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Solar space heating systems can be either active or passive. Passive systems use building components such as floors, walls, and sun spaces to collect and store heat. Often, small fans distribute heat, but mechanical equipment and the use of outside energy are kept to a minimum.
In contrast, active space heating systems rely on hardware such as rooftop collectors to collect and distribute heat. They use air or a liquid that is heated in the solar collectors and then transported by fans or pumps powered by a small amount of electricity. Solar heat is stored in rock bins or water tanks to provide heat during sunless periods.
Active solar energy systems are usually designed to provide from 40% to 80% of a home's yearly heating needs. But, data from systems installed through a government demonstration program indicates that active space heating systems are most economical when they are designed to handle about 50% of a home's heating requirements. A system sized to provide much more than this would not be economical since some of the extra capacity would only be used during the coldest days. The rest of the time, the extra equipment would be idle.
However, the cost of a system that provides less than 30% to 40% of a home's heat is seldom justified except in the case of window box, vertical wall, and other collectors designed to heat one room and not requiring storage.
Heat not provided by the solar system will have to come from the backup system which is usually a conventional furnace. (Backup systems are required by most building codes and mortgage lenders anyway - a requirement that makes active solar heating expensive because a homeowner must pay for and maintain two heating systems.) Still, individuals planning a new home can keep the extra expense of installing two systems at a minimum. For example, some furnaces can use two or three types of fuel giving the homeowner the option of using the fuel that is currently most economical. Also, a common heat delivery system shared by both the solar and backup heating system can be included.
A common heat delivery system can also be used when adding active space heating to an existing house. But in all cases, the backup heating system should be capable of supplying 100% of the home's heating requirements.
Solar collectors are usually mounted in rows, on the roof or the south wall of a house. They may also be ground mounted on a collector support structure. Collectors mounted on the south wall in areas covered by snow most of the winter will receive sunlight that is reflected from the snow. This will enhance the performance of the collectors enabling them to perform almost as well as if they were mounted on the roof.
Collectors should face true south (not magnetic south which is what a compass shows); although a deviation of 20 degrees or less from true south will not substantially reduce system performance. Collectors should also be tilted at an angle equal to your latitude plus 15 degrees. Between 9:00 A.M. and 3:00 P.M. collectors receive the most solar radiation and should not be shaded by trees, buildings, hills, or any other obstructions. Performance can be significantly reduced if even a small portion of the collector area is shaded.
Air systems consist of collectors, a rock storage bin, fans, ductwork and controls. They can operate in several modes depending on the amount of heat that's available. In the simplest mode, heated air moves directly from the collectors into the house. In another mode, which occurs when excess heat is collected, the system allows the rock storage bin to be charged. Finally, when no heat is being collected, the house is heated from the storage bin or by a backup heater.
When the system enters the heat storage mode hot air from the collectors is routed into the storage bin. The air first enters a plenum (an empty mixing space) at the top of the bin. As the air passes down through the bin it gives up most of its heat to the rocks. The air then returns to the collectors from a lower plenum for reheating.
To retrieve heat from the storage bin, house air is drawn into the lower plenum and blown upward through the rocks. Warm air is then drawn off the top of the bin and distributed to the house. If the air in the bin is too cool, an auxiliary heater boosts its temperature before it is distributed.
Warm air is delivered from the rock bin to the house through air ducts, sized for a velocity of 5 to 10 feet per second. Care should be taken to ensure there is no air leakage from the ductwork. The ducts should be insulated to a value of R-16.
Solar-heated air delivered through the ducts is not as warm as air delivered by conventional systems. Consequently, the ducts must be larger than ductwork in conventional systems because a larger amount of solar-heated air must be delivered to make up for its lower temperature.
Rock bins can be made from cinderblock, concrete, or wood. When treated plywood is used it should be lined with sheetrock and a vapor barrier to protect the rocks, and the entire system, from any gases released by the plywood. The greatest problem with air systems is air leakage from the storage bin as well as from ducts and dampers. Because leaks drastically reduce the efficiency of the system, the bins must be tightly constructed and sealed.
The rock bin should provide 1/2 to 1 cubic foot of storage for every square foot of collector. This is roughly 2.5 to 3 times the volume of a water tank providing equivalent storage for a liquid system.
