Development and comparison of two-bed silica gel–water adsorption chillers driven by low-grade heat source

A novel silica gel–water adsorption chiller (driven by hot water of 60–90 °C) with three vacuum chambers has been built in Shanghai Jiao Tong University (SJTU). This chiller was an improvement of an earlier deigned chiller and it integrated two single-bed systems (basic system) with only one vacuum valve. The performance of the chiller was tested and compared with the former adsorption chiller. The results show that the cooling power and COP of the chiller are 8.70 kW and 0.39 for the heat source temperature of 82.5 °C, cooling water temperature of 30.4 °C and chilled water outlet temperature of 12 °C. For a higher chilled water outlet temperature of about 16 °C, the COP increases to 0.43 while the cooling power is about 11.0 kW. Compared with that of the former chiller, the COP of this chiller increases by 20%.     

Performance analysis of a waste heat driven activated carbon based composite adsorbent – Water adsorption chiller using simulation model

This study aims at improving the performance of a waste heat driven adsorption chiller by applying a novel composite adsorbent which is synthesized from activated carbon impregnated by soaking in sodium silicate solution and then in calcium chloride solution. Modeling is performed to analyze the influence of the hot water inlet temperature, cooling water inlet temperature, chilled water inlet temperatures, and adsorption/desorption cycle time on the specific cooling power (SCP) and coefficient of performance (COP) of the chiller system with the composite adsorbent. The simulation calculation indicates a COP value of 0.65 with a driving source temperature of 85 °C in combination with coolant inlet and chilled water inlet temperature of 30 °C and 14 °C, respectively. The most optimum adsorption–desorption cycle time is approximately 360 s based on the performance from COP and SCP. The delivered chilled water temperature is about 9 °C under these operating conditions, achieving a SCP of 380 W/kg.     

采用复合吸附剂和高效吸附床的吸附制冷机性能测试

利用平行流换热器和自制的硅胶/氯化钙复合吸附剂研制了一台小型吸附式制冷样机,并对样机进行了试验测试。测试结果表明:相对于硅胶吸附制冷样机,复合吸附剂吸附制冷样机的COP和制冷功率都有了明显的提高;在热源温度为90℃,冷却水温度为35℃,冷冻水进口温度为16.5℃、出口温度为14.4℃,吸附10min,脱附5 min的运行工况下,在整个循环周期内(15 min),制冷功率为1.03 kW,SCP为128.3 W/kg,COP为0.29;在吸附周期内(10 min),制冷功率为1.54 kW,SCP为192.4 W/kg,样机的能量密度为10.3 kW/m3,平行流换热器的换热系数为472.3 W/(m2.K)。     

Experimental study on improved two-bed silica gel–water adsorption chiller

A novel silica gel–water adsorption chiller with two chambers has been built in Shanghai Jiao Tong University (SJTU). This chiller combines two single bed systems (basic system) without any vacuum valves. One adsorber, one condenser and one evaporator are housed in the same chamber to constitute one adsorption/desorption unit. In this work, the chiller is developed and improved. The improved chiller is composed of three vacuum chambers: two adsorption/desorption vacuum chambers (the same structure as the former chiller) and one heat pipe working vacuum chamber. The evaporators of these two adsorption/desorption units are combined by a heat pipe. So, no valves are installed in the chilled water sub system and one vacuum valve connects the two adsorption/desorption chambers together to improve its performance. The performance of the chiller is tested. As the results, the refrigerating capacity and the COP of the chiller are, respectively, 8.69 kW and 0.388 for the heat source temperature of 82.5 °C, the cooling water temperature of 30.4 °C and the chilled water outlet temperature of 11.9 °C. For a chilled water outlet temperature of 16.5 °C, the COP reaches 0.432, while the refrigerating capacity is near 11 kW. There is an improvement of at least 12% for the COP compared with the former chillers.     

The influence of heat exchanger parameters on the optimum cycle time of adsorption chillers

The research investigated the influences of heat exchanger parameters, such as heat capacity and NTU, on the optimum performance of a single-stage adsorption chiller. Silica gel–water pair was chosen as the adsorbent–adsorbate combination so that low temperature heat source under than 100 °C could be utilized as the driving force.The mathematical model of the adsorption chiller using dimensionless parameters was developed and a global optimization method called the particle swarm optimization was applied in the simulation to obtain the optimum cycle time. The results showed that the smaller heat capacity heat exchanger improved both the maximum specific cooling capacity (SCC) and the COP. While, the larger NTU of the adsorbent bed resulted in the decrease of the COP due to the short cycle time although the maximum SCC was enhanced.     

