THERMODYNAMICS OF PHASE-CHANGE ENERGY-STORAGE - THE EFFECTS OF LIQUID SUPERHEATING DURING MELTING, AND IRREVERSIBILITY DURING SOLIDIFICATION

This paper considers the question of whether the optimum phase-change temperature for maximum exergy storage is universally equal to the geometric mean of the heat source and environment temperature, T(m) = (T(infinity)T(e))1/2. The study consists of three parts. The first deals with the conduction-melting process, and shows that the optimum melting temperature is generally greater than the geometric mean of the source and environment temperatures. The second part covers the conduction-solidification process, and concludes that the irreversibility of solidification decreases monotonically as the phase-change temperature increases. The third part treats the complete cycle of melting (storage) followed by solidification (retrieval), and demonstrates that the optimum phase-change temperature is greater than the optimum temperature of the melting process alone.