Latent Heat Storage Systems

Scheme of of a solarthermal power plant with integrated storage capacity . T-s diagramm power plant with integrated storage capacity

Latent heat storage system use the energy absorbed or released during the isothermal phase change of materials. A first selection of suited storage materials is based on the melting temperature which should be close to the saturation temperature which is dependent on the pressure of the steam. Since systems using direct steam generation are operated at pressures between 30 and 100bar the melting temperature of phase change materials (PCM) should be in the range of 230°C and 330°C. Due to cost aspects metallic materials like tin or lead can’t be used, instead salts like sodium nitrate or potassium nitrate are typical candidate materials. A direct contact between storage material and steam is not possible, so thermal energy has to be transferred within the storage material. Cost effective storage systems demand high internal heat fluxes, which depend mainly on the heat conductivity of the storage material. Non-metallic storage materials usually show low heat conductivities, especially the solid phase behaves like a thermal isolator. Essential for the development of cost effective storage systems is the increase of the effective heat conductivity in the storage material. DISTOR include research on three different concepts for PCM-storage:

• innovative composite materials made from PCM and materials showing a high thermal conductivity
• minimization of the average distance for heat transfer within the storage material, e.g. by encapsulation of the storage material
• application of an intermediate heat transfer medium for transfer of thermal energy between the pipes of the steam system and the storage material