Saturday, June 25, 2011

Modifications to Sunboxes - Tape

25 June 2011: Chris Wood and I often debate the benefits of sunboxes and mirrors, and he is not as convinced as I am. His own use of the black polypropylene panels on his rig is to have them unglazed. If the ambient summer air temperature is above 21 degs and have a direct line of view to the sun, it is worth doing that. But I wanted something that would work in winter and equinox, and would not force the GSHP to work at very low temperatures.
  At this time of year there is a slight advantage to leaving the louvres open and letting air through them. The temperatures inside are not building up as high as they were in March/April, which is due I guess to the better Sun angle - I was seeing some daytime temperatures of over 40ºC in there, and now, on a good day, I am only seeing 30ºC. At the summer solstice, the declination of the sun at this latitude is 60º, very steep, with light bouncing off the face. On a sunny winter day, it is 14º, hitting the front face of the sunboxes almost directly.
  On the whole, averaged over a year, the boxes seem to do better if they are more airtight. So, I have applied duct tape along the bottom louvres to make them airtight along that face. I also added tape to the rooflets where there is an air gap. I will watch the daily temperatures and decide if that was right or wrong. It's easy to slit the tape if that was wrong.
   The risk here is that the black slabs act as chillers and without a source of fresh air, they cool the space more quickly than it can warm up from sky and sunlight. I see this happening, actually, with the air temperature falling once the slabs are working well.
  Notice that here, the foot mirrors are at reverse camber, pointing towards the panel faces, to catch the midday sun, when we have some!

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