Friday, April 15, 2011

Solar top up to System First energy foundations

System First foundations. These can be adapted
for underground thermal energy storage
15 April '11: I am having a correspondence with Chris Wood of Bullivants, who is contemplating ways to provide solar earth charging to his System First energy foundations. He has managed to get two of the black swimming pool panels that I have used in the Surya Sunboxes, and will build a small rig. They are donated by Ice Energy. He is at the moment intending to set them up unclothed in boxes, and this will be an interesting comparison with my enclosed boxes, and you can be sure that his data collection will be very thorough.

I don't think two of those panels will produce enough worth measuring, if they are uncovered. During summer when temps are regularly at about 20 degs, we get over 40 degs in boxes ... A good delta T ! I wasn't prepared to try mine without the sunbox enclosures as I want to move as rapidly as possible to low energy consumption of even to carbon zero. I don't have time to give a year to data collection from a system with reduced performance. If he wants a design and build Sunbox installation over his panels, I have invited him to ask for assistance. We could try 4mm polycarbonate instead of 6 to improve thermal entry. If his sponsors pay for the materials, I will provide my time and labour free, as it is part of my research.

Two of the Solar Focus panels
Chris replied with a message: "Hi David, the main heat transfer in the summer when we have direct beam sunlight is not through conduction with the air. It mainly due to the absorption of light i.e. irradiance on the black surface and thus heat transfer direct to the fluid. This effect has no relation to the delta T with the air. Effectively you could argue that your sunboxes are providing a boundary layer of still air, which in turn lowers the heat loss to the ambient i.e. an insulating effect but maintaining the pathway for irradiance. How much this is of  benefit is hard to quantify. Efffectively it's the same effect as the glass on front of a conventional flat plate solar collector,. Conversely the polycarbonate will have a certain reflective component which will reduce the irradiance upon the panel. Look up what the 'g' factor of polycarbonate is. I know some glass is 0.5 meaning that half the irradiance is reflected." Chris

I replied: "Chris, I'm aware that my panels take time to get going in the morning and I suspect that the steep azimuth angle of sun in early morning is causing most of easterly heat to reflect, hence I have fitted the corner mirrors. The GSHP often demands heat at that time of the morning for the DHW so they come on due to delta-T." I would also add that I am aware that a chilling of the air cavity can occur (due to the black chillers working too well), making the air cavity colder than the ambient air, for a short period - until the greenhouse effect works again and warms up the space again. I also suspect that the 'clear' polycarbonate that we have used is not completely clear as some of the labels on it suggest that they reduce sunlight. I cannot get completely clear polycarbonate without this solar feature. We seem to get enough heat already, but I wonder if 4mm polycarbonate would have worked? 6mm seems good for structural stiffness, and 4mm would require more aluminium framing.

What will this lead to? Well, my sunboxes were limited to 200mm projection from the wall so that they have free planning permission. This causes them to be shallow rectangles with a very small east and west surface area. A further development on another house would make the sunbox have a larger air cavity, with perhaps an octagonal plan (like a traditional bay window) so that it would pick up more morning heat - reducing the amount of heat lost through reflection. Perhaps a curved front surface would be possible, either with thinner polycarbonate clamped at either side, or by curving it thermally.
  The octagonal plan would be complicated to build with more edges, and the curved front panel would be difficult too, with the difficulty of sealing top and bottom. A flattened polycarbonate dome would be best, if it has a rectangular footprint. Some of these may be available as proprietary mouldings.

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