Experimental Study on Utilizing Silica Gel with Ethanol and Water for Adsorption Heat Storage

Adsorption heat storage is the most feasible technology for heating decarbonization, which can store large quantities of waste and renewable heat for an exceptionally long time. However, utilizing adsorption heat storage in geographical locations with sub-zero ambient conditions is challenging. Therefore, this paper experimentally investigates the use of ethanol as a working fluid paired with silica gel for adsorption heat storage and utilizes sub-zero ambient as the heat source. The heat storage characteristics, heat charging/discharging cyclic performance, and energy conversion performance via exergy analysis were determined under realistic operating conditions and benchmarked against the widely investigated silica gel/water. Ethanol adsorbate was successfully utilized as a working fluid to employ the evaporators operating under sub-zero ambient conditions. Silica gel/ethanol showed the most significant net cyclic uptake, twice that of silica gel/water. However, the physical characteristics of ethanol molecules led to a degree of non-desorbed fluid, which hampered such potential to store 18.08 kJ/kgads under a sub-zero evaporator temperature and 24.84 kJ/kgads for an above-zero evaporator temperature compared to silica gel of 155.12 kJ/kgads operating an above-zero evaporator temperature. On the other hand, silica gel/ethanol showed the fastest heat charging/discharging rate that can shorten the cycle time by 45%. The major contributor to exergy destruction was the exergy transferred by charging heat, which was five times the discharging heat due to the high charging temperature.