Here's a brilliant example of reuse. These guys reused the heat exchanger and a door from an old refrigerator to make a solar water heater. Every day, I see an old refrigerator being thrown out on the streets of Brooklyn, so this is a very powerful plan that nearly anyone could use to reduce their heating bills, at nearly no cost.
This design particularly struck me because it's one of those why didn't I think of that sorts. The heat exchanger is a simple, ready-made device for (guess what) heat-exchange.
But that's only half of the design. More importantly, check the diagram in the lower right. Here, we see that the designers hooked this device in series with a traditional water heater. In the worst cast, the boiler/burner does all of the heating (no worse off). In the best case, the solar dohickey does all of the work, and the boiler/burner's controller shuts off to save energy.
From: mother earth news