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.