(698c) Effects of Operating Conditions for a Silica Gel/Water Vapour Adsorption Thermal Energy Storage System

Thermal energy storage technology can be used to further integrate renewable energy sources into the current global energy portfolio to reduce/eliminate the energy supply and demand mismatch. However, conventional thermal energy storage processes often require toxic materials or occupy large volumes to store useful amounts of energy. Therefore, thermochemical energy storage systems, which are potentially less hazardous and more compact than conventional thermal storage techniques, should be studied and developed further.

In this study, a water-adsorption based thermochemical energy system is analyzed. Thermal energy is stored when water vapour desorbs from the surface of the adsorbent by heating the material and/or using a purge gas. The energy is then released by exposing the adsorbent to humid air. The adsorbent used in this study was silica gel, due to its favourable water-adsorption properties, commercial availability, and cyclical stability. Thermocouples and relative humidity probes were used to measure the temperature and humidity at the inlet and outlet of the column. This, along with the measurement of flow rate allowed the calculation of the amount of energy released based on the temperature change from the inlet to the outlet of the column.

Various operating parameters like particle size, flow rate, desorption temperature, and relative humidity were manipulated in order to observe their effects on system energy storage performance variables like energy storage density, thermal power, and temperature lifts. The effects of these operating parameters are quantified and discussed herein, and the energy storage system was subsequently optimized.

Particle size in the tested range appeared to have minimal impact on performance. Increasing the desorption temperature had positive effects until approximately 120°C, beyond which significant increases in performance were not observed. Higher relative humidity levels during adsorption were shown to drastically improve system performance, due to the greater water-adsorption capacity of silica gel at high humidity levels. Finally, the optimal flow rate was found to be around 20-24 SLPM. An optimum was likely observed due to the competing factors of pressure, residence time, and mass transfer, which are all affected by the flow rate. Ultimately, it was found that the system performed the best when the silica gel was desorbed at 120°C, the flow rate during adsorption was 24 SLPM and the relative humidity during adsorption was 90%. Under these conditions, an energy storage density of 200.7 kWh/m³_, a maximum temperature lift of 28.5°C and a maximum thermal power of 245.2 W/L was observed.