Theoretical analyses of a new two-stage absorption-transcritical hybrid refrigeration system

In this context, a two-stage absorption-transcritical hybrid refrigeration system is proposed. R744 is chosen as a refrigerant for the transcritical heat pump subsystem and LiBr-H₂O working pair for the two-stage absorption refrigeration subsystem. Based on the mathematical and physical models, theoretical investigation is carried out on its performance. The main effects are discussed on COP_net (the ratio of cooling capacity powered by low-grade heat to the low-grade heat consumption for the hybrid system) and COP_mt (the ratio of cooling capacity powered by mechanical work to the mechanical work consumption for the hybrid system). Comparing with the normal two-stage absorption refrigeration system, theoretical results show that COP_net could be improved up to about 55% when the refrigeration temperature is 7 °C. In addition, COP_mt are more than 50% higher than that of the conventional transcritical refrigeration system. It is also found that both 45–55 °C low-grade heat and condensing heat could be used as actuating heat of the two-stage absorption refrigeration subsystem.     

Performance parameters for the design of a combined refrigeration and electrical power cogeneration system

This paper discusses the conservation of energy in a cogeneration system. A steam power cycle (Rankine) produces electrical power 2 MW and steam is bleeded off from the turbine at 7 bar to warm a factory or units of buildings during the winter or to supply a steam ejector refrigeration cycle to air-conditioning the same area during the summer. In the summer this system can be as alternative solution instead of absorption. Certainly the ejector refrigeration unit is more economical than absorption unit. The ratio of electrical power/heat is varied into the region (0.1–0.4) and the evaporator temperature of the ejector cycle is varied into the region (10–16 °C). A computer program has been developed for the study of performance parameters of the cogeneration system.     

Experimental study of a miniature vapor compression refrigeration system with two heat sink evaporators connected in series or in parallel

A miniature vapor compression refrigeration system included two heat sinks connected in series (indicated as series system) or in parallel (indicated as parallel system) was built. The performance of the series system was studied and compared with that of the parallel system. The results indicate that the largest cooling capacity of the two systems is about 160 W and the optimal refrigerant charge is about 0.6 M_total in the miniature vapor compress refrigeration (VCR) system. There is no relation between the optimal refrigerant charge and the arrangement of the heat sinks. The coefficient of performance (COP) of the series system ranged from 1.81 to 3.22, while the COP of the parallel system was in the range of 1.51–2.92 under the cooling capacity of 100 W. The cooling of the heat sink 2 lag behind that of the heat sink 1 in the serial system, while the refrigerant is difficult to equally distribute in the parallel system.     

Two-stage heat pump system with vapor-injected scroll compressor using R410A as a refrigerant

Refrigerant vapor-injection technique has been well justified to improve the performance of systems in refrigeration applications. However, it has not received much attention for air conditioning applications, particularly for air conditioning in hot climates and for heat pumping in cold climates. In this study, the performance of an 11 kW R410A heat pump system with a two-stage vapor-injected scroll compressor was experimentally investigated. The vapor-injected scroll compressor was tested with the cycle options of both flash tank and internal heat exchanger configurations. A cooling capacity gain of around 14% with 4% COP improvement at the ambient temperature of 46.1 °C and about 30% heating capacity improvement with 20% COP gain at the ambient temperature of −17.8 °C were found for the vapor-injected R410A heat pump system as compared to the conventional system which has the same compressor displacement volume.     

Investigation of a mixed effect absorption chiller powered by jacket water and exhaust gas waste heat of internal combustion engine

CCHP (combined cooling heating and power) system based on ICE (internal combustion engine) has been widely used. A key issue is to efficiently recover the jacket water and exhaust gas waste heat for refrigeration. In this work, a mixed effect absorption chiller (AC), which couples single effect and double effect processes together, is investigated to recover these two kinds of waste heat simultaneously. The high pressure generator is powered by exhaust gas while one low pressure generator is powered by jacket water waste heat. Thermodynamic characteristics and off-design performance are simulated. Considering thermodynamic constraints, the start point temperature in low pressure generator should be 77°Cor lower. For a 16 kW ICE, the cooling output can reach 34.4 kW with COP of 0.96 and exergy efficiency of 0.186. Comparing with double effect or single effect AC, it can make a better use of different waste heat in CCHP system.     

Experimental study on cooling performance of air conditioning system with dual independent evaporative condenser

Evaporative condenser is an energy efficient and environmentally friendly air conditioning equipment. This paper proposed an air conditioning system using dual independent evaporative condenser and investigated the cooling performance. Many factors, such as evaporator water inlet temperature, compressor frequency, air dry-bulb temperature, air velocity and water spray rate, which influenced the cooling performances of air conditioning system with evaporative condenser have been investigated. The results indicated that cooling capacity and coefficient of performance (COP) increased significantly with the increasing of evaporator water inlet temperature (12–25 °C), the air velocity (2.05–3.97 m s⁻¹) and the water spray rate (0.03–0.05 kg m⁻¹ s). However, COP decreased with the increasing ambient air dry-bulb temperature (31.2–35.1 °C) and the compressor frequency (50–90 Hz). Furthermore, the heat transfer coefficient (K₀) was 232–409 W m⁻² K⁻¹ in different air velocity and water spray rate.     

