Experimental System of Solar Adsorption Refrigeration with Concentrated Collector

Summary

With solar energy as the driving force, a novel adsorption refrigeration system has been developed and experimentally investigated. Water vapor and zeolite formed the working pair of the adsorption system. This manuscript describes the setup of the experimental rig, the operation procedure, and the important results.

Abstract

To improve the performance of solar adsorption refrigeration, an experimental system with a solar concentration collector was set up and investigated. The main components of the system were the adsorbent bed, the condenser, the evaporator, the cooling sub-system, and the solar collector. In the first step of the experiment, the vapor-saturated bed was heated by the solar radiation under closed conditions, which caused the bed temperature and pressure to increase. When the bed pressure became high enough, the bed was switched to connect to the condenser, thus water vapor flowed continually from the bed to the condenser to be liquefied. Next, the bed needed to cool down after the desorption. In the solar-shielded condition, achieved by aluminum foil, the circulating water loop was opened to the bed. With the water continually circulating in the bed, the stored heat in the bed was took out and the bed pressure decreased accordingly. When the bed pressure dropped below the saturation pressure at the evaporation temperature, the valve to the evaporator was opened. A mass of water vapor rushed into the bed and was adsorbed by the zeolite material. With the massive vaporization of the water in the evaporator, the refrigeration effect was generated finally. The experimental result has revealed that both the COP (coefficient of the performance of the system) and the SCP (specific cooling power of the system) of the SAPO-34 zeolite was greater than that of the ZSM-5 zeolite, no matter whether the adsorption time was longer or shorter. The system of the SAPO-34 zeolite generated a maximum COP of 0.169.

Introduction

With the ozone-depletion problem of traditional vapor compressed refrigeration growing more serious, substituting traditional refrigeration with green technology has become a hot topic in recent years. Among those green technologies, the solar adsorption refrigeration has attracted much of the attention of researchers. Driven by low-grade thermal energy, the adsorption refrigeration system has the advantages of being environmentally friendly, small, and flexible. This adsorption system can also be driven with non-solar energy, for instance by waste heat discharged from thermal equipment or by engine exhaust gases from vehicles, as mentioned by Hu et al.¹

In an adsorption cooling system, the adsorption bed is the key component. Its work directly affects the performance of the whole system. Therefore, the design of the adsorption bed is the most important issue as pointed out by Sutuki.² A decade ago, the flat bed was mostly used in the adsorption cooling system.^3,4,5 Without any solar concentrating device, the flat bed temperature was usually low and hence the COP of the system was unsatisfactory. In contrast, the tubular adsorption bed improved the COP. It was reported that the COP could reach 0.21 in sub-Sahara region by Hadj Ammar et al.⁶ Furthermore, Wang et al.⁷ developed a spiral plate adsorber that was distinguished by the characteristic of continuous heat regeneration. The novel design of the adsorption bed shortened the cycle time of the system. Abu-Hamdeh et al.⁸ reported their study on the solar adsorption refrigeration system with a parabolic trough collector. Their test results showed the COP of the system varied from 0.18 to 0.20. El Fadar et al.⁹ studied an adsorption refrigeration system that was coupled with a heat pipe and powered by parabolic trough collector, which showed an optimum COP of 0.18.

To enhance the heat transfer of the tubular bed, some finned tube adsorbers were considered and the effect of the enhancement was examined. An innovative bed that took the form of the shell and tube heat exchanger was presented by Restuccia et al.¹⁰. The internal finned tube was coated with a zeolite layer so that the contact transfer resistance of heat/mass between the metal surface and the adsorbent material could be reduced. The system produced an output of 30-60 W/kg of specific cooling power in the cycling time of 15-20 s. Al Mers et al.¹¹demonstrated that the enhanced adsorber with 5-6 fins could significantly reduce the heat loss of the adsorber to ambiance and thereby improving the COP by 45%. The effect of a finned tube adsorber on the performance of the solar driven system was also studied by Louajari et al.¹². Using activated carbon-ammonia as the working pair, they showed that the cycling mass transfer in the adsorber with fins was greater than the one without fins.

In the current study, we experimentally studied an improved solar adsorption refrigeration system, in which a solar tracking parabolic trough collector was applied and an internal cooling tunnel was deployed. With the SAPO-34/ZSM-5 zeolite and the water vapor as the working pair, the system showed interesting characteristics in terms of thermodynamics and refrigeration. The experimental methodology as well as the typical test results will be presented and discussed in this report.

Protocol

1. Experimental Setup NOTE: The adsorption refrigeration system was composed of the adsorption bed, the evaporator, the condenser, the vacuum pump, and the solar trough collector (Figure 1). An automatic solar tracking device with a parabolic trough was manufactured and applied in the system to improve the efficiency of the solar collector. The automatic solar tracking trough was driven by the worm-gear device as shown in Figure 2. The …

Representative Results

Mass transfer characteristic of the bed through the adsorption process The adsorption bed is always the most important component in an adsorption refrigeration system, and the heat and mass transfer characteristic are the main factors affecting the performance of the whole system. By analyzing the recorded temperature change in the cross sections as shown in Figure 3(bottom panel), it is possible to know the heat and mass transfer …

Discussion

As a thermodynamic system, the performance of a solar adsorption refrigeration device depends on the optimum design and the proper operation of the system. Both the heat supply and the cooling method of the bed are important to guarantee the system works well. Water cooling is preferred to air cooling because of the high strength of convection heat transfer of water. The poor conductivity of the adsorbent material has usually determined the limited heat transfer rate of the bed. To improve the heat transfer of the bed, m…

Materials

evaporator	home-made		finned heat exchange
condenser	home-made		finned heat exchange
evaporator water tank	home-made		volume:9L
condenser water tank	home-made		volume:9L
vacuum pump	Beijing Jing Rui Ze Xiang Instrument Co. Ltd.		rotation speed:1400 motor pover:370W
condenser pressure sensor	Beijing Li Nuo Tian Sheng Instrument Co. Ltd.	16P2623	maximum:2200Pa
bed pressure sensor	Beijing Li Nuo Tian Sheng Instrument Co. Ltd.		maximum:2200Pa
adsorption bed	home-made		cylundrical glass tube
parabolic trough	home-made		high reflective aluminum sheet
water pump	home-made		motor pover:250W, water head:8m
water tank	home-made		volume:500L
DRT-2-2 direct solar actinometer	Beijing Tian Yu De Technology Co. Ltd.	03140132	sensitivity:13.257μV/W•m2
TBQ-2 solar pyranometer	Jinzhou Sunshine Technology Development Co., Ltd., China	209079	sensitivity:12.733μV/W•m2
SAPO-34 zeolite	Langfang Peng Cai Co., Ltd., China		20mm in length and 2.2mm in diameter
ZSM-5 zeolite	Langfang Peng Cai Co., Ltd., China		5.7mm in diameter