Choosing the appropriate solar energy system depends on factors such as the site, design, and heating needs of your house. Local covenants may restrict your options; for example homeowner associations may not allow you to install solar collectors on certain parts of your house. If you are unsure about what type of solar energy system to install, contact a solar energy specialist or engineer. No matter what system you choose, you should learn about it before making a purchase.

How Much Heat Should Active Systems Provide?
The local climate, the type and efficiency of the collector(s), and the collector area determine how much heat a solar heating system can provide. It is usually most economical to design an active system to provide 40% to 80% of the home’s heating needs. Systems providing less than 40% of the heat needed for a home are rarely cost-effective except when using solar air heater collectors that heat one or two rooms and require no heat storage. A well designed and insulated home that incorporates passive solar heating techniques will require a smaller and less costly heating system of any type, and may need very little supplemental heat other than solar. There are computer programs/software available to assist in properly siting and designing solar heating systems.

Supplementary Heating
Besides the fact that designing an active system to supply enough heat 100% of the time is generally not practical or cost effective, most building codes and mortgage lenders require a back-up heating system. Supplementary or back-up systems supply heat when the solar system can not meet heating requirements. They range from a wood stove to a conventional central heating system.

Positioning Collectors to Perform Optimally
In general, the optimum collector orientation is true south. True south is the highest apparent point in the sky that the sun reaches during the day. (True south should not be confused with magnetic south as indicated on a compass.) Collector orientation may deviate up to 20° from true south without significantly reducing the performance of the system. Collectors should be tilted at an angle equal to your latitude plus 15° for optimum performance. A collector receives the most solar radiation between 9:00 a.m. and 3:00 p.m. Trees, buildings, hills, or other obstructions that shade collectors reduce their ability to collect solar radiation. Even partial shading will reduce heat output.

You can position collectors in different locations. Collectors usually receive the most sunlight when placed on the roof. In some cases, however, the roof may be too shady or you (or your neighbors) may not like the look of collectors on the roof. If this is the case, you may mount the collectors on a supporting structure on the ground, or on the south wall of the house, where there is enough sunlight for the collectors to perform satisfactorily.

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Types of Active Heating Systems
There are two basic types of active solar heating systems based on the type of fluid heated in the collectors: liquid or air. “Liquid systems” heat water or an antifreeze solution in a “hydronic” collector, whereas “air systems” heat air in an “air collector.” Both of these systems collect and absorb solar radiation, then transfer the solar heat directly to the interior space or to a storage system, from which the heat is distributed. If the system cannot provide adequate space heating, an auxiliary or back-up system provides the additional heat. Liquid systems are more often used when storage is included.

Air Systems
An air system uses air as the working fluid for absorbing and transferring solar energy. Solar air collectors can directly heat individual rooms or be integrated into a central heating system. Depending on your needs and location, you may find the advantages of an air system outweigh its disadvantages. Air collectors produce heat earlier and later in the day than liquid systems. Therefore, air systems may produce more usable energy over a heating season than a liquid system of the same size. Also, unlike liquid systems, air systems do not freeze, and minor leaks in the collector or distribution ducts will not cause problems. (Do not, however, ignore leaks; they will reduce the overall performance of the system.)

Room Air Heaters – Not everybody wants to invest in a large system that supplies most of the heat for the home. Air collectors can be installed on a roof or an exterior (south facing) wall for heating one or more rooms. These systems are easier and less expensive to install than a central heating system. They do not have a dedicated storage system or extensive ductwork. The floors, walls, and furniture will absorb some of the solar heat, which will help keep the room warm for a few hours after sunset. Masonry walls and tile floors will provide more thermal mass, and thus provide heat for longer periods. A well-insulated house will make a solar room air heater more effective. Factory-built collectors for on-site installation are available. If you are a do-it-yourselfer, you may choose to build and install your own air collector.

The collector has an airtight and insulated metal or wood frame and a black metal plate for absorbing heat with glazing in front of it. Solar radiation heats the plate that, in turn, heats the air in the collector. An electrically powered fan or blower pulls air from the room through the collector, and blows it into the room(s). Roof mounted collectors require ducts for supplying air from the room(s) to the collector and for distribution of the warm air into the room(s). Wall mounted collectors are placed directly on a south-facing wall. Holes are cut through the wall for the collector air inlet and outlets.

Simple “window box collectors” fit in an existing window opening. They can be active (using a fan) or passive. In passive types, air enters the bottom of the collector, rises as it is heated, and enters the room. A baffle or damper keeps the room air from flowing back into the panel (reverse thermosiphoning) when the sun is not shining. These systems only provide a small amount of heat, since the collector area is relatively small.

