Solar Power

Energy (Whr) received by the Solar Panel. Location Milton Keynes, England.  52N 0W

November 1999

November 1999

December 1999

December 1999

January 2000

January 2000

February 2000

February 2000

March 2000

April 2000


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 2C 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 40C but was wiped out by the cold water at 5C 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 > 50C per hr. Successful "firings" have slopes of 60 - 100 C per hr. The "bad negative power" day had a mere 2 C 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 15C, with water at 50C.

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 52N for a 1m2 flat surface.

Jan  Feb  Mar  Apr  May  Jun  Jul  Aug  Sep  Oct  Nov  Dec
MJ/m2 day 2.3 4.2 7 11.6 15 18 16 13 10 6 2.8 1.7
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

Jan  Feb  Mar  Apr  May  Jun  Jul  Aug  Sep  Oct  Nov  Dec
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 80C, with an outside temperature of 10C.  

Moorhouse Weather Station Data

Pitsford Hall Weather Station     (This is a weather station 30 miles north of here.) 

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