Thermal Energy Storage Using Zinc as Encapsulated Phase Change Material

Concentrating solar power is inherently intermittent and thus thermal energy storage is an essential component of a successful baseload solar power plant. Phase change materials (PCM) have the potential to decrease the cost of thermal energy storage systems for these plants since the latent heat contribution can be significant. The present work deals with certain aspects of using zinc as PCM for storing solar energy at high temperatures from 300°C to 500°C. The objective is the storage of hundreds of mega-watt-hours equivalent of solar energy in systems using zinc as encapsulated phase change materials (EPCM). Stainless steel and nickel have been considered as encapsulation materials for zinc. The present work describes several aspects of this technology that need consideration in designing the EPCM; such as thermal analysis, materials issues, proof of storage and retrieval of energy and cost analysis. EPCM of several sizes and shapes subjected to many cycles of energy storage/retrieval have been considered as part of the calorimetry tests here. Thanks to the large thermal conductivities of the metals involved, storing and retrieving energy into/out of the PCM is not an impediment. Potential interaction of PCM with encapsulation materials and their impact on storage capabilities are also discussed. Lastly the cost estimates ($/kWh_th) of these large thermal storage systems based on procedures used by NREL are presented. Though zinc based EPCM storage systems can be expensive at around $54/kWh_th, they are more economical than the current two-tank storage systems that use sensible heat only for thermal energy storage.