Experimental investigation on composite adsorbent – Water pair for a solar-powered adsorption cooling system

doi:10.1016/j.applthermaleng.2017.12.053

Applied Thermal Engineering

Volume 131, 25 February 2018, Pages 649-659

https://doi.org/10.1016/j.applthermaleng.2017.12.053 Get rights and content

Highlights

•
An adsorber coated with composite adsorbents has been investigated.
•
The effect of pre-heating phase on SCP and EER has been studied experimentally.
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The effect of solar collectors’ area on SCP, COP and EER has been studied.
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A SCP of 208 W/kg, a COP of 0.24 and an EER of 4.5 have been achieved.
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The SCP is improved by 92.5% after conducting the pre-heating phase for 2 h.

Abstract

In this study, a solar-powered adsorption cooling system (ACS) using vehicle radiators as an adsorbent bed was built and the system performance was studied experimentally in the Guangzhou climate. 6 single-glazed flat plate solar collectors with the total area of 12 m² were utilized to collect solar energy. Zeolite 13X/CaCl₂ composite adsorbent – water was used as the adsorbent – adsorbate working pair. The composite adsorbent was coated on the fins of the vehicle radiators using an electrostatic coating method. The results show that an adsorbent coating layer with a thickness of 0.5 mm was evenly distributed, and strongly adhered. The effect of the duration of the pre-heating phase and solar collector area on the cooling performance of the ACS was investigated. A pre-heating phase of 2 h was proposed and a minimum area of solar collectors of 6 m² was recommended for a 1–2 kW scale ACS. A specific cooling power (SCP) of 208.2 W/kg of the ACS and an energy efficiency ratio (EER) of 4.5 driven by solar energy were achieved with a pre-heating phase of 2 h, and a maximum solar intensity of 880 W/m².

Introduction

Global warming and energy shortage issues have been receiving much attention in recent years all over the world. More and more electricity is being consumed, especially for air conditioning systems. Taking Hong Kong as an example, air conditioning systems driven by vapor compression refrigerant cycles account for 30–40% of a typical commercial building’s electricity consumption [1]. An adsorption cooling system (ACS) is an excellent supplementary system for any size of chilled water system where space and low grade heat sources are available. ACSs can be compared to conventional vapor compression cooling systems, with the compressor being replaced by a thermally driven adsorber. It utilizes low grade heat sources such as solar energy [2] and waste heat [3] to drive the refrigerant cycle and consumes very little electricity for the entire system [4]. Also, the adsorption/desorption cycles can be operational without the need for moving parts other than magnetic valves, thus leading to low vibration, mechanical simplicity, high reliability and a very long life time. Depending on the performance of the adsorbent, it is possible to use water as the refrigerant (the adsorbate), in contrast to the vapor compression system which uses ozone depleting refrigerants [5]. ACSs are thus more environmentally friendly [6]. As the technology becomes mature and the performance of ACSs continues to be enhanced, this will encourage building owners to adopt ACSs [7].

The adsorbent – adsorbate working pair is a key part of the ACS. Ahmed et al. [8] have summarized all available adsorbent – adsorbate working pairs and separated them into three categories, namely physical adsorbents, chemical adsorbents and composite adsorbents. Solmus et al. [9] developed an ACS with an adsorbent – adsorbate working pair of natural zeolite and water and tested with a 150 °C desorption temperature. The coefficient of performance (COP) and specific cooling power (SCP) were recorded at 0.25 and 7 W/kg, respectively. Boelman et al. [10] experimentally and numerically studied a commercially available silica gel – water ACS. A COP above 0.4 were obtained with a hot water inlet temperature of 50 °C. Kayal et al. [11] experimentally investigated the adsorption characteristics of AQSOA zeolites and water for ACSs by various methods. The results showed that the adsorption capacity of AQSOA-Z02 was 0.1 g/g at the desorption temperature of 65 °C. Wang et al. [12] evaluated the ACSs from five different working pairs and found that the SCP of the silica gel – water working pair was not as high as composite adsorbent – water pairs. Tso et al. [13] developed a silica activated carbon/CaCl₂ composite adsorbent, and numerically calculated that the SCP and COP were 378 W/kg and 0.7, respectively. Chan et al. [14] developed a zeolite 13X/CaCl₂ composite adsorbent and investigated this numerically. The results showed that the difference in equilibrium water uptake between 25 and 75 °C at 870 Pa was a 0.4 g/g, which was 420% of that of zeolite 13X. However, the zeolite 13X/CaCl₂ composite adsorbent has not been investigated in a solar-powered ACS experimentally yet.

