SOLAR TODAY - Fall/Winter 2009

Scott clarK

atascadero, Calif.: Classic southwest style is a natural for this passive solar straw bale home.

By KEn HaggarD
and DavID BaInBrIDgE
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MANY oF THE RENEWABlE energy devices described in this Get Started 2010 primer are “active” systems. That is, they use mechanical devices to gather energy from the environment. Passive design relies on natural energy flow through the building with a minimum of moving parts. Passive design uses architectural form to gather energy from on-site sources (chiefly the sun) and also to store and dissipate it (for instance, by letting in cool night air). Using massive materials that stabilize interior temperatures, like masonry and water tanks, allows the building to be the energy system.

In temperate climates, a small building — a house for instance — loses and gains heat through its skin and usually has a relatively equal heating and cooling requirement. Its size also makes it easy to ventilate and daylight naturally. By contrast, large buildings — commercial and office blocks — have a smaller ratio of skin to volume, so they tend to be dominated by their internal loads. Therefore heating becomes less important than cooling. Providing ventilation and natural lighting is more complicated.

Heating and cooling: In designing a new house, judicious use of insulation, south-facing glazing, cross ventilation and thermal

mass combined with the architectural form can provide most of the heating, cooling and ventilation needs. The minimal load left, usually in deep winter or high summer, can be met with a small, efficient mechanical backup system. The only unusual element in this mix is the thermal mass. In North America, thermal mass is traditional only in New Mexico. It can consist of masonry (optimally about two inches thick), glazed water tanks facing south (about nine inches thick), glazed concrete walls facing south (about 12 inches thick) or one of the new phase-change materials incorporated into drywall, masonry or concrete, forming the walls, floors or ceilings. By storing heat during the day and releasing it slowly at night, thermal mass moderates the indoor temperature. In reverse, the same mass can store the “coolth” of night air to moderate the heat of a summer day.

Lighting: Natural light from windows reaches only 12 to 15 feet (about 4 or 5 meters) into the interior. This can be increased with higher sloped ceilings, with higher windows and translucent interior walls. Adding light shelves (reflective horizontal surfaces outside windows positioned to reflect sunlight onto the ceilings) is another option. Horizontal skylights are counterproductive because they accentuate hot summer sun and minimize welcome winter sun. To bring daylight into interior rooms, use vertical light monitors or

dormers instead. light tubes, which are inexpensive, are also effective for daylighting.

The Wolken Education Center, a small school in los Altos Hills, Calif., was designed on these principles. It uses just 5 percent as much energy for heating as a standard office building of the same size, needs no air conditioning at all and saves 71 percent on its lighting bill. Using passive techniques along with an integrated photovoltaic roof, it produces as much energy as it uses and has a net-zero utility bill at the end of the year. This reduces the carbon dioxide load by 24. 5 tons per year while providing superior comfort and a feeling of well being to the building’s occupants. GS