MoWiTT: Mobile Window Thermal Test Facility

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 requires accurate measurement of the flow of energy through windows in realistic conditions, a capability provided by the Mobile Window Thermal Test facility. Consisting of a pair of outdoor, room-sized calorimeters, MoWiTT measures the net energy flow through two window samples in side-by-side tests using ambient weather conditions. MoWiTT characterizes the net energy flow as a function of time and measures the temperatures, solar fluxes, and wind conditions to which the samples are exposed.

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-value and solar heat gain coefficient, 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. By using the solar gain available through a window, either for winter heating or for daylighting, buildings stand to save a significant amount of energy. To pursue this strategy successfully, we need to know more about the time-dependent energy flows.

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.

Most window developers are interested in measuring the overall average or peak energy demands resulting from time-dependent energy flows.

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:

- provide direct comparisons of alternative systems

- determine effective U-values and solar heat gain factors under realistic conditions

- provide verification of energy-savings calculations

- measure the energy performance of windows for which calculations are unavailable or unreliable.

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 film 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