Hydration of magnesium carbonate in a thermal energy storage process and its heating application design

First ideas of an application design using magnesium (hydro) carbonates mixed with silica gel for day/night and seasonal thermal energy storage are presented. The application implies using solar (or another) heat source for heating up the thermal energy storage (dehydration) unit during daytime or summertime, of which energy can be discharged during night-time or winter and used to increase the temperature of the heat source side of a heat pump, increasing the COP. The application can be used in small houses, or at the user end of district heating systems. Experimental data are presented, determining and analysing kinetics and operating temperatures for the applications. In this paper the focus is on the hydration part of the process, which is the more challenging part, considering conversion and kinetics. The excess water is stored in a tank separate from the solid material storage reactor, from where the hydration water vapour is evaporated. Various operating temperatures for both the reactor and the water tank are tested and the favourable temperatures are presented and discussed. The thermal energy storage system with mixed nesquehonite (NQ) and silica gel (SG) can use both low (25-50%) and high (75%) relative humidity (RH) for hydration. The hydration at 40% RH gives a thermal storage capacity of 0.32 MJ/kg and at 75% RH gives a capacity of 0.48 MJ/kg.