HEAT TRANSFER CHARACTERISTICS OF LATENT HEAT THERMAL ENERGY STORAGE

Abstract

Latent heat thermal energy storage (LHTES) systems can be used to reduce electric demand when used in conjunction with Combined Heat and Power Plants or HVAC(Heating, Ventilation, Refrigeration and Air-Conditioning), as they can regulate the demand and supply of thermal energy. They can also be used to integrate renewable energy sources with the grid. A design procedure and performance modeling is required for designing and using thermal energy storage systems effectively. We propose hypotheses about the performance of an LHTES device with different operating conditions and material properties, for devices that are governed by different modes of heat transfer. We study the validity of this hypotheses numerically and experimentally for two types of LHTES devices, namely those that are governed by conduction and those that are governed by convection. A rectangular conduction driven LHTES device consisting of a phase change composite made of Tetradecane and expanded graphite was studied under constant heat transfer rate conditions. The results show that the thermal resistance of the device increases linearly with the discharged state Θ. Thus, the inlet temperature must be changed in order to maintain a heat transfer rate into the device. For a convection driven LHTES, buoyancy driven melting has been studied experimentally in a rectangular LHTES device of aspect ratio 2 at Rayleigh numbers between 2.5×109 to 6.3×109 and Prandtl numbers between 19 and 24. Based on the device geometry and operating Ra, it is hypothesized that due to convective mixing, the dimensionless time τ taken to reach discharge state Θ should increase linearly with Θ, until the amount of solid PCM is small and does not control the nominal temperature difference. A similar behavior is expected for the liquid fraction η, which is analogous to Θ, but as a measure of latent energy absorbed to total latent energy. The value of melted fraction at which this happens is termed ηcritical, and from numerical results, is observed to be fixed for a given geometry and material with fixed viscosity. This is in contrast to a conduction driven LHTES, where the melting process causes the thermal resistance to increase linearly with Θ, thus, τ should increase non-linearly with Θ. Our numerical and experimental results are consistent with these hypotheses. In the convective LHTES, the calculation of η based on images shows a linear variation of η with dimensionless time for majority of the melting process. Thus, convective effects result in improved mixing of heat in LHTES and allow for sustained heat transfer over the discharging period, even when the discharging temperature is unchanged.