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Passive Solar Design Concepts: A Brief History and Case Study |
 Passive Solar Design Strategy is nothing new. Cliff dwellers of the southwestern United States oriented their homes towards the south to maximize solar gain.
Viking ships used crude solar reflectors to burn the sails of enemy ships, and in 700AD, Persians used solar power in the sailing of ships as well as incorporating solar strategy into their building designs.
Passive Solar Design Strategies have long been used for drying bricks for buildings, a technique still used in many parts of the world.
While true Passive Solar Strategies are age old and time tested, basic principals must be correctly understood and applied. For instance, in the late 70’s and early 80’s, many builders and designers wanted to take advantage of the growing number of home buyers trying to harness the sun’s free energy.
Tax incentives under President Jimmy Carter along with high oil prices spurred a Passive Solar Design resurgence in the US. Many builders, however, did not understand true Passive Solar Design concepts. Many homes overheated in the summer and experienced major heat loss during the winter.
The buildings that were built correctly, however, provided comfortable living and work space year round while saving on energy costs as promised, thus proving the sustainability of these strategies.
Case Study/ Design Concepts
The sun travels from east to west, sitting lower on the south side of the horizon. A solar structure should take advantage of proper location to harvest this free energy. Many people say “face” your structure to the south, but this can be misleading. The term orientation refers to the direction that a building faces yet is not limited to that definition. Generally, homes are built to take advantage of a particular view and are usually facing a road. While these structures may not be facing south, they still have the proper ratio of windows, or glazing, per square footage. In other words, one can orient a building to the south, while still taking advantage of a view that faces in a different direction.
 The home described in the following case study had a beautiful view to the west, but had a great solar site as well. The house was to be a small, 24’ X 24’ rough framed, straw bale hybrid home and square in shape. A square is the second most efficient use of space; a circle is said to be the most efficient use of space. Most passive solar buildings use the shape of a rectangle to collect the sun’s energy on one of the longer sides of the house. This house was cut into the side of a gentle north hillside. The square was also chosen to eliminate major excavation as the hillside grew steeper toward the east.
The side facing south had the most windows (about 12 to 20 percent to first floor square footage). The west side (gable side, in this case) had the fewest number of windows (about two percent), despite the fact that the view was much better. To take advantage of the view, a large lower deck of locally milled black locust (a rot- and insect-resistant wood) was built to provide outdoor living space for warmer times of the year, thus the view was still enjoyed while not overheating the house in the hot months of the summer. This is one critical aspect of solar design that is sometimes ignored when windows are added to the west side of a building to take advantage of a good view. With more than two percent (576 x.02= 11.32 sq.ft of windows in this case) of windows to floor space, a home can easily overheat unless shaded by an external device… such as exterior blinds or trees. Overheating can also occur on the south side of the house as well. Employing properly sized overhangs are the most common strategy to prevent overheating on the south exposure. The sun sits lower in the winter when you really want to harvest the energy and higher in the summer when you want to block the sun’s heat. By using your city’s Latitude (for this case, Boone, NC at 36.2 degrees), a protractor, and some graph paper, one can easily learn to determine how long the overhang has to be in order to block heat in the summer, while letting winter sun penetrate deep into the house.
This house had an open floor plan which helped in a couple of ways: first, it enabled a fairly small space to seem much bigger. Secondly, open floor plans let the air move freely creating a convection cycle and spreading the warmth, so to speak. The house is two stories with a gable intersecting roof forming two large gable dormers on the south and north sides. The roof pitch is 12/12, or 45 degrees, which is a pretty steep pitch. Traditionally, steep pitched roofs were used in wet climates to ensure a quick departure of water from the roof. The overhangs were two feet, allowing the sun to be blocked in the warmer months while allowing winter sun to penetrate deep into the house. Longer overhangs also help protect the house from moisture by diverting water away from the foundation.
Another solar concept is thermal mass, which can be compared to a battery that stores energy. Dense materials such as concrete, rock, tiles, and even water can be used as thermal mass. Ever noticed how a big boulder or rock that’s been exposed to sun all day can still be warm even after the sun’s set? The thermal mass of the rock has stored energy from the sun and is still releasing it. This house was built on a monolithic slab for thermal mass. The slab was actually thickened on the south side of the house to provide more solar storage.
Flat, dark colors act as good absorbers. In this house, dark green tile was used in the kitchen dining area (south east/west) for maximum absorption. Other strategies such as reflectors are used in other passive solar applications, but normally not with buildings. Passive solar can also be used to cook food, dehydrate food and distill water, and passive solar greenhouses can be used to extend the growing season in colder climates. The small structure in this case study was built for around $35,000 in Bethel, NC in 2000/2001. The house also used photovoltaics to produce electricity. Photovoltaic systems are a form of active solar, because of the electrical components involved. Usually, a passive solar system is defined by have no mechanical devices or moving parts.
This house was not expensive to build and used far less energy than a “normal” house its size. North Carolina has several tax incentives in place which can actually make a solar home cheaper than a non-solar home in some cases. This house is very comfortable year round and despite having a view to the west, still used proper solar glazing ratios, while taking advantage of the beautiful view. Passive solar concepts and technologies have been around for hundreds of years, proving their effectiveness. Cheap energy and cheap politics have helped suppress this construction/design technique for a long time. It’s harvest time for sunshine.
Heath Moody is the head instructor for A-B Tech’s Carpentry Department, part of the A-B Tech Construction Science program. |
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