Sunday, May 29, 2011

Sunbox curves to Degree Days and PV combined

29 May '11: I have been comparing the Sunboxes over the year with the PV and the Degree days.
   It's annoying that February is a month of 28 days sandwiched between two of 31, because that always caused a blip in curves.
   The PV is performing the same as a year ago, the Sunboxes are gathering more per month than a year ago.

The curve reveals that as we pass the Equinox, the PV and SB curves swap positions. That is for two main reasons,
• Between 1st October and and of April, the GSHP is providing some heating to the house, so the delta-T improves and the SBs work more. This is also the explanation supported by Chris Wood.
• As the sun angles go past the Equinox, the effect of the two hills south of the house come into play, reducing the sunshine hours in winter by delaying sunrise and advancing sunset. So the PV capture is reduced more than if we were on flat ground.

More reflections on these curves:
• The air temperatures are still relatively warm in the boundary months of October and April, improving their performance further in those two months, so May dips below April in both years.
• Colder air temperatures but high sun angles in May seem to be beneficial for the PV capture. Following that thought, it is alarming how much the PV performance drops off in July and August when the panel surface temperatures get so high.
• When the sun is really high in June, there is a lot of reflection off the front face of the panels, and the thermal capture is more helped by long hours of warm air temperature than by direct sunlight. At this time, it might be better to open the vents and let the air in, rather than let the black chillers cool the space.

Reflection on Geometry and Mirrors:
• I wish I had not been so neat and tidily determined to stay within the 200mm sizing of the boxes, resulting in vertical front sunbox faces. Any angle would have been better, to reduce summer sun angle reflection - say 85-75º (5-15º from vertical). This was the idea in the Sunbox mark 2 that I built last year, but never erected onto the wall. See illustration, right.
• With a better sloping angle, there would be no need for the foot mirrors - their effect is to simulate a sloping front face by enlarging the angle of incidence to the sun. The top mirrors seem to have been effective in the winter months, but do nothing in summer.
• I have tilted the foot mirrors of the existing sunboxes to have a reverse camber - really tilting the midday sun right into the box. All I need now is some midday sun!

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