Study on a compact silica gel–water adsorption chiller without vacuum valves: Design and experimental study

A compact silica gel–water adsorption chiller without vacuum valves was manufactured and experimentally studied. This chiller contains two adsorption/desorption chambers and one chilled water tank. Each adsorption/desorption chamber consists of one adsorber, one condenser and one evaporator. The chilled water tank is adopted to mitigate the variation of the chilled water outlet temperature. A mass recovery-like process, which is a heat recovery process between the two evaporators, was carried out in this chiller. A novel heat recovery process was also fulfilled after the mass recovery-like process to improve the coefficient of performance (COP). The cooling power and COP were 9.60 kW and 0.49 respectively when the average hot water inlet temperature, cooling water inlet temperature, and chilled water outlet temperature were 82.0, 31.6 and 12.3 °C, respectively.     

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.     

Experimental study on performance improvement of a four-bed adsorption chiller by using heat and mass recovery

The efficacy of a four-bed adsorption chiller has been studied experimentally with respect to a simple but yet effective passive heat and mass recovery schemes. It substantially improves the adsorption chiller COP by as much as 30% over a broad range of cycle time with a wide heat source, coolant and chilled water temperatures. Two schemes have been considered here: Firstly, only the mass recovery is achieved by pressure equalization between the concomitantly cooled adsorber and heated desorber, exploiting the intrinsic vapor-uptake potential by pressure swing that remains in the adsorbent at the end of a half-cycle. Secondly, when both the heat and mass recovery schemes are employed at a rating point of maximum cooling capacity, the chiller COP could increase further to as much as 48%. These improvements are performed without additional hardware changes to the adsorption chiller.     

The effects of operation parameter on the performance of a solar-powered adsorption chiller

A solar-powered adsorption chiller with heat and mass recovery cycle was designed and constructed. It consists of a solar water heating unit, a silica gel-water adsorption chiller, a cooling tower and a fan coil unit. The adsorption chiller includes two identical adsorption units and a second stage evaporator with methanol working fluid. The effects of operation parameter on system performance were tested successfully. Test results indicated that the COP (coefficient of performance) and cooling power of the solar-powered adsorption chiller could be improved greatly by optimizing the key operation parameters, such as solar hot water temperature, heating/cooling time, mass recovery time, and chilled water temperature. Under the climatic conditions of daily solar radiation being about 16–21 MJ/m², this solar-powered adsorption chiller can produce a cooling capacity about 66–90 W per m² collector area, its daily solar cooling COP is about 0.1–0.13.     

Study on a twin‐bed adsorption refrigeration system using R134a‐activated carbon pair

A low capacity twin‐bed adsorption refrigeration system has been built with R134a as a refrigerant and activated carbon as the adsorbent. Simple tube‐in‐tube heat exchangers have been fabricated and have been used as the adsorber beds. Activated carbon (granular type) has been filled in the annular space of the inner tube and outer tube. A plate heat exchanger has been used as the condenser and the temperature of cooling water has been maintained between 25°C and 30°C, also the evaporator has been custom designed as per requirements. A mathematical model has also been developed and the results obtained have been found to be comparable. While operating the system in the single‐bed mode a cooling power of 250.4 W has been obtained with a coefficient of performance (COP) of 0.38 with an average evaporator temperature of 18.4°C against a predicted value of 263.7 W with a COP of 0.41. While operating in the twin‐bed mode a cooling power of 281.3 W with a COP of 0.47 with an average evaporator temperature of 17.6°C has been obtained against a predicted value of 294.5 W with a COP of 0.52.     

Study of a two-bed silica gel–water adsorption chiller: performance analysis

In this study, a lumped parameter simulation model has been developed for analysis of the thermal performance of a single-stage two-bed adsorption chiller. Since silica gel has low regeneration temperature and water has high latent heat of vaporisation, silica gel–water pair has been chosen as the working pair of the adsorption chiller. Low-grade waste heat or solar heat at around 70–80°C can be used to run this adsorption chiller. In this model, the effects of operating parameters on the performance of the chiller have been studied. The simulated results show that the cooling capacity of the chiller has an optimum value of 5.95?kW for a cycle time of 1600?s with the hot, cooling, and chilled water inlet temperatures at 85°C, 25°C, and 14°C, respectively. The present model can be utilised to investigate and optimise adsorption chillers.     

