Optimum thermoeconomic and thermodynamic performance characteristics of an irreversible three-heat-source heat pump

The coefficient of performance and specific heating load of an irreversible three-heat-source heat pump are given by using a general cycle model affected by the finite-rate heat transfer, heat leak and internal irreversibility of the cyclic working fluid. The heat pumping load divided by the total cost per unit time is taken as a new objective function and used to investigate the performance of the heat pump. The thermoeconomic and thermodynamic performance characteristics of the heat pump are discussed in detail. Some important performance parameters such as the thermoeconomic objective function and coefficient of performance are optimized. The optimally operating regions of the heat pump and the bounds of several performance parameters are determined. Finally, it is pointed out that the Carnot heat pump may be taken as a special case of a three-heat-source heat pump and consequently its optimal performance can be directly derived from the results obtained here.     

Performance analysis and parametric optimum criteria of an irreversible Atkinson heat-engine

Multi-irreversibilities, mainly resulting from the adiabatic processes, finite-time processes and heat loss through the cylinder wall, are considered in the cycle model of an Atkinson heat engine. The power output and efficiency of the cycle are derived by introducing the pressure ratio and the compression and expansion efficiencies. The performance characteristic curves of the cycle are presented. The bounds of the power output and efficiency are determined. The optimum criteria of some important parameters, such as the power output, efficiency and pressure ratio are given. The influences of the various design parameters on the performance of the cycle are analyzed in detail. The results obtained may provide a theoretical basis for both the optimal design and operation of real Atkinson heat engines.     

Performance analysis and parametric optimal design of an irreversible multi-couple thermoelectric refrigerator under various operating conditions

The performance of a thermoelectric refrigeration device, consisting of multi-couple thermoelectric elements and operating between two heat-reservoirs at constant temperatures, is investigated. The influence of the external and internal irreversibilities of the thermoelectric refrigeration device on the performance of the system is analyzed. The general expressions of the coefficient of performance and power input are derived by introducing some dimensionless parameters and variables. The coefficient of performance of the refrigeration device is maximized for a given cooling-load and total heat-transfer area of the system, and consequently, the structure parameters of the device and the area ratio of the heat exchangers of the system are determined optimally. The effects of the various parameters on the optimal performance of the device are discussed further. The results obtained here will be useful for a more detailed investigation and for the optimal design and manufacture of real thermoelectric refrigeration devices.     

A new analytical approach to model and evaluate the performance of a class of irreversible fuel cells

An irreversible model of a class of hydrogen–oxygen fuel cells working at steady-state is established, in which the irreversibilities resulting from electrochemical reaction, electrical resistance, and heat transfer to the environment are taken into account. The entropy production analysis is introduced and applied to investigate the physical and chemical performances of the fuel cell by using the theory of electrochemistry and non-equilibrium thermodynamics. Expressions for the power output and efficiency of the fuel cell are derived by introducing the equivalent internal and leakage resistances. With the help of the model being applied to high temperature solid oxide fuel cells, the performance characteristic curves of the fuel cell are presented and the influence of some design and operating parameters on the performance of the fuel cell are discussed in detail. Moreover, the optimum criteria of some important parameters such as the power output, efficiency, and current density are given. The results obtained may provide a theoretical basis for both the optimal design and operation of real fuel cells. This new method can also be used in the investigation and optimization of similar energy conversion settings and electrochemistry systems.     

Effects of combined heat transfer on the thermo-economic performance of irreversible solar-driven heat engines

A thermo-economic performance analysis and optimization has been carried out for an irrversible solar-driven heat engine with losses due to heat transfer across finite temperature differences, heat leak and internal irreversibilities. In the considered heat engine model, heat transfer from the hot reservoir is assumed to be simultaneous radiation and convection mode and the heat transfer to the cold reservoir is assumed to be convection mode. The effects of the technical and economical parameters on the thermo-economic performance have been investigated in order to see the collective effects of the radiation and convection modes of heat transfer. Also the optimal performance parameters of the heat engine, such as the thermal efficiency, temperatures of the working fluid and the ratio of heat transfer areas have been discussed in detail.     

A general method for the optimum design of heat recovery steam generators

The optimization of the heat recovery steam generator (HRSG) is one of the key elements for increasing the efficiency of combined plants. According to the current technical practice, it can be organized at different levels of complexity with objectives sequentially defined: operating parameters, geometrical details and technological elements.

According to this point of view, in the paper a complete strategy for the optimum design of the HRSG is outlined. The optimization is organized at two levels: the first one enables to obtain the main operating parameters of the HRSG, while the second involves the detailed design of the component concerning the geometric variables of the heat transfer sections. The output of the first-level optimization is the input of the second level. In particular, the second level of the optimization can be articulated in two different steps. The first step can be aimed to the minimization of the pressure drop for a given heat flow. The second step leads to a reduction of the overall dimensions, maintaining the imposed performance of the HRSG in terms of heat flow and pressure drop. The whole procedure is tested with reference to a case of existing HRSG structures; it shows the possibility of improving performance maintaining a constrained packaged size.     

