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Solar Energy systems don't have to be complicated or expensive to do an effective job of saving energy. The familiar active systems and their associated hardware - pumps, thermostats, rooftop collectors, etc. - represent only one method of converting a home to solar heat. There are simpler and cheaper alternatives. Passive solar systems, unlike active solar systems, require no significant mechanical equipment and operate without any external power. Instead, they rely on natural heat movement through convection, conduction and radiation.

Passive systems sometimes require more work on the part of the owner than active systems do. For instance, to ensure the greatest possible energy savings you may have to cover glass areas at night and uncover them in the morning. Still, their low cost and simplicity makes them especially good candidates for do-it-yourself projects. So if you've been thinking of going solar but have hesitated because of the expense, consider the systems described in this fact sheet.

Before You Begin (410, 448, 937)

Introducing a new source of heat into a leaky, drafty house may not make good economic sense. Granted, the thought of weatherstripping, caulking and insulating may not enchant you as much as adding a solar system does - but taking conservation measures first will save you more money than solar systems. Such measures can save more energy at a lower cost than most solar energy systems.

Before looking into a solar retrofit, take an energy tour of your house to determine where heat can readily escape through conduction and infiltration. (Conduction is how heat moves through solids such as windows and doors, and infiltration occurs when heat exits through cracks and gaps.) Once you determine where heat is escaping, weatherstrip, caulk, and insulate to the recommended levels. Your local utility or energy service company can provide assistance in identifying and installing appropriate energy conservation measures.

The Five Passive Elements

The systems featured below are somewhat different in their design and capabilities. However, each includes the same elements which must work together for the system to be successful: a collector which is the large glass (or plastic) area that sunlight passes through; an absorber which is the dark surface that sunlight strikes after it passes through the collector; a heat distribution method which in a strictly passive application means reliance on natural methods of heat transfer (some passive systems are "hybrids" and use fans, ducts, and blowers to boost air movement); and a control (or heat regulation device) to prevent overheating in summer and heat loss during sunless periods. Passive systems usually include storage in the form of materials that absorb and hold heat well, such as masonry or water.

Space Heating

Most homeowners use more energy for space heating than for lighting, air conditioning, and water heating combined. Adding one of the following systems won't mean you can shut down your conventional furnace, but it will allow you to rely on it less.

Thermosiphoning Air Panels (442, 508, 940)

One of the simplest ways to bring solar heat into your home is through a thermosiphoning air panel. Basically, a TAP (sometimes called a vertical wall collector) is a well-insulated enclosure, covered with glass or plastic glazing. It also contains an absorber plate, usually made of corrugated aluminum. TAPs include upper and lower vents to permit heat distribution through the thermosiphoning effect. (Thermosiphoning is natural air movement that occurs because hot air is buoyant and therefore rises, while cold air is denser and falls.)

A TAP is usually mounted flush against the south-facing wall. When sunlight strikes it the absorber plate is heated, warming the surrounding air. The air rises, entering the house through the top vent. Simultaneously, cool house air is drawn into the collector through the lower vent. This cool air is heated, and the cycle continues as long as the sun is shining.

At night, reverse thermosiphoning will occur unless the lower vent includes a back draft damper. (Reverse thermosiphoning happens when warm house air is drawn into the collector through the top vent forcing cool air from the collector into the house.) A back draft damper will seal the vent automatically when air flows in the wrong direction.

TAPs usually don't contain any provision for storing heat. Many of the TAPs sold commercially or built from plans are relatively small - 24 or 32 sq. ft. - which is only enough collector area to provide heat for a medium-sized room during the day. However, more heat can be provided if two or more TAPs are combined. Such a system may require the use of a fan to circulate heat beyond the adjoining room. A slight variation on the TAP are window box collectors which are very similar except they are designed to hang from south-facing windows. They are small (usually about 20 sq. ft.) and like TAPs, contain no provision for storing heat. Passive window box collectors rely on natural convection to circulate the air through the collector; active versions include a fan that draws room air through the collector and forces the air back into the room.

