Innovative methods for optimized design and state-of-charge control of latent heat thermal energy storage systems

The Pillow Plate (PP) geometry for heat exchanger applied to latent heat thermal energy storage (LHTES) is rapidly gaining interest in the scientific community. Thanks to the swelled shape and manufacturing simplicity it can enhance the heat transfer performance, guarantee high mechanical resistance and ensure low production costs with respect to other heat exchanger geometries. Abundant and relevant studies investigated the thermo-fluid dynamic characteristics of PP heat exchangers (HEXs) but very few studies concentrated in understanding PP technical and economic potential when applied to LHTES. This thesis aims at developing an innovative procedure for the optimal design of PP-LHTES. The methodology consists in the development of two models: a 1D analytical discretized stationary model, called the design model, and a 1D analytical discretized dynamic model, called the dynamic model. The former is be used to determine the design parameters and costs, while the latter is used to validate the designed system under dynamic conditions. The two models are validated against relevant experimental studies taken from literature and show good performances with errors in the order of 10% for the design model and of 2% for the dynamic model. Moreover, 27 combinations of systems which differ for PCM, temperature level and heat transfer fluid (HTF) temperature difference across the HEX are analyzed using the design model in order to investigate the design space and economic potential of PP-LHTES for industrial applications, specifically for energy values in the range 5 to 25 MWh and power values in the range 1 to 5 MW. The result is be presented in the form of design and cost maps. An exemplar case study is developed, starting from a reference case from literature, to demonstrate the potential of the developed methodology, showing how the input parameters should be collected and how the output of the two models should be elaborated to obtain the final design. This innovative methodology resulted to be very robust and reliable in multiple conditions, from very small to very large systems and the dynamic model is able to correctly capture the dynamic nature of PP-LHTES. Finally, PP-LHTES represents an interesting and valid solution for medium-large industrial applications with an energy capacity cost varying in the range 30 – 90 $/kWh.