Energy (Whr) received by the Solar Panel. Location Milton Keynes, England. 52°N 0°W
These graphs show the useful power obtained from the 2m2 panel. They show power in Watt hours per day per metre squared. I'm trivially dividing the measured power by 2 to give a "gut feel" for comparison with published data that quotes in m2.
This "useful power" is the portion of power delivered to the hot water cylinder from the solar panel. The pipe-work wastes around 20% of the energy received from the panel - so I could be optimistic and include that. I haven't: I'm only showing the useful energy obtained.
Note that the controller doesn't run the pump if there is insufficient sunlight to maintain a 2ºC temperature across the heat exchanger. This accounts for days where there is no power recorded.
The 11th November recorded a negative power! 2Wh was extracted from the warm cylinder: The manifold had water at 40ºC but was wiped out by the cold water at 5ºC in the pipe work. The controller decided it was on a road to nowhere and stopped the pump after 2 minutes.
December the 4th enjoyed solid sunshine. (3.5kWh - 4.5p (11d)) Gas costs 1.3p per kWh.
Perhaps a design change: don't only look for a temperature difference between manifold and coil to turn on the pump. Look for a manifold temperature rise of > 50ºC per hr. Successful "firings" have slopes of 60 - 100 ºC per hr. The "bad negative power" day had a mere 2 ºC per hr. OTT? Should I be using a DSP for the controller?
20% pipe work waste energy was calculated on 12/11/99 when the attic was at 15°C, with water at 50°C.
As a general guide 4.4kWh from the panel per day is required for our hot water requirements. (i.e. more than 2.2kWh in the above graphs)
These figures are reported programmatically.
Published Figures for average day for each month at 52°N for a 1m2 flat surface.
|w hr day||638||1166||1944||3222||4166||5000||4444||3611||2777||1666||777||472|
(Source: notes from the CAT solar heated water course)
Measured Figures for average day for each complete month
|MJ/m2 day||2.75||4.0||5.7||6.86|| || || || || || ||2.81||1.74|
|whr day 2m2||1525||2259||3162||3812|| || || || || || ||1500||967|
|for 1m2||762||1129||1581||1906|| || || || || || ||750||483|
|after gain /loss of 96%||732||1084||1518||1827|| || || || || || ||720||463|
|"error"||+14%||-7%||-21%||-43%|| || || || || || ||-7%||-2%|
The figures for November and December are close to the published values. Take into consideration that a 30° tilt from the horizontal will give a 100% gain. Panel and pipe losses will counteract this with 20% pipe loss and 60% panel efficiency.
The "error" will change during the year as the sun elevation changes; also pipe losses will change as the ambient temperature changes.
March figures are showing increased inefficiency. Some of this can be attributed to no water being drawn off. (A trip away.) A couple of days the circulating water touched 80°C, with an outside temperature of 10°C.
Moorhouse Weather Station Data
Pitsford Hall Weather Station
(This is a weather station 30 miles north of here.)
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