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Skylight Well Reduces Solar Heat Gain

It is well known that daylight is rapidly attenuated as it is reflected, multiply and diffusely, while passing through a skylight well that has a depth comparable to, or greater than, the size of its opening. The same is true of solar energy that strikes the walls of the light well. The diffusely reflected energy is transported downward by multiple reflections. On each reflection, a portion of the energy is absorbed in the well walls. This absorbed energy appears as heat. Building energy calculations have generally assumed that all this heat enters the building space below, creating a cooling load.

 Heat rejection by a skylight well.

Figure 1. Heat rejection by a skylight well. Measurements made on a clear double-glazed skylight tilted 20 degrees to the south at the top of a vertical-sided light well are shown. Solar energy absorbed in the well walls causes vertical temperature stratification in the well, resulting in an air temperature (upper plot) at the top of the well that is always above outdoor air temperature. (Peak air temperature, 34 °C (93 °F), peak well temperature, 60°C (140 °F.)) The measured energy flow (lower plot, points) into the space is considerably smaller than the energy flow entering the skylight (the energy that would enter the space if the skylight behaved exactly like a window; it is calculated from measured solar incidence and the interior and exterior temperatures). The difference is due to heat rejected through the well walls and the skylight, as indicated.

Old and new views of the thermal function of a skylight well.

Figure 2. Old and new views of the thermal function of a skylight well. In the conventional picture (a) the skylight in a well behaves thermally as though the well were not there. By contrast, the picture (b) that emerges from MoWiTT measurements sees the well as a "thermal diode" heat trap, which during the daytime rejects all energy converted to heat in the well and allows only that which remains in the form of radiation (mostly solar) to enter the space below.

Measurements on a skylight/light well combination made in EETD's Mobile Window Thermal Test Facility (MoWiTT) reveal that this is not the case. Energy absorbed in the skylight well is carried upward by convection and results in stable temperature stratification of the well air. Heat is trapped in the air at the top of the well and can only reach the space below by thermal radiation, which turns out to be a comparatively small effect. Figure 1 shows that this results in the air at the top of the well remaining always at a higher temperature than the outside air even on a very hot day. The heat transfer (as opposed to solar radiation) through the skylight is directed outward. The skylight/well combination rejects part of the solar gain that has entered through the skylight.

The measurements in Figure 1 show that in these tests approximately 25% of the solar energy admitted by the skylight (that is, the energy that would enter the space if the skylight behaved exactly like a window) was subsequently rejected, leaving only 75% to impose a cooling load on the space below. Of the rejected energy, about one-third was rejected by conduction through the walls of the well, with the remainder rejected by thermal transfer through the skylight. This leads to the altered view of skylight performance shown in Figure 2b.

These measurements mean that several new issues need to be considered in buildings designed with skylights. For example, heat rejected through the well walls could add to the cooling load or not, depending on the nature of the adjacent space. The amount of heat trapped by the light well depends on the geometry and reflectance of the well. Further research is needed to develop a method of calculating the trapped heat and the expected temperatures from the well geometry and incident solar flux on the skylight.

Our new insights into the thermal behavior of skylight wells will lead to new ways of optimizing skylight performance. It seems safe to say that with careful design of the light well (e.g., venting in summer, use of selective surfaces) skylight well systems could provide daylight without heating the space, other than heat contained in the light itself.

— Joseph Klems

For more information, contact:

  • J.H. Klems
  • (510) 486-5564; fax (510) 486-4089

This work was supported by the Office of Building Technology, State and Community programs. Office Building Research and Standards of the U.S. Department of Energy.

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