Air Collector Panels with Fans

One or several collector panels, similar to TAPs, can be installed on the wall or roof to collect solar energy. Instead of using the thermosiphoning principle to circulate the heated air, these collector panels (sometimes called solar air space heaters) have fans installed in the wall or ceiling vents to draw air from the room into the collector where it is heated and blown back into the room. A thermostat is used to control the air flow from the collector. Like TAPs, these solar units do not usually have a heat storage system; therefore, they will only operate when the sun is shining.

Amount of Heat Provided

During a typical heating season, a 20 sq. ft. TAP, air collector panel or window box collector will produce on the average between one and two million Btu. A 40 sq. ft. unit would produce 2 to 3 million Btu per heating season. The amount of heat produced will vary according to the amount of solar radiation (sunshine) available, the length of the heating season, and the efficiency of the unit. The amount of heat produced by these small units is typically enough to heat a medium-sized room on a sunny day.

Before purchasing any unit, ask the dealer what the collector can provide in Btu in your area, and to certify the estimate in writing. In evaluating the cost of these units, you may want to compare the number of Btu you can expect from the solar collector to what you require to heat your home and to what it would cost to produce those Btu using a regular energy source. For example, if you assume that the solar unit has a lifetime of 20 years, then the lifetime output of a 20 sq. ft. unit would be about 30 million Btu. At today's energy prices, 30 million Btu would be worth $200 to $600, depending on the fuel source and the area of the country. The Btu output from a larger unit, say 40 sq. ft., would be worth $400 to $1,200 over the lifetime of the unit. These estimates are based on current energy prices and could be higher if the cost of conventional fuels increases.

A TAP or window box collector can be built in a few weekends for $300 to $500, or less if some materials can be obtained cheaply. It is important to use well-designed plans that provide for sufficient air flow and some type of insulation in the collector unit.

Trombe Walls (940)

Named for Felix Trombe, who developed the concept, a Trombe wall in principle is similar to the TAP. The major difference is that a Trombe wall also stores heat. To add a Trombe wall without extensive remodeling, your house must already have a thick, uninsulated south-facing masonry wall. Such a wall can easily be converted into a solar heating system.

To transform the wall into a solar collector it must first be painted a dark color, preferably flat black. The paint must be chosen carefully because it must be able to withstand temperatures of 200 degrees F or more. A frame that supports a double layer of glazing (the collector) is then erected a few inches in front of the wall. Double glazing should be used in most parts of the country to reduce heat loss.

The wall will also act as storage, holding absorbed solar heat for five to seven hours. After this time, the heat begins to migrate into the living area. If the wall adjoins a room that requires daytime heating, vents can be cut through the wall to allow heat to be provided immediately. As with a TAP, if vents are used they should be equipped with back draft dampers to prevent warm room air from exiting through the vents at night. In new construction, movable insulation between the glazing and the wall is sometimes included to minimize heat losses, but movable insulation is difficult to retrofit adequately. Instead, energy saving curtains or panels can be used at night to cover the outside of the glass.

While a Trombe wall retrofit is more complicated than adding a TAP, it will usually provide more heat. The cost varies greatly depending primarily on the source of the glazing. If you can use salvaged patio door glass for example, you'll save a good deal of money.

Sunspaces and Solar Greenhouses (769, 992)

It's easy to understand why sunspaces are the most popular passive retrofit. Besides reducing a home's energy consumption, they provide additional living space and allow you to grow vegetables year-round.

