Performance of LiCl Impregnated Mesoporous Material Coating over Corrugated Heat Exchangers in a Solid Sorption Chiller

Hongzhi Liu, Katsunori Nagano, Junya Togawa
2018 Energies  
The composite material made by impregnating 40 wt. % lithium chloride (LiCl) into the mesopores of a kind of natural porous rock (Wakkanai Siliceous Shale: WSS) micropowders (short for "WSS + 40 wt. % LiCl") had been developed previously, and can be regenerated below 100 • C with a cooling coefficient of performance (COP) of approximately 0.3 when adopted as a sorbent in a sorption cooler. In this study, experiments have been carried out on an intermittent solid sorption chiller with the WSS +
more » ... ler with the WSS + 40 wt. % LiCl coating over two aluminum corrugated heat exchangers. Based on the experimental condition (regeneration temperature of 80 • C, condensation temperature of 30 • C in the desorption process; sorption temperature of 30 • C and evaporation temperature of 12 • C in the sorption process), the water sorption amount changes from 20 wt. % to 70 wt. % in one sorption cooling cycle. Moreover, a specific cooling power (SCP) of 86 W/kg, a volumetric specific cooling power (VSCP) of 42 W/dm 3 , and a specific sorption power of 170 W/kg can be achieved with a total sorption and desorption time of 20 min. The obtained cooling COP is approximately 0.16. Energies 2018, 11, 1565 2 of 15 the condenser to release heat. For the opposite process, cooling energy can be obtained in an evaporator because of water evaporation, and the evaporated water vapor binds to the sorbent during the sorption process [15] . Several novel porous solids with water as refrigerant are being researched in sorption heating and cooling systems: silica gel, FAM-01 and FAM-02 developed by Mitsubishi Plastics Ltd. (Tokyo, Japan), metal-organic frameworks (MOFs), and various composite sorbents [16] . Among these sorbents, composite sorbents are considered to be promising to enhance the efficiency of the sorption cooling and heating system by means of a target-oriented design of the sorbent specified for a particular cycle. Composites with two components can offer the opportunity for nano-tailoring the sorption properties by varying the confined salt chemical nature and content, porous structure of host matrix, and synthesis conditions [16, 17] . Two basic configurations for the composite sorbents-water working pairs were described in the literatures; namely, (1) packed bed sorbent contacting with the heat transfer surface, and (2) sorbent coating on the heat exchanger surface. For the packed bed sorbent cooling systems, Restuccia et al. [6, 18] developed an adsorptive cooling system with packed SWS-1L (mesoporous silica gel impregnated with CaCl 2 ) inside a high-efficiency heat exchanger. A mean SCP of 20-40 W/kg of adsorbent and a cooling COP of 0.4-0.6 were obtained when the T re was 95 • C, T con was 35 • C, and T ev was 10 • C. Freni et al. [19] tested the performance of a packed bed sorption chiller using SWS-8L (silica modified by Ca(NO 3 ) 2 ). At the operation condition of T ev = 15 • C, T con = 30 • C, and T re = 90-95 • C, the cooling COP increased from 0.28 to 0.41, while the SCP decreased from 389 W/kg to 190 W/kg when the cycle time changed from 8 min to 30 min. The performance of the packed bed sorption chiller using LiNO 3 impregnated into silica gel (SWS-9L: silica modified by lithium nitrate) was tested by optimizing of the relative duration of the isobaric adsorption and desorption stages [20] . When the sorption chiller was driven by a lower temperature heat (<90 • C) (T re was 75 • C, T con was 30 • C, and T ev was 10 • C), the cooling COP of 0.155 and SCP of 193 W/kg still remained reasonable. Though the above researched composites adopted in the sorption chillers showed relatively good performance, the main matrix is silica gel, which is a type of amorphous silica. It has been proven that the pores of silica gel increase in size when more than 5 wt. % chloride is impregnated into the pores [21], making it inappropriate for long-term use. Therefore, a new composite with high stability was developed by impregnating LiCl into pores of mesoporous WSS [22] . LiCl is usually used as the liquid desiccant [23] [24] [25] or liquid absorption chiller [26, 27] ; its higher exergetic efficiency and long-term stability [28] is thought to be suitable as the other main component of the composite sorbent for a sorption chiller. For the prototype of the sorbent packed sorption cooler built in our previous research [29] , the WSS + 40 wt. % LiCl can contribute to getting a cooling COP of 0.3 and SCP of 70 W/kg, when it was regenerated at a temperature as low as 80 • C; T con = 30 • C, and T ev = 10 • C with a cycle time of 30 min. This indicated that the sorption cooling system using WSS + 40 wt. % LiCl can be applied in low grade heat (solar thermal energy and industrial waste heat) utilization systems. However, heat and mass transfer improvement of the system is still needed to get a higher efficiency. When compared with the introduced pelletized bed, the coated heat exchanger ensures better heat transfer properties [12] , which can ensure good dynamics of the sorption/desorption cycle accompanying a shorter cycle duration. The coated sorbents can sorb water more efficiently [30] because of their faster sorption kinetics [31] , and thus can deliver higher SCP. Freni et al. improved the adsorptive cooling system with packed SWS-1L (mesoporous silica gel impregnated with CaCl 2 ) [6,18] by developing a sorption chiller based on a heat exchanger coated with a compact layer of SWS-1L. Higher SCP of 150-200 W/kg and a cooling COP ranging between 0.15 and 0.3 were obtained within a cycle time of 10-20 min when the SWS-1L was regenerated at 90-100 • C, T con = 35 • C, T sor = 15-20 • C, and T ev = 10 • C [12] . The interesting result of the coated heat exchanger obtained from the above research and our previous research on the sorption cooler has stimulated us to take a necessary step for the development of the WSS + 40 wt. % LiCl coated type heat exchanger installed sorption chiller. Consequently, in this study, an innovative intermittent sorption chiller with WSS sorbent coating over corrugated heat exchangers is presented and tested. The developed sorption chiller consists of
doi:10.3390/en11061565 fatcat:galfo4armbeljk4lauxjjqneei