Heat exchangers are made to transfer heat from a source (solar collectors) to a delivery or storage system (tank). Heat exchangers are classified as liquid to liquid, air to liquid or air to air. Heat exchangers can be designed for cooling or heating, although in the solar industry they are used primarily for heating.Liquid to liquid heat exchangers can be as simple as a coil of tubing submerged in a tank of water. This is called a single wall heat exchanger since only one wall of tubing seperates the two liquids. A double wall heat exchanger is required if the heat is being transferred from a non-potable liquid (glycol) to a potable (drinking water) system. These exchangers are designed so the fluids can never mix.
One example of a double wall heat exchanger would be AAA Solar's Quad Rod heat exchanger. The quad rod is a double wall heat exchanger made completely of copper with brazed fittings. Here are some pictures of a Quad Rod heat exchanger.
3' Quad Rod heat exchanger
A simple way you would use a heat exchanger would be to heat water in an existing water heater. A system for heating an existing water heater would include one 4'x10' collector, a heat exhanger, two pumps and a controller. In this system, a heating fluid (glycol) is circulated through the collector and the heat exchanger with a pump. This can be done with an AC pump that is wired to a controller with sensors at the collector and water heater or is can be done with a DC pump that is wired to a small PV panel. the water from the tank is then circulated through the heat exchanger with a pump as well. The two fluids should be flowing in opposite directions for maximum heat transfer. While the sun heats the glycol and the heated fluid passes through the heat exchanger the heat is transfered to the water that is passing through the heat exchanger at the same time. As the water from the tank circulated the tank is heated and will only "fire" if the solar is not able to keep up with demand.
For more information on heat exchangers and specific products please visit http://www.aaasolar.com/.