An experimental comparison of thermal energy storage in directly and indirectly radiated adsorbent beds integrated with solar thermal collectors

Adsorbents heated using solar energy can be used to achieve thermal energy storage and sorption refrigeration with low environmental impacts. This research compares two different methods of heating adsorbents with solar energy to store thermal energy: (1) by exposing the adsorbents to incident light transmitted through a solar collector window, and (2) by heating a highly absorbing solar collector cover, and then transferring the heat from this solar absorber to adsorbents located beneath it. To carry out this comparison experiments are conducted for three cases of adsorbent beds using zeolite 13X and water as the adsorbent-adsorbate pair. In the first case, the top of the adsorbent bed is a polycarbonate sheet, and the zeolites are heated directly by solar-simulated light transmitted through this sheet. In the second case, a blackened aluminum sheet is placed beneath the polycarbonate sheet to generate heat by absorbing incident light. For the third case, the blackened aluminum absorber is placed directly on top of the zeolite beads and the absorber is isolated from the walls of the reactor to avoid heat losses. The outcomes reveal an energy storage density (ESD) of 43.6 kWh/m3 (63.4 Wh/kg) when light is directly incident onto the zeolite 13X and an ESD of 33.3 kWh/m3 (48.4 Wh/kg) when light is incident onto a blackened absorber plate that transfers heat to Zeolite beads residing beneath it. However, ESD values were improved to 48.9 kWh/m3 (71.0 Wh/kg) when the blackened absorber plate was thermally insulated from the walls of the adsorbent bed. These results demonstrate the importance of an optimal absorber arrangement in enhancing the adsorption process for the purpose of elevating energy storage densities.