Experimental study of a low temperature heat driven re-heat two-stage adsorption chiller

The performance of an advanced adsorption chiller, namely, ‘reheat two-stage’ has been investigated experimentally in the present study. The performances in terms of specific cooling power (SCP) and COP are compared with those of conventional single and two-stage chiller. Results show that the reheat two-stage chiller provides more SCP values than those provided by conventional single-stage chiller while it provides better COP values for relatively low heat source temperature. The reheat two-stage chiller also provides almost same cooling capacity comparing with two-stage chiller for the low temperature heat source, while it provides higher COP value than that provided by two-stage chiller. Experimental results also show that the overall performance of the reheat two-stage chiller is always higher than that of conventional single and two-stage adsorption cycle even the temperature of the heat source is fluctuated between 55 and 80 °C.     

Performance study of a high efficient multifunction heat pipe type adsorption ice making system with novel mass and heat recovery processes

The purpose of this paper is to present the performance analysis of a multifunction heat pipe type adsorption ice maker with activated carbon–CaCl₂ as compound adsorbent and ammonia as refrigerant. For this test unit, the heating, cooling and heat recovery processes between two adsorbent beds are performed by multifunction heat pipes. A novel mass and heat recovery adsorption refrigeration cycle is developed. When mass recovery process is implemented before heat recovery process, the performance of the cycle with novel mass and heat recovery processes is much better than that for the cycle with the conventional mass and heat recovery processes. The experimental results show that the former cycle can increase the coefficient of performance (COP) and specific cooling power (SCP) by more than 17% compared with the latter cycle. In comparison with the basic adsorption cycle, the mass and heat recovery cycle can enlarge the cycled refrigerant mass and reduce the power consumption of boiler; the COP and SCP were improved by more than 11% when the mass recovery time was 20 s, while at the optimal mass recovery time of 40 s, the COP improvements for conventional and novel mass and heat recovery cycles are 43.8% and 68.7%, respectively. It was concluded that the novel mass and heat recovery processes are more beneficial to improve the performance of adsorption refrigeration system in comparison with the conventional mass and heat recovery processes.     

Experimental performance of a silica gel–water adsorption chiller

A newly developed adsorption water chiller is described and tested. In this adsorption refrigeration cycle system, there is no refrigerant valve. Thus, the problem of mass transfer resistance occurring in the conventional systems when methanol or water is used as refrigerant and resulting in pressure drop during the flow of refrigerant inside the tubing is eliminated. To make the utilization of low heat source with temperature ranging from 70 to 95 °C possible, silica gel–water was selected as working pair. The experimental results proved that it is able to produce a cooling power of 6.3 kW with a COP of about 0.4. The test results demonstrate that, through the heat recovery, the COP can be increased by 34.4% while mass recovery has the effect of increasing the cooling power by 13.7% and the COP by 18.3%. The performances of the system were analyzed for varied condensation temperature and for varied evaporation temperature. Based on the first prototype, the second prototype is designed and manufactured to improve the performance. Primary test results demonstrate that the performance is highly improved. It has a COP of about 0.5 and cooling power 9 kW for 13 °C evaporation temperature.     

A comparison of the performances of adsorption and resorption refrigeration systems powered by the low grade heat

In order to study the refrigeration performances of the resorption refrigeration technology, the resorption working pair of BaCl₂–MnCl₂–NH₃, which has the similar working requirements for the heat source and cooling source, and also could satisfy the similar refrigeration requirements with the adsorption working pair of CaCl₂–NH₃, is studied by simulation and experiments. In the simulation the mass transfer resistance is not considered for the systems, and the refrigeration performances related with heat transfer performances are studied, results show that the resorption refrigeration system has a higher refrigeration power and COP (coefficient of the refrigeration performance) because the refrigeration effect is generated by the reaction heat compared to the latent heat of evaporation. After the simulation the experimental test unit is constructed, and the experimental data are analyzed. Results show that the resorption rate is influenced by the critical mass transfer performance very much, and the refrigeration performance is lower than that of adsorption system. The resorption system also has the problem of the larger refrigeration power loss for the reason of the sensible heat requirement of low temperature adsorber. How to improve the mass transfer performance of resorption system and decrease the influence on the refrigeration power by the sensible heat requirement of low temperature adsorber will be the key research directions for the application of resorption refrigeration systems.     

