[Porsche-O-Phile]

I've read someplace about coupling these with passive solar heating systems - very efficient, low-cost - all you pay for is the circulation pumps and possibly some small level of heat in the storage tank to keep the temps up if necessary. I'm not 100% sure (you should check a specialist) what the circulating medium is - probably not just water since you could use a closed-loop system, probably more likely some sort of glycol mix, which would be better long-term against corrosion anyway - not to mention freezing if the system ever sat idle.

I'll see if I can dig up some links - kinda' busy right now though, but these absolutely are very, very cool systems to implement.