The performance of an ACS highly depends on the performance of the adsorber. Improving the method of filling adsorbent material in adsorbers is one area to be developed to enhance the COP and SCP. Wang et al. [15] experimentally investigated an ACS with fluidized-beds. Using fluidized adsorbent and fluidized-beds increased the average mass variation rate of adsorbate adsorbed or desorbed by 630% compared to the conventional fixed adsorbent beds. Sharafian et al. [16] experimentally investigated a waste heat-powered ACS using FAM-Z02 as adsorbent. The FAM-Z02 adsorbent particles were packed in between the fins of a heat exchanger which was used as the adsorbent bed. The results showed that more adsorbent packed, the poorer heat transfer ability was, which led to a lower SCP. When the amount of adsorbent was increased from 0.5 kg to 1.9 kg, the SCP of the ACS was decreased from 119.4 W/kg to 65.8 W/kg. Chan et al. [17] computational study the performance of an ACS using cylindrical shell units which was copper tubes with circular fins covered by stainless metal meshes as adsorbent beds. The silica gel adsorbent was filled in between the circular fins. A SCP of 81.4 W/kg was achieved. The performance of the integrated unit of the adsorbent material and heat exchanger (AdHex) [18], can significantly influence the adsorption capacity and heat and mass transfer, which can affect the performance of the systems [19]. Coating technology can highly enhance the performance of the AdHex [20]. Schnabel et al. [21] studied the adsorption kinetics of zeolite coatings. The results showed that the hydrated masses of the zeolite X coatings were 1.03 g on a 5 cm × 5 cm stainless steel plate and the mass equivalent thickness was 230 µm. The heat transfer resistance between the crystal and metal layer of the zeolite X coatings was lower than that of a sample consisting of a polymer-zeolite structure glued on a metal support. Tatlier [22] developed an ACS using heat exchangers coated with zeolite and a metal-organic framework as adsorbent beds. The results showed that the power values of the metal-organic framework coatings were 4–5-fold higher than those of the zeolite coatings under the desorption temperature of 110 °C. The optimum coating thickness of metal-organic framework coatings were between 130 µm and 170 µm, and a maximum power of 80.2 kW could be achieved. Calabrese et al. [23] developed and investigated an adsorption heat pump coated with silane/SAPO 34 composite adsorbent. The adsorption capacity of the composite adsorbent coating increased by 18–19% compared to that of the pure SAPO 34 coating. Although the cooling performances of the ACSs using coating technologies was very high, the cost of using most of the coating technologies was very high. Meanwhile, although the cost of using packing methods was low, the cooling performances of the ACSs using packing methods was also low. An electrostatic coating method is a coating technology using a high voltage electrostatic electric field to force the particles charged negatively to move in the opposite direction of the electric field and coating on a plate. The cost of using this method is low, however, the performance of using this method to coat composite adsorbent on an adsorber is not well investigated yet.

In terms of application, how to use the heat sources efficiently become a key [24]. Recently, the heat sources, especially solar energy, have been investigated numerically and experimentally [25]. Saha et al. [26] experimentally investigated a double-stage, four-bed, non-regenerative solar-powered ACS. The results showed that flat plate solar collectors could effectively produce the required hot water in any tropical climate conditions. The prototype produced chilled water at 10 °C and had a cooling power of 3.2 kW with a COP of 0.36, when the desorption was 55 °C. Lu et al. [27], [28] developed a heat pipe type solar-powered ACS and experimental studied its performance under typical summer weather conditions in Dezhou City. A cooling capacity and COP of 17.9 kW and 0.63 were achieved under the operating condition of a hot water inlet temperature of 79.0 °C and 65 kg adsorbent. Tso et al. [29], [30] developed a numerical model to investigate the cooling performance of a solar-powered ACS. The results showed that the COP and SCP of the ACS increased significantly with the increase of the solar collector area and the ACS had the best cooling performance with double glazed cover collectors. Koronaki et al. [31] developed a numerical model to study a solar-powered ACS using silica gel – water as working pairs in Mediterranean climate conditions. The results showed that flat plate collectors coated with chromium selective coating had a better performance than other collectors. A cooling capacity of 16 kW, and a COP of 0.51 were achieved with 47 kg adsorbent. Ambarita and Kawai [32] developed a solar-powered ACS using activated carbon – water as working pairs and experimentally investigated under Medan’s climate conditions. A solar COP of 0.085, a hot generator temperature of 110.1 °C, and an evaporator temperature of 6.03 °C were achieved. However, the effect of the area of solar collectors on the cooling performance of an ACS is seldom investigated experimentally under Guangzhou’s climate conditions in summer, where the weather is usually windy, hot (ambient temperature of 25–35 °C) and humid (relative humidity of 70–80%).

Optimizing the operating parameters and conditions is also a main research interest for ACSs [33]. Miyazaki et al. [34] developed a novel dual evaporator type adsorption cooling system with three adsorbent beds and tested the effect of operating temperatures and cycle time on the SCP and COP. It was found that a longer pre-heating/cooling time could reduce the temperature fluctuation of the delivered chilled water. Although the ACSs powered by solar energy have been previously studied by others numerically and experimentally, the effect of a pre-heating phase on the cooling performance of the ACSs has rarely been investigated experimentally. Besides, the effect of the duration of the pre-heating phase on the cooling performance of the ACS powered by solar energy has also not been investigated experimentally.

