Modelling of an adsorption chiller for dynamic system simulation

Adsorption chillers are periodically working chillers with fast temperature changes at the outlets of the hydraulic loops at the beginning of a new adsorption cycle. The scope of the current study is to predict the consequences of these temperature changes for the components in the hydraulic system in order to optimize the system's design. Therefore, a model of an adsorption chiller has been created in the object-oriented simulation language Modelica. The model follows a component approach for each part of the chiller based on fundamental equations for heat and mass transfer. Compared to an effective model all equations have a physical significance. Simulation results are validated by measurement values coming from an adsorption chiller tested. Measured temperatures and volume flows at the inlet of the hydraulic loops are given as input to the simulation model. The simulated output temperatures show good agreement with measured temperatures, heating and cooling power and coefficient of performance (COP).     

Thermoeconomic optimization of a two stage combined refrigeration system: a finite-time approach

A finite-time thermoeconomic performance analysis based on a new kind of optimization criterion has been carried out for a two-stage endoreversible combined refrigeration cycle model. The optimal performances and design parameters that maximize the objective function (cooling load per total cost) are investigated. In this context, the optimal temperatures of the working fluids, the optimum performance coefficient, the optimum specific cooling load and the optimal distribution of the heat exchanger areas are determined in terms of technical and economical parameters. The effects of the economical parameter that characterizes the investment and energy consumption costs on the general and the optimal performances have been discussed.     

Amélioration de la performance des machines frigorifiques á absorption par l'utilisation de cycles à absorption et désorption étagés

This study deals with staged absorption and desorption cooling systems which increase the performance of absorption cycles that are driven by only low-grade energy, particularly when the working fluids are NH₃H₂0. Instead of working with only one absorber, these systems use a cascade of absorbers composed by one operating at the evaporator pressure, followed by a series of absorbers operating at staged pressures P_j, between P_ev and P_c In the same way, a cascade of generators is used for desorption. For the same operating parameters for other equipment and the same COP, the systems that we propose permit the generators to run at temperatures below those of all other systems offered up to now and using the same working fluids. When T_ev = −10°C, T_a = T_c = 30°C, the temperature of the generators can be as low as 65°C while the COP of the system is 0.258 and the COP_ex 0.317. By increasing the temperature of generators to 85°C while maintaining the other parameters at the same values, COP becomes 0.374 and the COP,, 0.336. These results improve the performance of absorption systems using only low-grade energy (T < 100°C). Particularly, they are better than the performance of two-stage absorption systems which consist of two single-stage absorption cycles coupled with each other through the evaporator of the first cycle and the absorber of the second cycle. With the same operating parameters indicated above for our system at the evaporator, the condenser, and the absorber, these coupled cycles need temperatures at generators of 80 and 100°C, whereas they give a COP of only 0.270     

Pulse-Tube Dilution Refrigeration Below 10 mK

We report the design, realization and performance of dilution refrigerators using a pulse-tube cooler as a first cryogenic stage. The absence of a Dewar containing cryogenic fluids makes this new type of refrigerators particularly versatile. The system provides relatively high cooling power, and reaches temperatures well below 10 mK.     

Performance of ejector cooling systems using low ecological impact refrigerants

The theoretical behaviour of an ejector cooling system, using as working fluids propane, butane, isobutane, R152a and R134a, is obtained. The ejector works as a thermo-compressor that is simulated with a validated one-dimensional mathematical model, whose errors are lower than 6%. For a system unitary cooling capacity, a parametric study is carried out varying the generation, condensation and evaporation temperatures. From the obtained data, a complete analysis of the system performance can be achieved when the ejector and system operation parameters are considered. The best performance corresponds to the system using propane, because has the highest system coefficient of performance and its ejector has the maximum entrainment ratio value, the least area ratio value and the highest efficiency value. The considered generation temperature ranging from 70 °C to 95 °C is appropriate for low-grade energy sources assisting thermal cooling systems. After this system performance, come those in which R152a and R134a are employed, with isobutane and butane at the end. The obtained results represent potential design points of an efficient ejector cooling system operation, because to each combination of the above mentioned temperatures corresponds one and only one ejector geometry.     

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.     

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。     

Cycle analysis of an air-cooled LiBr/H2O absorption heat pump of parallel-flow type

A gas-fired absorption heat pump with cooling capacity of 2 RT was analysed as an air-conditioner for domestic use during the summer. The absorption heat pump considered was an air-cooled, double-effect, LiBr/H₂O system of parallel-flow type. The performance of the absorption heat pump in the cooling mode of operation was investigated through cycle simulation to obtain the system characteristics depending on the inlet temperature of air to the absorber, the working solution concentrations, the solution distribution ratio of the mass of solution into the first generator to the total mass of solution from the absorber, and the LTDs (leaving temperature differences) of the heat-exchanging components. When the predicted results were compared with the measured data for similar design conditions, reasonable agreement was observed. The optimum design and operating conditions of an air-cooled absorption system are suggested based on this cycle simulation analysis.     

