Experimental study on an adsorption chiller employing lithium chloride in silica gel and methanolEtude expérimentale sur un refroidisseur à adsorption utilisant le chlorure de lithium dans du gel de silice et du méthanol

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Abstract

A novel composite adsorbent – methanol adsorption chiller was proposed and manufactured. It was filled by the adsorbent composed by Lithium Chloride and silica gel. Methanol was used as adsorbate and refrigerant. Experiment results showed that compared with silica gel-water chiller, SCP (specific cooling power) and COP (coefficient of performance) of this novel chiller were improved by 16.3% and 24.2% separately when the temperatures of hot water inlet, cooling water inlet and chilled water outlet were 85 °C, 30 °C and 15 °C.

Highlights

► We have produced a novel composite adsorbent–methanol adsorption chiller and done some experiments on it. ► The performance of the chiller is improved greatly by using the new adsorbent. ► The proper cycle time for this chiller was 680 s/60 s/20 s. ► The best temperature of heat source is about 80–85 °C.

Introduction

Now energy consumption is one of the most popular topics. People are facing more challenges to balance sustainable development and environmental protection. Adsorption technology, which is a clean tech attracts more and more researchers. It employs friendly refrigerants (water, methanol, etc.) instead of the ozone depleting substances such as CFCs and HCFCs. In addition, the adsorption chiller can be driven by industry waste heat or solar energy with the temperature of 55–90 °C. As a result, this tech cuts a part of the greenhouse gas emitted from coal plants.

A family of adsorption working pairs has been studied (Aristov et al., 2002; Restuccia et al., 2004; Bauer et al., 2009; Veselovskaya and Tokarev, 2011). The typical working pairs are silica gel–water, zeolite–water, activated carbon–ammonia, activated carbon–methanol, activated carbon fiber–ammonia, inorganic salt-ammonia, etc. The utilizations of water and methanol as refrigerants in adsorption system make it possible to use the heat source below 100 °C (Saha et al., 2007). However, typical water/methanol working pairs do not have ideal characteristics. When water is used as an adsorbate, the low working pressure increases the risk of leakage problems and implies severe mass transfer limitation through the component of the machine through the adsorbent bed itself (Gordeeva et al., 2009). Methanol has a higher operation pressure. However the sorption capacity is rather low and usually does not exceed 0.15–0.4 g g−1 (Gordeeva et al., 2008). And these working pairs do not guarantee adequate performance when desorption temperature is lower than 90 °C (Freni et al., 2012). Some composite adsorbents have been proposed. Novel adsorbents with metal materials have been developed to enhance heat and mass transfer of adsorption material (Hu et al., 2009). The composites with hydrophilic salt largely improve the adsorption capacity (Aristov et al., 2002; Gordeeva et al., 2008). However, it is quite possible that those additives bring some new problems, for instance, corrosion, which hinders the real application in industry. As a result, a new way to produce the composite adsorbent which employing Lithium Chloride in silica gel has been proposed by the researchers in SJTU (Shanghai Jiao Tong University). The salt on the surface is removed and the characteristics of the new composite adsorbent have been studied (Gong et al., 2010a).

Now a chiller with this kind of adsorbent has been manufactured and tested. Usually the adsorption working pair for conventional adsorption chiller is water–silica gel. However the working pressure of water is very low. As a result, the mass transfer in the adsorbent bed is limited. Furthermore, small amount of air or gas, which are leaked into the machine or emerged by some unknown reactions, weaken the performance of the chiller significantly (Gordeeva et al., 2008). Methanol is used as an alternative adsorbate in this chiller. Its high working pressure enhances the stability performance of the chiller; also it has a lower freezing temperature. So tentative experiments for cold-storage or ice making are allowed to do. But the latent heat of evaporation of methanol is lower than that of water. It can only be compensated by the improved adsorption capacity of the composite adsorbent. The theoretical adsorption refrigeration cycle of working pair (composite adsorbent-methanol) has been studied (Gong et al., 2010b).

Though there are many research papers about the composite adsorbents (Aristov et al., 2002; Wang et al., 2009) and the chillers filled with typical working pairs (silica gel–water, activated carbon–methanol, etc.) (Anyanwu and Ezekwe, 2003; Freni et al., 2012 and Wang et al., 2005), there are few papers about the experimental study on the chiller filled by composite adsorbent. This paper presents the design and the experimental study of a chiller using the composite adsorbent and methanol. The objects of this paper are (1) analyzing the dynamic performances of the novel chiller. It is compared with that of silica gel-water adsorption chiller in similar structure. The result gives an experimental verification for real application of the composite adsorbent; (2) using methanol as refrigerant and heat pipe fluid. The working pressure is higher than that of water so that the system leakage risk can be reduced. As a result, the performance of the chiller and mass transfer performances should be improved; (3) finding an optimal operation strategy for the chiller, including the proper cycle time, mass recovery time and the water inlet temperatures. How these factors affect the chiller's performance is analyzed; (4) testing the chiller in both air conditioning condition and cold-storage condition.

Section snippets

System description

The schematic diagram of the composite adsorbent-methanol chiller is shown in Fig. 1a. The chiller is composed of three chambers. Both chamber 1 and chamber 2 contain one adsorber, one condenser, one evaporator. Chamber 3 is a heat-pipe evaporator. By using the loop heat pipe in evaporator (shown in Fig. 1b), the vacuum valves are not used inside the chiller, so that the reliability of the chiller could be enhanced. This heat pipe evaporator is similar to the evaporator described before (Wang

Working principle

The working cycle is made up of adsorption/desorption process, mass recovery process and heat recovery process. The operating states of the valves are shown in Fig. 3. The working processes are described in the following:

  • (1)

    Adsorber 1 works in adsorption process while adsorber 2 is in desorption process

Hot water flows into adsorber 2. Adsorber 2 is heated and the methanol in adsorber 2 is desorbed. As the adsorber 2, condenser 2 and evaporator 2 are in the same chamber and they are all connected,

Temperatures profiles of the working fluids

Fig. 4 shows the temperature histories of the inlets and outlets of hot water, cooling water, and chilled water:

  • (1)

    Hot water:

The volume flow rate of chilled water is 2 m3 h−1. An average hot water inlet temperature Ti,hot can set by the boiler. At the beginning of the cycle, the hot water flows into the desorber which is in adsorption process in last cycle, To,hot decreases sharply because the metal and adsorbent need to be heated up. After a short time, To,hot increases. In mass recovery process,

Conclusions

A novel composite adsorbent-methanol chiller has been tested. Based on the experiments, these conclusions are achieved:

  • (1)

    The new composite adsorbent, which is dealt with to eliminate the corrosion effect, can be applied in the chiller and the adsorption performance is improved. It enables the usage of methanol in the adsorption chiller. When the temperatures of hot water inlet, chilled water outlet, cooling water inlet are 84.8 °C, 15.3 °C, 29.8 °C, heating/cooling time is 680 s, mass recovery

Acknowledgments

This work was supported by the Key project of the Natural Science Foundation of China under the contract No.51020105010.

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