Solar / Alternative power.

There is a blog on renewables, but I guess you are suggesting one with more technical points.

There are some suppliers of solar tiles - which are fitted instead of standard roofing tiles on sloped roofs (rooves just doesn't look right). On flat surfaces, the collectors would need to be strong enough to cope with foot traffic - 8mmØ copper tube, closely laid?

Parallel!!

Hi Del the cat. I constructed such a contraption years ago, it is now on the roof of my house giving me all the hot water I need. I placed an utra-thin radiator painted mat black in a wooden case also painted black which was 50mm high on my roof. The radiator was copled to my warm water tank. Then I covered the wooden case with a sheet of glass (very important) as it keeps the heat in and stops air movement within the case. I did this first in 1979 and have now got a larger one one my roof. Spencer.

Hmmm, since you already have the photovoltaic you could add an insulated backer to the panels to cool them.

The backer panel would be a thin sheet of metal with some pipes bonded to the back surface. The pipes would lead to an insulated reservoir higher than the panels so you could get away with convection circulation.

As to the antifreeze idea, cheap but dangerous if it can come in contact with domestic water. I would use a light mineral oil that has a higher heat capacity and wont corrode your piping. Bonus would be if you got a leak there is little chance of poisoning.

As a heat exchanger the basic loop of pipe in the preheat tank is simple and cheap. If you can snag one a plate exchanger is much more efficient though expensive if bought new.

I used to hear more about propylene glycol as being much safer than ethylene glycol.