The front could be done of one enormous sheet, but there are transport and erection problems with that, and a bigger problem if one has to hinge it out for maintenance. So it is in two easily hinging out panels, just a bit larger than on the Peveril House.
We will not use the reflective solar concentrators as the aspect ratio of the Sunbox is vertical and the mirrors would be less useful than on a wider aspect ratio, as on the Peveril. If the mirrors on my house prove to add 5% or 10% to the harvest (this will be difficult to tell as we haven't got the previous Winter's figures to compare), they can be added to David's Sunbox next year.
We have a finalised cutting list for Polycarbonate and David is about to order. Then I will order the aluminium sections. As David's company is Carbon Legacy, heat pump installers, I can safely leave the fixing of the radiators and the connections to his team, providing they follow the simple circuit diagram.
The illustration below shows what Three 60m deep boreholes look like at 5 metre spacing, relative to the size of the building that the Sunbox is attached to. These have a combined volume of 6,500 m3, about 13,000 tons of soil.
Meanwhile we are having a lively discussion about value engineering.
- Prototypes always cost more than production models, so how cheaper can this be made? Does it need to be? - considering that it is a fraction of the cost of a solar water heating array of 4sqm, and producing more kWh/yr than that.
- Although the metal radiators might capture more energy, we are storing it in a very fuzzy unmeterable store, the deep earth, so why not accept and use the much cheaper plastic panels for production models? In the meantime, use the metal rads in the interests of research.
- Glass is the same or slightly cheaper than Polycarbonate, but there are about 5-6 reasons why we can't do it with glass.
- What happens to the 3,000-3,800 kWh that these may produce in a year? Well if David has to PULL 12,000 kWh/yr out of the ground, and these PUT down 3,000 kWh, then those poor little kWh do not have time to escape before they are being pulled back up - making it 100% efficient (unless there is a nasty groundwater bearing gravel layer that may lose some of the heat).
- There has been a lot of bad press recently about GSHPs (since the Energy Saving Trust report of early Sept) and it is rumoured that they are not as good as expected, nor as well installed. Do they deserve grants or any kind of Feed in Tariff in such an inefficient form?
- So, if Heat pumps need a bit of help to improve their credibility, do these Sunboxes represent a lifeline, perhaps an 'essential' add-on that should be included to make them more cost effective, and attract a higher Feed in Tariff?