The window has come a long way since the days when it was a single pane of
glass in a wood frame. Low-emissivity windows were designed to help buildings retain some
of the energy that would have leaked out of less efficient windows. Designing efficient
window-and-frame systems is one strategy for reducing the energy use of buildings. But the
net energy flowing through a window is a combination of temperature- driven thermal flows
and transmission of incident solar energy, both of which vary with time. U-factor and
solar heat gain coefficient (SHGC), the window properties that control these flows, depend
partly on ambient conditions. Window energy flows can affect how much energy a building
uses, depending on when the window flows are available to help meet other energy demands
within the building, and when they are adverse, adding to building energy use. This leads
to a second strategy for reducing building energy use: using the beneficial solar gain
available through a window, either for winter heating or for daylighting, while minimizing
adverse flows.
To pursue either strategy successfully requires accurate knowledge of the window energy flows. In simple cases this can be deducted from the values of the U-factor and SHGC obtained from either a calculation or a measurement under static laboratory conditions. Sometimes, however, this is not sufficient, and one must turn to some form of field testing. The LBNL approach to this is to make an accurate measurement of the time-varying flow of energy through windows in realistic conditions, a capability provided by the Mobile Window Thermal Test (MoWiTT) facility, which has a number of unique features.
Most window developers are interested in measuring the overall average or peak energy demands resulting from these time-dependent flows. They obtain energy demands from a complex set of calculations such as a building simulation model; in many cases, however, the information available to characterize a fenestration may be limited or inadequately described in available simulation models, or the user may be skeptical of the calculation's results. In these cases, direct performance measurements under well- characterized outdoor conditions provide performance information with higher confidence levels.
MoWiTT researchers at the Center carry out a DOE-funded program of research to characterize fenestration systems and develop publicly available calculation methods for predicting fenestration thermal performance. Their goals are to assess the performance of energy-efficient windows currently available or under development, such as the newly emerging superwindows; to identify opportunities for new development; and to create a knowledge base that will encourage the rational selection of optimal windows in the design process. Organizations that have collaborated on these projects include Andersen Corp., ASAHI Glass Co., ASHRAE, Cardinal IG, CIEE, LOF Glass Co., The Moore Co., PG&E, Rolscreen Co., and Southwall Technologies. In addition to its DOE-supported research, MoWiTT is also available for privately-funded studies of specific products or window improvement options. It can:
MoWiTT users under this arrangement have included the Bonneville Power Administration, LOF Glass Co., Cardinal IG, and Andersen Corporation. Recent MoWiTT accomplishments include:
- completion of a multi-year study of the effective exterior heat transfer coefficient experienced by windows in winter conditions, and testing of their dependence on wind speed
- testing of solar heat gain calculation schemes under consideration for the National Fenestration Ratings Council's voluntary performance ratings
- testing, in cooperation with Andersen Corp., ASAHI Glass Co., ASHRAE, Cardinal IG, LOF Glass Company, The Moore Co., Rolscreen Co., Southwall Technologies, and other companies, of the performance of new window technology
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For more information contact Joseph Klems