Numerical Investigation of a U‐Tube Solar Collector Including Phase‐Change Material Enhanced by Metal Foam

ABSTRACT To improve the thermal performance and storage capacity of evacuated‐tube solar collectors (ETSCs), phase‐change materials (PCMs) have been widely integrated into ETSC systems. However, the low thermal conductivity of PCMs limits heat transfer within the storage domain. In this study, a three‐dimensional numerical model of a U‐tube ETSC integrated with PCM enhanced by copper metal foam (PCM–MF) is developed to investigate the influence of thermal conductivity enhancement on system performance. Three operating conditions based on the heat‐transfer fluid (HTF) flow are examined: continuous HTF circulation, a 1‐h zero‐flow charging period, and an intermittent operation mode that combines charging and discharging periods. The results show that under continuous HTF circulation, the PCM–MF configuration enhances heat transfer, increasing the thermal efficiency from 89.27% to 91.10%. However, this enhancement accelerates heat extraction from the PCM, suppressing melting and reducing the stored thermal energy, whereas a pure PCM stores approximately 39.7% more energy under the same conditions. In the absence of HTF flow (i.e., zero flow), the role of the MF shifts from heat extraction to heat distribution. The PCM–MF exhibits faster, more uniform melting, reducing melting time by about 10 min while maintaining storage performance comparable to that of pure PCM. Under intermittent HTF operation, in which a heating period without flow is followed by fluid circulation, the PCM–MF configuration shows improved performance compared with continuous‐flow conditions. The stored thermal energy of PCM–MF increases from 63.63 kJ under continuous flow to 74.6 kJ under intermittent operation, while the storage efficiency rises from 5.90% to 6.91%. However, the pure PCM configuration still exhibits a higher total energy storage capacity. These results demonstrate that the effect of MF is strongly dependent on the operating mode, with a trade‐off between heat‐transfer enhancement and thermal energy storage capacity in PCM–MF solar collectors.