Energy storage through adsorption and desorption of water vapour in raw Saudi bentonite

Adsorption/desorption of water vapour on raw Saudi bentonite (RB) is proposed as a heat energy storage. This is most readily achieved by adsorption and desorption of water vapour on RB at different temperatures as a function of time. The RB subjected to preheating temperature of 200 °C, before subjecting to the adsorption process carried out. The IR spectra of RB before adsorption of water vapour at 298 and 313 K were studied. The adsorbed and desorbed water vapour from bentonite surfaces at 298 and 313 K was determined at different time. The adsorptive capacities of RB sample at 298 and 313 K were 0.0097 and 0.0141 mol/g of dry RB, respectively, after 72 h. The desorbed amounts are 0.0085 and 0.01 mol H2O/g of RB at 298 and 313 K, respectively after 72 h. A kinetic models of second order of the adsorption and desorption of water vapour fitted well the experimental data. Application of Van't Hoff's law at two temperatures (298 and 313 K) yields the adsorption and desorption enthalpy. The adsorption enthalpy (stored energy) of RB increased with increasing contact time up to 5 h. At this time the maximum enthalpy was about 30 kJ/g dry bentonite, at which the clay has lost all the energy that could be released due to adsorption of water vapours. Then it shows a decrease in sorption energy when the time increases. On the other hand, the desorption enthalpy increases gradually with the increase of the time up to 72 h then become constant, maximum enthalpy was 14.99 kJ/g. The rate of water vapour adsorption was found to be very high so that the extracted energy from the bentonite surface would not be a problem in any practical utilization of this system.