About 20 years ago I bought a 2,000 square foot uninsulated house in upstate New York which got me interested in solar space heating. After the usual insulating and weather-stripping, I added an attached greenhouse and built a window box solar space heater. These worked well enough to make me want to explore further solar possibilities.
After reading everything I could on the subject, I decided to build a passive solar earth sheltered home in Tennessee. I researched the subject by reading magazines and books and visiting every solar building I could. The book that gave me the best perspective on the history of solar design and its uses was A Golden Thread; 2,500 Years of Solar Architecture and Technology. I’d like to give you a summary of what I’ve learned.
The earth-sheltered passive solar Kalmer home is topped by a solar water heater on the roof. The house is constructed of cedar cordwood covered with hand-split oak shakes.
The evolution of solar technology
Solar powered home and water heating technologies have been evolving for thousands of years. The effectiveness of many of even the oldest solar technologies, especially the simpler ones like passive solar architecture, have been adequate for centuries.
The steady evolution of solar architecture and technology has been periodically interrupted by the discovery of apparently plentiful and cheap fuels, such as new forests or deposits of coal, oil, natural gas and uranium. Successive civilizations have short-sightedly treated this energy capitol as income. This attitude persists today.
We speak of “producing” oil, as if it were made in a factory, but we don’t produce oil; all we do is mine it and burn it up. Neglecting the interests of future generations who are not here to bid on this oil, we have been squandering, in the last few decades, an inheritance of hundreds of millions of years. Only recently have we begun to come full circle to the same realization that similar boom and bust cultures have reached before us: that we must turn back to the sun, and seek elegant ways to live within the renewable energy income that it bestows on us.
It is very important to appreciate the lessons of earlier cultures, lest we repeat their mistakes.
From the wood-short Greeks and Romans onward, people became aware of the limits of their dwindling fuel resources. They then rediscovered much of the earlier knowledge of permanent, practical solar energy.
At several points in history-the latest being today-observers of the energy scene have bemoaned the absurdity of having to rediscover and reinvent what should have been practiced continuously. Today we stand precisely where several earlier cultures have stood. We have suddenly learned the transitory nature, the vulnerability, and the high social, ecological, and even economic costs of depending on nonrenewable hydrocarbons to hold our society together. But we still play elaborate games of self-deception by giving these precious fuels (and the electricity made from them) tax and price subsidies which in the US alone total roughly $100 billion a year. Although some available solar technologies are more expensive than oil and gas, almost all cost several times less than what we would have to pay to replace them with nuclear power or synthetic fuels.
Perhaps this is the last time the inevitable solar age will be temporarily forestalled by a false sense of abundance. For unless some new form of energy now wholly unknown is discovered soon, there are no long-term energy alternatives other than nuclear reactions kindled artificially or the natural energy flows driven by nuclear fusion sited at the appropriate distance of 93 million miles.
The Greeks ravaged forests for fuel and building materials 2,500 years ago. By the fifth century BC many parts of Greece were totally stripped of trees. This led to the earliest examples of solar architecture based on the changing seasonal position of the sun.
The Greeks knew that in winter, the sun travels in a low arc across the southern sky; in summer it passes high overhead. They built their homes so the winter sunlight could easily enter the house through a south facing portico, similar to a covered porch. Overhanging roofs and eaves shaded the house from the high summer sun.
Socrates said, “In houses that look toward the south, the sun penetrates the portico in winter, while in summer the path of the sun is right over our heads, and above the roof, so that there is shade.”
The Greeks planned cities so that each house had good southern exposure. In the first century AD Romans had solar rights laws.
About 500 BC the great Greek playwright Aeschylus noted that a south facing orientation was a normal characteristic of Greek homes. It was a sign of a modern or civilized dwelling, he declared, as opposed to houses built by primitives and barbarians, who, “though they had eyes to see, they saw to no avail, they had ears, but they understood not. But like shapes and dreams, throughout their time, without purpose they wrought all things in confusion. They lacked knowledge of houses turned to face the sun, dwelling instead like ants in sunless caves.”
