Performance of double effect absorption compression cycles for air-conditioning using methanol–TEGDME and TFE–TEGDME systems as working pairs: Performances de cycles à compression absorption à double effet pour le conditionnement d'air utilisant les couples méthanol–TEGDME ou TFE–TEGDME

The organic working pairs trifluoroethanol (TFE)–tetraethyleneglycol dimethyether (TEGDME or E181) and methanol–TEGDME have some advantages over classical water–LiBr and ammonia water working pairs in absorption cycles. One of the most important features is the wide working range caused by the absence of crystallization, the low freezing temperatures of the refrigerants and the thermal stability of the mixtures at high temperatures.The performance of a double effect absorption cycle for these organic mixtures can be improved if a compression stage is introduced between the evaporator and the absorber. The coefficient of performance (COP) and primary energy ratio (PER) values in the cooling mode are significantly increased over a wide working range: the cycle can work with temperature lifts of 50ºC at 5ºC in the evaporator or it can also be powered by low grade heat. For these conditions COP and PER values are higher than 1.0 and 0.7 respectively, and the power supplied to the compressor represents up to 15% of the thermal energy supplied to the generator. As it is possible to work at high temperatures lifts, the absorber and condenser can be air cooled.     

空气制冷机的研究和发展

空气是对环境最友好的制冷剂,空气制冷系统可以自由地从环境中获取或从系统向环境排放制冷剂;同时,空气制冷机组对系统密封性要求较低,因此其可靠性高,维护性好,在飞机环境控制系统中得到了广泛应用。本文介绍了空气制冷系统的循环方式和基本特性,总结了系统的性能分析、实验研究及系统关键部件的研究现状,对空气循环制冷系统在空调、热泵、冷冻冷藏、飞机环境控制系统及储能领域的应用进行了综述,并对关键技术的发展应用进行了分析。总体而言,近几十年来,随着涡轮机械设计制造技术、高效换热技术、高速电机技术的进步,空气制冷技术得到快速发展,系统结构更加紧凑,在列车空调、冷冻/冷藏运输领域得到了越来越广泛的应用。在制冷温度为-100～-50℃范围内空气制冷系统的COP高于传统复叠式制冷系统,因此,空气制冷系统在食品速冻领域具有显著优势,同时,在储能、热泵等领域具有较大的应用潜力。     

Simulation of a solar driven aqua-ammonia absorption refrigeration system Part 1: mathematical description and system optimization

A theoretical thermodynamic analysis was performed on an aqua-ammonia refrigeration (AAR) cycle coupled to a solar water heating system using CPC-collectors and augmented with auxialiary energy. Thirteen AAR cycles were considered in the search for the maximum coefficient of performance. Also investigated was the exergetic performance of the AAR cycle. The adopted temperature and mass flow rate control strategy on the storage tank-auxiliary heater-AAR generator loop ensured operation of the refrigeration systems at design conditions. Consequently, the ratio between the pumps' electrical power and the designed evaporator heat transfer rate was kept to a minimum.     

大型风冷冷水机组性能评价系统节能性研究

针对大型风冷冷水机组性能评价系统能耗高的现状,分析评价系统的主要能耗构成,指出传统大型风冷冷水机组性能评价系统的高能耗成因,提出一种新型变容量全回收风冷冷水机组性能评价系统。新型评价系统省去样机恒温水箱、冷冻水箱和换热水泵,将变容量水冷乙二醇冷冻机组与经济器(冷却塔协同功能板换)串入闭式回路实现一级温控,结合变频泵与调节阀实现二级温控。实证表明:相比传统评价系统,应用双级温控的新型评价系统相近工况过渡耗时减少为前者的70%,综合能耗降为传统的75%,运行稳定性良好,节能效果显著。     

Performance of a metal hydride cooling system

The performance of a single-stage metal hydride cooling system working with the ZrMnFe/MmNi_4.5Al_0.5 pair has been evaluated based on heat transfer and reaction kinetics considerations. Results show that the initial and operating costs of the system have to be minimized by optimizing the bed thickness, effective thermal conductivity and overall heat transfer coefficients. Of the three operating temperatures (heat source, heat sink and refrigeration temperatures), refrigeration temperature has the greatest influence on system performance, as desorption during the refrigeration process normally controls the cycle time and specific alloy output. However, heat source and heat rejection temperatures assume importance at high refrigeration temperatures. The average COP of the above system lies between 0.45 and 0.50.     

