Solar Water Heater Test

Solar Water Heater Electricity Saving Test

We decided to test how much electricity was saved by a solar water heater over a period of 33 days from 8 November 2010 to 12 December 2010.  The system tested was an Optimum Energy ST150 solar water heating kit retrofitted to a standard, newly installed Kwikot 150 litre geyser, in Johannesburg.

The ST150 solar water heating kit uses a flat plate solar collector to heat the water.  A Geyserwise Max differential controller controls a small 220V AC circulation pump that extracts colder water from the bottom of the geyser, pumps it through the solar panel, where it is heated, and then returns the heated water to the top of the geyser.   Whenever the solar panel water temperature is 7 degrees higher than the geyser water temperature, the Geyserwise Max controller switches on the circulation pump to circulate the water.  It switches the pump off again when the temperature difference falls to within 2 degrees.  So the pump switches on and off frequently on a hot day as the heated water is continually transferred into the geyser.

The Geyserwise Max controller also allows one to set times during which the electric element in the geyser becomes active.  During these periods, if the geyser water temperature is below the set point temperature, the controller will switch on the geyser’s electric element to heat the water to the set point temperature.  If the water is hotter than the set point, the electric element will not be switched on.  Most solar systems depend quite heavily on this “back-up” electric heating to ensure that water is hot enough after an overcast day, or if you use too much hot water during the night.  We set the Geyserwise Max to activate the element from 16:30 to 18:00 in the afternoon, and from 04:30 to 06:00 in the morning.  We selected a set point temperature of 55 degrees celsius.

Our test procedure was as follows:

We fitted a water meter to the cold water inlet of the geyser so we could establish precisely how much cold water flowed into the geyser during the period.  All cold water flowing into the geyser was heated to at least the set point temperature of 55 degrees (if heated by the electric element, or even higher if heated by the solar panel).

We also attached a data logging electricity meter to the power line feeding the system so that only the power consumed heating water, running the circulation pump and the Geyserwise differential controller was measured.

 We also noted what the weather was like on most days during the test period, how many showers were taken each day (hot water was used basically only for showering), as well as what the geyser temperatures were at the start, and end, of the solar heating period each day.

Test Data

The table below shows those notes.  We used the data from the data logger to establish whether the electric heating element was switched on in the morning and/or the afternoon on each day, which is also shown in the table.

Date (2010)	Weather	No. of Showers	Element on in the morning?	Element on in the afternoon?	Tank temp. at 09:00	Tank temp. at 16:00
8 Nov	Very windy but sunny	1	Yes			65
9 Nov	Overcast and raining	1		Yes		50
10 Nov	Cold, windy but sunny	2	Yes			68
11 Nov	Sunny	2	Yes			67
12 Nov	Sunny with some wind	2			43	70
13 Nov	Sunny and very hot	1	Yes		47	69
14 Nov	Sunny very hot, afternoon thunderstom	1			59	70
15 Nov	Completely overcast	2		Yes	39	42
16 Nov	Overcast and drizzling	1	Yes	Yes	40	50
17 Nov	Completely overcast	2	Yes	Yes	42	40
18 Nov	Overcast	2	Yes	Yes
19 Nov	Overcast with some sun (power failure in the morning)	1	Yes (twice)	Yes	43	54
20 Nov	Very hot and sunny	1	Yes		39
21 Nov	Very hot and sunny, with afternoon thunderstorm	0				74
22 Nov	Overcast	2		Yes	42	49
23 Nov	Partly cloudy, rain in afternoon	2	Yes		45	61
24 Nov	Overcast in morning, partlycloudy thereafter	1	Yes		41	60
25 Nov	Not recorded	2	Yes		52
26 Nov	Overcast, sunny late afternoon	1	Yes		45	55
27 Nov	Sunny	2	Yes		48	58
28 Nov	Overcast with rain	1	Yes	Yes	45	54
29 Nov	Sunny	2				70
30 Nov	Overcast in morning, some sun in afternoon	1	Yes	Yes	42	50
1 Dec	Overcast, sunny late afternoon	2	Yes		46	65
2 Dec	Mostly sunny	2	Yes		44	66
3 Dec	Mostly overcast, occassional sun	1	Yes	Yes	42	50
4 Dec	Partly cloudy	1	Yes			68
5 Dec	Sunny	1			42	67
6 Dec	Overcast in the morning, sunny thereafter	2			50	62
7 Dec	Sunny	1			45	67
8 Dec	Sunny	1	Yes		46	65
9 Dec	Sunny	1
10 Dec	Sunny, thunderstorm in afternoon	1				68
11 Dec	Sunny, thunderstorm in afternoon	2			42	65
12 Dec	Sunny	2		Yes	43	48

The results of the measurements taken were as follows:

Total kWh consumed during the test period	36.96kWh
Total water heated during the period	2010.3 litres
Number of showers during period	56

If we use the standard specific heat equation to calculate how much heat energy would be required to heat that amount of water from 19 degrees celsius (which is a good estimate of the average cold water temperature for the Highveld), to 55 degrees celsius, then we get 84.1kWh.  This is a reasonably accurate estimate of the amount of electrical energy that would have been used by an electric element in a geyser to heat that amount of water to 55 deg. C.  So if we compare the 36.96 kWh used by the solar system to the estimated electrical energy of 84.1kW required to heat the same amount of water using a conventional electric geyser alone, we get  a savings estimate of 56.1%.

In the table of notes above, one can see that on some days the electric element switched on in the morning, on others it switched on in the afternoon, and on some days when either little water was used, or when the sun was hot for a few days in a row, the element never switched on at all.  When it was really overcast on successive days, the element was switched on sometimes both in the morning and the afternoon on the same day.  The graphs below show examples of each situation.

The graph above shows a day when all the water heating was done by the solar panel alone.  You can see the power drawn by the circulation pump as it switches on and off.  As can be seen, the circulation pump uses very little power, which is why it is not visible on the scale of the other graphs above, in comparison to the much larger power consumed by the electric element on those other days.

Test Conclusions

The amount of cloud cover has a large effect on the effectiveness of a solar water heating system, as can be expected.  This means however that your monthly electricity bill will vary according to the weather, due to the large effect heating water using electricity usually has on most household electricity bills.

A solar water heating system will use electricity to heat water sometimes even on bright sunny days, depending on various factors such as; exactly what time of the day you use hot water, how much hot water you use, and how hot the water got the day before.

During the test period, which had a fair proportion of overcast days, we achieved an estimated saving of 56%, in comparison to heating water using an electric element alone.  However, if one studies the table of notes taken during the test, it will be clear that the savings achieveable using a solar water heating system will vary widely according to the weather.

A solar water heating system is capable of heating the water to quite high temperatures on a hot sunny day.  The highest geyser water temperature we recorded during this test was 74 deg. C on 21st Nov 2010.