Hi,
I continue to flail around trying to find the best way to cover as much as possible of our primary energy costs at our small home in London, even in mid-winter. (Roof and ground) space is a very significant limit to microgeneration here.
I have been looking at solar DHW, and of course one of the irksome things about it is that it provides little support in winter and if sized to get good spring/autumn support then it is throwing away lots of energy and stressing the system in summer.
While pondering ASHP (air-source heat-pump) here:
http://www.earth.org.uk/air-source-heat-pump.html
I discover that commercial R744 (eg Eco cute) ASHP units to cover space heating and DHW are achieving CoPs (coefficient of performance) of 5 or more, even in non-ideal circumstances, and even as much as a CoP of 4 with external air temps below zero and DHW of 45--55C. (Installs of Eco cute systems in Japan have passed 1 million: http://www.r744.com/news/news_ida286.php )
I note that the most efficient retail solar PV on the market has an efficiency of roughly the reciprocal of that, ie ~20% for SunPower SPW-315 and the Sanyo HIT stuff I already have on my roof.
That means that a combination of PV and ASHP can end up extracting just about as much useful CH/DHW heat per square metre/yard/whatever from my available roofspace as plain solar thermal, albeit going though a helluva ride to get there. It may even do better in cold air than plain solar thermal could. And the maximum PV that we are allowed to connect without massive red-tape (3.7kW/16A) could make enough power on average to cover DHW demand even in December.
The nice thing about the PV/ASHP combo is, of course, no overheating in summer, and no 'rejected'/'wasted' energy, as anything that we don't use as electricity or via the heat-pump for heating can get pushed back into the grid (to displace some fossil-fuel burn elsewhere).
(GSHP would probably beat ASHP but we just don't have the ground area, and what we do have is riddled with sewers.)
Rgds
Damon
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