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In the current study pre-formed geopolymer half shells were fabricated from a combination of fly ash and black slag. Five (5) geopolymer shells were filled with a chloride-based phase change material (PCM) with a melting point of 540 °C for use in a high temperature thermal energy storage system (TESS). Additionally, three (3) of these shells contained an inner coating of alumina (various manufacturers) while another geopolymer shell was pre-fired at 610 °C. The half shells were cycled around the melting point of the PCM (500–580 °C) for five (5) cycles, with the mass change of the samples recorded. Of the five (5) samples only the Aremco-coated sample suffered less PCM loss than the control. Furthermore, the melting point and latent heat of PCM samples from the geopolymer half shells were measured by differential scanning calorimetry (DSC) to find if degradation occurs. In all samples the PCM melting point was not affected while the latent heat was significantly affected. Additionally, the Aremco-coated sample resulted in PCM segregation. Refereed/Peer-reviewed

The search for optimal phase change materials (PCMs) for latent heat thermal energy storage systems (LHTESS) focuses almost exclusively on the properties of the PCM. This neglects the significant contribution of the cost of the containment vessel on the total cost of the LHTESS. Thus, to accurately assess the thermoeconomic performance of various PCMs, the relationship between the cost of the containment vessel and the properties of the PCM must be understood. This paper presents an analytical method for optimizing the design of tube and shell or tube and fin containment vessels for the least cost, subject to geometric and performance constraints. One of the key performance constraints is the time to charge, whose evaluation typically requires computationally-expensive simulations which are unsuitable for optimization algorithms. To enable efficient optimization, a novel closed-form approximation of the charge time is developed and validated through numerical simulations. Subsequently, the optimization methodology is used to investigate the relationship between the optimal vessel geometry and configuration, fin properties, PCM properties, and cost of two PCMs. For these PCMs, the use of high thermal conductivity fins is shown to dramatically reduce the total cost of the system. The new methodology is an efficient way to compare the thermoeconomic performance of PCMs as latent heat storage solutions, allowing for the accurate assessment of PCMs. Keywords PCM heat exchanger design; Latent heat thermal energy storage system optimization; Phase change material; Thermal energy storage

According to the SunShot initiative, one sixth of the levelized cost of electricity for Concentrated Solar Thermal Power is thermal energy storage. For this power generation paradigm to be successful, the cost of every sub-system must be dramatically reduced. However, the search space for possible storage mediums is too large for a brute force experimental search to be feasible. Thus, a more refined approach is necessary. In this paper, eutectic salt combinations are considered as storage medium. The state of the selection process for these eutectics is discussed. Various methods to predict the important thermophysical properties are reported and applied to eutectics whose physical properties are known. Based on single salt properties, the density of molten salt eutectics can be predicted, around their melting point, to within 5%. Prediction of the melting point and composition is accurate to within 7%. However, the estimation of latent heat for multi-component eutectics is not always accurate, and requires more work. Finally, the thermal conductivity of multi-component eutectics has not been well studied; further research is required to corroborate the predictions. Refereed/Peer-reviewed