The technical performance of a solar water heater depends on the water consumption profile over the day and the weather conditions.
According to the renewable energy study of EMSD, a solar water heating installation with a 4-m2 solar collector and storage tank capacity of 250 to 270 litres can produce about 8,200 MJ of thermal energy per year. The total solar radiation for the collector is about 18,980 MJ per year, hence giving an overall efficiency of 43% for the solar water heating installation, taking into account the auxiliary power consumption.
The result indicates that solar water heating systems can work effectively in Hong Kong and that significant energy savings can be achieved. Systems with smaller storage tanks and smaller solar collector areas will have slightly lower performance than the 250-litre system, as small tanks usually have higher loss per unit storage volume. Conversely, larger systems such as those for hotels and hospitals will be more efficient, as the storage tanks could be as big as 3000 liters in these facilities.
For design of solar water heating installations, an important design parameter is the daily mean horizontal solar irradiation for each month of the year. A typical set of figures for Hong Kong is given below. The figures were derived from the solar irradiation data gathered by Hong Kong Observatory's King's Park weather station over the past twenty years.
Monthly averaged daily horizontal solar irradiation