Solar energy is often used for the hot water needs of families in sunny climates. In temperate climates with little winter sun, solar water heaters can serve as preheating systems, with backup from a standard system. The solar water heater raises the temperature of the water before it enters the standard water-heating tank, so that the electric element or gas burner consumes less fuel. Solar water heaters can cut the average family's water-heating bill by 40 to 60 percent annually, even in a cold climate. Heavy water users will benefit the most. Although initial costs of solar water heaters may be higher than for conventional systems, they offer long-term savings. A complete system costing under $3000 can provide two-thirds of a family's hot water needs even in New England. This is competitive with the still less expensive gas water heater. Some states offer income tax credits, and some electric utilities give rebates for solar water heaters. Solar water heaters are required on new construction in some parts of the United States.
Solar water heating isn't always the best choice. When considering a decision to go solar, the existing water heater should first be made as efficient as possible. A careful analysis of the building site will determine if there is adequate sun for solar collectors, which will need to face within 40 degrees of true south. Trees, buildings, or other obstructions should not shade the collectors between 9 a.m. and 3 p.m.
Solar water heaters use either direct or indirect systems. In a direct system, the water circulates through a solar collector (Fig. 9-1). Direct systems are simple, efficient, and have no piping or heat exchanger complications. In an indirect system, a fluid circulates in a closed loop through the collector and storage tank. With an indirect system, the fluid is not mixed with the hot water, but heat is passed between fluids by a heat exchanger. This allows for the use of nonfreezing solutions in the collector loop.
Solar water heater systems are categorized as either active or passive. In passive systems, gravity circulates water down from a storage tank above the collector. The heavy tanks may require special structural support. These systems tend to have relatively low initial installation and operating cost and to be very reliable mechanically. Active systems use pumps to force fluid to the collector. This leaves them susceptible to mechanical breakdown and increases maintenance and energy costs. Active systems are more common in the United States.
Solar energy can heat outdoor swimming pools during the months with most sun. Solar pool heating ex-
Figure 9-1 Solar water heater.
tends the swimming season by several weeks and pays for itself within two years. The pool's existing filtration system pumps water through solar collectors, where water is heated and pumped back to the pool. More complex systems are available for heating indoor pools, hot tubs, and spas in colder climates.
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