Monday, June 18, 2012

Sunbox versus Tubes, again

17 June 2012: We've had a full five weeks of running since the heat exchanger was installed and both systems have ran satisfactorily in identical weather conditions. It's time to get back to the new big question, which is to compare the performance of the Surya sunbox versus the Kingspan Tubes.

For this discussion, Time starts on May 10th 2012 when the heat exchanger was finally in place. From 10 May to 17 June, the Tubes have captured 101 kWh and the Sunbox collected 350 kWh. The Tubes seem to react only to sunlight (to build up sufficient delta-T to start the pump) whereas the Sunbox is happy to work on overcast days, just if the air temperature in the box goes up and creates a bit of delta-T. The large mass of liquid in the sunbox means that it does run for some hours a day, and there is time for the energy to reach the borehole before it cools. (I now wish I had plumbed the original sunboxes in using 22mm pipe). Hence there is a major performance difference, a ratio of 1 to 3.5. If the tubes were on the south wall, they would fare no better (less insolation than on the roof). 

Double the quantity on a south facing roof at 37ยบ might get closer to it, 11-12% more insolation over the whole year compared with the present position. 101 kWh times 2 times 1.12 = 226 kWh in that time period - still not enough.

On a simple linear calculation, it would seem that 3.2 times the area of the present tubes might be enough to equal the Sunbox, i.e. 6.4 sqm of tubes. But there is never a linear relationship between surface area and energy capture, because much depends on the admittance of the storage medium, rate of circulation, distance to thermal store. A water tank reaches optimum temperature, reduces its admittance as the tank temperature rises and the final result is stasis, no further admittance. A deep borehole is the best, because its volume is close to infinite in comparison to the size of the house and the solar panels. Double, treble or quadruple the panels and it could still take as much energy as we can send it. But would 6.4 metres of tubes cost a lot, plus a much larger heat exchanger? Yes they would, more than a Sunbox using solar-focus panels. The solar-focus panels are so cheap (less than £100) that one could cheerfully add another two, and make it 6 square metres, if one had enough remaining surface area.

Comparison of Sunboxes, new to old: I'm glad to see that last year (2011) in very sunny and warm conditions, the old Sunbox captured 350 kWh in the same time space of 2012 that the new one captured 356 kWh, which indicates that the new design works better, having endured 5 weeks of rain and cold with only one week of sunshine in all that time, whereas 2011 at the same time had remarkably warm and sunny conditions.

(My next big question will be to compare PVT with the two earlier installed systems.)

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