If you ever stepped barefoot on a sunbaked rock on a July afternoon, you know what solar energy is all about. Heat generated by the sun would melt a sphere of ice the size of the earth in 16.6 minutes. While only a small portion of that energy is intercepted by the earth, there is still enough to provide 646,000 horsepower for every square mile of its surface.
People have been trying to capture and use this enormous reservoir of free solar power since the dawn of history. The Greeks designed their houses around central sun-gathering courtyards 3,000 years ago. In pre-Columbian America, Indians of the Southwest carefully oriented their cliff pueblos to trap the warmth of the winter sun. During colonial times prudent homeowners built their homes with stove-warmed kitchens on the north side so that living areas would have sunny south walls. Thomas Jefferson, in designing Monticello, was keenly aware of the sun’s potential for providing heat and used special windows to help trap its warmth.Some of our most modern solar technology has its roots in the past. Functional solar systems were producing domestic hot water in 30 percent of the homes in Pasadena, California, before 1900, and by 1940 Miami had 60,000 of them. A flat-plate solar collector array built in 1907 used a sheet-iron absorber plate topped by glass. By 1914 solar collectors using copper tubes soldered to copper sheets were heating homes in California.
Today, solar systems for home heating are working almost everywhere in the country, not only in the Sun Belt but in the North too. Wherever local codes permit, you can save money and fuel with a do-it-yourself solar installation.
Solar collectors are becoming more and more common as their prices go down and oil prices go up.
Climax solar collector, patented in 1891, consisted of a series of black tanks in a glazed box. sun-heated water moved through tanks by convection for use in bath or kitchen below.
Solar panels on this south-facing roof and large windows on the east side allow the sun to warm the house during the day and provide consistent power. Costs for solar electric cells like these have dropped dramatically the last few decades and continue to become more affordable.

Where Solar Stands Today

The energy crisis of the 1970s spawned a kaleidoscope of dreams and schemes in solar home design. Some are solidly practical, some are innovative and radical, and a few resemble Rube Goldberg contraptions.
Among the most simple contrivances is a hot water system for a beach house. Tap water is run through 100 feet of black plastic hose coiled on the roof. The sun-warmed water feeds into an attic tank that is tapped as needed for showers or dishwashing. Almost as simple is a supplementary heating system that is not much more than a box sloping from the ground to the bottom of a south-facing first floor window. A black-painted divider of foam insulation suspended down the box’s center permits cool air from the house to slip down the channel beneath the foam panel and up its sun-warmed topside.
At the other end of the technological scale are houses with towering glass walls and ingenious devices for trapping and storing the sun’s heat. One New Hampshire home has two-story double-glazed plastic panels covering a black foot-thick concrete wall. During the day sun pouring through the panels heats the concrete. Ports at floor and ceiling allow cool interior air to circulate by convection up the wall and then reenter the building at the top after being warmed. At night and on cloudy days bushels of tiny plastic beads are blown into the space between the double glazing to prevent escape of stored heat. Come morning, the beads are sucked out by a vacuum pump to canisters in the garage until needed again. Another unique solar home in New Mexico has south-facing walls of water-filled drums stacked in racks like wine bottles. Ends facing outward are black to soak up sun and warm the water. At night windowless walls of insulating material hinge upward to seal in the heat.
Many technological improvements are on the way, including vacuum insulated collectors and cells that can convert sunlight into electricity. Engineers in New Mexico have built a “power tower,” topped by a boilerlocated at the focus of almost 2,000 mirrors, which has produced temperatures of 3000°F.

Will It Work in Your Home?

Solar water heaters have long been competitive with electricity, fuel oil, and natural gas in many parts of the country. Solar space heat is usually competitive wherever there is moderate to high solar radiation (see map). Even in low radiation locales, solar home heating can be a big money saver if fuel and utility costs are high.
Most successful systems take over part, not all, of the heating load. Such systems are cheaper to build and will usually pay for themselves sooner than those that rely exclusively on the sun.
For solar heating to be effective a house must have a large surface—typically a roof—facing within 10 degrees of true south. The building should also be well insulated (10 or more inches of high quality insulation in the attic and six inches in the walls for northern parts of the nation). Heating ducts or pipes should be wrapped with insulation. Storm doors and storm windows are important. All outside joints should be caulked and fireplace dampers snugly fitted and closed when not in use.
As a rule of thumb, solar heating makes good economic sense if it amortizes, or pays for itself, within0 years. In other words, your 10-year savings in home fuel consumption should equal or exceed the cost of installing a solar system. Generally speaking, solar heating will provide the biggest savings in homes with high fuel bills: if your fuel bills run more than $3,000 a year, a $15,000 solar system can be amortized in 10 years by cutting fuel consumption in half; but if you only spend $500 to heat your home, it is doubtful that solar heating will pay. Another consideration is the relative cost-effectiveness of a switch to solar versus a simple modernization of your present heating system. For example, you can often save more money by installing a fuel-efficient oil burner in place of an old one than you can by going solar.

