But its problem is that it complicates things.
My original idea was beautifully simple, the low grade solar-air panels to send heat deep into the ground - the delta-T (temp difference) would work perfectly because the ground is always cold, and the circuit could be a single large circuit. Easy. However, the idea is principally useful to people who have deep boreholes, ie. very very few. And it's fair to accept that some or much of the heat will drain away, as all my doubting friends say.
The thermal battery idea is very good (although it's a different idea) and could be applied to hundreds of houses, perhaps many more in 5 yrs time. So it's worth researching, and worth prototyping.
However, it needs a high temperature delivery, eg perhaps a pro quality flat-plate solar-thermal panel, so that there is a clear delta-T between panels and battery. The swimming pool panels would only be significantly hotter than the battery in summer and when the sun shines.
It also requires to redesign the circuit into two looped circuits with the battery as the hub (each circuit with their own pump), and thus prevent the heat-pump simply moving heat from the battery to the outside air. Using ball valves, I can change the circuit so that the battery is either connected, or the circuit is a single one as per my original plan. (I have designed something, but need to draw it an put it on the blog - also need to discuss it with the dissertation students.) I cannot depart too far from the original idea as I would leave them with a topic different from the one they started with.
I could just make my life easier and keep the 4 panels as proposed (and some of the piping is in. However, its worth considering David Atkins' earlier idea and have a mixture - two of the Swimming pool panels, and a single 2 sqm Flatplate solar-thermal panel.
For the thermal battery, I will use Rainwater as the sharing medium for the balls in the thermal battery, although BASF are interested in helping with a Micronal emulsion, and I also know that cooking oil would not evaporate - but it might get smelly. Water is easier to dispose of if that is necessary, and is adequate for proving the concept, and is free, and therefore more inspiring and possible to others as an idea to emulate.
I would test the amount of evaporation weekly in the summer and know if water is unsuitable.