Thermodynamics of Phase-Change Storage in Series With a Heat Engine

This paper addresses thermodynamics of phase-change storage elements in series with heat engines. It is assumed that the duration of the heat storage and the discharge are equal. It is also assumed that the same heat transfer fluid (HTF) with a constant flow rate is used for the whole cycle. The major constraint imposed on these systems is the stability of the temperature of the HTF supplied to the engine during the storage-discharge cycle. It is shown, for this setup, that the freezing point of the phase-change material (PCM) is defined by the First Law. Maximal stability corresponds to the freezing point equal to the arithmetic mean of the inlet temperatures of the hot and the cold streams. An analytic expression is developed for the Second Law efficiency of the heat storage-removal cycle for the phase-change element in series with an engine. It yields maximal entropy production in the absolute stability limit. Two analytically tractable models of a phase-change storage in series with a heat engine are studied in detail. One involves a PCM slab, and the second involves a PCM tube-and-shell heat exchanger.