Solar Hot Water Systems-gctid52484

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Solar Hot Water

   Adding a solar hot water system to a home is one of the best ways to incorporate renewable energy. Solar water heaters can be very cost-effective and relatively simple to install. A properly designed system operates efficiently regardless of outside temperature, and when used in conjunction with a backup gas or electric water heater the system provides ample hot water while cutting down fossil fuel use by up to 80% or more.

Residential solar hot water systems can range from simple to quite complex, using multiple fluid loops and heat exchangers to serve a variety of needs - from hot showers to radiant floor space heating to driveway snowmelting. The technology is established and proven; the sun can heat water for any need we might have.

The simplest solar hot water systems are used in warm climates, where freezing is never a problem. In these areas, a batch system is a good option. The hot water storage tank is actually a part of the collector on the roof. When insulated properly, the tank holds heat overnight.

In colder climates, measures must be taken to avoid freeze damage to the system. One design, called a drainback system, pumps water up into the rooftop collectors only when the sun provides enough warmth to prevent freezing. When night falls, the pump shuts off and water drains back into an indoor holding tank for the night. Another design uses heat transfer fluid, usually propylene glycol, that does not freeze in cold weather. The heat from this fluid is passed on to the domestic hot water system through a nifty gadget called a heat exchanger - without actually mixing the fluids.

Most domestic solar hot water systems can be thought of as solar pre-heaters. They do not instantaneously turn cold water (about 60°F) to hot water (about 120°F) in just one pass through the collector. Instead, water slowly moves through the collector and gains some heat through the collector; perhaps a 10°F increase. That thermal energy is then deposited into a hot water storage tank. This continual accumulation of heat slowly raises the temperature in the tank. Then, if necessary (usually in winter), the backup heater further boosts the temperature of this water. It is important to note that solar hot water systems must be sized properly to avoid overheating in summer - or excess heat may need to be dumped to prevent damage to the equipment or to the heat transfer fluid itself.

The fluid in a solar hot water system circulates, either through the passive rising of the fluid as it heats (thermosyphon) or with the use of an active electric pump. Some systems use simple PV-powered pumps that circulate fluid only when the sun is shining on the collector.

After short wavelength visible light passes through the glass on a solar thermal collector, it strikes the dark absorber plate and becomes long wavelength (infrared) heat. Long wavelengths can’t penetrate the glass and heat gets stuck inside the collector, creating an intense greenhouse effect.

Solar Hot Water Collectors
Solar hot water collectors consist of two main types. Traditional flat-plate collectors provide several parallel copper-pipe paths for the fluid to pass through. These pipes are attached to a dark-colored absorber plate and the whole arrangement sits inside an insulated frame beneath a layer of glazing. A typical home system might use two 4’ x 8’ collectors of this type. A newer alternative is an array of multiple evacuated-tube collectors. These long glass tubes contain no air - so they are insulated very well from the surrounding environment. Inside each tube is a dark metal absorber plate which transfers heat upwards through a thin copper tube to be absorbed by passing water or glycol (anti-freeze).

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