PCM-Graphite Latent Heat Storage Systems for Industrial Process Heat Recovery

Abstract

Increasing energy prices and shortage of fossil fuels lead to a growing interest in alternative energy sources. In combination with energy storage systems the generation of solar process heat can be provided independent from the weather leading for example to a cost efficient stabilization of power output. For this application latent heat storage units with phase change materials (PCMs) can be designed to store solar process heat within a narrow temperature interval utilizing the high storage density of the different PCMs. This is achieved using the latent heat of melting in the melting / solidification process, or the latent heat of re-crystallization in a solid / solid phase transition. However, this advantage can only be used in technical applications if the heat transfer in the PCM is sufficiently high. As most pure PCMs exhibit a low thermal conductivity (about 1 W/(m•K) or less), methods to improve heat transfer in PCMs have been under investigation for decades. The heat transfer in a PCM can be increased by addition of highly thermal conductive materials. Due to its superior properties - high thermal conductivity, good processability, and chemical inertness - graphite has distinct advantages for this purpose. Depending on the requirements of the respective application, various routes to combine PCM and graphite are used. For example, besides the fabrication of PCM/graphite composite materials, the increase of heat exchanger surface by highly thermal conductive graphite plates is a favorable method for large scale applications, in particular. Effective thermal conductivities up to 30 W/(m•K) have been realized. This paper gives an overview of actual and potential applications of PCM/graphite heat storage systems focusing on storage of solar heat for high temperature applications such as process heat generation and solar thermal power plants.