Sophisticated solar communities were built by the Pueblo Indian tribes of the American southwest. The Anasazi built sky city Acoma with full sun exposure.
The use of solar heat in horticulture also has a long history dating back to early Rome, where the earliest glazing materials were thinly split stone, such as mica or selenite.
During the 1600s, the French and English developed a technique of constructing fruit walls, vertical or sloping masonry walls facing south or southeast, to which they attached the branches of fruit trees or grape vines. The walls absorbed solar heat, and would greatly lengthen the growing season, even allowing tender blossoms to survive a hard freeze.
Solar water heating has a shorter history, starting with bare metal tanks painted black and tilted to face the sun. An 1891 patent combined that technique with the solar hot box, increasing the tank’s ability to collect and store heat. This was our nation’s first commercial solar water heater, called The Climax.
Visible light makes up only 46 percent of the total energy emitted from the sun, while 49 percent is in the infrared band, which we experience as heat. The remaining portion is in the ultraviolet band. Earth intercepts only a tiny 2 billionths of the sun’s total radiant output, but this is the equivalent of 35 thousand times the total energy used by all people.
Anatomy of a direct gain passive solar home
For the past 15 years, I’ve been living in a solar collector-otherwise known as a direct gain passive solar home. It is naturally well lit, thanks to many large, evenly spaced windows on the south wall. These appropriately shaded windows allow direct sunlight to reach the back of the building in winter, but allow no direct sunlight inside in summer. The light which does enter strikes the textured, brown concrete floor, slip formed stone walls, and large stone fireplace, gently warming these surfaces which absorb and store heat, moderating temperature fluctuations. Having insulation on the exterior of the building allows these thermal masses to remain at or near room temperature, absorbing heat during sunny days and radiating warmth at night. This makes interior temperatures very stable, naturally staying warm in the winter and cool in the summer. Because the floor and walls are doing double duty as thermal flywheels, temperatures also remain very even throughout the house.
This simple system is effective enough to require backup heat only after cloudy days in December, January, and February. My only backup heat is a large stone fireplace, modeled after the high thermal mass Russian and European designs. Mine also provides domestic hot water. My space and water heating bills are near zero.
Passive solar systems are simple in concept and use, have few or no moving parts, require little or no maintenance, and require no external energy input. Passive systems collect and transport heat by non-mechanical means.
Active systems employ hardware and mechanical systems to collect and store heat, often using some outside energy source such as electricity for fans and pumps.
The greenhouse effect is most commonly demonstrated by leaving a parked car in the sun with the windows rolled up. We all know how hot that can get because it lacks storage. In a passive solar building, your windows are your collectors, your walls and floor are your absorbers and storage. Water and phase change materials can also provide storage. Typically one half to two thirds of total surface area is masonry. An open design aids heat and light distribution.
A direct gain building is the simplest live-in solar collector-heat storage and distribution all in one. They work well on sunny days and in cloudy climates by collecting and using every bit of energy that passes through the glazing, direct or diffuse. Masonry thermal storage materials include concrete, concrete block, brick, stone and adobe.
Indirect gain is when sunlight first strikes a thermal mass, located between the sun and the living space, commonly called a Trome wall.
An attached greenhouse is a combination of direct and indirect gain.
This attached greenhouse was built for about $250. Barrels filled with water provide thermal mass.
Isolated gain is when collector and storage are isolated from the living space.
Temperature fluctuation can be controlled by operable windows or vents, shading devices, and a backup heating system.
Windows can still receive 90% of possible gain when oriented within 25 degrees east or west of true south.
Incorporating solar design into a new structure is fairly simple and low cost. My passive solar, earth sheltered, post and beam framed house cost about $8 per square foot to build, not including labor. There are no special materials required-just a rearranging from typical design. Looking at ordinary homes, I usually see picture windows facing the road, when it would have been a simple change initially to orient to the south, providing a lasting energy gain in winter, instead of a loss. South facing windows provide a net energy gain; all other directions are an energy loss.