Performance of diffusion absorption refrigeration cycle with organic working fluids

A diffusion absorption refrigeration (DAR) cycle is driven by heat and utilizes a binary solution of refrigerant and absorbent as working fluid, together with an auxiliary inert gas. Commercial DAR systems operate with ammonia–water solution and hydrogen or helium as the inert gas. In this work, the performance of a simplified DAR system working with an organic absorbent (DMAC – dimethylacetamide) and five different refrigerants and helium as inert gas was examined numerically, with the aim of lowering the generator temperature and system pressure along with a non-toxic refrigerant The refrigerants were: chlorodifluoromethane (R22), difluoromethane (R32), 2-chloro-1,1,1,2-tetrafluoroethane (R124), pentafluoroethane (R125) and 1,1,1,2-tetrafluoroethane (R134a). The results were compared with the performance of the same system working with ammonia–water and helium. Similar behavior was found for all systems, regarding the coefficient of performance (COP) and rich and poor solution concentrations as functions of generator temperature. It was found that typical generator temperature with the new substances was 150 °C, yet lower COPs, higher evaporator temperatures and lower condensation temperature of about 40 °C governed these systems.     

Ejector expansion refrigeration system: Ejector design and performance evaluation

Use of a two-phase flow ejector as an expansion device in vapor compression refrigeration systems is one of the efficient ways to enhance its performance. The present work aims to design a constant-area two phase flow ejector and to evaluate performance characteristics of the ejector expansion refrigeration system working with R134a. In order to achieve these objectives, a simulation program is developed and effects of operating conditions and ejector internal efficiencies on the system performance are investigated using EES software. Comparison between present results and published experimental data revealed that the developed model can predict the system COP with a maximum error of 2.3%. The system COP increased by 87.5% as evaporation temperature changed from −10 °C to 10 °C. Finally, correlations to size ejector main diameters as a function of operating conditions, system cooling capacity and ejector internal efficiencies are reported.     

Effects of liquid refrigerant injection on the performance of a refrigeration system with an accumulator heat exchanger

Liquid refrigerant injection technique can be a very effective method for controlling subcooling and the compressor discharge temperature of a refrigeration system at high ambient temperatures. In this study, the effects of liquid refrigerant injection on the performance of a refrigeration system with an accumulator heat exchanger were investigated by varying the liquid injection rate at the conditions of constant expansion valve opening in the evaporator and constant total flow rate. During the tests, the ambient temperature was maintained at 43 °C. With the increase of the liquid injection rate, the subcooling at the inner heat exchanger outlet increased and the superheat at the accumulator outlet decreased. However, unacceptable results such as the increase of the compressor discharge pressure and decrease of the system performance were also observed depending on the control method applied. To obtain high system performance and reliability, optimum control methods for liquid injection in the accumulator heat exchanger are suggested. The liquid injection technique for the refrigeration system with an accumulator heat exchanger was found to be an effective method for controlling adequate subcooling and the compressor discharge temperature of the refrigeration system at high ambient temperatures.     

Potential energy benefits of integrated refrigeration system with microturbine and absorption chiller

This paper presents and analyzes the performance potential of a refrigeration system that is integrated with a microturbine and an absorption chiller (RMA). The waste heat from the microturbine operates the absorption chiller, which provides additional cooling. This additional cooling capacity can be utilized either to subcool the liquid exiting the condenser of the refrigeration system or to precool the air entering the condenser in the refrigeration system. Moreover, any surplus cooling capacity not utilized in the subcooler can be utilized to precool the microturbine intake air. The additional assistance to the refrigeration system enhances the efficiency of the refrigeration cycle, which in turn reduces the required microturbine size. The smaller size of the microturbine enhances the part load efficiency, especially in lower ambient temperatures. With increased microturbine efficiency, RMA with subcooler, RMA with subcooler and microturbine intake air precooler, and RMA with condenser air precooler can reduce the annual energy consumption by 12, 19, and 3%, respectively, as compared to a refrigeration system operating without any waste heat utilization from the microturbine. Therefore, RMA with subcooler and microturbine intake air precooler has the best potential of energy savings. The payback period of RMA with subcooler and microturbine intake air precooler is estimated in 3 years, which facilitates it as an economically feasible solution among the options investigated.     