计及温度条件下的微穿孔板结构优化设计

微穿孔板吸声体是由穿孔直径在1 mm以下的薄板和板后空腔组成的共振吸声结构,其结构通常可利用经典的微穿孔板理论来设计。但在温度变化条件下,经典的微穿孔板理论已经不足以设计出满足要求的微穿孔板结构。文中在设计微穿孔板吸声结构时,不仅考虑了结构参数孔径d、板厚t、孔间距b及空腔深度D对微穿孔板吸声特性的影响,又计入了温度T这一参数。拟采用改进的粒子群优化算法,分别对一定温度下的单层和双层微穿孔板吸声体的结构参数进行优化设计,搜索得到最优的参数组合,使其在给定的频带范围内平均吸声系数最高。优化结果表明:利用改进的粒子群算法设计出的微穿孔板吸声结构在给定频率范围内吸声系数较大,且符合给定温度的要求。     

电动客车用余热回收型热泵的换热器优化及性能实验研究

本文针对热泵空调系统在冬季低温工况下制热能力衰减问题,通过换热器设计优化,研发出基于喷射补气的余热回收型热泵空调系统,并进行了性能实验研究.结果表明:研制的准二级压缩电动客车热泵空调系统在低温条件下具有较好的制热性能.在环境温度为-20℃,车内温度为20℃,余热量为1.8 kW的制热工况下,相比于无余热回收工况,系统制...     

Theoretical analysis and optimization of a hybrid CO2 transcritical mechanical compression – ejector cooling cycle

The paper provides the results of a design-theoretical study of a hybrid carbon dioxide (CO₂) transcritical mechanical compression – ejector cooling cycle. The hybrid cooling cycle is a combination of a CO₂ transcritical mechanical compression refrigeration machine (MCRM) powered by electricity, and an ejector cooling machine (ECM) driven by heat rejected from the CO₂ cooling cycle. Refrigerants R245ca, R601b (neopentane) and R717 (ammonia) are investigated as the working fluids of ECM in the present study. A method to determine the optimal design parameters and performance of the hybrid cooling cycle is presented. It is shown, that efficiency growth of the transcritical CO₂ cooling cycle due to ejector cooling cycle use is higher as evaporating temperatures are lower.     

A Dry Dilution Refrigerator for the Test of CUORE Components

We report the design, the construction and performance of a dilution refrigerator using a pulse-tube cooler as a first cryogenic stage. Both the pulse tube and the dilution unit are commercial. The absence of a Dewar containing cryogenic fluids makes this new type of refrigerator particularly versatile: it was built to carry out cryogenic tests on CUORE components. The system provides a cooling power of about 0.2 mW at 100 mK, and reaches temperatures as low as 6 mK.     

Performance enhancement of a diffusion-absorption refrigerator

An effort was undertaken to improve the current cycle performance of the diffusion-absorption refrigerator (DAR). The thermodynamic basis for the design changes is to reuse waste heat from the rectifier to heat the weak absorbent from the absorber. A new generator with heat exchanger (GWHX) was designed and fabricated. The test results of the DAR with GWHX are compared with baseline tests. The new generator design demonstrated a significant improvement in the cooling COP of as much as 50% compared to the baseline tests while the cooling capacity was unchanged.     

Sorption cooling: A valid extension to passive cooling

Passive cooling has shown to be a very dependable cryogenic cooling method for space missions. Several missions employ passive radiators to cool down their delicate sensor systems for many years, without consuming power, without exporting vibrations or producing electromagnetic interference. So for a number of applications, passive cooling is a good choice. At lower temperatures, the passive coolers run into limitations that prohibit accommodation on a spacecraft. The approach to this issue has been to find a technology able to supplement passive cooling for lower temperatures, which maintains as much as possible of the advantages of passive coolers.Sorption cooling employs a closed cycle Joule–Thomson expansion process to achieve the cooling effect. Sorption cells perform the compression phase in this cycle. At a low temperature and pressure, these cells adsorb the working fluid. At a higher temperature they desorb the fluid and thus produce a high-pressure flow to the expander in the cold stage. The sorption process selected for this application is of the physical type, which is completely reversible. It does not suffer from degradation as is the case with chemical sorption of, e.g., hydrogen in metal hydrides. Sorption coolers include no moving parts except for some check valves, they export neither mechanical vibrations nor electromagnetic interference, and are potentially very dependable due to their simplicity. The required cooling temperature determines the type of working fluid to be applied. Sorption coolers can be used in conjunction with passive cooling for heat rejection at different levels.This paper starts with a brief discussion on applications of passive coolers in different types of orbits and on the limitations of passive cooling for lower cooling temperatures.Next, the working principle of sorption cooling is summarized. The DARWIN mission is chosen as an example application of sorption and passive cooling and special attention is paid to the reduction of the radiator area needed by the sorption cooler.The application field of this type of sorption cooling in space missions is currently being expanded by examining the performance of alternative working fluids, suitable for different cooling temperatures.     

