Design and Evaluation of a High Temperature Phase Change Material Carnot Battery

In the current study, a high temperature thermal storage system with a hybrid of phase change material and graphite as the storage materials is designed and evaluated as to its applicability for use as a utility-scale Carnot battery. The design includes an externally heated liquid sodium tank, which is used as the heat transfer fluid. This is used to charge and discharge the storage system consisting of a graphite storage medium sandwiched by two phase change materials. Finally, electrical generation is by way of a supercritical carbon dioxide Brayton cycle operated at 700 °C. Detailed modelling of these designs was conducted by way of a previously validated numerical model to predict performance metrics. Using the aforementioned designs, a preliminary cost estimate was undertaken to better determine applicability. From these results, it was found that while the graphite system was the most effective at storing energy, it was also the highest cost due to the high cost of graphite. In total, 18 storage tanks containing nearly 17,400 tons of storage material were required to store the 1200 MWht required to run the sCO2 power block for 10 h. Under the study conditions, the cost of a PCM-based Carnot battery was estimated to be $476/kWhe, comparable to other storage technologies. Furthermore, it was found that if the cost of the graphite and/or steel could be reduced, the cost of the system could be reduced to $321/kWhe.