Simulation and validation of a hybrid-power gas engine heat pump

Hybrid-power gas engine heat pump (HPGHP) combines hybrid power technology with gas engine heat pump, which can keep the gas engine working in the economical zone. In this paper, a steady-state model of the HPGHP in heating condition has been established, the optimal torque curve control strategy is proposed to distribute power between the gas engine and battery pack. The main operating parameters of the HPGHP system are simulated on Matlab/Simulink and validated by experimental data, such as operating temperature, coefficient of performance (COP), fuel-consumed rate, etc. Heating capacity and COP of the heating pump system are validated under different ambient temperatures and water flow rates. The simulation and experiment results shows acceptable agreement, the maximum difference is respectively 8.9%, 5.9%, 9.5% and 8.2% for engine torque, motor torque, reclaimed heat and fuel-consumed rate. Based on the simulation results, HPGHP has the lowest fuel-consumed rate of 283 g (kWh)⁻¹ at engine speed of 3000 rpm; the PER of HPGHP system is about 15.9% and 11.4% higher than the GHP under the same load in Mode C and D.     

吸收式制冷(热泵)循环流程研究进展

吸收式制冷作为最早的人工制冷方法,诞生至今已有200多年。在民用和工业中的实际应用有60多年。近20余年来,吸收式制冷在理论与应用等方面都取得了迅速发展,并在制冷机市场上占有相当的份额,得到国内外厂商和学者的广泛关注与研究。随着人类能源消耗量的不断增加,需要进一步深入研究新能源、分布式能源及能源的高效利用。余热、废热、可再生的太阳能、地热能等的利用使得热能驱动的吸收式制冷(热泵)技术得到越来越多的关注。与采用电驱动蒸气机械压缩式制冷(热泵)系统不同,吸收式制冷(热泵)技术可利用采用低品位热源的热能直接驱动,运行成本远低于电驱动系统。吸收式系统多采用H2O-LiB r溶液、NH3-H2O溶液等自然工质作为制冷剂,具有环境友好特性,同时具有安全、可无噪音运行、可靠性高等显著优点。但也具有占地面积大、初投资高,冷却负荷高,一次能源效率低(直燃形式)等不足。针对这些特性,现阶段的主要研究方向包括:循环设计优化、工质对选择、系统部件热质传递强化、系统控制策略优化等。狭义的吸收式循环是指闭式、溶液吸收制冷剂蒸气的吸收式制冷(热泵)循环。该类循环按照循环形式分类包括单吸收循环、多吸收循环和复合循环。单吸收循环主要包括基本单效吸收循环、扩散吸收循环、膜吸收循环、热变换器循环、重力驱动的阀切换循环以及自复叠循环;多吸收循环主要包括再吸收循环、多效循环、中间效循环、多级循环、中间级循环以及GAX循环;复合循环主要包括喷射-吸收复合、压缩-吸收复合和膨胀-吸收复合等复合形式。现有吸收式制冷技术研究热点主要包括且不局限于太阳能、中低温余热利用、冷热电联产、储能(蓄冷、蓄热),膜交换材料、高温下耐腐蚀材料,塑料热交换器等方面。吸收式循环现有循环结构的提出针对的是一定温度和浓度下循环,面对新的应用场景、新材料以及新吸收工质对,吸收式循环可以提出多种更高效、更宽热源驱动温度范围和溶液浓度范围的新循环。     

Experimental analysis of a novel cooling system driven by liquid refrigerant pump and vapor compressor

This study introduced a novel energy saving cooling system, i.e. a combined cycle coupled with a traditional vapor compression cycle with a pumped liquid two-phase cooling cycle. The system has two operation modes, i.e. the compression cycle mode driven by compressor and the pump cycle mode driven by refrigerant pump. A multi-purpose test bench was constructed to experimentally evaluate the performance of the integrated cycle system under various operation conditions. The effects of cycle working condition and the shift temperature between the two operation modes on the overall cycle performance were investigated in detail. It is found that the novel cycle system has a higher EER compared to the traditional compressor system when the ambient temperature is relatively low. The further experimental results and comparative annual energy saving analysis also indicated that the proper shift temperature is about −5 °C from the system EER and cooling capacity point of view.     

Development of microwave assisted zeolite–water adsorption heat pump

A microwave assisted zeolite–water adsorption heat pump system was designed, manufactured and investigated experimentally. The influence of operation time of microwave oven on performance of the adsorption heat pump was studied. The performance criteria: coefficient of performance, specific cooling power and volumetric cooing power, were calculated for the designed and tested adsorption heat pump system. The regeneration of adsorbent bed was achieved very rapidly (35 min) by using microwave heating system. The poor thermal conductivity of adsorbent did not affect the periods of isosteric heating and isobaric desorption processes.     

