Solar Domestic Hot Water Heating
Electric and natural-gas water heaters are among the biggest users of energy in the home, but solar water-heating systems are a green alternative that's gaining popularity. These systems rely on the sun for all or most of their power and rely on fossil fuels only as a backup.
Solar heating systems can be used effectively in any climate, even if the region isn't particularly warm and sunny. As long as a home site has an unshaded area that faces south, it's a good candidate. The key to selecting and installing a successful system is understanding the system types available and the household's hot-water needs.
The two types of solar domestic water-heating systems are passive and active (also known as forced circulation). The main difference between the two is that a passive system relies on convection and an active system relies on a pump to circulate household water or a heat-transfer fluid within the system. All systems have these additional basic components: a solar collector to absorb solar energy, a hot-water storage tank and a backup energy source. Most solar water heaters in the United States are the active type because they’re the most efficient, but passive solar water heating systems typically are less expensive than active systems, can be more reliable and may last longer.
Both passive and active systems can be further divided into the following additional types, each with benefits and drawbacks:
- Active, direct circulation. In these systems, solar energy directly heats household water. Pumps circulate household water from the cold-water supply, through the solar collectors and into the storage tank for use in the home. This type of system works well in climates where it rarely freezes, because there's little chance that the water will freeze as it circulates to and from the solar collector.
- Active, indirect circulation. In these systems, pumps circulate a non-freezing, heat-transfer fluid through the solar collectors and into a heat exchanger within the storage tank. Cold-water supply runs directly into the storage tank, where it's heated, stored and ready for household use. These systems are popular in cold climates because circulating the heat-transfer fluid instead of water prevents freezing.
- Passive, integrated collector storage (ICS). In this system, also called a batch water heater, water is heated and stored in the solar collector. ICS systems are suitable only for warm climates where there’s no risk of freezing.
- Passive thermosiphon. The solar collector is located below the storage tank. The cold-water supply runs into the solar collector. Water is warmed in the solar collector and then flows by convection to the storage tank. A potential drawback of this system is that the roof design must appropriate to supporting the weight of the storage tank.
Sizing the System
When sizing a solar water-heating system, the most important components you'll need to determine are the size of the solar collection area and the size of the storage tank. The number of people in the household drives both of these. For a household of two, use a collection area of about 20 square feet. For each additional person in the household, add 8 square feet for homes in the southern United States and 12 to 14 square feet for homes in the northern U.S. Regarding the storage tanks, a typical sizing ratio is 1.5 gallons of storage tank per square foot of solar collector. For a household of up to three people, install a 60-gallon storage tank; for a household of three or four, install an 80-gallon tank; and for a household of up to six people, install a 100-gallon tank.
Costs and Savings
On average, solar heating systems can reduce water-heating costs by 50 percent to 80 percent. The actual savings a homeowner realizes over conventional water heating depend on the amount of hot water used, the cost of conventional fuels in the area and the geographic location of the home.