|210 litre double coil|
|117 litre single coil|
The area of south facing wall that I could fit evacuated tubes to is limited, so there is no point in having a tank bigger than 120-160 litres. One could store heat in a water tank by day, and purge it during the night to the deep borehole.
I will do a drawing to show an initial scheme. Twin coil tanks are all too large (180-300). The benefit of a twin coil is that it could also preheat the DHW - but I am against that on health and plumbing reasons.
There is a way to do what I am aiming for with a single coil tank. The solar panel circuit would fill the tank entirely (yes, that's a lot of glycol), and the ground loop would go through the built in coil, controlled by a thermostat and solenoid valves. The tubes can heat this up to something like 60ºC on a good day, and this would be dumped at night. I will work out the thermal capacity.
Unfortunately, this might mean some shopping with Navitron (the first company who come to mind for evacuated tubes), but I would like never to have to put any business in their direction after experiencing the manically hostile management style of the user forum. Nearer home, our department is always setting up rigs with tubes, and if any of these experiments have completed, I could offer a home to the redundant tubes.
Also, the loft hatch is 515mm wide and the standard tank with insulation is 520mm wide. It is possible to remove some woodwork to make enough width to get a tank up.