Solar hot water - Heat exchange

Some things allow heat to move from them to other things really well and some things don't.

The rate of heat exchange between one item and another makes us say words like insulation, and conductor. The reason people selling you insulation can claim that it keeps you warm in winter and cool in summer is because it does. Insulation achieves this by denying heat exchange between the sun and your house, or your house and winter. A perfectly insulated house would always be the average yearly temperature of your region.(unless you add or remove heat (Actually a perfectly insulated house would always be whatever temperature it happened to be on the day it was built.))

Heat conductors, on the other hand, love passing heat around and sharing it with anything that also conducts heat. An example of a good conductor of heat would be an aluminium cooking pot. Or in my house, copper based stainless steel cooking pots. The reason the copper is on the bottom of the pot is because it conducts heat well and the stainless steel doesn't. The reason the stainless steel is there is due to a scandalous error from within the scientific community, and some ill-informed law. A poor heat exchanger makes for a poor pot. When you heat a pure stainless steel pot the heat doesn't spread evenly and your food burns because all the heat is in just a few spots where the flame or electric element touches it.

If you want to collect heat, it turns out another thing to do is to stop reflecting it. Those silver things people put up against the inside of car windscreens work quite well at reflecting heat, as do white things. Black things on the other hand absorb heat. Matt black is even better.

For 10 years or so, from when I was about 5 years old, we had a black and white cat. Once, when it had been lying in the sun for a few hours one hot summer's day, I remember patting it. It tore strips of flesh off my arms. It was a starving stray when we took it in and it simply hated humans.

If you want to collect heat from the environment, it's best to use something that is good at heat exchange to collect heat, and then its best to use something that isn't good at heat exchange to store it and stop it escaping. If you want to collect heat from a nuclear reactor,  its best to use someone else's country.

I wont be exercising the nuclear hot water option at my house. Although at first glance it seems safe enough, I'm told it's prohibitively complicated and expensive, and could take longer than the two months I've allotted to each project to gain local council approval.