Experimental evaluation of a variable effect LiBr–water absorption chiller designed for high-efficient solar cooling system

A variable effect LiBr–water absorption chiller is studied in this paper based on a real developed 50 kW prototype. The chiller is designed specifically for the high-efficient utilization of the solar power with variable temperature. It can obtain the optimized COP and cooling power under different heat source temperatures. The construction, circulation and testing system of the chiller were introduced. A typical running condition of the chiller from the starting to the steady operating was given to show the dynamic performance. Several groups of the temperatures and COPs were given to show the steady state performance. These data showed that the COP increased from 0.69 to 1.08 under generation temperature from 95 °C to 120 °C. Besides, the effects of chilled water temperature, cooling water temperature, pump frequency and opening of valve on COP and cooling power were analyzed respectively.     

Carnot-COP for sorption heat pumps working between four temperature levels

The theoretical efficiency limits of heat driven heat pumps operating between three and four temperatures are derived from the fundamental thermodynamical laws, i.e. the energy balance and the entropy balance. While in the three temperatures case the system is fully determined by specification of the three temperatures and the cooling capacity, a four temperature heat pump needs, in addition to the four temperatures and the cooling capacity, specification of an additional operating parameter. This can be, for example, the ratio of the two heat flows which are released at the two different intermediate temperatures. Various assumptions regarding this proportion are discussed with respect to their relevance for both the combination power cycle/vapor compression cycle as well as for single-effect sorption cycles. The present analysis shows that a single-effect sorption heat pump is principally not able to operate reversibly in an environment of four externally specified temperatures unless the four temperatures follow, incidentally, a correlation that is given by the equilibrium properties of the employed working fluids. Therefore, in endo-reversible models for four-temperature sorption cycles only three rather than four operating temperatures may be specified independently.     

Heat transfer performance of cryogenic oscillating heat pipes for effective cooling of superconducting magnets

The cryogenic oscillating heat pipe (OHP) for conduction cooling of superconducting magnets was developed and the function was demonstrated successfully. OHP is a highly-efficient heat transfer device using oscillating flow of two-phase mixture. The working fluids that are employed in the present research are Nitrogen, Neon and Hydrogen, and the operating temperatures are 67–91 K, 26–34 K and 17–27 K, respectively. The estimated effective thermal conductivities from the measurement data of the OHP were higher than one of the solids such as copper at low temperature. These results revealed that the cryogenic OHP can enhance the performance of cooling system for magnets.     

Experimental study on key geometric parameters of an R134A ejector cooling system

Ejector geometric parameters largely depend on condensation pressure of working fluids. The area ratio of the ejector is much smaller than that of ejector cooling systems using lower condensation pressure working fluids when R134A is used as working fluid. Consequently, it deserves to study other key geometric parameters when using this working fluid. In this paper, the ejector cooling system with R134A was established first. Next, the influence of three geometric parameters such as nozzle exit position, the length of constant-area section and diverging angle of primary nozzle on the ejector performances was investigated via experimental methods. The results showed that the influence of nozzle exit position on ejector performance is evident as compared to the other two. Moreover, optimal geometric parameters and dimensionless parameters between them were obtained and compared to those of lower condensation pressure systems. This provided simple but practical ejector design guidelines for real engineering applications.     

Calculation methods for comparing the performance of pure and mixed working fluids in heat pump applications

Three methods for comparing cycle performance of working fluids, pure as well as non-azeotropic mixtures, are investigated for two applications and for two mixture pairs, HCFC22-CFC114 and HCFC22-HCFC142b, and their pure components. The methods differ in the way of calculating the heat exchange processes. They assume, respectively, equal minimum approach temperatures, equal mean temperature differences and equal heat transfer areas. Changes of coefficient of performance (COP) with composition are explained for all methods. It is shown that transport properties must be taken into account when making rigorous comparisons between working fluids. To predict the relations between fluids with high accuracy, one must use the method with equal heat transfer areas. By the method with equal mean temperature differences, the COP can be estimated with the same accuracy for mixtures as for pure fluids, and can be used for rough estimations of the COP level with different fluids. The method of equal minimum approach temperatures should be avoided for non-azeotropic mixtures.