Properties of Granular Materials as Heat Transfer and Storage Medium for CSP Plants with Thermal Energy Storage and Moving Bed Heat Exchanger

Kurzfassung

The dispatchability of Concentrating Solar Power (CSP) plants and a further reduction of levelized electricity costs are strong incentives for the development of cost-efficient technical solutions for thermal storage systems. A heat storage system using movable fine-grained particles as storage material is a technology option that offers novel low-cost solutions. To deploy such a storage concept, the application of a Moving Bed Heat Exchanger is likely the most efficient way to recover the stored heat from the particles and to run a Rankine steam cycle. The granular inventory has to meet solar-specific requirements. Also, the materials’ properties and their long-term behavior provide important information on the optimal design and operation of the storage/heat-exchanger system. To that end, granular natural and ceramic materials have been experimentally characterized with regard to their application as sensible heat transfer and storage media in a dynamic high temperature operation. Those materials potentially offer high heat capacities and high operation temperatures. Thermo-physical, thermo-mechanical, tribological and rheological measurements have been conducted. Thermal bulk conductivity is found to be only marginally affected by the solid’s conductivity. Specific heat is almost the same for all solids. Ceramics entirely withstand thermal cycling, while quartz sand is prone to severe degradation. Most materials are found to attain a saturated state of attrition when they are sheared under mechanical load, where quartz sand offers the lowest mass fraction of debris at saturation level. In the grain size range regarded here, all materials show an excellent flowability. Nevertheless, the presence of debris affects the flowability and thus requires consideration in the design of the device. Concluding, ceramic materials are found to have advantages, but natural products offer a considerably more economical solution.