Passive Systems: The Soft Side Of Solar Heating

There are two basic types of solar heating systems: active and passive. Active systems use liquid or air to absorb and transfer the heat to its destination. They require pumps and piping or fans and ducts to do the job but are relatively easy to install in existing buildings as well as new ones. An active system is the type most often used when an older home is being remodeled and upgraded with energy-saving features.
Passive solar systems, sometimes referred to as the soft approach because they require little in the way of hardware, let nature do most of the work. They do notneed pumps, blowers, or plumbing and usually have no leakage or winter freeze-up problems. They use large, heat-absorbing masses, such as concrete walls and water-filled drums, to trap solar heat as it passes through south-facing windows. Heat transfer can be by natural radiation or convection, or warmed air can be channeled to where it is needed with the help of vanes, dampers, and blowers. Because a passive system is a basic element of the house, it works best when planned as part of a new construction. However, there are many features of passive systems that can be incorporated to advantage in any house. Large glass areas on a south wall (shaded by arbors in the summer) can cut heating bills substantially in locations with moderate solar radiation. A simple greenhouse (see image on following page) can be an effective solar supplement to the home-heating system. Another passive heat collector is the thermosiphon air panel. Heat absorbed by a black metal sheet under insulated glass vents through a duct at the top. Cool room air enters the panel through a bottom duct.
Water-filled oil drums and large pulley-operated shutters of lightweight insulating material form south 

Instead of a Furnace Try a Greenhouse

Greenhouses are among the oldest and most familiar solar heating devices, but because they warm vegetables and flowers rather than men and women, most people do not think of them as replacements for a conventional home-heating device. However, what warms a plant can also warm a house, and lean-to greenhouses are being used more and more as passive solar collectors in homes with unobstructed south walls. In a typical design, such as the one shown at right, the original frame wall has been replaced by a cinder block collector wall, painted black to improve heat absorption.
The greenhouse, made with double-glazed panels of transparent plastic, is butted directly against the wall. Vents cut through the cinder blocks allow the circulation of heated air. The vents have dampers, but under most conditions the dampers are left open: the small amount of heat that escapes from the main house on cloudy days or at night helps to keep the greenhouse—and the plants growing in it—at the proper temperature. During an excessively warm day the vents would be closed and the greenhouse itself shielded from the sun.

rapping and Storing the Sun

The simplest passive systems use double transparent glazing to admit sunlight, wall and floor masses to soak it up, and some sort of drapery or shutter arrangement to prevent the trapped heat from escaping through the glass at night or on cloudy days. Temperatures produced by such systems are relatively low, but heat builds up significantly in good storage materials, such as concrete, adobe, and ceramics, which release it slowly to the building’s interior during nights and sunless days.

Making the System Fit the Environment

A passive solar heating system must be carefully tailored to local conditions so that the best use is made of available sunlight. The key element in any passive system is the collector wall. The generally accepted rule is that the wall should face directly south, although some experts recommend angling it slightly to the east to catch more sun in the early morning when outside temperatures are lowest. The ideal collector should be aligned so that the rays of the sun strike it perpendicularly. At 40°N, which is roughly the latitude of New York, Philadelphia, Indianapolis, Chicago, Kansas City, Des Moines, Denver, and Salt Lake City, a solar wall sloping at an angle of 60 degrees to the horizontal comes close to the ideal. Farther north, the ideal angle is greater; farther south, it is smaller. Nevertheless, considerations involving snow accumulation, summer shading, high initial cost, and structural integrity generally lead solar architects to specify vertical walls despite the loss of about 10 percent in efficiency.
Collector walls are almost always sheathed in double-layered glass with an insulating airspace between the layers. This type of glass provides the best combination of transparency to sunlight plus the ability to insulate against the loss of house heat. Reflecting glass and heat-absorbing glass both provide relief from overheating in the summer but only at the cost of reduced heating ability in the winter. Intelligent landscaping and carefully designed top and side shading surfaces solve the problem even more efficiently and effectively. Proper planting can moderate summer and winter temperatures by as much as 20°F. A curving row of evergreens bracketing the north and northeast quadrants of a building will serve as a windbreak against winter gales. Tall deciduous shade trees with high canopies placed on the south and southwest sides of the house will form a parasol against searing afternoon sun in summer. During winter, when the leavesVacuum collector is like a thermos bottle. Fluid-carrying pipes are surrounded by an evacuated insulating cylinder. Collectors can be placed on roof or against south wall.
Focusing collector is a trough, capped by lenses that bend the sun’s rays so they concentrate on a water-carrying pipe along the trough’s bottom. Unit swivels to track the sun.
Reflector-type solar collector has a curved mirrored surface that concentrates solar rays onto an absorber plate. The plate contains pipes that enclose liquid for heat transfer.

