Design considerations for refrigeration cycles

The Carnot COP, which assumes a thermodynamically ideal cycle in which no irreversibilities exist, is often considered to be a design goal for actual cycles. However, the Carnot COP does not consider heat transfer mechanisms. Heat transfer at a finite rate is necessarily an irreversible process and unavoidable in a refrigeration cycle. The lack of consideration of rate processes reduces the usefulness of the Carnot COP as a realistic design goal. In this paper, the limitations of both thermodynamics and heat transfer are considered to identify a more realistic design goal for the COP of refrigeration cycles. The consideration of heat transfer limitations leads to a design rule for the optimum distribution of heat exchange area between the low- and high-temperature heat exchangers.     

Equivalent Carnot cycles for sorption refrigeration: Cycles de Carnot équivalents pour la production de froid par sorption

Temperature–entropy diagrams are usually used to describe heat-driven engines as well as vapour compressor refrigerators, but they have not yet been used for sorption refrigeration. Such Carnot cycles are introduced here to describe three sorption refrigeration technologies (liquid absorption, solid adsorption, chemical reaction). This is performed for basic cycles (without heat recovery) and for simple advanced cycles. Equivalent four-temperature Carnot cycles are obtained for basic cycles, whereas equivalent six-temperature Carnot cycles are obtained for advanced cycles. This approach shows that the quantity ΔH-TΔS (where ΔH and ΔS are the enthalpy and entropy variations associated with mass transfer during a cycle in/out of the sorbers) plays the same role as the mechanical energy in vapour compression cycles.     

不可逆卡诺制冷机的最佳制冷率,制冷系数特性

在考虑制冷剂与热源间热阻损失的内可逆卡诺制冷机模型基础上，用一常数项表示热漏损失，用一常系数项表示循环中除热阻和热漏外的其余不可逆性（如摩擦、涡流、非平衡等），提出了一个新的不可逆制冷机模型。导出了其最佳制冷率、制冷系数关系和最大制冷系数及其相应的制冷率。     

Performance optimization of an irreversible Carnot refrigerator with finite mass flow rate

The present study develops a theoretical model for the optimization of an irreversible Carnot refrigerator subject to a constraint of finite mass flow rate, which includes internal as well as external irreversibilities. By introducing two dimensionless parameters as indicative of the mass flow rates for the refrigerator, the new model allows detailed analyses on the finite mass flow rate allocation problem of working fluids among the hot- and cold-side heat exchangers of Carnot refrigerators. The analytical solutions of the maximum coefficient of performance (COP) for irreversible Carnot refrigerators are obtained under the equivalent of the finite-flow rate constraint. Furthermore, the influences of major parameters on the maximum COP and the corresponding mass flow rate allocation are examined and shown by numerical examples. The obtained results may provide a theoretical guidance for the optimal design and operation of real refrigerators.     

Study of different internal vapour transports for adsorption cycles with heat regeneration

A ‘three-temperature’ model of the adsorption cycles with heat regeneration is used for investigating and analysing the influence of different parameters on the performance of such cycles. The influence of the heat source temperature on the thermodynamic efficiency (COP/COP_Carnot) is investigated. The result is that the thermodynamic efficiency of the cycle is always limited. In order to reduce the internal irreversibilities, different internal vapour transports for pressurising (depressurisation) the adsorber are investigated: first, adiabatic direct pressurisation (depressurisation) with the condenser (evaporator) instead of pressure changes by heat transfer; second, adiabatic internal vapour recovery between the adsorbers (partial pressurisation/depressurisation); third, separation of the adsorber into separate compartments between which vapour cannot be redistributed during pressurisation or depressurisation. Results show that the first process significantly reduces the COP, while the second one enhances the cooling power, and the third one does not change the performance. Analysis gives satisfactory explanation of these results.     

Review of advanced absorption cycles: performance improvement and temperature lift enhancement

The objective of this study is to propose and evaluate advanced absorption cycles for the coefficient of performance (COP) improvement and temperature lift enhancement applications. The characteristics of each cycle are assessed from the viewpoints of the ideal cycle COP and its applications. The advanced cycles for the COP improvement are categorized according to their heat recovery method: condensation heat recovery, absorption heat recovery, and condensation/absorption heat recovery. In H₂O–LiBr systems, the number of effects and the number of stages can be improved by adding a third or a fourth component to the solution pairs. The performance of NH₃–H₂O systems can be improved by internal heat recovery due to their thermal characteristics such as temperature gliding. NH₃–H₂O cycles can be combined with adsorption cycles and power generation cycles for waste heat utilization, performance improvement, panel heating and low temperature applications. The H₂O–LiBr cycle is better from the high COP viewpoints for the evaporation temperature over 0°C while the NH₃–H₂O cycle is better from the viewpoint of low temperature applications. This study suggests that the cycle performance would be significantly improved by combining the advanced H₂O–LiBr and NH₃–H₂O cycles.     

