot- and cold-side heat exchangers and the regenerator, the irreversible compression and expansion losses in the compressor and turbine, the pressure drop losses at the heater, cooler and regenerator as well as in the piping and the effect of the finite thermal capacity rate of the heat reservoirs. The maximum power output optimization is performed in two aspects. The first is to search the optimum heat conductance distribution corresponding to the optimum power output among the hot- and cold-side of the heat exchangers and the regenerator for a fixed total heat exchanger inventory. The second is to search the optimum thermal capacitance rate matching corresponding to the optimum power output between the working fluid and the high-temperature heat source for a fixed ratio of the thermal capacitance rates of 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 heat reservoirs, the efficiencies of the compressor and the turbine, and the pressure recovery coefficient, are provided by numerical examples. The power plant design with optimization leads to a smaller size, including the compressor, turbine, and the hot- and cold-side heat exchangers and the regenerator. When the heat transfers between the working fluid and the heat reservoirs are carried out ideally, the pressure drop loss may be neglected, and the thermal capacity rates of the heat reservoirs become infinite. The results of this paper become those obtained in recent literature.

Optimum distribution of heat exchanger inventory and optimum thermal capacitance rate matching for power optimisation of a regenerated closed Brayton cycle

The power output of the cycle is taken as objective for performance analysis and optimisation of an irreversible regenerated closed Brayton cycle coupled to variable-temperature heat reservoirs. The analytical formulae about the relations between power output and pressure ratio are derived with the heat resistance losses in three heat exchangers, the irreversible losses in the compressor and turbine, the pressure drop losses in the piping and system, and the effect of the finite thermal capacity rate of the heat reservoirs. The power output optimisation is performed by searching the optimum heat conductance distribution among the three heat exchangers for the fixed total heat exchanger inventory, and by searching the optimum thermal capacitance rate matching between the working fluid and the heat reservoir.

Optimization of cooling load and coefficient of performance for real regenerated air refrigerator

The performance analysis and optimization of a real regenerated air refrigerator is carried out by using finite-time thermodynamic method in this article. To maximize the cooling load and the coefficient of performance of the refrigerator, the heat conductance distribution between the hot- and cold-side exchangers and the regenerator for the fixed total heat exchanger inventory and heat capacitance rate matching between the working fluid and heat reservoirs are optimized, respectively. The influences of the pressure ratio, the total heat exchanger inventory, the efficiencies of the compressor and expander, the pressure recovery coefficient, and the ratio of the heat capacitance rates of heat reservoirs on the optimum characteristic of the refrigerator are shown by detailed numerical examples. The results obtained may provide guidance for the design of practice air refrigeration plants.

Comparative performance analysis for endoreversible simple air refrigeration cycles considering ecological, exergetic efficiency and cooling load objectives

SYNOPSIS On the basis of exergetic analysis, the performance analysis and optimisation of endoreversible simple air refrigeration cycles with constant-temperature heat reservoirs is carried out by using the finite-time thermodynamic method in this paper. The expressions for cooling load, ecological function and exergetic efficiency of the refrigeration cycle are derived. The influences of pressure ratio of the compressor and allocation of heat exchanger inventory on the optimal performance of the cycle are investigated by detailed numerical examples. Performance comparisons between ecological optimisation objective, exergetic efficiency optimisation objective and traditional cooling load optimisation objective are performed. The results may provide guidelines for the design and optimisation of practical refrigeration plants.

Cooling load and coefficient of performance optimizations for real air-refrigerators

Based on a simple irreversible variable-temperature heat reservoir air (Brayton) refrigeration cycle model, a performance analysis and optimization of a real air refrigerator is carried out using finite-time thermodynamics. To maximize the cooling load and the coefficient of performance (COP) of the cycle, the allocation of a fixed total heat-exchanger inventory and thermal-capacity rate matching between the working fluid and heat reservoirs are optimized, respectively. The influences of pressure ratio, the total heat-exchanger inventory, the efficiencies of the compressor and expander, the thermal capacity rate of the working fluid and the ratio of the thermal-capacity rates of the heat reservoirs on the performance of the cycle are shown by numerical examples. The results obtained provide guidances for the design of practical air-refrigeration plants.