A two-stage deep freezing chemisorption cycle driven by low-temperature heat source

A two-stage chemisorption cycle suitable for deep-freezing application driven by low- temperature heat source was proposed. Through two-stage desorption processes, the two-stage cycle can break through the limitations of the heating temperature and ambient cooling temperature. The two-stage cycle based on CaCl₂/BaCl₂-NH₃ working pair can utilize the heat source with a temperature of above 75°C, and simultaneously realize deep-freezing all the year round. Experimental results and performance prediction show that the adsorption quantity of calcium, theoretical coefficient of performance (COP) and optimized specific cooling power (SCP) of the CaCl₂/BaCl₂-NH₃ chemisorption system are 0.489 kg/kg (salt), 0.24 and 120.7 W/kg, when the heating temperature, ambient cooling temperature, pseudo-evaporating temperature and mass ratio of reacting salt and expanded graphite are 85, 30, -20, and 4∶1, respectively.     

Thermodynamic analysis and performance study for adsorption refrigeration cycle driven by a fuel cell electric vehicle waste heat

Three kinds of adsorption refrigeration cycles are analyzed in this paper, a two‐bed continuous cycle, an adiabatic mass recovery cycle, and an isothermal mass recovery cycle. Operating parameters (including desorption temperature, adsorption temperature, cycle adsorption rate, COP, and period refrigerating capacity) with the change of the evaporating temperature, condensing temperature, heat capacity ratio, and heat resource temperature are discussed. The analysis indicates that performance differences between the mass recovery cycle and the two‐bed continuous cycle are reduced with an increasing of evaporating temperature and heat source temperature. By increasing the heat capacity ratio, COP values for the three kinds of cycle decrease. When the heat source temperature is between 70 and 90°C, the performance of the isothermal mass recovery cycle is best. Through study, this paper puts forward that the isothermal mass recovery cycle is the best cycle for adsorption refrigeration systems driven by fuel cell electrical vehicle waste heat. © 2010 Wiley Periodicals, Inc. Heat Trans Asian Res, 39(7): 523–538, 2010; Published online 16 July 2010 in Wiley Online Library ( wileyonlinelibrary.com ). DOI 10.1002/htj.20315     

Study on solar/waste heat driven multi-bed adsorption chiller with mass recovery

The study deals with a solar or waste heat driven three-bed adsorption cooling cycle employing mass recovery scheme. A cycle simulation computer program is developed to investigate the performance of the chiller. The innovative chiller is driven by exploiting solar/waste heat of temperatures between 60 and 90 °C with a cooling source at 30 °C for air-conditioning purpose. The performance of the three-bed adsorption chiller with mass recovery scheme was compared with that of the three-bed chiller without mass recovery. It is found that cooling effect as well as solar/waste heat recovery efficiency, η of the chiller with mass recovery scheme is superior to those of three-bed chiller without mass recovery for heat source temperatures between 60 and 90 °C. However, COP of the proposed chiller is higher than that of the three-bed chiller without mass recovery, when heat source temperature is below 65 °C.     

Numerical investigation of a two-stage air-cooled absorption refrigeration system for solar cooling: Cycle analysis and absorption cooling performances

Two-stage air-cooled ammonia–water absorption refrigeration system could make good use of low-grade solar thermal energy to produce cooling effect. The system simulation results show that thermal COP is 0.34 and electrical COP is 26 under a typical summer condition with 85 °C hot water supplied from solar collector. System performances under variable working conditions are also analyzed. Circular finned tube bundles are selected to build the air-cooled equipment. The condenser should be arranged in the front to get an optimum system performance. The mathematical model of the two-stage air-cooled absorber considering simultaneous heat and mass transfer processes is developed. Low pressure absorber should be arranged in front of middle pressure absorber to minimize the absorption length. Configuration of the air-cooled equipment is suggested for a 5 kW cooling capacity system. Temperature and concentration profiles along the finned tube length show that mass transfer resistance mainly exists in liquid phase while heat transfer resistance mainly exists in cooling air side. The impacts on system refrigeration capacities related to absorption behaviors under variable working conditions are also investigated. Both cycle analysis and absorption performances show that two-stage air-cooled ammonia–water absorption chiller is technically feasible in practical solar cooling applications.     

