Performance evaluation of evacuated tube solar domestic hot water systems in Hong Kong

Evacuated tube solar water heaters are increasingly in use in Hong Kong because of their good thermal efficiency and high water temperature achievable as compared to the flat-plate solar water heaters. But so far their thermal performance has not been systematically evaluated and therefore not well known to the users. This paper reports our experimental and numerical works on evaluating the performance of the two common types of evacuated tube solar water heaters for domestic hot-water applications. These are the single-phase open thermosyphon system and the two-phase closed thermosyphon system. Our results show that the daily and annual thermal performance of the two-phase closed thermosyphon solar collector is slightly better than the single-phase open thermosyphon design. But the payback periods of the two are relatively the same because of the higher initial costs of the two-phase closed thermosyphon collector system. Although economically they are less attractive than the flat-plate type collector system, they are suitable for applications in advanced systems with higher temperature demands.