In our three previous studies, Tso et al. [35] developed and experimentally investigated an adsorber consisting of cylindrical shell units which were covered with metal mesh. The zeolite 13X/CaCl₂ composite adsorbent was packed between the circular fins of the cylindrical shell units. A SCP of 106 W/kg was achieved with a constant hot water inlet temperature of 85 °C. Zhu et al. [36] developed a solar-powered ACS using vehicle radiators covered by metal mesh as the adsorbent bed. Silica gel was used as the adsorbent and packed between fins. A SCP of 52.2 W/kg was achieved with conducting a pre-heating phase of 2 h. Chan et al. [37] developed and experimentally investigated an adsorber consisting of finned heat exchangers. Zeolite 13X/CaCl₂ composite adsorbent was coated on the fins of the heat exchangers. A SCP of 401 W/kg was achieved under an operating condition of a constant hot water inlet temperature of 85 °C. However, the performance of the composite adsorbent has not been investigated in an ACS driven by solar energy, which is not as stable as electrical heaters and is closer to practical application.

This study aimed at building and investigating a solar-powered ACS. In this study, the zeolite 13X/CaCl₂ composite adsorbent was coated but not packed on an adsorber, and investigated experimentally in a solar-powered ACS. The electrostatic coating method was used to coat the zeolite 13X/CaCl₂ composite adsorbent on vehicle radiators and its practicability was investigated and discussed. More importantly, the effect of the duration of the pre-heating phase and area of solar collectors on the cooling performance of the ACS was investigated experimentally under Guangzhou’s climate conditions in summer.

Section snippets

Basic working principle

The basic working principle of an ACS is that the refrigerant, which is water in this study, can continuously evaporate rapidly, produce water vapor and produce a cooling effect in a low pressure evacuated container, named an evaporator. A large amount of adsorbent in an adsorber adsorbs the water vapor from the evaporator to maintain a low pressure condition. Cooling water is supplied to the adsorber to remove the adsorption heat. After the adsorbent is saturated with water vapor, hot water

Test unit

A photograph of the ACS built in the laboratory in Nansha, Guangzhou, is shown in Fig. 2(a). The main components of the ACS located indoors were a water-cooled condenser, an evaporator, two adsorbers, and an overall controller. The condenser, evaporator, adsorbers and overall controller were located indoors, and the diameter of the indoor part of the ACS was 1 m × 1 m × 1.5 m (length × width × height). There were two components located outdoor, which were a water cooling tower shown in Fig. 2

Results and discussion

The operating condition is shown in Table 1. In Table 1, $T_{cool, in}$ represents the cooling water inlet temperature to the adsorber; $t_{ph}$ represents the pre-heating phase time; $t_{ads, des}$ represents the adsorption/desorption phase time; $t_{mr}$ represents the mass recovery time; $t_{hr}$ represents the heat recovery phase time; and ${\dot{m}}_{cool}$ represents the adsorber cooling water mass flow rate. The hot water inlet temperature depended on the area of the solar collectors and solar intensity. The cooling water

Conclusions

In this study, a double-bed ACS was built. 8 vehicle radiators were assembled as an adsorbent bed and a total of 5.6 kg of zeolite 13X/CaCl₂ composite adsorbents were coated using electrostatic coating method in the adsorber. The effects of the duration of pre-heating phase, the area of solar collectors and the electrostatic coating method on the SCP of the ACS were experimentally investigated. The following conclusions are drawn.

(1)
Compared to the SCP of the ACS without conducting the pre-heating

Acknowledgement

Funding sources for this research are provided by the Hong Kong Research Grant Council via General Research Fund accounts 16201114, and the HKUST Initiation Grant via the account code of IGN16EG05.

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Research PaperExperimental investigation on composite adsorbent – Water pair for a solar-powered adsorption cooling system

Highlights

Abstract

Introduction

Section snippets

Basic working principle

Test unit

Results and discussion

Conclusions

Acknowledgement

Int. J. Refrig.

Renew. Sust. Energy Rev.

Renew. Sust. Energy Rev.

Energy Convers. Manag.

Int. J. Refrig.

Appl. Energy

Int. J. Heat Mass Transf.

Int. J. Refrig.

Int. J. Heat Mass Transf.

Int. J. Refrig.

Energy

Energy

Appl. Therm. Eng.

Energy Convers. Manag.

Appl. Therm. Eng.

Appl. Therm. Eng.

Appl. Therm. Eng.

Appl. Therm. Eng.

Prog. Energy Combust. Sci.

Renew. Energy

Energy Convers. Manag.

Appl. Therm. Eng.

Int. J. Heat Mass Transf.

Appl. Therm. Eng.

Case Studies Therm. Eng.

Energy

Research Paper
Experimental investigation on composite adsorbent – Water pair for a solar-powered adsorption cooling system