Dynamic characteristics of a novel adsorption refrigerator with compound mass‐heat recovery

A three‐effect heat pipe (heat pipe heating, heat pipe cooling and heat pipe heat recovery) adsorption refrigeration system using compound adsorbent (calcium chloride and activated carbon) was designed. The dynamic characteristics of mass and heat pipe heat recovery were studied. The results show that mass recovery and heat pipe heat recovery can improve (specific cooling power) SCP and (coefficient of performance) COP greatly. The averaged SCP of the cycle with mass recovery and the cycle without mass recovery is 502.9 W/kg and 436.7 W/kg at about 30 °C of cooling water temperature and ?15 °C of evaporating temperature. The corresponding COP is 0.27 and 0.24 respectively. Copyright © 2011 John Wiley & Sons, Ltd.     

Study on a dual-mode, multi-stage, multi-bed regenerative adsorption chiller

In this paper, a detailed parametric study on a dual-mode silica gel–water adsorption chiller is performed. This advanced adsorption chiller utilizes effectively low-temperature solar or waste heat sources of temperature between 40 and 95 °C. Two operation modes are possible for the advanced chiller. The first operation mode will be to work as a highly efficient conventional chiller where the driving source temperature is between 60 and 95 °C. The second operation mode will be to work as an advanced three-stage adsorption chiller where the available driving source temperature is very low (between 40 and 60 °C). With this very low driving source temperature in combination with a coolant at 30 °C, no other cycle except an advanced adsorption cycle with staged regeneration will be operational. In this paper, the effect of chilled-water inlet temperature, heat transfer fluid flow rates and adsorption–desorption cycle time effect on cooling capacity and COP of the dual-mode chiller is performed. Simulation results show that both cooling capacity and COP values increase with the increase of chilled water inlet temperature with driving source temperature at 50 and 80 °C in three-stage mode, and single-stage multi-bed mode, respectively. However, the delivered chilled-water temperature increases with chilled-water inlet temperature in both modes.     

Development of a metal hydride refrigeration system as an exhaust gas-driven automobile air conditioner

Aiming at developing exhaust gas-driven automobile air conditioners, two types of systems varying in heat carriers were preliminarily designed. A new hydride pair LaNi_4.61Mn_0.26Al_0.13/La_0.6Y_0.4Ni_4.8Mn_0.2 was developed working at 120–200 °C/20–50 °C/−10–0 °C. P-C isotherms and reaction kinetics were tested. Reaction enthalpy, entropy and theoretical cycling coefficient of performance (COP) were deducted from Van’t-Hoff diagram. Test results showed that the hydride pair has flat plateau slopes, fast reaction dynamics and small hystereses; the reaction enthalpy of the refrigeration hydride is −27.1 kJ/mol H₂ and system theoretical COP is 0.711. Mean particle sizes during cycles were verified to be an intrinsic property affected by constitution, heat treatment and cycle numbers rather than initial grain sizes. Based on this work pair, cylindrical reactors were designed and a function proving metal hydride intermittent refrigeration system was constructed with heat conducting oil as heat source and water as heat sink. The reactor equivalent thermal conductivity is merely 1.3 W/(m K), which still has not meet practical requirement. Intermittent refrigeration cycles were achieved and the average cooling power is 84.6 W at 150 °C/30 °C/0 °C with COP being 0.26. The regulations of cycling performance and minimum refrigeration temperature (MRT) were determined by altering heat source temperature. Results showed that cooling power and system COP increase while MRT decreases with the growth of heat source temperature. This study develops a new hydride pair and confirms its application in automobile refrigeration systems, while their heat transfer properties still need to be improved for better performance.     

