Unlike photovoltaic systems, which convert the sun's energy into electricity, active solar thermal systems transform sun-heated fluid into space heat and hot water. Systems consist of electric pumps, fans, complex controls, storage tanks, and collectors.
Two types of collectors gather solar rays: flat-plate and evacuated-tube. Flat-plate is the more common type and has been around longer. Evacuated-tube is a more recent design.
A flat-plate collector consists of an insulated, waterproof metal box. The top side faces the sun and has a glass or plastic cover that acts like a greenhouse, letting in the rays and holding in the heat. Inside the box is a specially coated dark-colored absorber plate and pipes containing circulating fluid. Evacuated-tube collectors include a dark-colored absorber plate and parallel rows of vacuum tubes, all connected to one header pipe. Tube designs include glass, glass-metal, and glass with fluid flow paths.
Absorber plates typically include long strips of metal covered by a special coating. New technologies now produce coatings that are highly efficient. The absorbed radiation heats the circulating fluid.
Solar System Concerns
Flat-plate collectors have a simple, sturdy design. However, that design does allow them to lose heat through convection and radiation. Wind loads, which can carry away the heat, must be calculated.
Evacuated tubes can operate even on cloudier days. They use somewhat fragile glass - think of an insulated vacuum-sealed bottle. However, they may have some problems where snow or ice may accumulate and need to be removed. There can also be problems if breakage or damage destroys the all-important vacuum or from heat buildup if there is not an adequate draw on the system.
Collectors can be open- or closed-loop types. In open-loop systems, household potable water is pumped through the collector. The collector must be drained during long periods of freezing weather or use a drain-back system. While efficient, open systems may have corrosion problems if hard water damages components.
In closed-loop systems, an antifreeze-like solution passes through a heat exchanger mounted near the home's solar water heater. Closed loop can lose a bit of efficiency during the heat exchange process and there is some maintenance required with the fluid.
Solar System Requirements
When selecting a solar system, homeowners need to learn how much solar energy is available to their home, a term known as “solar constant.” Other considerations are a home's latitude; available surface for the collector, trees, or buildings that would shade collectors; the collector's location and how much it deviates from magnetic South; and the distance from the collector to the solar storage tank.
At certain latitudes, radiation in winter and in summer are quite different because of the angle of the sun in relation to the Earth. Even the pitch of the surfaces where the solar panels will be placed must be checked with tilt angles and orientations varying considerably between locations. Most solar panels are mounted on roofs and the angle is preset by the roof angle. Angles greater than the optimum will reduce efficiency.
A home's roof space may not even face south or may not be strong enough to support the system. If roof areas will not work, homeowners can also consider exterior walls, free-standing or ground systems.
Another factor is the home heating system. Solar thermal systems are well suited for radiant floor heating systems and boilers with hot water radiators. Forced-air systems using a heat exchanger work but lose some efficiency. Solar collectors, however, do provide or assist with hot water needs for a household.
In northern climates with very cold temperatures or long periods of cloudy skies, a backup system will be needed. A house that is drafty and lacking in energy efficiency will need backup. Even the comfort levels of household members - especially elderly members who prefer a higher thermostat setting - need to be kept in mind. Lenders and building codes may also require a backup system.
Check out local codes or covenants that may restrict options. Some municipalities have objected to systems obstructing side yards, unlawful height additions on roofs, violation of historic district regulations, and excessive roof loads.
Cost may be a factor. To make a solar system cost-effective, it should be used most of the year and not sit idle in the summer. Year-round hot water operation improves the cost effectiveness of the system.
A system works best for a budget if it can provide 40 to 80 percent of a home's heating needs. An active system that supplies less than 40 percent of a home's heating needs does make much economic sense.
When shopping for a system, compare certified solar collector equipment by checking ratings stickers from the Solar Rating and Certification Corporation.
Costs for an active solar heating system vary greatly and, in part, with competition in the marketplace. In Wisconsin, for example, where there is little competition, an evacuated-tube system for hot-water delivery only in a single-family residence costs between $9,000 and $12,000.