Stable minor hysteresis loops in interrupted solid–liquid transitions of a composite phase change material

Abstract

The existence and properties of minor hysteresis loops in solid–liquid transitions of a composite phase change material (PCM) are investigated. Experiments are carried out on a cuboidal sample (60 × 50 × 50 mm) made from graphite and a commercial organic PCM. Time-varying temperature boundary conditions are imposed on two opposite faces of the cuboid using heating rates between ±0.0625 K/min and ±0.25 K/min. Temperature is measured at five locations in the direction of heat flow. The phase change is found to be non-isothermal. Thermal hysteresis is also found, expressed by shifts in the temperature ranges in which melting and solidification occur. Melting is observed between 40 ∘C and 44 ∘C, solidification is observed between 33 ∘C and 43 ∘C. Four experiments are carried out with periodic heating/cooling, where the temperatures do not sweep through the entire phase transition temperature range. In these experiments the melting and solidification processes are interrupted. It is found that the hysteresis is reduced but does not disappear. Instead the temperature shifts and the absorbed and released heat are reduced. A rate-independent hysteresis model is proposed to describe this behaviour. It implements (down)scaling of the major hysteresis loop and forms stable and closed minor hysteresis loops after sufficient heating/cooling cycles. In comparison to models that neglect hysteresis, the model reduces deviations between measured and simulated temperature profiles by 31% to 54%. The model can be useful for analysing the cycle performance of thermal storage technologies operating with complete and interrupted phase change.