Central Heating Systems – It is possible to integrate solar air collectors into a central heating system, which generally requires a storage component. These types of systems are rarely installed now, but the concept received a great deal of attention in the late 1970s and early 1980’s, when federal tax credits spurred innovations in an industry that was in its infancy.

Many of these systems stored solar heat from the collectors in a large bin of rock. In a typical design, a blower circulates warm air from the collectors through a large duct into a plenum (a mixing space at the top or side of the bin), then through the rocks, which absorb most of the heat, and into a second plenum at the bottom or other side of the bin. The air then returns to the collectors for reheating. When the house requires heat, air is pulled from return ducts in the house in a reverse direction through the bin and is pushed into supply ducts for heating individual rooms. Thus, the rock bin serves as storage and as a heat exchanger. Bin temperatures can reach 140°F (60°C). If the air in the bin is too cool, a back-up system heats the air leaving the bin to the desired temperature before distributing it.

A rock bin requires 1/2 to 1 cubic foot (0.014 to 0.028 cubic meters) of volume for every square foot of collector area. (This is two to three times more space for storage materials than liquid system tanks.) Rocks of uniform size (3/4 inches to 1 1/2 inches [19 to 38 millimeters] in diameter, such as river rock, are ideal. The rock must be clean, dry, and without any dirt or gravel. The rocks must be kept dry inside the bin to prevent problems with mold, mildew, and insects.

An ideal location for a bin is a crawl space or basement, because warm air naturally rises to the living space, though they can be located outdoors, above or below ground. The bins can be made ocinder blockck, concrete, or wood. They must be tightly constructed and sealed to prevent air leaks and moisture intrusion, and well insulated. Treated wood needs to be sealed with a plastic liner to keep gases released by the plywood from entering the air.

Because of the difficulty in keeping the bin dry, and thereby avoiding the growth of mold and mildew on the rocks, rock bin storage is only appropriate in very dry climates. Even then, it may be difficult to control mold and mildew, posing potentially severe indoor air quality problems. Also, the electrical energy necessary to power the fans to move the air through the system reduces its overall efficiency and cost effectiveness. These issues and the large volume of storage area needed are the reasons why few if any of these systems are now installed.

One option is to store heat from the collectors in a water tank. This requires an air-to-water heat exchanger similar to a car radiator located exterior to the tank. A blower circulates warm air from the air collectors across/through the heat exchanger and back to the collectors when the air in the collectors is at a specified temperature. A thermostat controls the blower fan. The heat is then distributed from the storage in the same way as in a liquid system (see below).

Liquid Systems
Liquid solar collectors are most appropriate for central heating. They are the same as those used in solar domestic water heating systems. Flat-plate collectors are the most common, but evacuated tube and concentrating collectors are also available. In the collector, a heat transfer or “working” fluid such as water, antifreeze (usually non-toxic propylene glycol), or other type of liquid absorbs the solar heat. At the appropriate time, a controller operates a circulating pump to move the fluid through the collector. The liquid flows rapidly through the collectors, so its temperature only increases 10-to-20° F (5.6-to-11°C ) as it moves through the collector. Heating a smaller volume of liquid to a higher temperature increases heat loss from the collector and decreases the efficiency of the system. The liquid flows to either a storage tank or a heat exchanger for immediate use. Other system components include piping, pumps, valves, an expansion tank, a heat exchanger, a storage tank, and controls.

Storing Heat in Liquid SystemsLiquid systems store solar heat in tanks of water or in the masonry mass of a radiant slab system. In tank type storage systems, heat from the working fluid transfers to a distribution fluid in a heat exchanger exterior to or within the tank. There are different types and configurations of heat exchangers available, which are not discussed in detail here.

Most storage tanks require one to two gallons (3.8 to 7.6 Liters) of water for each square foot (0.093 square meter) of collector area. Tanks are pressurized or unpressurized — which type is used depends on the overall system design. Before choosing a storage tank, you should consider several factors, including cost, size, and durability, where to place it (in the basement or outside), and how to install it. You may need to construct a tank on-site if a tank of the necessary size will not fit through existing doorways. Tanks also have limits for temperature and pressure, and must meet local building, plumbing, and mechanical codes. You should also note how much insulation is necessary to prevent excessive heat loss, and what kind of protective coating or sealing is necessary to avoid corrosion or leaks.

Specialty or custom tanks may be necessary in systems with very large storage requirements. They are usually stainless steel, fiberglass, or high temperature plastic. Concrete and wood (hot tub) tanks are also options. Each type of tank has its advantages and disadvantages. All types require careful consideration for their location, due to their size and weight. It may be more practical to use several smaller tanks rather than one large one. The simplest storage system option is to use standard domestic water heaters. They are designed to meet building codes for pressure vessel requirements, are lined to inhibit corrosion, and it is easy to attach pipes and fittings.

