7 Sept 2009: Talking to my architect friend in Cardiff, Alan Gillard, he described how he has used reverse cycle heatpump in a scheme in Wales, that is in effect, charging the earth. Using the reverse cycle, the underfloor heating can be underfloor cooling, so that excess summer heat from the buildings is dumped into the borehole... where it is retrievable in the evening, or in the colder seasons. He knows of schemes in Europe that use reverse cycle this way.
The last two UK summers have not been hot enough for this to be activated, but its a great idea for hotter climates, instead of air conditioning.
He also discussed the idea of the solar thermal water tank being also a buffer for ground charging in my previous blog article. He suggested that it could be made even more efficient if the tank was charged with surplus heat from a winter / night-time source such as the backboiler pipes in a woodburning stove. So that all year round and in the evenings the tank could be getting heat from somewhere - (summer=solar, winter=woodburning stove) and that whenever the temperature rises above a target temp like 30 or 35, the remaining heat is sunk into the ground loop to charge the earth - this stops the tank ever from overheating.