An analysis of sensible and latent heat storage units is performed in both energy and exergy terms. The stores are compared, through variation of design parameters for both charging and discharging in order to find exergy extrema, i.e. operating conditions that maximize the exergy yield in the transfer of heat to and from the stores. Reference is made to the stratified sensible heat store.
The overall conversion efficiency for the thermal exergy supplied by the heat source to the heat load via the heat store is of the order of 10% at NTU = 1. The stratified store is the most efficient at high NTU values.
For the charging operation there exists exergy extrema for both the well-mixed sensible store and the PCM store. An energy analysis of the charging of the PCM store shows that the transition temperature should be low. From an exergetic point of view the transition temperature should be the geometric mean of the charging and the initial temperatures, if the charging temperature is fixed. When the end temperature of a store in a changing operation is fixed the transition temperature should be low. Charging with a constant heating rate is advantageous if the transition temperature is low. No optimal stopping time can be recognized for the discharging operation. The merit of a high exergy content is that of a high thermal power during discharging. This favours a PCM store with a high transition temperature.