Distributing Heat for Liquid Systems – There are different ways to distribute the solar heat: with a radiant floor, with hot water baseboards or radiators, or with a central forced-air system. In a radiant floor system a solar-heated liquid circulates through pipes embedded in a thin concrete slab floor, which then radiates heat to the room. Radiant floor heating is ideal for liquid solar systems because it performs well at relatively low temperatures. A carefully designed system may not need a separate heat storage tank, though most systems do for temperature control. A conventional boiler or even a standard domestic water heater can supply backup heat. The slab is typically covered with tile. Radiant slab systems take longer to heat the home from a “cold start” than other types of heat distribution systems. Once they are operating, however, they provide a consistent level of heat. Carpeting and rugs will reduce the system’s effectiveness.

Hot-water baseboards and radiators require water between 160° and 180° F (71° and 82° C) to effectively heat a room. Generally, flat-plate liquid collectors heat the transfer and distribution fluids to between 90° and 120° F (32 ° and 49° C). Therefore using baseboards or radiators with a solar heating system requires that either the surface area of the baseboard or radiators be larger, that the distribution liquid be heated further (with the backup system), and/or that a medium temperature solar collector, such as an evacuated tube collector, be used.

It is possible to incorporate a liquid system into a forced-air heating system, and there are different options for doing so. The basic design is to place a liquid-to-air heat exchanger in the main room-air return duct prior to it entering a conventional forced air heating system. Air returning from the living space is heated as it passes over the solar heated liquid in the heat exchanger. Additional heat is supplied as necessary by an additional heat exchanger or electric heating element in the conventional heater. The solar heat exchanger must be large enough to transfer sufficient heat to the air, especially when the heat exchanger liquid is at 90°F (32°C).

Controls for solar heating systems are usually more complex than those of a conventional heating system, because they have to analyze more signals and control more devices (including the conventional, backup heating system). Solar controls use sensors, switches, and/or motors to operate the system. The system uses other controls to prevent freezing or extremely high temperatures in the collectors.

The heart of the control system is a differential thermostat, which measures the difference in temperature between the collectors and storage unit. When the collectors are 10-to-20 ° F (5.6-to-11 C°) warmer than the storage unit, the thermostat turns on a pump or fan to circulate water or air through the collector to heat the storage medium or the house.

The operation, performance, and cost of these controls vary. Some control systems monitor the temperature in different parts of the system to help determine how it is operating. The most sophisticated systems use microprocessors to control and optimize heat transfer and delivery to storage and zones of the house.

It is possible to use a solar photovoltaic (PV) panel to power low voltage, direct current (DC) blowers (for air collectors) or pumps (for liquid collectors). The output of the PV panels matches available solar heat gain to the solar collector. With careful sizing, the blower or pump speed is optimized for efficient solar gain to the working fluid. During low sun conditions the blower or pump speed is slow, and during high solar gain, they run faster. When used with a room air collector, separate controls may not be necessary. This also ensures that the system will operate in the event of utility power outage. A PV system with battery storage can also provide power to operate a central heating system with a storage component, though this is expensive for large systems.

Performance and Maintenance
How well an active solar energy system performs depends on effective siting, system design, and installation, and the quality and durability of the components. The collectors and controls now manufactured are of high quality. The biggest factor now is finding an experienced contractor who can properly design and install the system.

Once a system is in place, it has to be properly maintained to optimize its performance and avoid breakdowns. Different systems require different types of maintenance, but you should figure on 8 to 16 hours of maintenance annually. You should set up a calendar with a list of maintenance tasks that the component manufacturers and installer recommends.

Costs and Benefits
The cost of an active solar heating system will vary. A simple window air heater collector can be made for a few hundred dollars. Commercial systems range from $30 and $80 per square foot of collector area, installed. Usually, the larger the system, the less it costs per unit of collector area. Commercially available collectors come with warranties of 10 years or more, and should easily last decades longer. Most systems offer the greatest return on investment in areas with high heating fuel and electricity costs. The economics of an active space heating system improve if it also heats domestic water, because an otherwise idle collector can heat water in the summer.

Some states offer sales tax exemptions, income tax credits or deductions, and property tax exemptions or deductions for solar energy systems. If you are not sure what benefits your state offers, contact your state energy office. (Check your local phone directory for the address and phone number of your state energy office.)

Heating your home with an active solar energy system can significantly reduce your fuel bills in the winter. A solar heating system will also reduce the amount of air pollution and greenhouse gases that result from your use of fossil fuels such as oil, propane, and natural gas for heating or that may be used to generate the electricity that you use.

If you are planning to purchase an active system you should learn as much as you can about the technology. In this way, you will know how to size and install your system properly, make a wise purchase, and maintain the components, and learn how to avoid problems or pitfalls that can occur.

Credit: U.S. Department of Energy