Performance characteristics of an irreversible solar-driven Braysson heat engine at maximum efficiency

A novel model of the solar-driven thermodynamic cycle system which consists of a solar collector and a Braysson heat engine is established. The performance characteristics of the system are optimized on the basis of the linear heat-loss model of a solar collector and the irreversible cycle model of a Braysson heat engine. The maximum efficiency of the system and the optimally operating temperature of the solar collector are determined and other relevant performance characteristics of the system are discussed. The results obtained here may provide some theoretical guidance for the optimal design and operation of solar-driven Braysson and Carnot heat engines.     

Performance and optimum design analysis of an absorber plate fin using recto-trapezoidal profile

This paper establishes a new profile, viz. recto-trapezoidal (RT) of an absorber plate fin on the basis of ease of fabrication as well as augmentation of heat transfer rate per unit fin volume. An analytical model has been developed for evaluating the thermal performance and optimum dimensions of an absorber plate fin using this typical profile. The present study is equally suitable for an absorber plate fin having a rectangular, trapezoidal or triangular profile also with consideration of their respective geometrical parameters. The optimization of the RT profile has been cast in a generalized form either by maximizing heat transfer rate for a given fin volume or by minimizing fin volume for a given heat transfer duty. From the optimum design analysis, significant results have been noticed when an additional constraint is imposed with the fin volume. Under this design condition, it may also be highlighted that for an optimal circumstance, the heat transfer rate through a RT profile absorber plate fin is greater than a trapezoidal or triangular profile for the same fin volume. However, this observation may be restricted to the limited values of fin volume only. The optimum design analysis for the RT profiled absorber plate fin has also been studied under the different design constants. Finally, for the variation of all the design variables, optimum design curves have been generated for a wide range of thermo-geometric parameters.     

Optimal operation conditions for a single-stage heat transformer by means of an artificial neural network inverse

Analysis based on first and second law of thermodynamics together with direct and artificial neural networks inverse (ANNi) have been used to develop a methodology to decrease the total irreversibility of an experimental single-stage heat transformer. With the proposed methodology it is possible to calculate the optimal input parameters that should be used in order to operate the heat transformer with the lower irreversibilities. Mathematical validation of ANNi was carried out together with the comparison between the total cycle irreversibility (I_cycle) obtained thermodynamically and the I_cycle determined by using the ANNi. The results showed a mean discrepancy of 0.9% of the I_cycle values. The proposed new methodology can be very useful to control on-line the performance of a single-state heat transformer obtaining lower I_cycle values.     

Finite-time thermodynamic modelling and analysis of an irreversible Otto-cycle

The performance of an air standard Otto-cycle is analyzed using finite-time thermodynamics. In the irreversible cycle model, the non-linear relation between the specific heat of the working fluid and its temperature, the friction loss computed according to the mean velocity of the piston, the internal irreversibility described by using the compression and expansion efficiencies, and the heat-transfer loss are considered. The relations between the power output and the compression ratio, between the thermal efficiency and the compression ratio, as well as the optimal relation between the power output and the efficiency of the cycle are indicated by numerical examples. Moreover, the effects of internal irreversibility, heat-transfer loss and friction loss on the cycle performance are analyzed. The results obtained in this paper may provide guidance for the design of practical internal-combustion engines.     

Effect of irreversibilities on the performance of a thermoelectric generator

Using an externally and internally irreversible heat engine model, the maximum power and thermal efficiency at maximum power output have been determined for a thermoelectric generator. The irreversibilities can be characterized by a single parameter named the device-design parameter. The efficiency and power decrease with an increase of the device-design parameter which appears in the equations for maximum power and efficiency.     

Performance evaluation and optimum design strategies of an acid water electrolyzer system for hydrogen production

The performance of a new acid water electrolyzer system for hydrogen production is investigated, based on semi-empirical equations of a phosphoric acid water electrolyzer. The circulating electrolyte concentrations under differently operating temperatures are optimized so that the minimum input voltages of the electrolyzer are determined for other given conditions. The optimum electrochemical characteristics of the electrolyzer are revealed. Moreover, it is expounded that the Joule heat resulting from the irreversibilities inside the electrolyzer is larger than the thermal energy needed in the water splitting process. The general performance characteristics of the phosphoric acid water electrolyzer system are discussed, from which the lower bound of the operating current density is determined. The upper bound of the operating current density is further determined by introducing a multi-objective function including the system efficiency and hydrogen production rate. Consequently, some optimum design strategies of a phosphoric acid water electrolyzer system are obtained and may be chosen according to different practical requirements.     

Modeling and optimum design of hybrid solid oxide fuel cell-gas turbine power plants

An integrated solid oxide fuel cell-gas turbine process is modeled and analyzed. A two dimensional finite difference model is developed for planar solid oxide fuel cells (SOFCs), considering co- and counter-flow configurations. In this model, the SOFC is divided into five control volumes. Mass and heat transfer and electrochemical processes are examined and such factors as electrochemical reactions, conduction in the solid structure, convection in channels and between channels and solid structures and polarization losses are discussed. Also, the influences of temperature, pressure, current density and fuel utilization on voltage and electric power for both configurations are investigated. The results show that voltage and electric power increase as pressure and temperature rise, for both configurations. The optimum current density is determined to be 9000 A m⁻² (for co- and counter-flow). Also, the voltage in both configurations decreases as fuel utilization increases. Furthermore, an investigation of the effect of input temperature, pressure and current density on system performance and number of cells required demonstrates that the power output should be fixed at 200 kW. With increasing temperature, pressure and mole fraction of H₂, the number of cells required decreases and the electrical and overall efficiencies both increase. The optimum number of cells (co-flow) and current density are found to be 2122 and 8000 A m⁻², respectively.     

