Long-term modeling of the performance of a solar still with phase-change material

Solar stills may be a small-scale and eco-friendly alternative to conventional, large-scale, and fossil-fuel-powered desalination plants due to growing worldwide water scarcity. However, solar stills suffer from low productivity which can be increased by optimizing the water mass and phase-change material (PCM) mass. Therefore, this paper aims at investigating the influence of water mass and PCM mass on the long-term performance of a solar still (SS) located in Poland. The long-term performance of SS is an important issue, as people need desalinated water all year round. Simultaneously, there is a limited number of publications concerning this issue in the open literature as most papers focus on the SS performance in a short-term period, usually no more than a couple of days. Therefore, research on the effect of water and PCM mass on SS performance in the context of long-term operation under various weather conditions, which is done in this work, is valuable and necessary. Additionally, this is the first time when the SS is investigated under Polish weather conditions. The study is carried out by conducting a theoretical analysis based on the mathematical modeling of the solar still. The mathematical model describing the thermal processes in the SS is established and validated using the experimental results. The results of simulations for average days for each month show that the maximum productivity of the SS is in July (3039 mL/m2/day with 5 kg of water), while the lowest is in October (900 mL/m2/day). The greatest annual SS productivity is 488.8 L/m2/year in Poland and is achieved with 5 kg of water and without PCM. The daily productivity is inversely proportional to the mass of water and the mass of PCM. The PCM reduces productivity by up to 11%. The results of daily simulations from April to October 2021 show that the daily productivity of the SS in Poland varies from 0.2 L/m2/day to 5.2 L/m2/day. The productivity of the SS investigated in this study in Poland is comparable to the productivity of solar stills investigated in countries with similar daily solar insolation and ambient air temperature.