High Temperature Thermal Energy Storage Utilizing Metallic Phase Change Materials and Metallic Heat Transfer Fluids

Cost and volume savings are some of the advantages offered by the use of latent heat thermal energy storage (TES). Metallic phase change materials (PCMs) have high thermal conductivity, which relate to high charging and discharging rates in TES system, and can operate at temperatures exceeding 560 degrees C. In the study, a eutectic aluminium-silicon alloy, AlSi12, is identified as a good potential PCM. AlSi12 has a melting temperature of 577 degrees C, which is above the working temperature of regular heat transfer fluids (HTFs). The eutectic sodium-potassium alloy (NaK) is identified as an ideal HTF in a storage system that uses metallic PCMs. A concept is presented that integrates the TES-unit and steam generator into one unit. As NaK is highly reactive with water, the inherently high thermal conductivity of AlSi12 is utilized in order to create a safe concept. As a proof of concept, a steam power-generating cycle was considered that is especially suited for a TES using AlSi12 as PCM. The plant was designed to deliver 100 MW with 15 h of storage. Thermodynamic and heat transfer analysis showed that the concept is viable. The analysis indicated that the cost of the AlSi12 storage material is 14.7 US$ per kWh of thermal energy storage.