Performance analysis of an Organic Rankine Cycle with superheating under different heat source temperature conditions

This paper presents an analysis of non-regenerative Organic Rankine Cycle (ORC), based on the parametric optimization, using R-12, R-123, R-134a and R-717 as working fluids superheated at constant pressure. A computer programme has been developed to parametrically optimize and compare the system and second law efficiency, irreversibility of the system, availability ratio, work output, mass flow rate with increase in turbine inlet temperature (TIT) under different heat source temperature conditions. The calculated results reveal that R-123 produces the maximum efficiencies and turbine work output with minimum irreversibility for employed constant as well as variable heat source temperature conditions. Hence, selection of a non-regenerative ORC during superheating using R-123 as working fluid appears to be a choice system for converting low-grade heat to power.     

Optimum performance characteristics of an irreversible solar-driven Brayton heat engine at the maximum overall efficiency

An irreversible cycle model of a solar-driven Brayton heat engine is established, in which the heat losses of the solar collector and the external and internal irreversibilities of the heat engine are taken into account, and used to investigate the optimal performance of the cycle system. The maximum overall efficiency of the system is determined. The operating temperature of the solar collector and the temperature ratio in the isobaric process are optimized. The influence of the heat losses of the solar collector and the external and internal irreversibilities of the heat engine on the cyclic performance is discussed in detail. Some important curves which can reveal the optimum performance characteristics of the system are given. The results obtained here are general, and consequently, may be directly used to discuss the optimal performance of other solar-driven heat engines.     

Thermal performance analysis and optimum design parameters of heat exchanger having perforated pin fins

This paper reports the heat transfer enhancement and corresponding pressure drop over a flat surface equipped with circular cross section perforated pin fins in a rectangular channel. The channel had a cross section area of 100–250 mm². The experiments covered the following ranges: Reynolds number 13500–42,000, clearance ratio (C/H) 0, 0.33 and 1 and interfin spacing ratio (S_y/D) 1.208, 1.524, 1.944 and 3.417. Correlation equations were developed for the heat transfer, friction factor and enhancement efficiency. The experimental results showed that the use of circular cross section pin fins may lead to heat transfer enhancement. Enhancement efficiencies varied between 1.4 and 2.6 depending on clearance ratio and interfin spacing ratio. Using a Taguchi experimental design method, optimum design parameters and their levels were investigated. Nusselt number and friction factor were considered as performance parameters. An L₉(3³) orthogonal array was selected as an experimental plan. First of all, each goal was optimized separately. Then, all the goals were optimized together, considering the priority of the goals, and the optimum results were found to be Reynolds number of 42,000, fin height of 50 mm and streamwise distance between fins of 51 mm.     

A critical review on design aspects and developmental status of metal hydride based thermal machines

Over two decades, research in the field of metal hydride based thermal machines has gained immense attention by the researchers of different fields. Because of its capability to store large volume of hydrogen per unit mass at near ambient condition, its utilization has been spread in numerous applications such as energy storage and other biological, chemical, aerospace and nuclear applications. Though there have been several review reports published on metal hydride based hydrogen storage, but the present work is focused on the thermal management issues and worldwide developmental status of various metal hydride based thermal machines such as thermal energy storage, heat upgradation, heat pump, cooling system, and hydrogen purification and compression. With a brief discussion about the basic understanding of metal hydride alloy formation, this paper also covers screening of metal hydride alloys, design considerations and evolution of different reactor geometries for various metal hydride based thermal management systems. The review also addresses the benefit of coupling of a metal hydride based hydrogen energy system with a conventional thermal system in order to a produce hybrid system with much higher performance and almost zero environmental pollution.     

Influence of multi-irreversibilities on the performance of a Brayton refrigeration cycle working with an ideal Bose or Fermi gas

An irreversible cycle model of the quantum Brayton refrigeration cycle using an ideal Bose or Fermi gas as the working substance is established. Based on the theory of statistical mechanics and thermodynamic properties of ideal quantum gases, expressions for several important performance parameters such as the cooling rate, coefficient of performance and power input, are derived. The influence of the degeneracy of quantum gases, the internal irreversibility of the working substance and the finite-rate heat transfer between the working substance and the heat reservoirs on the optimal performance of the cycle is investigated. By using numerical solutions, the cooling rate of the cycle is optimized for a set of given parameters. The maximum cooling rate and the corresponding parameters are calculated numerically. The optimal boundaries of the coefficient of performance and power input are given. The optimally operating region of the cycle is determined. The expressions of some performance parameters for some special cases are derived analytically.