|Illustration from Geotherm website|
He is a big fan of the 'Compact Collector' a grille of pipes that is hung in a trench as deep as you can plant it, and is a substitute for a borehole - although it still needs more space than our garden would allow (it is also an IVT product). It addresses a smaller volume of soil than a borehole, so he strongly recommends solar charging.
Link to D.Maritan's paper
This illustration off his website shows an example, although it doesn't show a solar panel. His paper was written in 2008 and reports on data collected since 2004. The addition of the solar panels (and array of evacuated tubes) had a significant effect in smoothing out the deep ground temperature over the seasons. It was on a building in Soave, NW Italy, of 160sqm, larger than the Peveril Solar house - and with a larger roof surface area.
If you look at Geotherm's web page on GSHP with different collector types, you should use Chrome as that will translate it into English. There's nothing I can find on his website about the Solar charging experiment, but I have emailed David Maritan to ask for more details of the house or building, and to find out if the project is continuing.
We would have had a compact collector if it wasn't for the tiny size of our garden and the closeness of the collector to the foundations. We consider ourselves lucky to have had a borehole because it still buries the heat efficiently, and avoids ground swell. Solar charging wasn't in our thoughts when the house was built in 2006-7.