Saturday, April 23, 2011

The walls are helping

23 April '11: By the way, with this summer weather, I am again seeing a special phenomenon, which is that hours after sunset, the Sunboxes are still about 4 degrees higher than the ambient air temperature, even though the daylight hours ended with rain and thunder after a hot day.
    Being at over 22º, the Sunboxes are still sending heat down into the borehole even though it is after 10pm at night. The brick wall has been absorbing sunshine all day and some of that must be conducting to the air space in the Sunboxes. The temperature inside the box has been over 40º for many hours of the day and behind the wall is 100mm of insulation, so the only way out for that heat is to be released to the air space, and a little bit outwards through the brickwork surrounding the boxes.
   This is the phenomenon that enraged the trolls on Navitron forum, who thought that the idea of getting heat from the wall was some 'malicious hoax'.  I got some very nasty criticism, with some casting 'nasturtiums' on me, my children, my wife and job, as if a bit of criticism based on Physics and Engineering wasn't enough.
   Sorry, but having watched the airspace temperature many times this week being 40º, 42º, sometimes 46º, for hours on end, day after day, it is no surprise to me that the brick wall returns this daytime heat. It is simple logic. And it is not taking heat from the house, as it is warmer than the house, and if anything, some of that heat may be migrating inwards to warm up the bedroom at night.

  For a quick calculation:
  • Area of Brickwork= 5.25 m2 behind the polycarbonate
  • Volume of Brickwork = 0.54 m3 assuming 102mm brick leaf.
  • Thermal capacity of brickwork = 0.48 kilowatt hours per ºK per cubic metre
  • Brickwork cools from 42º to 20º during afternoon, and for a couple of hours after sunset, by which time the ambient air temperature is around 14ºC. After the Sunbox goes below 20º, the pump is turned off to save on pump consumption.
  • Thermal storage = 5.7 kilowatt hours over that 22 deg range
  This cannot soak into the walls due to 100mm of insulation, so it has to return to the airspace. Some of it escapes through air leaks, and some is absorbed into the black panels if the glycol is circulating for a few hours after sunset.

It is for this reason, the summer night heating, that I would be reluctant to make Sunboxes that were a portable box with a plywood back, because one would lose this thermal capacity effect.
    Looking at this further, it seems that there is no need to try to make it into a Trombe, i.e paint the brickwork black to help it absorb more sunlight. There is little of it visible to the Sun, most of it is covered by large black chillers. The brickwork gets its heat by conduction from the very hot air in the box. My wife would complain terribly if I painted part of the wall black, even though she is unable to see it directly. If the appearance of the wall matters less than performance, then painting it black could be helpful.

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