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Performance
The output of the solar water will depend on the amount of solar irradiation that reaches the collector
throughout the day. On a good day, with 6-7 hours of sunlight, the glass tubes will collect enough
energy to heat the water up to 60ºC - 70ºC. You may even spot steam coming from the vent pipe as the
water reaches boiling point. This is quite normal if you have not been using water from the system. By
setting the tilt less than the latitude the system will work better in summer and using a tilt of more
than latitude will improve the performance in winter.
By facing the system a few degrees east of true north the water will heat earlier in the day and,
similarly, a little more west of north will mean the water will heat later in the day.
Shown below is the average solar irradiance for various cities in South Africa for collectors positioned
at true north and a tilt equal to the latitude:
Solar Irradiance MJ/m²/day (figures from NASA 22 year average)
Area
Johannesburg
Cape Town
Durban
Port Elizabeth
East London
Bloemfontein
Based on the above average solar irradiance, the average daily temperature rise of the thermal storage
water would be as shown in the table below. Note that these are average figures and, on any one day,
depending on the cloud cover and temperature, different results will be obtained:
Expected Average Temperature Rise of water in the Storage Tank from Solar Only
Area
Johannesburg
Cape Town
Durban
Port Elizabeth
East London
Bloemfontein
Note that one needs to consider the amount of hot water drawn from the tank each day (the energy
used) and the residual temperature each day to determine what the actual storage tank temperature
will be, and what the total available hot water from solar heating only will be.
Based on at least this hot water input being used every day, in other words, that tank is in
equilibrium, without boiling, or running out of hot water (the balance coming from electrical heating
if this is required), the average daily saving in electricity is shown in the table below:
11
Jan
Feb
Mar
Apr
21.5
21.0
20.7
20.5
26.2
25.6
23.5
19.9
18.6
19.1
19.0
19.1
22.1
20.9
19.4
18.1
21.0
20.3
19.2
18.6
23.9
22.8
21.6
29.8
Jan
Feb
Mar
Apr
17.0
16.6
16.3
16.2
20.5
20.0
18.4
15.8
14.8
15.1
15.0
15.1
17.4
16.5
15.4
14.4
16.6
16.1
15.2
14.7
18.8
17.9
17.0
16.4
Copyright © 2013 - 2016 Enervision. All Rights Reserved.
Issue 2.0 Mar 2013
Vision Solar Systems - 100 L Gravity Fed Type
May
Jun
Jul
Aug
20.6
20.3
21.5
23.1
16.0
15.0
15.5
17.2
18.6
18.1
18.8
19.8
16.7
15.9
16.9
17.9
17.9
17.0
17.6
19.1
20.1
19.4
20.4
22.9
May
Jun
Jul
Aug
16.3
16.1
16.9
18.7
12.8
12.0
12.4
13.7
14.7
14.4
14.9
15.6
13.3
12.7
13.5
14.2
14.3
13.5
14.0
15.1
15.9
15.3
16.1
18.0
Sep
Oct
Nov
Dec
23.5
21.3
21.4
21.5
20.5
23.4
25.2
25.6
19.2
17.3
17.7
18.4
19.4
19.7
21.0
22.1
20.2
19.4
20.3
21.3
23.3
23.0
23.4
24.2
Sep
Oct
Nov
Dec
18.5
16.8
16.8
16.9
16.2
18.3
19.7
20.0
15.2
13.8
14.1
14.6
15.3
15.5
16.6
17.4
15.9
15.3
16.0
16.8
18.3
18.1
18.4
18.9
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