低温制冷机的性能评价

作为对制冷系数COP概念的补充,建议采用yong效率（Exergy Efficien-cy）的概念来综合评价多级低温制冷机的性能,同时也为其性能优化过程提供理论指导。本文还以yong效率为基础,综合评价了4.2K和80K温区低温制冷机的发展水平,具有一定参考价值。     

包含五种循环的普适定常流内可逆制冷循环最优性能

用有限时间热力学方法分析了一类普适定常流内可逆制冷机循环,导出了存在传热损失时,由一个吸热过程、一个放热过程和两个绝热过程组成的一类普适的定常流内可逆制冷机循环的制冷率、制冷系数、(火用)损失率、(火用)输出率和生态学性能,并由数值计算分析了循环过程对循环性能的影响特点.所得结果包含了内可逆Carnot、Diesel、Otto、Atkinson和Brayton制冷循环的特性.     

广义不可逆卡诺热泵的有限时间(火用)经济性能优化

本文研究了牛顿定律系统广义不可逆卡诺热泵的有限时间炯经济性能，导出了存在热阻、热漏和其它内不可逆性时卡诺热泵的最优利润率解析式以及相应的供热系数界限，并用数值算例分析了热漏等因素对利润率和炯经济性能界限的影响。     

Thermodynamics of magnetic refrigeration

A comprehensive treatment of the thermodynamics of cyclic magnetic refrigeration processes is presented. It starts with a review of the work, heat and internal energy of a magnetized specimen in a magnetic field, and a list of the thermodynamic potentials is given. These are based on the very recent discovery of an alternative Kelvin force. It is shown that this force is compatible with the internal energy proposed by Landau and Lifshitz. New formulas for the specific enthalpies are presented. Cyclic processes are discussed in detail, e.g. the Brayton, Ericsson and Carnot cycles. Magnetic refrigeration and magnetic heat pump cycles are preferably designed by applying the cascade or/and regeneration principle. Cascade systems allow wider temperature ranges to be obtained. The main objective of this article is to yield a theoretical basis for an optimal design of new magnetic refrigeration and heat pump devices.     

Fundamental thermodynamics of ideal absorption cycles

Starting from the representation of a real absorption refrigeration cycle on a temperature-entropy diagram, step-by-step idealisations of the binary mixture, together with the thermodynamic transformations are considered, in order to derive the ideal thermodynamic absorption cycle performance and temperature formulae. It is demonstrated that the ideal absorption cycle is the combination of a Carnot driving cycle with a reverse Carnot cooling cycle. The resorption cycle is analysed in the same manner. Information is included on absorption cooling with heat recovery cycles, heat pumps and temperature amplifiers. From the analysis of single-stage cycles, and by superimposing absorption cycles operating at different temperatures and utilising specific residual heat of the higher temperature sub-cycles, the performance and temperature relations of double, triple and multistage cycles are derived. Special attention is given to three types of triple-stage cycle and their ideal equivalence is demonstrated and represted on the pressure-temperature-concentration (PTX) diagram. A simple hybrid absorption-compression cycle is analysed and the results are compared with those of ideal cold generation cycles (combinations of driving and cooling cycles). Consideration is also given to cold generation systems. Finally, the validation of the fundamental thermodynamics of absorption cycles is presented by applying an exergy analysis. This paper presents the thermodynamic principles involved to obtain simple formulae, in a similar way to the Carnot cycle in order to convey the ideal theoretical limitations.     

Cooling machine with integrated cold storage

The use of thermal solar energy systems in combination with thermal driven sorption chillers for climatisation gains increasing influence. For solar assisted cooling a backup system is necessary for times when no solar energy is available. Absorption chillers driven by a combination of thermal collectors and conventional furnaces, which supply the driving heat in times of no insolation, suffer from an abrupt drop of the system efficiency (COP) during the operation change. This drop in COP can be avoided by installing a combined heat buffer and storage feature. Various possibilities of heat storage features are compared. An experimental setup of a high-efficient absorption chiller which facilitates the supply of a constant load of coldness at constantly high COP in spite of periodically available driving heat is presented.     

Numerical optimisation of a two-stage ejector refrigeration plant

Jet-refrigeration cycles seem to provide an interesting solution to the increasing interest in environment protection and the need for energy saving due to their low plant costs, reliability and possibility to use water as operating fluid. A steam/steam ejector cycle refrigerator is investigated introducing a two-stage ejector with annular primary at the second stage. The steady_state refrigerator, exchanging heat with the water streams at inlet fixed temperatures at the three shell and tube heat exchangers, evaporator, condenser and generator, is considered as an open system. Heat transfer irreversibilities in the heat exchangers and external friction losses in the water streams are considered, ignoring the internal pressure drop of the vapor. A simulation program numerically searches the maximum COP at given external inlet fluid temperatures as a function of mass flows, dimensions and temperature differences in the heat exchangers. The code gives the ejector and heat exchangers design parameters.     