Preliminary theoretical analyses of thermal performance and available energy consumption of two-stage cascade cycle heat pump water heater

In this paper, the analysis model of thermal performance and available energy of two-stage cascade cycle heat pump water heater (HPWH) was established. For the first time, the equivalent transformation thermodynamic analysis method and available energy consumption equation were employed to carry out the theoretical analyses of thermal performance and available energy consumption for two-stage cascade cycle HPWH. And the correctness of the analysis approach proposed in this paper was verified by an example. The calculation results showed a significant agreement with the results derived by the first law of thermodynamics and energy conservation law. The available energy analysis of two-stage cascade cycle HPWH clearly revealed the available energy consumption rates for different working processes, including heat transfers of heat exchangers, flow of heat-carrying fluid, working medium cycle, output of hot water, etc. This approach could be conveniently used in the analyses of thermal performance and available energy consumption for actual complex thermal system with irreversible process. This paper provided the theoretical basis for the actual optimization design and operation analysis of cascade type heat pump systems, and furthermore, the results of the calculation instance would also have a certain reference value.     

高功率储能相变冷却系统实验研究

本文针对间歇工作的高功率电子设备对冷却系统的特殊需求,研制了一种以蒸气压缩制冷循环为基础的复叠式储能相变冷却系统,理论分析了系统的工作原理和工质选择原则,模拟计算了储能器的运行特性,通过实验测量了系统启动和稳定运行时的冷却能力和运行性能。结果表明:机械泵启动时,系统压力先降低后升高,幅值为30 kPa,热源启动时,系统压力先升高后降低并维持稳定,系统的主要压降发生在冷却器中;储液器在热源启动时,能有效储存系统多余的工质,并间接控制冷却器内的相变温度和压力;储能器在系统运行过程中,满足系统换热需求并维持机械泵入口5℃的过冷度,设计合理;系统的冷却能力随运行时间逐渐降低,当平均热负荷超过10 kW时,系统能稳定运行5 min,满足课题的设计要求。     

The CNPM thermal heat pump. Part 1 — general description and thermodynamic analysis

A research programme, funded by CNR (National Research Council), has been undertaken by CNPM since 1973. The aim of the programme is the construction and testing of a prototype thermal heat pump. The most significant component is an organic Rankine cycle engine, driving the compressor of a heat pump. Since the heat rejected by the engine is supplied to the user — water for domestic heating — the whole system performs as a ‘heat multiplier’, converting the high temperature heat given to the engine into a larger amount of low temperature heat, to be used for domestic heating.In this paper, the selection criteria for the working fluid — a completely fluoro-substituted hydrocarbon — and the main thermodynamic data of both power and heat pump cycles, are discussed; the finally adopted plant configuration is described, with particular emphasis on the influence exterted by the working fluid nature on the heat exchangers and turbo-machinery dimensions and performance. A discussion on the merits of the single fluid solution (ie the same working fluid in the power and the heat pump cycle) and dual fluid solution is also carried out. The feasibility of a low-temperature heat distribution, based on compact-surface, natural-draft convectors, with the relevant advantages on the Rankine and heat-pump cycles, is also investigated.Finally, the expected overal; system performance is given, both at design and part-load conditions. As a premium for the rather complex but efficient thermodynamicscv of the system, significant energy savings are obtained in all situations.     