A sunspace is a partially or totally glazed room that is added onto the south-facing wall of a building. Unlike a conventional greenhouse, a sunspace is well caulked and weatherstripped, and includes materials for storing solar heat. Consequently, a sunspace only occasionally requires auxiliary heating and collects extra heat which can be distributed to the main living area. Providing adequate heat storage is very important if the sunspace will be used for food production of living space since storage will moderate temperature swings. Heat can be stored in an uninsulated masonry wall common to the greenhouse and the house. Additional storage options include water-filled containers or a masonry floor. A fan or blower is sometimes used to enhance heat distribution. Summertime overheating can be prevented through ventilation, by whitewashing the glazing, or by using venetian blinds or energy-saving curtains. Winter heat loss can be minimized by using movable insulation or reflective coatings.

A do-it-yourselfer can build a sunspace for under $3,000. Kits for sunspaces are available although a basic kit may not include everything that's needed. Professionally designed and constructed sunspaces, like any home remodeling project, can be quite expensive - $5,000 to $10,000 or more.

A sunspace that is also used for food production or living space is probably a good investment. If you are only interested in reducing your heating costs, a sunspace may not be the best solar option. Depending on the size of the sunspace, its efficiency, and your climate it will only provide on the average, 20% to 40% of the heating load. A TAP, for example, may provide almost as much heat but for less money.

Water Heating

A significant portion of a home's water heating needs can be met through low-cost passive solar water heaters, either batch heaters or thermosiphoning water heaters.

Batch Water Heaters (832)

Batch water heaters (sometimes called breadbox heaters) are the simplest solar hot water systems. They consist of one or more tanks painted black and placed horizontally in a rather large, well-insulated collector box. Often, hot water tanks that have had their outer coverings removed are used for the tanks. Aluminum foil or other reflective materials are used to line the box and reflect sunlight onto the back and sides of each tank.

The south side of the enclosure consists of glass or plastic glazing to admit sunlight. A double layer is needed for most areas, and triple glazing may be required in extremely cold climates.

Water from the city enters from the bottom of the collector as solar-heated water is drawn off the top.

Freeze protection for batch water heaters is particularly important because the water tanks and plumbing connections are outdoors. Storage tanks generally don't freeze although nighttime heat losses can be high; the large thermal mass of the water prevents the water from freezing.

Tank freezing can occur during periods of low usage (such as vacations) or during periods of unusually cold weather. Plumbing connections are more susceptible to freezing and therefore all exposed connections must be heavily wrapped with insulation. Another option is to drain the collector at the first sign of frost. The cost of a batch water heater is affected greatly by the number of tanks required for the system, which in turn depends on your hot water requirements. Collectors can be built by do-it-yourselfers for under $650. They retail for about $600 to $2,000.

Thermosiphoning Water Heaters (688)

Thermosiphoning systems consist of a solar collector panel to absorb solar heat, and a separate storage tank to hold solar-heated water. The solar collector is faced true south at an angle equal to the latitude. It must be mounted at least a foot below the storage tank to permit thermosiphoning (upward movement of water by natural convection). When the water in the collector is heated, it becomes less dense, and rises to the top of the storage tank. At the same time, cool water from the bottom of the tank flows into the bottom of the collector.

Although thermosiphoning systems can be quite efficient, and supply 40 to 60 percent of your hot water, 2 problems keep them from being used more often. First, because the storage tank must be installed above the collector, it is often placed on an upper floor, or high in the attic - above the roof rafter. In some cases, the roof or flooring may have to be reinforced because water tanks are quite heavy.

Second, thermosiphoning collectors in their simplest form contain no safeguard against freezing. This is important because water remains in the collector whenever convection stops (during sunless periods). If this water freezes it can expand with enough force to burst the piping of the tank. Freezing can be prevented by using movable insulation, an antifreeze/water solution, or by installing a valve allowing the water to be drained at night. But operating a thermosiphoning water heater is cheaper and easier in areas where freezing does not occur. If used elsewhere, the simplest solution is to drain them before the first frost.

Thermosiphoning water heaters are more complicated to install than batch water heaters and therefore are usually more expensive. Local building regulations may require that a plumber do the installation for any type of water heating system.

Do-it-yourselfers should thoroughly investigate the requirements and concepts before tackling a water heater installation.