Active Solar Movers: Complex but Convenient

Active solar heating is an indirect process. Radiation from the sun warms an intermediate medium (either liquid or air) rather than heating the house directly. The medium is then piped into the house where its heat is extracted by heat exchangers. Excess heat is generally stored in a water- or rock-filled reservoir from which the house system draws as needed. Active solar systems are almost always paired with a conventional system that switches on automatically to tide the house through spells of cold, overcast weather. Typically, a forced hot air system is used as the backup.
The operating temperature of a typical active system is in the 120°F to 160°F range—twice as high as most passive systems. As a result, active systems can easily be put to use to produce hot water. They are also preferred by many engineers for space heating because they perform better than passive systems in areas with low to moderate solar radiation. Their adaptability to existing homes and compatibility with forced air heating systems are added advantages.initial cost, medium, the heat storage facility, a circulating pump, flow controls, and heat exchangers. Storage of sun-warmed water to supply heat at night and on sunless days requires a tank big enough to hold at least 1 1/2 gallons for each square foot of collector area plus an additional 2 percent allowance for expansion when the water heats up. The tank can be located in the basement, the garage, or behind a fence in the backyard. It can be aboveground or buried. It must, however, be well insulated against heat loss. If the system is to be used for domestic hot water heating only, an insulated 80-gallon tank will do the job for a family of four or five.

The Collectors Are the Key

The prime components of any active system are the collectors. These are the devices that trap the sun’s heat and carry it into the building in a stream of moving air or water. There are three basic types of collectors: vacuum, focusing, and flat plate.
Vacuum collectors consist of three concentric tubes. The two inner tubes carry the transfer liquid. A vacuum between them and a transparent outer tube insulates the fluid against heat loss. The tubes are highly efficient, producing fluid temperatures over 300°F in commercial use. For domestic application, an automatic drainback system prevents overheating. The collectors work well in overcast, and are seeing increasing use in the far north where their superior fuel-saving capabilities compensate for their high initial cost.
Focusing collectors use lenses or mirrored surfaces to concentrate sunlight on fluid-carrying pipes. Such collectors can produce temperatures of several thousand degrees Fahrenheit. They are more efficient per square foot than flat-plate collectors but require a complex tracking system to keep them pointed at the sun. Not only is the tracking system expensive, but the reflectors themselves are two or three times as costly per square foot as most flat-plate types. If they drift off the sun, or if skies are overcast, they stop working completely.
Flat-plate collectors are simple, inexpensive, and can be mounted on the roof or walls of an existing home or on the ground in angled frames. Most will collect some heat in light overcast.
For maximum heat production the surface area of an array of flat-plate collectors should be equal to about one-half the floor area of the house. However, a collector array with only one-third the area of the floor will handle 40 to 60 percent of the heat load in most regions, provided the house is well insulated and has a south-facing roof on which the collectors can be mounted. Most homeowners settle for such a system, since its lower cost results in faster payback of the investment.lectors, begins to circulate warm water from the storage tank to the heat exchanger in the return air duct. In most systems the storage tank will hold enough heat to maintain moderate indoor temperatures for two or three days. After that, the backup furnace must take over.

Underground Residences And Backyard Cookery

Underground architecture is attracting more and more attention from solar heating experts and others interested in squeezing the last drop of heating potential from the sun’s output. The temperature at a depth of 10 feet or more is a nearly constant 55°F summer and winter, day and night, in the cold north and the warm south. As a result, only enough energy is needed to raise indoor temperatures by 10 or 15 degrees in order to have a comfortable year-round living climate—a requirement that is well within the reach of even the simplest solar heating systems.
Architects employ a wide variety of techniques aimed at eliminating any dampness or a cavelike atmosphere in their underground dwellings. Skylights, dropped gardens, solaria, and light wells are common features. Two particularly successful approaches are illustrated on the next page: a hillside house and an atrium-type house.
Solar homes that are built into a hillside almost always feature an exposed south side that accommo
dates, at the very least, double-glazed conventional windows or sliding glass doors. These south-facing expanses of glass serve as large passive solar collectors. Buried walls on the north, east, and west sides act as storage masses, soaking up solar heat during the day and releasing it to the house interior at night and on cloudy days, much as in any passive solar home. The more sophisticated and elaborate in-hill design shown at right, below, includes a broad sweep of south-facing glass along the roofline, windows backed by a concrete heat storage structure along the south-facing side of the second floor, and a solar greenhouse stretched out along the first floor’s south wall. Concrete floors and walls inside the home store part of the sun’s warmth; the excess is fanned through ducts to a bed of stone rubble beneath the ground floor. At night or on sunless days the same system of fans and ducts pulls stored heat from the rubble for use in warming the house. Shutters of insulating material hinge down from ceilings to prevent the loss of heat through the glass at night.