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Components of Passive Solar Heating Designs

10/19/2007 by Andy Walker, PhD PE

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All three types passive solar designs -- direct gain, sunspaces, or Trombe walls -- have the same major components:

  • windows to admit the solar radiation
  • mass to store the heat and avoid nights-too-cold and days-too-hot by smoothing out the temperature fluctuations
  • a superior level of insulation in walls, roof, and foundation.

Windows

Advances in window technology have revolutionized passive solar heating design. Excessive heat loss from large window areas used to limit the application of passive solar heating to moderate climates. The well-insulated glass assemblies available today allow large windows even in very cold climates and high elevations, albeit at higher cost.

The designer may now select glass with a wide range of optical and thermal properties. The heat loss from a glazing assembly is described by the loss coefficient, or U-value in units of (W/m2/C or BTU/SF/hour/F). The lower the U-value of a window, the less heat loss. Manufacturers construct windows with multiple layers of glass separated by gaps of air or other low-conductivity gas to reduce convective heat loss, and apply a low-emissivity (low-E) coating to reduce radiative heat loss. The U-value of a window ranges from 1.23 for single-pane with metal frame to as low as 0.24 for triple-pane with low-E coating and gas fi ll. Standard double-pane glass has a U-value between 0.73 and 0.49, depending on the type of frame.9 A low U-value is of benefit in both warm and cold climates.

Other properties to consider include the solar heat gain coefficient (SHGC), and the visible transmittance. The SHGC is the fraction of solar heat that is transmitted directly through the glass, plus the fraction absorbed in the glazing and eventually convected to the room air. SHGC varies from 0.84 for single-pane clear glass to as low as zero for insulated opaque spandrel glass. Standard double-pane clear glass has an SHGC of 0.7. A high SHGC is of benefit on the south side to admit solar heat in winter, but on east and west sides, or in warm climates, a low SHGC is best. The visible transmittance of glass is an important consideration for daylighting goals. New developments in glass technology include photochromic (changes with light level), thermochromic (changes with temperature) and electrochromic (changes with application of an electric voltage). These new glass products will offer a versatile palette to the designer when commercially available.

Vertical south-facing windows are recommended over sloped or horizontal glazing for passive solar buildings in the northern hemisphere. Sloped glazing provides more heat in the cool spring, but this benefit is obviated by excessive heat gain in the warm autumn and also the additional maintenance caused by dirt accumulation and leaks. Overhangs admit the low winter sun while blocking the high summer sun, but since the ambient temperature lags behind the sun’s position in the sky (cool on the spring equinox, warm on the autumnal equinox), there is no single fixed window overhang geometry that is perfect for all seasons. Therefore movable external awnings, plant trellises (which are usually fuller in autumn than in spring), or internal measures, such as drapes and blinds, are often used to improve comfort.

Thermal Storage Mass

Thermal storage mass is often provided by the structural elements of a building. It is important that the mass be situated such that the sun strikes it directly. Mass may consist of concrete slab floor, brick, concrete, masonry walls, or other features such as stone fireplaces. A way to add some mass to a sun-tempered space is to use a doublethickness of drywall. There are some exotic thermal storage materials including liquids and phase-change materials, such as eutectic salts or paraffin compounds, that store heat at a uniform temperature. These materials are not commonly used, however, due to their cost and the need to reliably contain them over the life of the building.

Optimum levels of insulation in a passive solar building are frequently double those used in standard construction, not only to reduce backup fuel use, but also to help limit the size of the required passive solar heating features to reasonable proportions. The need to add insulation has implications for selection of wall section type and choice of cathedral versus attic ceiling, since an attic can accommodate more insulation. Insulation on slab edges and foundation walls is especially important, because these massive elements are often used to store solar heat. In all cases, the insulation should be applied to the outside of the mass in order to force the mass to stabilize the interior temperature. The mass should not be insulated from the occupied space, so that it easily heats the room air. Furring out from the mass wall or carpeting the floor slab is not recommended. Finished concrete or tile floors are preferred. Durable insulated finish systems are available for exterior application to concrete or block walls. Although advanced glazing assemblies are already well-insulated, drapes and movable insulation are sometimes used to provide additional insulation at times when solar gain is not a factor, such as at night.

It is not reasonable to expect passive solar energy to heat mass that is not directly in the sun, or to distribute widely throughout a building. The reason is that natural (passive) convection is caused by the temperature difference between the hot area and the cold area, and we want that temperature difference to be minimized for comfort reasons. Distribution to other parts of a building requires a mechanical solution involving pumps or fans.

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