Design and performance analysis of a latent thermal storage system with erythritol for concentrated solar thermal systems

Several renewable thermal energy sources, such as solar energy, are characterized by being unstable and intermittent. To compensate for these adverse characteristics and increase the efficiency of the systems, the adoption of energy storage systems is necessary. An interesting solution is represented by latent heat thermal energy storage systems (LHTESs), which are used to store energy by means of phase change materials (PCMs). However, since the thermal conductivity of PCMs is small, a careful design of these systems is crucial and the choice of the geometry of the involved heat exchangers is of particular importance. In this study, erythritol is selected as the principal phase change material (PCM) to be employed in a bar-and-plate heat exchanger, which will be coupled with a parabolic trough solar concentrator installed on the rooftop of the Department of Industrial Engineering at the University of Padova. Erythritol (ET) has been chosen - at least in the first analysis - due to its favourable melting temperature, which ranges approximately between 389 K and 394 K, making it suitable for medium-temperature thermal energy storage (100 ÷ 250 °C). The melting range of the PCMs considered is appropriate for the operating conditions of the selected heat transfer fluid (HTF), which is pressurized water at 4 bar with working temperature between 127 °C and 139 °C. However, although erythritol offers positive features such as higher latent heat of fusion compared to other organic polyalcohol PCMs and the possibility of being produced with zero impact starting from biomass, these advantages do not eliminate its technical drawbacks, particularly the subcooling. To assess whether this undesired effect persists, and eventually intensifies, as the amount of PCM involved during discharge increases, and to check for any unexpected thermal degradation, the phase change temperature ranges of the ET from the literature are qualitatively verified through a dedicated experimental campaign. During the discharge phase, a subcooling degree of 15 K was detected while the phase change ranges were confirmed during the melting phase. In this study, the potential use of erythritol as a PCM for a thermal energy storage system coupled with a solar concentrator is evaluated through numerical simulations. The storage unit consists of a bar-and-plate heat exchanger in which the PCM fills an aluminium finned cavity, while water acts as the heat-transfer fluid. A dedicated MATLAB® numerical model is developed to predict erythritol charging and discharging times under various operating conditions. The model is validated against experimental data previously acquired using RT42, and a sensitivity analysis is performed to select appropriate temporal and spatial discretization while ensuring numerical stability and manageable computational effort. The simulations also support the optimal configuration of the experimental setup to be coupled with the solar concentrator, considering the intrinsic intermittency of the solar resource. For the current geometry, increasing the HTF volumetric flow rate from 100 L h⁻¹ to 350 L h⁻¹ and the temperature difference from 6 K to 18 K reduces melting times by up to 79% (from 10615 to 2225 s), showing that the most significant performance improvements occur at the highest flow rates and temperatures. Two possible solutions to mitigate subcooling are also considered in the simulations: (i) an optimized encapsulation strategy for micro-erythritol, and (ii) the use of alternative paraffin-based PCMs (RT111HC and RT125) with comparable operating temperatures. Finally, leveraging previous measurement campaigns conducted by the STET research group, a MATLAB® tool was developed for sizing a simple steel-tube absorber to be implemented in the future experimental rig.