Also, many homes are built with brick or stone on the outside of the insulated shell, when a reversal of positions, with insulation exterior to the thermal mass, would greatly improve the homes’ thermal stability and comfort at little or no added cost. Having heat absorbing masonry materials as part of the home’s interior structure, such as floors and walls, can reduce overall building costs, especially when, if things are designed correctly, the need for a furnace or boiler can be eliminated, or at least downsized.
Sometimes there are minor problems with having sunlight entering your home. At times I find a certain chair too brightly lit for comfort, but I just move to another. This is the advantage of spreading the windows out along the southern wall. You have some solid wall in between windows, to minimize glare and provide some shaded areas. I suppose the sunlight also helps fabrics fade, although I haven’t noticed this occurring. People in more populated areas may have some privacy concerns with a lot of large windows facing their neighbors, but this can be designed around, possibly going to a Trome wall, or indirect gain system.
Solar design in existing structures
Even though it’s simpler to incorporate into a new structure, a lot can be done with existing structures. Energy conservation must be the first step, since there is little use collecting solar heat if you can’t hold onto it. Weather-strip and insulate, possibly adding exterior insulation. Consider moveable insulation for doors and windows at night. A small addition of a double door or airlock entry way can increase energy efficiency, and give you a place for all those shoes and coats.
An attached solar greenhouse, or sunspace, can provide heat, food, beauty, and additional room. Plants thrive in them. My 8′ x 18′ attached solar greenhouse cost $250 to build, and my wife enjoyed it and what it can do for plants so much that we now have a 22′ x 48′ freestanding greenhouse for her plant business.
Properly placed vegetation is also important, even for houses with no solar aspect. Deciduous trees, shrubs or vines on the east, west, or south sides will lose their leaves in winter to allow sunlight in, while providing cooling shade in the summer. Evergreen foliage on the north side will buffer winter winds.
Solar hot water
Solar hot water can be added to existing structures, as I did to my house nine years ago. I am now past the point where the money I invested in the solar water heater equals the money I would have spent on electricity to heat water. Consider the fact that in the next five to eight years you are going to pay the cost of a solar water heater, whether you buy one or not. It’s your choice-you can invest in solar now, demonstrating your support for sustainable energy, and getting free hot water after your payback period, or continue to pay ever-increasing energy bills, which indicates your support for maintaining the status quo.
Passive solar design is not just about heating. Many solar design considerations also help with summer cooling. Thermal mass resists overheating, direct earth contact through slab-on-grade, and earth sheltering all contribute to cooling in hot weather. The most efficient shape of building for maximum winter solar gain is elongated along the east-west axis, giving a large south facing wall and smaller east and west facing walls. This design also minimizes unwanted summer heat gain on the hot east and west sides.
Radiant barrier placed in the attic or roof system can reflect 97% of radiant heat, keeping the excess solar gain in summer from the living spaces. Light colored roofing also helps. During the hottest months, I place soaker hoses along the ridge of my roof, to trickle water slowly enough so it evaporates before reaching the edge, for a solar evaporative cooling effect. Vegetation is usually the best shade, because it is later arriving in the spring, when we need more solar gain, and usually provides shade into fall, as well as proving its own evaporative cooling effect.
There are several low-cost, low-tech devices that anyone can use, such as an integral passive solar water heater which is basically a tank in a box. Window box collectors, window greenhouses, and attached greenhouses can help heat the house. Solar food dryers, cookers and ovens can also reduce utility bills. Many of these can be homemade, inexpensively.
Some typical questions that I get about my solar home are:
- Doesn’t it get hot inside in the summer? No, proper orientation and shading prevent sunlight from entering the building during the summer months, keeping it cooler than the average home.
- Doesn’t it cost more to build? No, properly done it can cost less.
- Does it have to face south? Yes, in this hemisphere it does-within 25 degrees east or west of south.
As sure as the sun will rise tomorrow, our energy costs will also continue to rise. Getting heat from sunlight is economical, ecological, dependable, readily available, time tested, powerful and empowering. This free and equally distributed energy source arrives at our homes almost daily. Let’s all try to make better use of it, for our own well-being as well as the planet’s.