Modelling of heat exchangers based on thermochemical material for solar heat storage systems

In solar thermal energy storage systems the operation modes involve charging and discharging. This paper focuses only on the charging leading to an endothermic reaction and therefore an efficient heat exchanger is required to transfer the heat for fast and complete charging. Two different heat exchangers are studied in this paper. A plate fin and helical coil heat exchangers embedded in a magnesium chloride bed is modelled and solved using the software Comsol 4.3a based on finite element method. Meshing analysis is performed for parameters sensibility and the results show a temperature variation of 13 °C (helical coil) and 19 °C (plate fin) in the material bed during the charging mode of the thermochemical heat storage system. The pressure distribution in the heat transfer fluid and the temperature distribution in the material bed are presented and the calculated overall heat transfer coefficient of 173 W/m2•K (helical coil) and 236 W/m2•K (plate-fin) are obtained on the base of the total heat transferred (Q) to through the system. The fluid flow is in turbulence regime (Re = 13200) in the fin-plate, but in laminar mode (can be kept up to Re = 20000) [1] in the coil because the flow is affected by secondary flow cause by centrifugal forces. This study allows the choice of the heat exchanger wherein with first experiment has been made and compared.