Performance optimization of a two-circuit cycle with parallel evaporators for a domestic refrigerator-freezer

A two-circuit cycle with parallel evaporators (called a “parallel cycle”) for a domestic refrigerator-freezer (RF) shows energy saving potential compared with a conventional cycle with a single loop or serial evaporators because of a low compression ratio in the fresh food compartment (R)-operation. The objective of this study is to investigate the effects of the refrigerant charge, R-capillary tube, and refrigerant recovery operation on the performance of a parallel cycle. In addition, design guidelines for the heat transfer area and air flow rate of an R-evaporator are proposed. When the parallel cycle was optimized in terms of the refrigerant charge and R-capillary tube diameter, the energy consumption was reduced by 7.8% over a bypass two-circuit cycle with the same RF platform. In addition, an additional energy saving of 1.8% was obtained by the optimization of the operating sequence and refrigerant recovery operation.     

Development of an Adaptive Food Preservation System for food quality and energy efficiency enhancement

This paper proposes a system design of an Adaptive Food Preservation System (AFPS). It is motivated by the fact that constant storage condition in today's refrigerators has deficiency. Customized storage with fast freezing capability can better manage food quality and energy efficiency. Key components include AFPS packages and vortex tubes. Superior fast freezing capability is demonstrated by comparisons with benchmarks from refrigerator tests and regulatory standards, and experimentation with a simulated test setup at various settings of input. Theoretical and analytical models are proposed to predict package inlet and exit temperatures, freezing capacities, and freezing efficiencies. At 7 bar inlet pressure, the maximum available freezing efficiency, mean efficiencies with vacuum insulation and with ABS insulation of the AFPS Package and Vortex Tube Assembly are 6%, 4.66%, and 2.80%, respectively. AFPS technology consumes 0.18% in time and 45% in energy during fast freezing comparing with what a typical household freezer does.     

Water-side reversible CO2 heat pump for residential application

This paper presents the energy assessment of a water/water R744 chiller/heat pump, working according to a transcritical cycle, used for winter heating, summer cooling and tap water production. The different functions (heating, cooling, hot water) are managed water side. The analysis of the R744 chiller/heat pump is based on an original simplified method, which is able to predict the energy performance of the unit based only on its performance data at the nominal rating conditions. The method was validated against experimental data. A comparison with a state-of-the-art R410A unit is presented. The monthly analysis shows that the CO₂ unit is very efficient in hot water production, but penalised in heating and cooling service. The adoption of an ejector in place of the expansion valve makes the CO₂ system reach the same energy consumption as the R410A unit, despite the presence of the water loop only in the R744 lay-out.     

氨蒸气压缩高温热泵系统研究现状

高温热泵是解决冷热双利用、实现节能减排的重要方法.由于工质使用的限制,自然工质成为研究的重点,氨作为自然工质,具有优良的热力性能.本文通过热力计算对氨和其它常用热泵工质进行了性能对比分析,并对氨蒸气压缩式高温热泵进行了综述,发现氨在80～95℃范围内综合性能最佳,适合用于60～110℃高温工况.同时本文基于单级压缩热泵...     

Air cycle combined heating and cooling for the food industry

In the food industry there is often a need to both cook and cool food. Heating and cooling processes are rarely directly linked due to heat from refrigeration processes being insufficient to cook food. Therefore cooling and cooking is usually provided by separate pieces of equipment. This paper presents an air cycle system where the hot air was used for heating and the cold air used for fast freezing.The apparatus used a bootstrap unit developed for aircraft air conditioning which was unable to run at the low temperature required for food freezing, so a parallel compressor was added to enable the system to operate at the low temperature required for food freezing. This approach allowed temperatures as low as −140 °C and as high as 234 °C. The system cooked and froze beef burgers, and at the same time heated water up to 98 °C.     

