There are two types of active solar hot water systems:
Direct Circulation Systems
A direct circulation system pumps water through the collectors directly into a storage tank. A direct circulation system is more efficient, and is cheaper than to install, than an indirect system. However, as the collector and water in the pipes are outside, this type of system is susceptible to freezing. These are best in warmer areas that do not have hard or acidic water.
Indirect Circulation Systems
An indirect circulation system is better for colder climates that experience freezing conditions. A non-freezing heat-transfer fluid, rather than potable water, is pumped through the collectors. Then the fluid flows through a heat exchanger to heat the residential water supply. The following components are required:
Photo Credit: U.S. Department of Energy
Heat-Transfer Fluids carry the heat from solar collectors to water storage tanks. These heat transfer fluids are non-toxic and protect the collector and piping from freezing. The right fluid depends on the type of solar thermal system and the climate. The primary factors are freezing point, boiling point, and viscosity (which determines how much energy is needed to pump the fluid).
Heat Exchangers transfer solar heat from the transfer fluid to the home water supply. Heat exchangers are often made from metals such as copper and stainless steel. The main type of heat exchanger is liquid-to-liquid, which uses transfer fluid, with one or two barriers between the transfer fluid and the water supply. These are described below:
single-wall: the heat transfer fluid and the potable water are separated by the heat exchanger material.
double-wall: the heat transfer fluid and the potable water are separated by two layers of heat exchanger material with air in between. If the heat exchanger has a problem the two fluids will not touch each other.
The most common designs of heat exchangers are:
coil (shown above) where the exchanger is a tube in the storage tank. If you have an indirect oil or gas water heater, you are already are using a coil heat exchanger!
shell-and-tube, where two tubes with water and heat-transfer fluid circulating in opposite directions are encased in a metal shell outside the storage tank.
tube-in-tube, with the two tubes of water and heat-transfer fluid in direct thermal contact with each other, is the most efficient type.