Passive Solar heating

Steve Moore - solar farmer
Steve Moore, solar farmer

One of the easiest ways to start getting off-grid is to build your home, or a home extension, using passive solar design. This kind of structure uses the way the sun interacts with the building itself to capture the sun’s energy and use it for heating system, cooling, or lighting. All that’s needed is a careful attention to building site, architectural design, and building materials. Not only is passive solar energy use affordable and efficient, but can result in beautiful living spaces full of natural light and pleasant landscaping.

Pennsylvania cultivator Steve Moore and his wife Carol have been farming organically for 26 years, using horses for the farm work up until last year. They also had dairy heifers and were raising pigs for market. The farming operation included a greenhouse. Twelve years ago, needing more income, they were considering expanding the greenhouse.

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But they started being aware of all the gadgets the greenhouse contained and that it was taking 1,500 gallons of propane every 10 days to heat it. They realized they were missing the mark. They changed their operation and since then have been growing commercially in a passive solar greenhouse, using no fossil fuel at all.

Instead, the greenhouse retains its warmth with double-glazing made of multi-year plastic sheeting. At night, an additional sheet of plastic is currently placed over all the crops to hold in further heating. Initially, the greenhouse was heated by a thermophilic (hot) compost pile, but now a more carbon-efficient mesophilic (warm) pile is used. In addition, the compost pile has seedling flats placed on top of it and serves as a heating table.

At night, this is covered by plastic sheeting as well. The greenhouse has grown lettuce in an average of 5-week cycles (rather than the normal 8-week ones experienced by most people) – except during the three hottest months of the year – with temperatures dipping as low as -17 degrees Fahrenheit in its southern Pennsylvania location.

Moore stated that ten years ago they were struggling, and following the current advice to “get big,” they rented more ground and bought more horses. Then a neighbor lent Moore a copy of How To Grow More Vegetables. He thought it was good, but totally impractical. However, he attended one of John Jeavons’ Three-Day Workshops and has become more and more involved with the method.

He has found that the basic precepts of GROW BIOINTENSIVE growing – improved soil structure, raised permanent beds, increased organic matter, close spacing and transplanting – all work well in a passive solar greenhouse.

Moore said they are experimenting with nutrition and growing most of their own food. They are focusing on extending the growing season of plants rather than growing out of season. After 4 years of growing GROW BIOINTENSIVEly, the soil in the greenhouse now contains 14% organic matter. And this was soil which was compacted by machinery while the greenhouse was being built.

Moore feels that using transplants gives them security, because they know they will have plenty of seedlings at the right time. He mentioned that they use alfalfa in their compost to minimize the nitrate level accumulation that can happen in greenhouse growing. And in the 12 years since they have been using an unheated greenhouse, they have never had a disease problem.

During the warmer weather, hot weather crops, such as sorghum, peppers, eggplant and onions, are grown – some of them in troughs hanging from the ceiling of the greenhouse. Peppers have produced 6.0 to 11.1 times the U.S. average, eggplant 5.0 to 6.9 times, onions 2.0 times and bok choi 7.4 times.

The first decision to make is where to build. Obviously enough, the building must be in reach of sunlight. But if the aim is to warm the house in winter and cool it in summer, the landscaping around the building site must be carefully considered. The U.S. Department of Energy estimates that appropriate landscaping can save 25% of a household�s energy towards heating and cooling. The shelter of trees provides cooling shade in summer and a helpful wind break in winter, regulating temperatures in and around the home. In addition, water evaporating from trees, a lawn, or other vegetation can cool the neighboring air up to 5 degrees Celsius. Unfortunately the welcome shading and cooling characteristics of trees in summer may not be as welcome in winter, when even a bare tree can significantly limit the penetration of sunlight to a building. Because of this one might consider limiting trees on the south side of buildings, or ensuring that large trees don’t block solar energy collecting units on the home.

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Indeed, in the Northern Hemisphere, large, south-facing windows are a key ingredient in building design. They collect warming sunlight in winter, and, unlike east- or west-facing windows, don’t take in excessive light during long, hot summer days.

As critical as location and architecture are, to take full advantage of passive solar design’s potential for affordable and efficient energy use, one must choose proper materials. With passive solar systems, the building materials collect, store, and transfer energy. Of course many materials have upsides and downsides. One of the most basic materials in passive solar design is glass. Windows are an excellent way of bringing the energy of the sun into the home, but if constructed improperly or out of less than ideal materials, they can be a nemesis to passive energy use, over-heating rooms in the summer and allowing precious warmth to trickle out in the winter if the frame is not well-constructed. Tight frames and good quality workmanship are important for maximizing the efficiency of the windows, but the actual building materials play a significant role in how solar energy acts within a building.

The choice of glass, then, is critical. Translucent materials permit short-wave solar radiation into the building and prevent long-wave heat radiation from exiting. The amount of light that penetrates glass depends on what angle it strikes the glass itself. At a perpendicular, or 90 degree angle, the most light is allowed to enter. And at small angles of 30 degrees or less, the majority of light is reflected by the glass and does not enter. After the house is built there is not much to do to change the angle at which light strikes the glass, but there are other variables that can be changed.

Glazings can alter how light is allowed to penetrate and escape buildings through the windows. They can be made of a variety of materials, including glass, acrylics, fiberglass, and other materials.

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