Solar houses part one
by Paul Kando
Part One: Passive solar
I am often asked about solar options, even by people whose homes lose much of their heating energy through leaks and lack of insulation. The next several articles will be devoted to solar energy. First, however, I must emphasize that switching to another energy source, including solar, makes no economic sense unless the efficiency of the building itself is improved first.
All of our energy ultimately comes from the Sun. It arrives on Earth as electromagnetic radiation, the product of the nuclear fusion of two hydrogen isotopes – deuterium and tritium – into helium, a process that releases copious amounts of energy. The most common use of solar energy in nature is the photosynthesis of plants which produces carbohydrates from water and carbon dioxide and releases oxygen for animals, including ourselves, to breathe. Plants and fellow animals provide us with energy in the form food, and indirectly with fuel to warm us, power our transportation and our economy. Bio-fuels, including firewood, are solar energy stored in plants. Fossil fuels are solar energy stored in the remains of prehistoric animals that once were feeding on plants.
Man-made solar systems collect and utilize the sun’s energy as either heat or electricity. Solar thermal systems take advantage of the fact that all matter naturally warms as it absorb solar radiation. Photovoltaic (PV) systems, on the other hand, convert the sun’s electro-magnetic rays directly to a flow of electrons, thus generating electricity. In houses, passive solar and water heating, as well as PV systems find practical application.
Passive solar – free solar heat collected by passive means, i.e. not requiring the use of pumps or fans, is already a part of every house because all windows collect solar energy in the form of heat. Windows facing the equator (south in our hemisphere) collect both direct and diffuse radiation, while those that do not “see” the sun collect diffuse radiation only. Direct radiation provides the highest temperature and therefore the bulk of useful heat. South facing windows, in spite of large heat losses that characterize all windows, are often net heat gainers. Over a heating season passive solar heat can provide as much as 10% of the heating needs of a Maine house.
The trouble is that, along with all other forms of heating energy, such houses waste much of this passive solar heat through their under-insulated, leaky building envelope. Furthermore in a typical house insect screens block a significant amount of sunshine – 15% to 40% of the solar heat gain, depending on the screen’s mesh size and cleanliness. In our cold climate all window screens should be removed in winter to maximize solar heat gain – an easy to implement, no cost improvement.
Technically speaking, passive solar heating is very efficient. Direct-gain collectors, like south facing windows in full sunlight, convert into useful heat as much as 65-70% of the solar radiation that strikes them. In contrast, photosynthesis has a theoretical maximum efficiency of around 11%.
Like any solar thermal system, passive solar heating relies on the following six working components: aperture, absorber, distribution, storage, insulation and control. In a building the aperture is the south-facing window; the mass of the building’s interior functions as both absorber and storage; distribution is by natural convection; and control consists of overhangs and shades that exclude unwanted summer heat gain. Naturally, for the system to work, the building itself must be well sealed and insulated or any energy collected will be for naught.
Built from scratch, passive solar buildings optimize the above design elements for maximum performance. The percentage of the required heat load met by passive solar heat – known as the passive solar fraction – translates to a corresponding reduction in heating costs. In well designed passive solar houses heating cost reductions as high as 75% have been reported. In buildings that meet the international Passive House standard, energy savings as high as 97% have been achieved. However these savings are not due primarily to passive solar heat gain, but to a very high degree of energy conservation.
In the typical existing Maine house a passive solar contribution of 15% is a reasonable expectation. This is achievable by common sense measures to minimize heat losses and remove obstacles (such as bug screens) that block solar heat gain. Assessments of how much passive solar contributes to heating your home as well as your home’s year round hourly solar access, are part of every energy audit offered under the auspices of the Midcoast Green Collaborative.
Next week: Seasonal solar water heaters