Carnot comparison of multi-temperature level absorption heat cycles

Comparison of different absorption heat cycles is not always made on the correct manner. This also includes comparison of an ideal absorption cycle with a mechanical analogy. A new Carnot model operating with two heat engines and two mechanical heat pumps is defined to be the correct and logical way to describe the mechanical analogy for an absorption heat pump and an absorption heat transformer. General equations for the Carnot coefficient of performance, COP_r, are exemplified and simulated for an absorption heat pump and an absorption heat transformer, and an entropy flow fraction diagram is introduced. The important fact that the absorption heat cycles must operate under the same conditions when they are compared is discussed.     

具有热阻、热漏和内不可逆性的联合卡诺热机生态学优化

用有限时间热力学的方法分析具有热阻、热漏、内不可逆性的定常流联合卡诺型热机循环．导出了在傅立叶导热定律下联合循环功率、效率和生态学指标的性能，并进行优化；得到功率、效率和生态学指标之间的优化关系，并由数值计算分析了功率、效率和循环熵产率之间的关系．所得的结果表明，最大生态学指标下的效率十分接近于联合循环可以达到的最大效率；相应的熵产率也要低于以输出功率为优化目标时的熵产率．     

A computational study about the types of entropy generation in three different R134a ejector mixing chambers

The purpose of this investigation is to study the efficiency of a R134a ejector operating with three different mixing chambers by means of a CFD based entropy generation analysis. With the aid of the differential equation for entropy, the local entropy generation is pursued. Using this method, the areas where the irreversibilities occur are identified and geometry improvements suggested. The novelty of the numerical procedure presented herein is that the bulk entropy generation is analysed by means of four sources related with viscous dissipation and heat transfer, divided in mean and fluctuating terms. Entropy generation within the boundary layer has also been considered. The latter has proved to be small, as well as the heat transfer contribution. The fluctuating viscous dissipation accounts for more than 75% of the added entropy, being its main sources the shear layer after the nozzle exit and the shock wave trains, independently of their position.     

Thermodynamic based comparison of sorption systems for cooling and heat pumping: Comparaison des performances thermodynamique des systèmes de pompes à chaleur à sorption dans des applications de refroidissement et de chauffage

A comparison of thermodynamic performances of sorption systems (liquid absorption, adsorption, ammonia salts and metal hydrides) is carried out for typical applications (deep-freezing, ice making, air-conditioning and heat pumping) with either air-cooled or water-cooled heat sink. The results are given in terms of cooling coefficient of performance (COP) (heating COP or coefficient of amplification (COA) for the heat pump), cooling (heating) power versus reactor volume or weight and thermodynamic efficiency. LiBr–water systems show the best results for air-conditioning except when small units are required (metal hydride systems lead to more compact units). Other systems, however, show better results for other applications (chemical reaction with ammonia salts for deep-freezing, adsorption for heat pumping).     

Comparitive analysis of an automotive air conditioning systems operating with CO2 and R134a

This paper evaluates performance merits of CO₂ and R134a automotive air conditioning systems using semi-theoretical cycle models. The R134a system had a current-production configuration, which consisted of a compressor, condenser, expansion device, and evaporator. The CO₂ system was additionally equipped with a liquid-line/suction-line heat exchanger. Using these two systems, an effort was made to derive an equitable comparison of performance; the components in both systems were equivalent and differences in thermodynamic and transport properties were accounted for in the simulations. The analysis showed R134a having a better COP than CO₂ with the COP disparity being dependent on compressor speed (system capacity) and ambient temperature. For a compressor speed of 1000 RPM, the COP of CO₂ was lower by 21% at 32.2°C and by 34% at 48.9°C. At higher speeds and ambient temperatures, the COP disparity was even greater. The entropy generation calculations indicated that the large entropy generation in the gas cooler was the primary cause for the lower performance of CO₂.     

The effect of refrigerant combinations on performance of a vapor compression refrigeration system with dedicated mechanical sub-cooling

Performance characteristics due to use of different refrigerant combinations in vapor compression cycles with dedicated mechanical sub-cooling are investigated. For scratch designs, R134a used in both cycles produced the best results in terms of COP, COP gain and relative compressor sizing. In retrofit cases, considering the high sensitivity of COP to the relative size of heat exchangers in the sub-cooler cycle and the low gain in COP obtained due to installation of a dedicated sub-cooling cycle when R717 is the main cycle refrigerant, it seems that dedicated mechanical sub-cooling may be more suited to cycles using R134a as the main cycle refrigerant rather than R717. With R134a as the main cycle refrigerant, no major difference was noted, by changing the sub-cooler cycle refrigerant, in the degradation of the performance parameters such as COP and cooling capacity, due to equal fouling of the heat exchangers.