Power density optimisation of an endoreversible closed variable-temperature heat reservoir intercooled regenerated Brayton cycle

SYNOPSIS Taking power density, defined as the ratio of power output to the maximum specific volume in the cycle, as the objective function, this paper applies the theory of finite time thermodynamics to find the optimal distribution of heat conductance of the hot- and cold-side heat exchangers, the optimal intercooling pressure ratio, the optimal total pressure ratio and the optimal heat capacity ratio between working fluid and heat reservoir of an endoreversible closed intercooled regenerated Brayton cycle coupled to variable-temperature heat reservoirs with heat resistance losses in the hot- and cold-side heat exchangers, the intercooler and the regenerator, by using detailed numerical calculation. The maximum power density, the double-maximum power density and the triple-maximum power density are obtained by optimisation. The effects of some design parameters, including the cycle inlet heat reservoir temperature ratio, the inlet temperature ratio of cooling fluid in the intercooler and the cold-side heat reservoir, and the total heat exchanger inventory, on the maximum power density and the corresponding efficiency, optimum intercooling pressure ratio, and optimum heat conductance distributions between the hot- and cold-side heat exchangers, the double-maximum power density and the corresponding efficiency and optimum total pressure ratio, as well as the triple-maximum power density, are analysed.

Simulation of a transcritical CO 2 heat pump cycle for simultaneous cooling and heating applications

A steady state simulation model has been developed to evaluate the system performance of a transcritical carbon dioxide heat pump for simultaneous heating and cooling. The simulated results are found to be in reasonable agreement with experimental results reported in the literature. Such a system is suitable, for example, in dairy plants where simultaneous cooling at 4 °C and heating at 73 °C are required. The optimal COP was found to be a function of the compressor speed, the coolant inlet temperature to the evaporator and inlet temperature of the fluid to be heated in the gas cooler and compressor discharge pressure. An optimizing study for the best allocation of the fixed total heat exchanger inventory between the evaporator and the gas cooler based on the heat exchanger area has been carried out. Effect of heat transfer in the heat exchangers on system performance has been presented as well. Finally, a novel nomogram has been developed and it is expected to offer useful guidelines for system design and its optimisation.

Efficiency optimization of an irreversible closed intercooled regenerated gas-turbine cycle

Thermal efficiency is optimized for an irreversible closed intercooled regenerated gas-turbine cycle coupled to variable-temperature heat reservoirs in the viewpoint of the theory of thermodynamic optimization (or finite-time thermodynamics). It is first performed by searching the optimum intercooling pressure ratio and the optimum heat conductance distributions among the four heat exchangers (the hot- and cold-side heat exchangers, the intercooler, and the regenerator) for fixed total heat exchanger inventory. Secondly, the optimization is performed further with respect to the total pressure ratio of the cycle, the maximum efficiency is maximized twice and the double-maximum efficiency is obtained. Thirdly, the optimization is performed additionally with respect to the thermal capacitance rate matching between the working fluid and the heat reservoir, the double-maximum efficiency is maximized again and a thrice-maximum efficiency is obtained. In the optimization, the following effects are taken into account: the irreversibility of heat transfer in the four heat exchangers, the irreversible compression and expansion losses in the compressors and the turbine, the pressure drop loss in the piping, and the finite thermal capacity rates of the three heat reservoirs. The optimization method is applied to the conceptual design of a closed-cycle intercooled regenerated helium turbine power plant for high-speed warship propulsion. The numerical example shows that the method herein is valid and effective.

Exergy-based ecological optimisation for an endoreversible Brayton refrigeration cycle

The optimal exergy-based ecological performance of an endoreversible Brayton refrigeration cycle with the loss of heat-resistance is derived by taking into account an exergy-based ecological optimisation criterion as the objective function, which consists of maximising a function representing the best compromise between the exergy output rate and exergy loss rate (entropy generation rate and environment temperature product) of the refrigeration cycle. The expressions of the optimal cooling load, entropy generation rate and exergy-based ecological function of the cycle are derived by optimising the allocation of a fixed total heat exchanger inventory. The exergy-based ecologically optimum isentropic temperature ratio, coefficient of performance (COP), cooling load and entropy generation rate at maximum exergy-based ecological function point are also presented. Moreover, numerical examples are given to show that the effects of the temperature ratio of two reservoirs, the total heat exchanger inventory, and the temperature ratio between heat sink and surroundings on the optimal performance of the cycle.

Performance optimisation for an irreversible variable-temperature heat reservoir air refrigerator