Performance evaluation of multi-stage, multi-bed adsorption chiller employing re-heat scheme

This paper deals with the performance investigation of a silica gel/water-based multi-stage, multi-bed, six-bed adsorption chiller employing re-heat scheme. The innovative chiller is powered by waste heat or renewable energy sources of temperature between 50 and 70 °C along with a coolant of inlet temperature at 30 °C for air-conditioning purpose. The performance of the six-bed adsorption chiller using re-heat scheme is compared with that of the six-bed chiller without re-heat. With the same operating conditions, such as the heat transfer fluid inlet (HTF) temperatures, HTF flow rates, adsorption/desorption cycle time and same chiller physical dimension, it is found that both the cooling capacity (CC) and the coefficient of performance (COP) of the three-stage chiller with re-heat scheme are superior than those of the three-stage chiller without re-heat scheme.     

Study on a re-heat two-stage adsorption chiller – The influence of thermal capacitance ratio,overall thermal conductance ratio and adsorbent mass on system performance

Silica gel/water based adsorption cycles have a distinct advantage in their ability to be driven by heat of near-ambient temperature so that waste heat below 100 °C can be recovered. One interesting feature of refrigeration cycles driven by waste heat is that they do not use primary energy as driving source. From this context, some researchers investigated the performance of multi-stage adsorption refrigeration cycles those can be operated by heat source of temperature 60 °C or lower which are usually purged to the environment, with a heat sink of temperature at 30 °C. However, the performances of multi-stage systems are low. To improve system performance, an analytic investigation on a re-heat two-stage chiller is performed to clarify the effect of thermal capacitance ratio of the adsorbent and inert material of sorption element, overall thermal conductance ratio of sorption element and evaporator along with silica gel mass on the chiller performance. Results show that cycle performance is strongly influenced by the sorption elements overall thermal conductance values due to their severe sensible heating and cooling requirements resulting from batched cycle operation. The effect of thermal capacitance ratio (C_s/C_m) becomes significant with relatively higher mass of silica gel. It is also found that the chiller performance increases significantly in the range of silica gel mass from 4 to 20 kg.     

余热制冷用氯化钙_硅胶复合吸附剂的制备及性能研究

为了开发出利用余热进行吸附制冷的高性能吸附剂,采用浸渍法在真空下将氯化钙担载于粗孔硅胶上,制备了硅胶/氯化钙复合吸附剂,测试了复合吸附剂的吸附等温线和吸附速率,测试结果表明:浸渍法得到的复合吸附剂对水具有更大的吸附能力,在20%的湿度下,复合吸附剂在2h的吸附量为15.64 g/100 g吸附剂,是单一硅胶在相同条件下吸附量的8.06倍。用制备的复合吸附剂制作了一台小型吸附制冷机并进行了测试,当热源温度为90℃,冷却水温度为35℃时,在整个循环周期内(15 min),制冷功率为0.705kW,单位质量吸附剂的制冷功率(SCP)为70.51 W/kg,COP为0.25。     

CO2跨临界双级压缩/喷射制冷循环热力学分析

建立了同时采用双级压缩和利用喷射器代替节流阀的CO2跨临界双级压缩/喷射制冷循环模型,在系统稳定运行的条件下,分析了高压压力、气体冷却器出口温度、蒸发温度和高、低压压缩机吸气过热度对循环性能的影响,并与CO2跨临界单级压缩/喷射制冷循环和双级压缩制冷循环进行了比较.结果表明:在给定条件下,双级压缩/喷射循环的性能系数明显优于其他两种循环;随着气体冷却器出口温度的升高和蒸发温度的降低,循环的性能系数分别降低了54.9%和43.2%,并且其下降速度大于双级循环的性能系数下降速度;高、低压压缩机吸气过热度升高均导致双级压缩/喷射循环性能系数降低.     

吸附制冷中回质过程的新模型及循环性能的研究

吸附式制冷在余热回收方面具有很好的前景，设计了用于大客车发动机余热回收的吸附式制冷系统，进行了分布参数的模拟计算，结果显示吸附床设计是比较合理的。同时建立了一种新的回质计算模型，提出了新的回质操作方法。模拟计算显示，回质是改善吸附循环性能的重要手段，回质过程提高了循环SCP和COP达一倍以上，回质过程非常迅速，5s即已完成，可以有效地缩短循环时间，提高循环性能。     

Numerical analysis of an advanced three-bed mass recovery adsorption refrigeration cycle

