What is solar heating?
Solar heaters, or solar thermal systems, provide environmentally friendly heat for household water, space heating, and swimming pools. The systems collect the sun’s energy to heat air or a fluid. The air or fluid then transfers solar heat directly or indirectly to your home, water, or pool.
Solar water heaters, sometimes called solar domestic hot-water systems, may be a good investment for you and your family. Solar water heaters are cost effective for many applications over the life of the system. Although solar water heaters cost more initially than conventional water heaters, the fuel they use—sunshine—is free. Solar heating technologies can be used in any climate. To take advantage of solar energy, you usually need to have an unshaded area that faces south, southeast, or southwest, such as a roof. In some cases, a solar professional may recommend west-facing roofs for solar collectors.
The type of system you choose, including the type of collector and whether it is active or passive, depends on several factors. These include your site, the climate you live in, installation considerations, cost, and how you would like your solar heating system to be used.
What are the basic components of a solar thermal system?
Solar water heaters and solar space heaters are made up of solar collectors, and all systems except pool heaters have some kind of storage. In pool systems, the swimming pool itself is the storage, and the pool’s filtration pump circulates the pool water through the collectors.
Active systems also have circulating pumps and controls; passive systems work without this added equipment.
Three types of solar collectors are used for residential applications: flat-plate, integral collector-storage (ICS), and evacuated-tube collectors.
Flat-plate collectors are the most common type. Glazed flat-plate collectors essentially are insulated, weatherproofed boxes that contain a dark absorber plate under one or more glass or plastic (polymer) covers. Unglazed flat-plate collectors are simply a dark absorber plate, made of metal or polymer, without a cover or enclosure. Unglazed flat-plate collectors made from polymer materials are typically used in solar pool-heating systems.
Integral collector-storage systems, also known as ICS or “batch” systems, are made of one or more black tanks or tubes in an insulated, glazed box. Cold water first passes through the solar collector, which preheats the water, and then continues to the conventional backup water heater. ICS systems are simple, reliable solar water heaters. However, they should be installed only in mild-freeze climates because the outdoor pipes could freeze in severely cold weather.
Evacuated-tube solar collectors
are usually made of parallel rows of transparent glass tubes. Each tube contains a glass outer tube and metal absorber tube attached to a fin. The fin is covered with a coating that absorbs solar energy well, but which inhibits radiative heat loss. Air is removed, or evacuated, from the space between the glass tubes and the metal tubes to form a vacuum, which eliminates conductive and convective heat loss. In the United States, evacuated-tube collector systems are used most frequently in commercial applications.
Most solar water heaters require a well-insulated storage tank. Solar storage tanks have an additional outlet and inlet connected to and from the collector. Active solar systems usually include a storage tank along with a conventional water heater. In two-tank systems, the solar water heater preheats water before it enters the conventional water heater. In a one-tank system, like the one shown on page 4, the backup heater is combined with the solar storage in one tank.
Active solar water heaters use pumps to circulate water or a nonfreezing heat-transfer fluid from storage tanks through the collectors. Active systems are usually more expensive than passive systems, but they are also usually more efficient.
Direct circulation systems use a pump to circulate household water through the collectors and into the home; they work well in climates where it rarely freezes. Indirect circulation systems use pumps to circulate a non-freezing heat-transfer fluid through the collectors and a heat exchanger. This heats water that then flows into the home. Indirect systems are popular in climates prone to freezing temperatures.
Passive direct solar water heaters, like the one shown on page 5, move household water or a heat-transfer fluid through the system without using pumps or electricity. Passive systems work during power outages, but they should not be used in climates where temperatures often go below freezing. Passive systems are typically less expensive to purchase and maintain than other types of solar systems. They are also inherently more reliable and may last longer. However, passive systems are not usually as efficient as active systems.
ICS passive solar systems may be best in areas where temperatures rarely go below freezing. They are also good in households with significant daytime and evening hot-water needs.
Thermosyphon systems work because water flows through the system when warm water rises as cooler water sinks. In this system, the collector must be installed below the storage tank so that warm water will rise into the tank. These systems are reliable, but contractors must pay careful attention to roof design because the water in the storage tank is heavy. Thermosyphon passive solar systems are usually less expensive than active systems, but more expensive than ICS systems.
How have solar systems improved?
Since the early 1970s, the efficiency and reliability of solar heating systems and collectors have increased greatly and costs have dropped. Improvements to materials, a rating system for consumers, and more attractive designs have all helped to make systems more successful.
Low-iron, tempered glass is now used instead of conventional glass for glazing. Improved insulation and durable selective coatings for absorbers have improved efficiency and helped to reduce life-cycle costs.
The Solar Rating and Certification Corporation (SRCC) and the Florida Solar Energy Center (FSEC) certify and rate solar thermal systems and equipment. SRCC evaluates product reliability and rates the performance of solar water-heating systems by subjecting them to technical reviews. SRCC has a directory of certified systems on its Web site along with system performance ratings. FSEC publishes similar information specific to Florida that is useful in other states with similar climates. See the “Getting help” section for contact information.
The appearance of the systems has also improved. Today’s collectors can usually be mounted flush with the roof for a streamlined system that looks like skylights. Unglazed polymer collectors for solar pool heating are now available in terra cotta colors as well as black, so homeowners can choose the color that will best match their home.