AirToob Lightning
Possibly the most untapped source of energy saving is the heat pump (familiar to most people in the form of an air conditioner or a refrigerator, but it applies to heating as well as to cooling).

Many people don't realize that it can be cheaper to move heat than to generate it. Even "cold" water (by which I mean water we wouldn't necessarily want to swim in!) contains heat that can be moved (by refrigerating a river, say, in the case of a conveniently situated concert hall - the river becomes slightly colder and the concert hall becomes hotter).

The popularity of this concept has been steadily growing for domestic housing, as well as for large commercial buildings.

You can do a neat experiment at home to demonstrate one of the main methods of moving heat, used in a conventional refrigerator or air conditioner. Just blow on the back of your hand with your mouth wide open. It doesn't matter how hard you blow, the air will feel warm - yes? Now blow on the back of your hand through pursed lips. It doesn't matter how gently you blow, the air will feel (and is) cooler. That's refrigeration (well, adiabatic cooling to be precise) in action.

(I invented that simple experiment to explain one reason why aircraft piston engines need "carb heat".)

There are now many domestic heating systems that refrigerate the ground (or water in the ground), moving heat from the ground into your house. If the conditions are right, it is cheaper to do this than to generate the heat.

If you're interested, see here.

BTW: I was prompted into writing this by a nice message from oilhand99, who is interested in this kind of thing. Thanks, Ben!

[Many applications of heat pump technology - save money and help the environment!]

and if you like this kind of thing...

[Solid state refrigeration (for small applications at present, e.g. water chillers and electronic components)]
[My environment page]