PCM-GRAPHITE COMPOSITES FOR HIGH TEMPERATURE THERMAL ENERGY STORAGE

Storage systems based on Phase Change Materials (PCMs) with solid-liquid phase transition are considered to be an efficient alternative to sensible thermal storage systems. 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. Furthermore, these storages have high energy densities compared to sensible heat storages assuming such energy efficient operation. However, there are heat transfer limitations on the storage design due to the low thermal conductivity of PCMs. These limitations lead to a low charging and discharging power of the storage. Various methods have been proposed to enhance the heat transfer of the poorly conducting PCMs, such as additional fin structures or dispersed particles with a high thermal conductivity, as well as microencapsulated PCMs [Zalba 2003]. Another major approach is the use of composite latent heat storage materials (CLHSM). Here, the properties of a high latent heat of the PCM and the good thermal conductivity of an additive are combined. For low temperature applications, paraffin-graphite CLHSM have been reported [Py 2001, Marín, 2005]. Our ongoing research focuses on high temperature storage designs for industrial process heat utilization and solar power generation using direct steam technology in parabolic through plants. For both applications, the heat transfer fluid is water-steam. The concept is as follows: During the charging period, heat from the condensing steam is transferred to the melting PCM; during the discharging process, heat from the solidifying PCM is used to generat