Is there a Payback aspect to Sunbox augmentation of a ground source heatpump?
I have been thinking more about Payback, in respect of Photovoltaic. But does that apply to Sunboxes?
Only a couple of winter's testing will prove that conclusively, in terms of reduced electricity consumption. More significantly, in our article for Shanghai, Blaise and I have pointed out that if the sunboxes are effective and permanent installations, then the size of the ground loop can be reduced - perhaps saving the entire cost of the sunboxes.
Our ground loop is presently calculated to get 9,800 kWh from the ground annually. If we put 2,500-3,000 kWh of solar heat down in the same time and recover most of it, then the ground loop or borehole depth can be recalculated for only 7,000 kWh. The original boreholes were 48m deep and cost £4,500. Could we have reduced this by £1,000?
However..... I would say No..... the deeper the ground loop, the more effectively you can bury the heat from the sunboxes - well away from the seasonally changing upper surface. So if you can afford it, have the full depth borehole or horizontal ground loop, and just enjoy the reduced electricity bills and faster heating cycle by the GSHP. The full size ground loop is a precaution against a future occupier misusing the sunboxes or not repairing a failed thermostat or solenoid valve.
Could we improve the performance of the Sunbox system by doubling the number? We presently have 4 sqm of black panel. I don't reckon we could. Heat is transferred by 'delta-T' that is the transfer of heat by temperature difference. So just doubling the area would not make the liquid hotter, just more of it.
One idea could be to use a professional quality solar thermal panel, delivering temperatures of up to 40-60ºC which could be better for Summer charging. I am against that on technical and cost grounds, but mostly because the performance in Winter would be very poor compared with the solarium style panels that can tolerate freezing. The pipe diameter in a solar thermal panel is small, so the GSHP would not like the pressure drop, and you would need an intermediate hot water storage tank with a larger diameter coil going through it - costly! My sunboxes were performing well in the cold spells of March - and I look forward to the coming winter to prove that they will work well all through the cold season.