Daylight Redirecting Window Films
Dr. Raghunath Padiyath | 3M Company
Objectives of the Demonstration
Since the 1970s, the United States Congress has mandated improvement in building efficiencies and a reduction in energy consumption by all federal agencies. Several studies have shown that better use of daylight can reduce energy demands by 20-40%, while reducing emissions and carbon footprint. In addition to providing a connection to the outdoors, daylight can provide visual comfort, stimulate healthy circadian rhythm, reduce stress, and improve productivity and attentiveness. A variety of products such as light shelves, light redirecting blinds, and prismatic panels are available in the marketplace to address the need for better daylighting. Most of these products are either not suitable or cost prohibitive for retrofitting to an existing window to make better use of daylight.
The overall objective of this project was to evaluate daylight redirecting films (DRFs) or systems under a variety of conditions and conduct a thorough evaluation to better assess the potential for energy savings in Department of Defense (DoD) buildings. Specific objectives were to verify the performance of DRFs, scale-up the prototype DRF in a factory setting, quantify the potential for energy savings, and qualitatively assess occupant satisfaction.
DRFs were produced in a roll-to-roll format that consisted of acrylic micro-prismatic elements on a clear polyester (polyethylene terephthalate [PET]) substrate and coated with a pressure sensitive adhesive (PSA) on the backside. The microstructures are designed to maximize reflection of incident sunlight towards the ceiling to allow the ceiling surfaces to redistribute the light more uniformly in the space. The films were installed in six different DoD buildings across three different climate zones. The sites were selected based on user profile, building location, access, window design and structure as well as availability of similar, if not identical, space that could be designated as “control” space in order to perform a side-by-side comparison.
The team installed luminance monitors, utilized simulation techniques, and conducted surveys of the occupants. The surveys evaluated occupant comfort in terms of glare, light quality, and aesthetic quality of the installation. Surveys were conducted before and after installation of the window film to determine the effect of the application. Energy savings resulting from the DRF were not tracked during the project due to a number of reasons. Instead, a simulation exercise based on the measured optical characteristics was used to predict the potential energy savings in three different climate and geographical regions.
Following are some of the significant findings from this demonstration:
- Energy savings that can be achieved as a result of the installation of DRFs on clerestory windows are a function of building location, window orientation, and type of photocontrols. The savings can range from 0.39 - 2.11 kilowatt hour (kWh)/square foot (ft2) of the floor area.
- With photocontrols alone, the savings are restricted to a lighting zone within 8 feet from the window wall and may be significantly reduced if the occupants keep the blinds closed, as frequently observed during this and other studies. With the application of DRFs, there is no risk of reduced energy savings from closed blinds. Furthermore, the savings with DRFs can be higher than optimally adjusted blinds.
- The daylit zone can be extended to at least 24 feet from the window wall compared to about 8 feet for a space with no DRF.
- Spaces with DRFs were perceived to be brighter and more cheerful.
- It was necessary to position an optically diffusing surface in front of the microstructured film adhered to the glazing surface to minimize the occasional glare. Due to the vagaries of the window design at each site, different methods were adopted to install the diffuser. The diffuser characteristics were carefully chosen to have no discernible impact on the optical characteristics of the system.
- The increase in illuminance due to DRF was not accompanied by a corresponding increase in glare. In some instances, glare was in fact reduced or eliminated as a result of application of DRFs.
The DRF was intended to be a single film applied to the existing window using a standard window film installation process. However, as the prototype film was developed and tested, it became apparent that a diffusing film must be positioned in front of the redirecting film to reduce or eliminate glare. Factors such as transmission, haze, and clarity were used to select the optimal diffuser that reduced the glare while minimizing any loss in light transmission. Different application techniques had to be used in order to achieve the same effect in the buildings where demonstrations were conducted.
Several facility managers inquired about the blast resistance properties of the film. At this time, the blast resistance properties have not been tested. However, the team believes that the film should be able to achieve a ‘3a’ rating in General Services Administration protection standard based on knowledge of other window film materials and constructions.
A number of different ways to make the technology commercially available are being worked on. These include insulated glass units with the DRF laminated to the inside surface of the outer pane and a diffusing glass as the inner pane; triple pane windows with a removable sash where DRF is laminated to the outer surface of the inner pane of IGU and diffusing film on the removable sash; and DRF applied to a single pane window of buildings designed with monitor roof. In addition, DRF film with integrated diffusing film is currently in the final stages of product development.