SYNOPSIS The performance analysis and optimisation of an irreversible air refrigerator with variable-temperature heat reservoirs is carried out by taking the cooling load density, i.e., the ratio of cooling load to the maximum specific volume in the cycle, as the optimisation objective using finite-time thermodynamics (FTT) or entropy generation minimisation (EGM) in this paper. The analytical formulae for the relationships between cooling load density and pressure ratio, as well as between coefficient of performance (COP) and pressure ratio are derived with the heat resistance losses in the hot- and cold-side heat exchangers, and the irreversible compression and expansion losses in the compressor and expander. The influences of the effectiveness of the heat exchangers, the inlet temperature ratio of the reservoirs, and the efficiencies of the compressor and expander on the cooling load density versus and pressure ratio are provided by numerical examples. The cooling load density optimisation is performed by finding the optimum pressure ratio of the compressor, the optimum distribution of heat conductance of the hot- and cold-side heat exchangers for a fixed total heat exchanger inventory, and the optimum heat capacity rate matching between the working fluid and the heat reservoirs. The influences of some design parameters, including the effectiveness of the heat exchangers between the working fluid and heat reservoirs, the efficiencies of compressor and expander, the inlet temperature ratio of heat reservoirs, the heat conductance distribution and the heat capacity rate matching between the working fluid and the heat reservoirs on the maximum cooling load density are provided by numerical examples. Optimisation of refrigeration plant design leads to a reduction in size of the compressor, expander, and the hot- and cold-side heat exchangers.

Optimum Heat Conductance Distribution for Power Optimization of a Regenerated Closed Brayton Cycle

In this paper, the power output of the cycle is taken as objective for performance optimization of an irreversible regenerated closed Brayton cycle coupled to constant-temperature thermal energy reservoirs in the viewpoint of finite time thermodynamics (FTT) or entropy generation minimization (EGM). The analytical formulae about the relations between power output and pressure ratio are derived with the heat resistance losses in the hot- and cold-side heat exchangers and the regenerator, the irreversible compression and expansion losses in the compressor and turbine, and the pressure drop loss in the piping. The maximum power output optimization is performed by searching the optimum heat conductance distribution corresponding to the optimum power output among the hot- and cold-side heat exchangers and the regenerator for the fixed total heat exchanger inventory. The influence of some design parameters, including the temperature ratio of the heat reservoirs, the total heat exchanger inventory, the efficiencies of the compressor and the turbine, and the pressure recovery coefficient, on the optimum heat conductance distribution and the maximum power output are provided. The power plant design with optimization leads to smaller size including the compressor, turbine, and the hot- and cold-side heat exchangers and the regenerator.

Power optimization of an endoreversible closed intercooled regenerated Brayton-cycle coupled to variable-temperature heat-reservoirs

In this paper, in the viewpoint of finite-time thermodynamics and entropy-generation minimization are employed. The analytical formulae relating the power and pressure-ratio are derived assuming heat-resistance losses in the four heat-exchangers (hot- and cold-side heat exchangers, the intercooler and the regenerator), and the effect of the finite thermal-capacity rate of the heat reservoirs. The power optimization is performed by searching the optimum heat-conductance distributions among the four heat-exchangers for a fixed total heat-exchanger inventory, and by searching for the optimum intercooling pressure-ratio. When the optimization is performed with respect to the total pressure-ratio of the cycle, the maximum power is maximized twice and a ‘double-maximum’ power is obtained. When the optimization is performed with respect to the thermal capacitance rate ratio between the working fluid and the heat reservoir, the double-maximum power is maximized again and a thrice-maximum power is obtained. The effects of the heat reservoir’s inlet-temperature ratio and the total heat-exchanger inventory on the optimal performance of the cycle are analyzed by numerical examples.

Transcritical CO2 heat pump systems: exergy analysis including heat transfer and fluid flow effects

This paper presents the exergetic analysis and optimization of a transcritical carbon dioxide based heat pump cycle for simultaneous heating and cooling applications. A computer model has been developed first to simulate the system at steady state for different operating conditions and then to evaluate the system performance based on COP as well as exergetic efficiency, including component wise irreversibility. The chosen system includes the secondary fluids to supply the heating and cooling services, and the analyses also comprise heat transfer and fluid flow effects in detail. The optimal COP and the exergetic efficiency were found to be functions of compressor speed, ambient temperature and secondary fluid temperature at the inlets to the evaporator and gas cooler and the compressor discharge pressure. An optimization study for the best allocation of the fixed total heat exchanger inventory between the evaporator and the gas cooler based on heat transfer area has been conducted. The exergy flow diagram (Grassmann diagram) shows that all the components except the internal heat exchanger contribute significantly to the irreversibilities of the system. Unlike a conventional system, the expansion device contributes significantly to system irreversibility. Finally, suggestions for various improvement measures with resulting gains have been presented to attain superior system performance through reduced component irreversibilities. This study is expected to offer useful guidelines for system design and its optimisation and help toward energy conservation in heat pump systems based on transcritical CO2 cycles.