Performance research of self regenerated absorption heat transformer cycle using TFE-NMP as working fluids

A heat transformer is proposed in order to upgrade low-temperature-level energy to a higher level and to recover more energy in low-temperature-level waste heat. It is difficult to achieve both purposes at the same time using a conventional heat transformer cycle and classical working pairs, such as H₂O–LiBr and HN₃–H₂O. The new organic working pair, 2,2,2-trifluoroethanol (TFE)-N-methylpyrolidone (NMP), has some advantages compared with H₂O–LiBr and NH₃–H₂O. One of the most important features is the wide working range as a result of the absence of crystallization, the low working pressure, the low freezing temperature of the refrigerant and the good thermal stability of the mixtures at high temperatures. Meanwhile, it has some negative features like NH₃–H₂O. For example, there is a lower boiling temperature difference between TFE and NMP, so a rectifier is needed in refrigeration and heat pump systems. Because TFE–NMP has a wide working range and does not cause crystallization, it can be used as the working pair in the self regenerated absorption heat transformer (SRAHT) cycle. In fact, the SRAHT cycle is the generator–absorber heat exchanger (GAX) cycle applied in a heat transformer cycle. In this paper, the SRAHT cycle and its flow diagram are shown and the computing models of the SRAHT cycle are presented. Thermal calculations of the SRAHT cycle under summer and winter season conditions have been worked out. From the results of the thermal calculations, it can be found that there is a larger temperature drop when the waste hot water flows through the generator and the evaporator in the SRAHT cycle but the heating temperature can be kept the same. That means more energy in the waste heat source can be recovered by the SRAHT cycle.     

A new model and analytical solution for the heat conduction of tunnel lining ground heat exchangers

In order to solve the freezing damage problem of tunnel in a cold region, an innovative tunnel lining ground heat exchangers (GHEs) heating system was introduced in China for the first time and applied to Linchang tunnel in Inner Mongolia. This new-type system utilized the tunnel linings as heat exchanger to extract geothermal energy from the surrounding rock in the middle part of the tunnel so as to heat tunnel lining and the drainage system at the tunnel entrance. As for the thermal analysis and design of it, the classical line heat source model and “hollow” cylindrical heat source model for borehole GHEs are not applicable. Therefore, a new line source model was put forward in this paper, which considered both composite medium and time-dependent boundary conditions. Its analytical solution was obtained using the superposition principle and finite integral transfer method. The calculated results from the new model were validated by the experimental data of thermal response tests. Furthermore, the new model was applied to analyze the impact factors of heat transfer performance for design optimization, including the ground temperature as well as the flow rate and the inlet temperature of heat carrier liquid. It was proven that the new model can provide adequate tools for the design of tunnel lining GHE heating system.     

Modelling the freezing of butter

Butter is a water-in-oil emulsion so its behaviour during freezing is very different from that of most food products, for which water forms a continuous phase. The release of latent heat during freezing is controlled as much by the rate of crystallization of water in each of the water droplets as by the rate of heat transfer. Measurements of the freezing of butter show that the release of latent heat from the freezing water depends on the degree of supercooling, which, in turn, depends on the cooling medium temperature, the size of the butter item, the packaging and the type of butter. Four modelling approaches were tested against the experimental data collected for a 25 kg block of butter. A “sensible heat only model” accurately predicted the butter temperature until temperatures at which water freezing becomes significant were reached. An “equilibrium thermal properties model” predicted a temperature plateau near the initial freezing point of the butter in a manner that was inconsistent with the measured data. A third model used a stochastic approach to ice nucleation based on supercooling using classical homogeneous nucleation theory. The predicted temperatures showed that supercooling-driven nucleation alone is not sufficient to predict the freezing behaviour of butter. A fourth approach took account of time-dependent nucleation and ice crystal growth kinetics using classical Avrami crystallization theory. The relationship between the ice crystal growth rate and the supersaturation was assumed to be linear. The model predicted the experimental data accurately, particularly by predicting the slow rebound in the temperature following supercooling that is found when freezing butter under some conditions.     

