Abstract

This study experimentally compares the thermal performance of two vertically oriented passive solar water heating systems under arid outdoor conditions in Gabes, Tunisia: a thermosiphon solar water heater (TSWH) and an integrated solar water heater (ISWH). Both prototypes were installed side by side, facing south, with identical collector areas and the same climatic exposure. Experiments were conducted over two consecutive clear-sky days under no-load and load conditions. The systems were evaluated in terms of water-temperature evolution, thermal stratification, thermal efficiency, heat-retention behaviour, overall heat-loss coefficient, and useful hot-water production. The results showed that the ISWH achieved higher peak water temperatures, reaching 59.8 °C and 57.5 °C during the two test days, compared with 48.55 °C and 53.65 °C for the TSWH. The ISWH also showed higher peak thermal efficiencies of approximately 49% and 48%, while the TSWH reached approximately 41% and 40%. Under hot-water extraction conditions, the ISWH delivered about 25 L of usable hot water at 45 °C, compared with 19 L for the TSWH. However, the TSWH exhibited better thermal retention, with a lower overall heat-loss coefficient of 1.763 W/m2K compared with 2.38 W/m2K for the ISWH. These findings demonstrate a clear trade-off between rapid daytime heat capture and non-solar heat preservation. The ISWH is more suitable for applications requiring higher daytime hot-water production, whereas the TSWH is preferable when improved heat retention after solar input decreases is required.

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