The most common type, these heat a fluid that flows through tubes attached to a dark-colored absorber plate in an insulated box. These consist of double-walled glass tubes that have a vacuum between the outer glass and the inner absorber tube. The vacuum in the tube makes them more efficient and more expensive than the typical flat-plate collector. A batch system is just a water tank enclosed in a glazed, insulated, dark-colored box that's exposed to the sun. Sunlight strikes the batch tank, pre-heating the water inside before it flows to the conventional hot water tank.
Heat is transferred from the collector to water in one of two ways.
The heat from the fluid is then transferred to the water through a heat exchanger. This fluid contains antifreeze in cold climates. Also, hard water will form scale on these collectors, reducing their efficiency. Most systems include a conventional water heater as a storage and backup heat source. During sunny periods, your solar heater can provide all of your hot water.