The evacuated tube solar water heaters are widely in use today. But so far their thermal performance has not been systematically evaluated, despite the increased popularity. An experimental solar hot water system including two collector types, namely water-in-glass and heat-pipe, of evacuated tube solar water heater has been set up. Their capacity is typical for serving single family domestic hot water system. Performance tests were carried out in Hong Kong and under almost identical boundary conditions by placing the two collectors side-by-side. The energy performance has been analysed and the daily efficiency, transient efficiency and night-time heat loss were compared. While the energy acquisition performance of the heat-pipe collector design is found slightly better, the heat loss performance is found weaker than the water-in-glass collector design.
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