Then it goes back and repeats the cycle, over and over. If you keep your starting heat exchanger at room temperature, say by circulating tap water through it, Zhang says, your ending heat exchanger will soon cool off nicely. It's important to use the right material for your plate, Zhang adds. You want it to hold a lot of heat, but not conduct it very well, so you don't ruin the temperature gradient you're trying to create. Ceramic would be ideal for the purpose, he suggests, except that ceramics cannot easily be made thin enough to function efficiently.
Most designs use plastic or stainless steel.
One persistent design problem, Maynard says, has to do with improving the interface between the stack and the heat exchanger. The typical heat exchanger is made of copper tubing and fins, with a gauge of three millimeters. Copper tubing can't be made much smaller. So the ratio is 30 to one. This size disparity means a lot of heat is lost in transfer.