The sun has been used for 1000s of years to passively heat water. In fact over 40,000,000 homes in China use solar as the primary energy source for hot water heat. In the US new home builder are being required to install solar water heating in southern states such as Hawaii.
Domestic solar water heating uses basic principles of heat absorption. However special selective coatings are now being used which greatly increases the effectiveness of solar water heater. Selective coatings are special materials that they have both very high absorption properties and very low emissive properties. Black paint was commonly used as it has high absorption, however it fails in that it has equally high heat emission. (it losses the heat it absorbs very quickly). Modern solar collectors have up to 94% absorption properties while only emitting 6% back to the environment.
The principle design of a solar water heater is very simple. Sun radiation hits the selective coating on a solar collector which absorbs the radiation and converts it into thermal heat. This heat then is used to heat water or another fluid that flows through a series of tubes. The water is then directly used for domestic hot water. A secondary heater can, if necessary, supplement the solar heater.
Soar water heaters can work equally as well in freezing climates. However to avoid damage that can happen with frozen water, a fluid capable of withstanding colder temperatures such as glycol is used. In this type of system, a heat exchanger is used to transfer the heat from the glycol to the potable water system.
Older systems used a drain back system to prevent damage from freezing. In this solar system, temperature sensors triggered drain valves to open thus draining the entire system and preventing freezing. However newer collectors can heat water in any temperatures so this method of draining the water before it would freeze is not so popular anymore and glycol is used instead.
The use of a Solar Pre-Heat tank is quickly becoming the most popular solar water heating configuration. With a pre heat tank system, solar heat is used to heat the main water supply before it enters the existing hot water heater. In this type of system any temperature gain in the supply water is positive and contributes to cost savings.
While special solar water heater tanks can be found, a regular water heater tank will also serve well as a pre heat tank. Sizing a solar pre tank is very important as most system can fail because of over sizing. As a general rule, one gallon of water is needed per square foot of solar collector.
It is a false expectation to replace 100% of a homes hot water energy with solar. The reasons are clear; the sun does not always shine and the summer months have more sun available than the winter months. For these reasons, the optimal solar hot water system will supplement about 60% to 80% of a homes regular hot water energy consumption.
Solar energy is becoming more affordable than ever. Governments are offering low cost loans, tax breaks and even financial support. The solar industry is at an all time high and the future looks even brighter. Contact your local utility agency to see what programs are offered in your region.