Friday, September 3, 2010

Solar Oven ideas for Surya Sunboxes

Where does this Solar Oven idea lead?
I have established that the 'greenhouse effect' works, but can something be done to enhance this?

I have access to bright aluminium sheeting from the engineering faculty stores, so it has set me thinking for ways to enhance the performance of my sunboxes - I propose to instal mirror/ reflector/ concentrators around the sunboxes to capture even more solar heat, as it is not possible to enlarge the boxes themselves.
• I am reluctant to get scaffolding back again, but if the changes are modest, they could be done from a well set up builders' ladder.
• I wish I had made the existing boxes more airtight. The upper polycarbonate panel is too perfect a fit, and would have been better with about 2cm of overhang all round. The lower louvre is surrounded by a 5mm air gap, and in my Mark2 design, I have an overhang on the upper panels, and on the lower, I have rebated the detail to remove this gap. I am thinking of buying more polycarbonate to replace the upper louvres, but it will be difficult to do from ladders.
• But I could also modify the bottom louvre to be upwardly reflective aluminium, bouncing light to the solar collectors. Equally, I could make them black painted aluminium, so they got very hot, and increased the air temperature. What a pair of extremes I have to balance there! 
• The back surface is presently Brick Wall, and that does reasonably work well to retain heat. But should one instead fit bright reflective sheets over it to reflect heat back, which would remain inside the box because the waveform is changed and it does not reflect through the front polycarbonate panels. Alternatively black painted sheets of metal fastened to the wall might build up much higher air temperature (this is how solar ovens work, the interior has to be blackened.) Being light in weight, this would be high performance when heat is available, and cool quickly in the dark - could be a good thing as it would reduce the amount of wasted pump time in the darkness. But the boxes fit so tightly to the black panels that there is not really enough wall visible for this to be worth doing. And the metal sheet would stop the brickwork warming up.

There is a similar argument in the Solar Oven community. The theory is that if the Pot with the food is dark, and if the interior of the box is reflective, the heat bounces around until it is absorbed by the pot. It doesn't escape through the glass because the wavelength is changed by reflection.
• For the polycarbonate top louvre of the sunboxes, I already built an elaborate hinging mechanism that had a summer, equinox and winter setting for air flushing to prevent over-heating, but I have found this all to be unnecessary - simply, SHUT is the best option. But I could adapt the same idea and have bright metal reflector hinging above the sunbox that has three angle settings - the midday sun angle varies from 14º (winter solstice) to 37º (equinox) to 60º (summer solstice), so the reflector could be adjusted at intervals to improve the performance. This would reflect sunlight vertically downward, thus not shining onto the flat surface of the black collectors, but it could considerably raise the air temperature in the boxes. If this did not confuse the thermal probe in the upper zone, it could be useful to have higher air temp.
    I would definitely like to try this on the NEXT one I do, but feel that without scaffolding, this would be too difficult to apply to the ones on my house. It would be nice if these had an effect like those winglets on the end of aeroplane wings - a small change, but one that produces real economies. In fact, the bottom louvre could be replaced with reflective aluminium. There is no case for vertical reflectors at the sides as sun is orbiting 15º an hour, and with a solar cooker, the people adjust the angle, but I have to put up with fixed angles.
   Actually... thinking about it.... if they are simply 90º bracketed to the existing tilting rooflets, that would do for averaging winter and summer angles - and with my riveting tool instead of screwing, it would be easy to fix from a Ladder! Bright Winglets of 300mm by 1800 long would add one full additional square metre to the theoretical area of the panels! So I will order some Aluminium sheeting next week!

April '11 Postscript: Despite my comments above, I did add some vertical corner mirrors in December '10, to help with morning capture. It costs almost nothing to do and is worth trying, to overcome morning reflection off the front faces of the boxes. And I have been round the boxes with clear tape, making them more airtight at the junctions.

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