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 a silica gel–water adsorption chiller integrated with a closed wet cooling tower

A silica gel–water adsorption chiller integrated with a closed wet cooling tower is proposed. This adsorption chiller consists of two vacuum chambers, each with one adsorber, one condenser and one evaporator. Vacuum valves were not adopted in this chiller in order to enhance the reliability. A novel heat recovery process was carried out after a mass recovery-like process to improve the coefficient of performance (COP). Integration of the closed wet cooling tower into the chiller could ensure the cleanliness of cooling water circulating in the chiller and also promote the convenient setup of the chiller. A transient one-dimensional mathematical model was adopted to study this adsorption chiller. The simulated results showed that the cooling power and COP were 10.76 kW and 0.51 respectively when the hot water inlet temperature, the chilled water inlet temperature, the air inlet wet bulb temperature and dry bulb temperature were 85, 15, 28 and 30 °C respectively.     

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.     

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.     

Experimental study of a solid adsorption cooling system using flat-tube heat exchangers as adsorption bed

In this paper, a solid adsorption cooling system with silica gel as the adsorbent and water as the adsorbate was experimentally studied. To reduce the manufacturing costs and simplify the construction of the adsorption chiller, a vacuum tank was designed to contain the adsorption bed and evaporator/condenser. Flat-tube type heat exchangers were used for adsorption beds in order to increase the heat transfer area and improve the heat transfer ability between the adsorbent and heat exchanger fins. Under the standard test conditions of 80 °C hot water, 30 °C cooling water, and 14 °C chilled water inlet temperatures, a cooling power of 4.3 kW and a coefficient of performance (COP) for cooling of 0.45 can be achieved. It has provided a specific cooling power (SCP) of about 176 W/(kg adsorbent). With lower hot water flow rates, a higher COP of 0.53 can be achieved.     

Experimental study of a solidified activated carbon-methanol adsorption ice maker

In this paper, the experiments are performed on an adsorption ice maker driven by waste heat, which uses up to two beds. Each bed uses methanol as refrigerant and solidified activated carbon (120 kg adsorbent totally, 60 kg adsorbent per bed) as adsorbent. This system is designed to be driven by the waste heat of a 100 kW diesel engine. The experiments show that the cooling power could be enhanced by the mass recovery process up to 11%, and the heating power could be lowered by the heat recovery process up to 30%. The optimal cooling power of this prototype is about 2.0 kW and corresponds to a specific cooling power (SCP) is about 17 W/kg with both heat and mass recoveries between two beds. Considering the optimal adsorption time is much longer than optimal desorption time at the condition of ice making, the experiments are operated on a single bed (60 kg adsorbent per bed) and the adsorption time used in experiments is two times of desorption time, then the performance of a three-bed adsorption ice maker (120 kg adsorbent totally, 40 kg adsorbent per bed) is predicted by the results of experiments on this single bed. The results of prediction show that both COP and cooling power of three-bed operation could be enhanced greatly compared to the two-bed operation; optimal SCP and COP are respectively 22 W/kg and 0.239 when mass and heat recoveries proceed between three beds. Optimal ice productivity of this three-bed system is 21 kg/h when the water temperature is 25 °C and ice temperature is −7 °C.     

A four-bed mass recovery adsorption refrigeration cycle driven by low temperature waste/renewable heat source

The study deals with an advanced four-bed mass recovery adsorption refrigeration cycle driven by low temperature heat source. The proposed cycle consists of two basic adsorption refrigeration cycle. The heat source rejected by one cycle is used to power the second cycle. Due to the cascading use of heat and cooling source, all major components of the system maintain different pressure levels. The proposed cycle utilize those pressure levels to enhance the refrigeration mass circulation that leads the system to perform better performances. The performance of the proposed cycle evaluated by the mathematical model at equilibrium condition and compared with the performance of the basic two-bed adsorption refrigeration cycle. It is seen that the cooling effect as well as COP of the proposed cycle is superior to those of the basic cycle. The performances of the cycle are also compared with those of the two-stage cycle. Results also show that though the cooling effect of the proposed cycle is lower than that of two-stage cycle for heat source temperature less than 70 °C, the COP of the cycle, however, is superior to that of two-stage cycle for heat source temperature greater than 60 °C.     

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.     

A new combined adsorption–ejector refrigeration and heating hybrid system powered by solar energy

In this paper the design scheme of a new continuous combined solid adsorption–ejector refrigeration and heating hybrid system driven by solar energy was proposed, the thermodynamic theory of this system was constructed, and the performance simulation and analysis were made under normal working conditions. In the combined hybrid system, zeolite–water working pair was chosen in view of environment protection and solar energy utilization. From simulation the combined hybrid system had a cooling capacity of 0.15 MJ per kg zeolite in the day-time and a cooling capacity of 0.34 MJ per kg zeolite in the evening, and could furnish 290 kg hot water at 45 °C for family use. Furthermore, under the same working conditions, compared with an adsorption system without an ejector with a COP of 0.3, the combined system's COP was improved by 10% totally and reached 0.33.