1.2 Performance of a solar domestic water heater
The temperature of the hot water inside a solar water heater at the end of a sunny
day depends on the available
solar radiation,
the
temperature of the water
supplied by the city network, the
ambient air temperature
and the temperature
of the existing water inside the storage tank at the beginning of the day.
It is noted that hot water must reach a temperature of at least 45
o
C to adequately
cover the needs of a household, without the use of the auxiliary heater.
•
Solar radiation:
The availability of solar radiation varies during the year.
It is high during summer, but can reach only 1/3 of summer values during
winter. It is noted that availability of solar radiation may also vary sometimes
significantly from year to year.
•
City water temperature:
This temperature is not steady during the year.
In winter, it is usually around 5
o
C - 10
o
C, while in summer it may exceed 25
o
C.
Therefore, in winter, the temperature of city water must be raised by 35
o
C
- 40
o
C, but in summer only by 15
o
C -20
o
C.
•
Ambient air temperature:
Thermal losses of a solar water heater depend
on outdoor air temperatures. The SUN&HEAT solar water heater that you
have bought is well-insulated to minimize heat losses.
When hot water from the solar heater is used in the afternoon of a sunny day,
its temperature remains steady for quite a long time. The temperature of the hot
water during usage remains steady until a large percentage of the total volume of
the water in the storage tank is consumed. This percentage also depends on the
season of the year.
When the storage tank is full of cold water from the city network on a summer
morning, the temperature of hot water at the end of the day is expected to reach
approximately 55
o
C - 65
o
C. Respectively, on a sunny winter day, the water will
reach a temperature of 35
o
C - 40
o
C.
These temperatures, at end of the day, depend on the city water temperature. The
higher they are (as in southern locations) the higher the hot water temperature.
Detailed data exist in the test report, based on relevant standards (EN 12976-2,
ISO 9459-2), available by SUN&HEAT.
Considering two cities of the same country, one in the north and one in the south,
the city water of the northern city is usually colder. Under these conditions, the
domestic hot water system in the south produces hotter water and it seems to be
more effective. Both solar domestic systems might deliver the same amount of
energy from the sun to the domestic water.