Investigation of dry ice blockage in an ultra-low temperature cascade refrigeration system using CO2 as a working fluid

Dry ice blockage in a CO₂-solid-gas-flow-based ultra-low temperature cascade refrigeration system is investigated experimentally by a visualization test and a system study of the liquid CO₂ blew into an expansion tube through a Throttle needle valve. The visualization test shows that dry ice sedimentation occurs in low flow velocity and the dry ice formation makes the heat transfer behavior of CO₂ complicated. The sedimentation also occurs at low condensation temperature and low heating power input. Based on the present investigation, it is found that the present ultra-low temperature cascade refrigeration system is better to work at heating power input above 900 W and condensation temperature above −20 °C. At suitable operating condition, the present ultra-low temperature cascade refrigeration system has been shown the capability of achieving ultra-low temperature −62 °C continuously and stably.     

An absorption based miniature heat pump system for electronics cooling

The development of an absorption based miniature heat pump system is motivated by the need for removal of increasing rates of heat from high performance electronic chips such as microprocessors. The goal of the present study is to keep the chip temperature near ambient temperature, while removing 100 W of heat load. Water/LiBr pair is used as the working fluid. A novel dual micro-channel array evaporator is adopted, which reduces both the mass flux through each micro-channel, as well as the channel length, thus reducing the pressure drop. Micro-channel arrays for the desorber and condenser are placed in intimate communication with each other using a hydrophobic membrane. This acts as a common interface between the desorber and the condenser to separate the water vapor from LiBr solution. The escaped water vapor is immediately cooled and condensed at the condenser side. For direct air cooling of condenser and absorber, offset strip fin arrays are used. The performance of the components and the entire system is numerically evaluated and discussed.     

Attempt of integration of a small commercial ammonia-water absorption refrigerator with a solar concentrator: Experience and results

The integration of a small commercial ammonia-water absorption refrigerator with a solar concentrator as heat source was analyzed theoretically and experimentally. Operation parameters of the fridge were determined by parametric fitting experimental data using a thermodynamic model of the cycle leading to a working pressure of 25 bar, weak and strong solution concentrations of 15% and 30% respectively and a COP of ∼0.18. As sold, the fridge can operate with a reduction in the electric power supply of up to 40%. Results showed that the Parabolic Concentrator (PC) used can deliver temperatures above 200 °C for almost 6 h with an average useful power delivery and efficiency of 530 W and 26% respectively. During laboratory test runs it was possible to hold the refrigeration cycle running in a very unstable regime despite the inappropriate heat distribution in the generator unit due to the lower temperature heat source used (220 °C). However, outdoor testing with the sun as energy source proved unsuccessful due to the higher heat losses that occur at the pipes connecting the PC with the fridge.     

一个用太阳能驱动的新型吸收制冷循环

提出了一个由太阳能驱动的新型吸收制冷循环。论文将热变器原理用于吸收制冷,从而大大提高了吸收制冷的循环效率。详细介绍该新型循环的热力学模型,同时以一个典型的太阳日照为例,计算了新循环的性能系数（ＣＯＰ）、冷凝热、理论极限制冷温度和制冷量,并与传统循环进行了比较。结果表明,新循环不仅克服了传统循环在热源工况不稳定时将导致系统工况不稳定甚至不能工作的缺点,而且还具有更高ＣＯＰ值。     

Performance of compression/absorption hybrid refrigeration cycle with propane/mineral oil combination

This paper discusses the feasibility of a vapor compression/absorption hybrid refrigeration cycle for energy saving and utilization of waste heat. The cycle employs propane as a natural refrigerant and a refrigeration oil as an absorbent. A prototype of the cycle is constructed, in which a compressor and an absorption unit are combined in series. The performance of the cycle is examined both theoretically and experimentally. Although the solubility of the propane with the oil is not enough as a working pair in the absorption unit, the theoretical calculation shows that the hybrid cycle has a potential to achieve a higher performance in comparison with a simple vapor compression cycle by using the waste heat. In the experiment, the prototype cycle is operated successfully and it is found that an improvement of an absorber is necessary to achieve the good performance close to the theoretical one. The application of an AHE (absorber heat exchanger) can reduce the heat input to a generator. Further examinations on some other combinations of refrigerant/refrigeration oil and additives are desirable.     

Experimental investigation on R134a vapour ejector refrigeration system

The experimental investigation of the performance of a vapour ejector refrigeration system is described. The system uses R134a as working fluid and has a rated cooling capacity of 0.5 kW. The influence of generator, evaporator and condenser temperatures on the system performance is studied. This kind of system can be operated with low grade thermal energy such as solar energy, waste heat, etc. The operating conditions are chosen accordingly as, generator temperature between 338 K and 363 K, condenser temperature between 299 K and 310.5 K, and evaporator temperature between 275 K and 285.5 K. Six configurations of ejectors of different geometrical dimensions are selected for the parametric study. The performance of the refrigeration system at different operating temperatures is presented.