Numerical analysis of an adsorption cycle employing advanced three-bed mass recovery cycle with and without heat recovery is introduced in this paper. The cycle consists of three silica gel adsorbent beds with different heat utilization functions. The beds can be divided into two cycles with different desorption mechanisms. The working principle of the cycle is introduced, and performances of three-bed, single stage, and mass recovery adsorption cycles are compared in terms of coefficient of performance (COP) and specific cooling power (SCP). The paper also presents the effect of adsorber mass distribution and desorption time on performance. The results show that by applying heat recovery to the cycle, better COP values will be produced compared to that without heat recovery. The results also show that there is an optimum point of adsorber mass distribution and desorption time that produces optimum performance. Furthermore, the paper also compares the performances of the proposed cycle, a single-stage cycle, and a mass recovery cycle.     

Particular characteristics of transcritical CO2 refrigeration cycle with an ejector

The present study describes a theoretical analysis of a transcritical CO₂ ejector expansion refrigeration cycle (EERC) which uses an ejector as the main expansion device instead of an expansion valve. The system performance is strongly coupled to the ejector entrainment ratio which must produce the proper CO₂ quality at the ejector exit. If the exit quality is not correct, either the liquid will enter the compressor or the evaporator will be filled with vapor. Thus, the ejector entrainment ratio significantly influences the refrigeration effect with an optimum ratio giving the ideal system performance. For the working conditions studied in this paper, the ejector expansion system maximum cooling COP is up to 18.6% better than the internal heat exchanger cycle (IHEC) cooling COP and 22.0% better than the conventional vapor compression refrigeration cycle (VCRC) cooling COP. At the conditions for the maximum cooling COP, the ejector expansion cycle refrigeration output is 8.2% better than the internal heat exchanger cycle refrigeration output and 11.5% better than the conventional cycle refrigeration output. An exergy analysis showed that the ejector expansion cycle greatly reduces the throttling losses. The analysis was also used to study the variations of the ejector expansion cycle cooling COP for various heat rejection pressures, refrigerant temperatures at the gas cooler exit, nozzle efficiencies and diffuser efficiencies.     

Effects of unequal finite piston speeds on the optimal performances of endoreversible Carnot refrigeration and heat pump cycles

The performances of endoreversible Carnot refrigeration and heat pump cycles with loss of heat resistance and finite piston speeds are analysed and optimized by using the combination of finite time thermodynamics, finite speed thermodynamics and direct method. The unequal finite piston speed model on four branches is adopted. Expressions of cooling load of endoreversible Carnot refrigeration cycle and of heating load of endoreversible Carnot heat pump cycle are derived with a fixed cycle period and unequal finite piston speeds on the four branches. Numerical examples show that there exist optimal expansion ratios, which lead to maximum cooling load and maximum heating load for the fixed coefficient of performance (COP), respectively. The maximum cooling load, maximum heating load, optimal ratios of finite piston speeds and optimal hot- and cold-side working fluid temperatures versus COP characteristics for the endoreversible Carnot refrigeration and heat pump cycles are obtained. Moreover, the effects of design parameters on the performances of the two cycles are discussed.     

Performance Comparison of Three-Bed Adsorption Cooling System With Optimal Cycle Time Setting

This article presents the optimal cycle time and performance of two different types of silica gel–water-based three-bed adsorption chillers employing mass recovery with heating/cooling scheme. A new simulation program has been developed to analyze the effect of cycle time precisely on the performance of the systems. The particle swarm optimization (PSO) method has been used to optimize the cycle time and then the optimum performances of two chillers are compared. Sensitive analysis of cycle time has been conducted using the contour plot of specific cooling power (SCP) with driving heat source temperature at 80°C. It is found that the center point of the contour indicates the maximum SCP value and optimal cycle time, which are comparable with the quantitative values obtained for the PSO method. Both three-bed mass recovery adsorption cycles can produce effective cooling at heat source temperature as low as 50°C along with a coolant at 30°C. The optimal SCP is similar for both cycles and is greater than that of the conventional two-bed adsorption system employing the same adsorbent–refrigerant pair. Consequently, the proposed comparison method is effective and useful to identify the best performance of adsorption cycles.