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how passive solar works

Operation

Passive solar architecture is a building design system that uses solar energy for heating and lighting to produce highly efficient sustainable buildings that have dramatically reduced heating and cooling costs. The passive solar building operates in two modes, one during the winter heating season, the other during the summer cooling season.

Winter Operation

During the winter, the sun’s path scribes a relatively shallow arc. Windows and skylights are oriented to admit as much of the solar energy as possible. This radiant energy, or insolation, shines on the thermal mass, usually the floors and walls, of the building, absorbing the heat energy during the day (figure 1). As the interior air cools, the thermal mass radiates its energy back into the building, heating the structure with the energy captured during the daylight hours (figure 2). Even during cloudy days, there is considerable insolation and subsequent heating of the thermal mass. Furthermore, in many climates, the coldest weather occurs during periods of high atmospheric pressures which typically are accompanied by clear, cloud free days.

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Figure 1. Winter Insolation
Figure 2. Winter Radiation

Summer Operation

In the summer, the sun’s path in the sky scribes a much higher arc. Roof overhangs along with various shading devices for skylights are precisely designed to prevent the sun from shining directly into the building (figure 3). The large thermal mass is shaded, and not warmed by the sun’s radiant energy. Therefore, it to remains cooler than and subsequently absorbs the building’s heat during the day and evening (figure 4).

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Figure 3. Summer Insolation
Figure 4. Summer Radiation

Design Considerations

At Design Northwest, our primary guiding force has always been to meet the needs and wants of the client, both fiscally and in the space and style of structure created. When a project becomes a passive solar project, along with meeting the client’s needs, certain additional solar design parameters must be woven into the design process.

Primary design elements in passive solar architecture include the orientation of the structure on the building site, correctly sizing windows and skylights, detailing roof overhangs with regard to summer and winter sunlight, the inclusion of thermal mass and careful detailing of insulation and drainage elements.

Solar design begins by considering the solar path and sun angles for the latitude of the building’s location. Precise placement of the windows and skylights determine when and how much and where light is allowed into the structure. Solar angles determine the location and sizing of roof overhangs and shading devices, manual and/or automatic, to limit solar gain during summer operation. Thermal mass is used as a heat sink. In the winter, it absorbs heat during the day, and radiates it back into the structure, offsetting heating costs. In the summer, as the interior of the building warms, the thermal mass absorbs the heat build-up, helping to cool the interior, reducing mechanical system costs for cooling. Design and planning the spaces in the building are carefully considered to optimize the passive heating and cooling characteristics.

Thermal Mass

The key to the system is the large thermal mass of the passive solar structure. Thermal mass is the ability of a body to store thermal energy, or heat. All objects have thermal mass (even air has thermal mass), however for passive solar structures we are interested in using materials that have a high thermal mass. Objects with high thermal mass are slow to heat up, and once warmed they are slow to cool down.

The thermal mass of a passive solar building is typically located in the floor slab and exterior walls, and usually constructed of concrete or other types of masonry. Water also has a high thermal mass and can be used successfully in some designs aspects, such as an internal trombe wall.

In the winter solar heating months, the large thermal mass allows the building to absorb solar energy during the day and release it back into the building as the structure cools. Conversely, in the summer cooling season the large thermal mass is shaded from direct sun and absorbs excess heat in the building helping to cool the structure. Due to the large thermal mass of the passive solar structure, the interior is less susceptible to temperature fluctuations making the building more comfortable to inhabit in both summer and winter.

The ideal material for creating thermal mass is dense and has a high specific heat capacity. Water, brick, adobe, earth, stone, and concrete are all examples of materials fitting these criteria, and thus are excellent for creating thermal mass in passive solar structures. In modern building construction, concrete is a readily available and easily worked material with well-known structural and operational characteristics. It has a proven track record both in conventional and passive solar construction. It is the material most commonly used as the thermal mass in contemporary passive solar structures.

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