PCM-Graphite Composites for High Temperature Thermal Energy Storage

Latent thermal energy storage systems use the heat absorbed during melting and released during solidification of phase change materials (PCMs). From an energy efficiency point of view, PCM storage systems have the advantage that they operate with a small temperature difference between charging and discharging of the storage. Also, these storage systems have high energy densities compared to sensible heat storages assuming such energy efficient operation. PCMs considered for temperatures between 100 and 300°C are mainly anhydrous salts. Applications include industrial process heat utilisation and solar power generation using direct steam technology. However, there are heat transfer limitations on the storage design due to the low thermal conductivity of the salts. Major approaches to overcome this inadequate heat transfer are the enlargement of the heat exchanger surface or the use of composite latent heat storage materials (CLHSM). This work selects the latter, where the properties of the high latent heat of the PCM and the good thermal conductivity of graphite are combined. As a PCM, this work uses the equimolar composition of potassium nitrate (KNO3) and sodium nitrate (NaNO3) with a melting temperature of about 220°C. There is a variety of potential preparation routes for CLHSM and these processes have a decisive impact on the interconnectivity of the graphite. Generally a highly interconnected graphite matrix is desirable in order to achieve a high effective thermal conductivity. We present in this paper the preparation of CLHSM from graphite and the eutectic KNO3-NaNO3 by the compression and the infiltration route using natural graphite flakes, ground expanded graphite particles and compressed expanded graphite plates. This results in composites with a different level of interconnectivity of the graphite. The composites are characterized in terms of their effective thermal conductivity as a function of temperature using the laser-flash method. The impact of thermal cycling on the effective thermal conductivity, segregation, form stability and expansion of the CLHSM is also discussed. The CLHSM show a considerable enhancement of the thermal conductivity compared with those of the single PCM

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

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