Power optimization of an irreversible closed intercooled regenerated brayton cycle coupled to variable-temperature heat reservoirs

In this paper, power is optimized for an irreversible closed intercooled regenerated Brayton cycle coupled to variable-temperature heat reservoirs in the viewpoint of the theory of thermodynamic optimization (or finite-time thermodynamics (FTT), or endoreversible thermodynamics, or entropy generation minimization (EGM)) by searching the optimum intercooling pressure ratio and the optimum heat conductance distributions among the four heat exchangers (the hot-and cold-side heat exchangers, the intercooler and the regenerator) for fixed total heat exchanger inventory. When the optimization is performed with respect to the total pressure ratio of the cycle, the maximum power is maximized twice and the double-maximum power is obtained. Further, as the optimization is performed with respect to the thermal capacitance rate matching between the working fluid and the heat reservoir, the double-maximum power is maximized again and a thrice-maximum power is obtained. In the analysis, the heat resistance losses in the four heat exchangers, the irreversible compression and expansion losses in the compressors and the turbine, the pressure drop loss in the piping, and the effects of finite thermal capacity rate of the three heat reservoirs are taken into account. The effects of the heat reservoir inlet temperature ratio, the total heat exchanger inventory and some other cycle parameters on the cycle optimum performance are analyzed by a numerical example. The optimum results are compared with those reported in recent reference for the conceptual design of a closed-cycle intercooled regenerated gas turbine nuclear power plant for marine ship propulsion. The numerical example shows that the method herein is valid and effective.

Power optimization of an endoreversible closed intercooled regenerated Brayton cycle

In this paper, power is optimized for an endoreversible closed intercooled regenerated Brayton cycle coupled to constant-temperature heat reservoirs in the viewpoint of finite-time thermodynamics (FTT) or entropy generation minimization (EGM). The effects of some design parameters, including the cycle heat reservoir temperature ratio and total heat exchanger inventory, on the maximum power and the corresponding efficiency are analyzed by numerical examples. The analysis shows that the cycle dimensionless power can be optimized by searching the optimum heat conductance distributions among the hot- and cold-side heat exchangers, the regenerator and the intercooler for fixed total heat exchanger inventory, and by searching the optimum intercooling pressure ratio. When the optimization is performed with respect to the total pressure ratio of the cycle, the maximum dimensionless power can be maximized again.

Cooling-load density optimization for a regenerated air refrigerator

A performance analysis and optimization of a regenerated air refrigeration cycle with variable-temperature heat-reservoirs is carried out by taking the cooling-load density, i.e., the ratio of cooling load to the maximum specific volume in the cycle, as the optimization objective using finite-time thermodynamics (FTT) or entropy-generation minimization (EGM). The model of a regenerated air refrigerator is presented, and analytical relationships between cooling-load density and pressure ratio, as well as between coefficient of performance (COP) and pressure ratio are derived. The irreversibilities considered in the analysis include the heat-transfer losses in the hot- and cold-side heat-exchangers and the regenerator, the non-isentropic compression and expansion losses in the compressor and expander, and the pressure-drop losses in the piping. The cycle performance comparison under maximum cooling-load density and maximum cooling-load conditions is performed via detailed numerical calculations. The optimal performance characteristics of the cycle are obtained by optimizing the pressure ratio of the compressor, and searching for the optimum distribution of heat-conductances of the hot- and cold-side heat-exchangers and regenerator for the fixed total heat-exchanger inventory. The effect of heat capacity rate matching between the working fluid and heat reservoirs on the cooling-load density is analyzed for the cycle. The influences of the effectiveness of the regenerator as well as the hot- and cold-side heat-exchangers, the efficiencies of the expander and the compressor, the pressure-recovery coefficient, and the temperature ratio of the heat reservoirs on the cooling-load density and COP are examined and illustrated by numerical examples.

Theoretical optimization of a regenerated air refrigerator

The performance analysis and optimization of a regenerated air refrigerator is carried out by taking the cooling load density, i.e. the ratio of cooling load to the maximum specific volume in the cycle, as the optimization objective using finite-time thermodynamics or entropy generation minimization in this paper. Analytical relationships between cooling load density and pressure ratio, as well as between coefficient of performance (COP) and pressure ratio are derived. The irreversibilities considered in the analysis include the heat transfer losses in the hot- and cold-side heat exchangers and the regenerator, the non-isentropic compression and expansion losses in the compressor and expander, and the pressure drop losses in the piping. The comparison of the cycle performances under maximum cooling load density and maximum cooling load conditions is performed. The optimal performance characteristics of the cycle are obtained by optimizing the pressure ratio of the compressor, and searching the optimum distribution of heat conductances of the hot- and cold-side heat exchangers and regenerator for the fixed total heat exchanger inventory. The influences of the effectivenesses of the regenerator as well as the hot- and cold-side heat exchangers, the efficiencies of the expander and the compressor, the pressure recovery coefficient, and the temperature ratio of the heat reservoirs on the cooling load density and COP are examined and shown by numerical examples.