DAM两相闭式热虹吸回路冷却系统实验研究

两相热虹吸回路由于较高的散热性能在高功率电子设备冷却领域有较好的应用前景。为了解决四通道数字阵列模块的冷却问题,本文设计了一套两相闭式热虹吸回路冷却系统样机并对系统启动特性、充液量和工作倾角对系统散热性能的影响进行了实验研究。研究结果表明,样机系统结构及散热性能满足指标要求,启动性能和工作性能良好。此外,该系统对热耗1600 W、局部热流密度接近100 W/cm~2的组件的冷却效果良好。     

Numerical simulation of an advanced energy storage system using H2O–LiBr as working fluid, Part 1: System design and modeling

The advanced energy storage technology proposed and patented by authors can be applied for cooling, heating, dehumidifying, combined cooling and heating, and so on. It is also called the variable mass energy transformation and storage (VMETS) technology in which the masses in one or two storage tanks change continuously during the energy charging and discharging processes. This paper presents an advanced energy storage system using aqueous lithium bromide (H₂O–LiBr) as working fluid. As one of VMETS systems, this system is a closed system using two storage tanks. It is used to shift electrical load and store energy for cooling, heating or combined cooling and heating. It is environmental friendly because the water is used as refrigerant in the system. Its working principle and process of energy transformation and storage are totally different from those of the traditional thermal energy storage (TES) systems. The electric energy in off-peak time is mostly transformed into the chemical potential of the working fluid and stored in the system firstly. And then the potential is transformed into cold or heat energy by absorption refrigeration or heat pump mode when the consumers need the cold or heat energy. The key to the system is to regulate the chemical potential by controlling the absorbent (LiBr) mass fraction or concentration in the working fluid with respect to time. As a result, by using a solution storage tank and a water storage tank, the energy transformation and storage can be carried out at the desirable time to shift electric load efficiently. Since the concentration of the working solution in the VMETS cycle varies continuously, the working process of the VMETS system is dynamic. As the first part of our study, the working principle and flow of the VMETS system were introduced first, and then the system dynamic models were developed. To investigate the system characteristics and performances under full-storage and partial-storage strategies, the numerical simulation will be performed in the subsequent paper. The simulation results will be very helpful for guiding the actual system and device design.     

Performance analysis of a solar cooling plant based on a liquid desiccant evaporative cooler

Summer air conditioning represents a growing market in buildings worldwide, with a significant growth rate observed in European commercial and residential buildings. Available heat driven cooling technologies can be used in combination with solar thermal collectors to reduce the load caused by air conditioning on the electric utilities and to reduce the environmental impact. This work reports a performance analysis of an open cycle solar cooling plant. The plant, installed in Northern Italy, is based on a liquid desiccant evaporative cooler coupled with a solar field. Experimental tests run during summer show average primary energy ratio and primary energy saving index of about 1.6 and 30%, respectively. Though this performance is satisfactorily, the regeneration unit always operated near the lower bound of the nominal temperature range. Therefore, optimization of the solar system design could lead to higher performance.     

Optimization of a CO2–C3H8 cascade system for refrigeration and heating

Conventional working fluids (refrigerants) are being phased out worldwide to combat with the twin menace of ozone layer depletion and global warming and natural refrigerants are fast gaining favour lately. Single stage and multi stage refrigeration systems fail to widen the gap between heat source and heat sink temperatures required in many industrial applications requiring simultaneous heating and cooling and cascaded systems appear to be the best alternative. Modest research has been done in cascaded systems based on natural refrigerants thereby offering good potential for research. In this paper, a cascaded system for simultaneous heating and cooling (refrigeration and heat pump system) with a carbon dioxide based HT cycle and propane based LT cycle for simultaneous refrigeration and heating applications has been analyzed. To facilitate prediction of optimum performance parameters, performance trends with variation in the design parameters and operating variables have been presented in this article. Relevant expressions have been developed to serve as guidelines to the user for selecting appropriate design parameters like intermediate temperature so that the system yields optimum performance. Independently developed property codes have been employed for both carbon dioxide and propane for higher accuracy.