Dense rock, such as river rock (which is predominantly quartz), performs best. The rocks should be of uniform size, roughly 3/4 inches to 1-1/2 inches in diameter. Before the rocks are put into the bin, they must be washed to remove dirt and insect eggs. Problems with mold, mildew, and insects can be prevented by keeping the rocks inside the bin dry.
Liquid solar systems consist of collectors, a storage tank, pumps, pipes, a heat exchanger and controls. Its operating modes are the same as an air system's except that in many cases the house cannot be heated directly from the collectors. For example, if the house has forced air heating, a heat exchanger is required to transfer the heat from the solar-heated liquid to the air.
Liquid systems usually use water to store solar heat. One to 1-1/2 gallons of water are needed for every square foot of collector. The tanks used to store water can be made of concrete, steel, or fiberglass-reinforced plastic and should be insulated to a value of R-19 or better.
The water in the storage tank is heated in one of two ways. In an open loop system it is circulated directly through the collectors. In a closed loop system the water is heated indirectly by a heat transfer fluid (often an antifreeze-water solution). The transfer fluid absorbs heat from the collector, and then passes through a heat exchanger that transfers the heat to the water inside the storage tank. (Thus the antifreeze-water solution and the water earmarked for household use are kept separate.) The heat exchanger is usually a coiled copper tube that conducts heat from the transfer fluid flowing through it to the storage water in which it is immersed. Because exposure to oxygen encourages corrosion, the heat exchanger should be completely submersed inside the tank.
Liquid solar systems also use heat exchangers that are external to the storage tank. Several designs of external heat exchangers exist, but they all transfer heat from fluid circulated through the collector to water that is pumped to storage. It is also possible to use small heat exchangers for individual rooms if the backup system is also on a room-to-room basis (such as in a house equipped with electric baseboard heating). These small heat exchangers are available as standard plumbing units in various sizes, and contain their own blowers. This type of heat delivery is often less expensive than central heating, since it allows heat in unused rooms to be turned off.
The temperature of water in liquid solar systems reaches 90 degrees F to 120 degrees F as opposed to the 160 degrees F to 180 degrees F reached in conventional heating systems.
Therefore, solar space heating systems are usually not used with hot water baseboards or radiators since they require water at a temperature 50 degrees or higher than a typical solar system provides. If baseboards or radiators are used with solar heating, their surface area should be significantly increased.
The method of heat distribution most compatible with active systems is radiant slab heating. This uses plastic or copper pipes embedded in a concrete floor and can operate effectively at a relatively low temperature. When solar-heated water circulates through the pipes, the floor heats up and then radiates its heat to the room.
An active solar heating system's controls are more complicated than those of a conventional heating system. They analyze more signals, make more decisions, and control more devices (including the conventional heating system). Solar controls use sensors, switches, and motors to operate the system, and to provide backup heating when the solar system cannot meet the requirements. Other controls are used to prevent extremely high temperatures or to protect against freezing.
The heart of the control system is usually the differential thermostat. This measures the difference in temperature between the collectors and storage unit. When the temperature in the collectors is from 10 to 20 degrees higher than the temperature in the storage unit, the thermostat will turn on a pump or fan to circulate heat to storage, or directly into the house.
Control systems vary in function, performance, and expense. A basic control system would perform the necessary functions to operate the solar system in three or four different modes. Some control systems monitor the temperature in different parts of the system which helps determine how the system is operating. The most sophisticated controls are microprocessors that can limit heating to where it is needed or desired, and if designed accordingly, can operate windows, window shades, dishwashers, and other devices.
Performing periodic maintenance on the system can help prevent major problems. Both liquid and air systems require 8 to 16 hours of maintenance per year. At this time pipes and ductwork should be checked for leaks, collectors should be checked for damage, and the glazing should be cleaned. Also, pumps and fans need to be lubricated, and filters in air systems should be cleaned.
Active space heating systems cost between $8,000 and $25,000. They offer the greatest return on investment to those who heat with electricity in sunny winter climates.
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NOTE: The relative advantages and disadvantages of air and liquid solar heating systems often depend on site-specific considerations such as geographic location, available storage space and the method of heat delivery.