Abstract
A numerical study was carried out to investigate charging and discharging processes of different phase change materials (PCMs) used for thermal storage in an innovative solar collector, targeting domestic hot water (DHW) requirements. The aim was to study PCMs that meet all application requirements, considering their thermal performance in terms of stored and retrieved energy, outlet temperatures, and water flow rate. Work was carried out for three flat-plate solar panels of different sizes. For each panel, a PCM tank with a heat exchanger was attached on the back plate. Simulations were conducted on a 2D domain using the enthalpy–porosity technique. Three paraffin-based PCMs were studied, two (A53, P53) with phase-change temperatures of approximately 53 °C and one of approximately 58 °C (A58H). Results showed that, during charging, A58H can store the most energy and A53 the least (12.30 kWh and 10.54 kWh, respectively, for the biggest unit). However, the biggest unit, A58H, takes the most time to be fully charged, i.e., 6.43 h for the fastest feed rate, while the A53 unit charges the fastest, at 4.25 h. The behavior of P53 lies in between A53 and A58H, considering stored energy and charging time. During discharging, all PCMs could provide an adequate DHW amount, even in the worst case, that is, a small unit with a high hot water consumption rate. The A58H unit provides hot water above 40 °C for 10 min, P53 for 11 min, and A53 for 12 min. The DHW production duration increased if a bigger unit was used or if the consumption rate was lower.
Highlights
Fossil fuel depletion and environmental pollution have shifted the production of energy towards renewable energy sources (RES)
Among the three phase change materials (PCMs) studied, A53 and P53 required less time for charging than A58H, which is desirable, considering the short intervals when high intensity solar radiation is available during a day
The larger the heat exchanger (HE) size, the more time is needed for a full charging, independently of the PCM type or the HTF flow rate, but this relationship is not linear as it will be discussed in the discharging section
Summary
Fossil fuel depletion and environmental pollution have shifted the production of energy towards renewable energy sources (RES). Khan have experimentally investigated a solar thermal storage unit with a shell-and-tube heat exchanger, with or without longitudinal fins They concluded that the charging or discharging duration is significantly shorter with the fin configuration [24]. It is a compact solution that consists of a commercial flat-plate solar collector and a thermal energy storage tank that contains a high efficiency heat exchanger (HE) immersed into the PCM. One of the issues that should be taken into account when developing a costeffective thermal energy storage solution is the PCM selection In this way, the study of the different PCMs contributed to the design of the system, as it assisted in understanding the phenomena taking place in the innovative collector when using them. Three different Solar Kit units were considered for simulations, based on the commercial solar collector’s gross area, as indicated by the manufacturer DIMAS SA [30]
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