ASHRAE Journal - July 2009 - 106

A 2006 California Energy Commission study on electrochromic windows estimated a lighting energy savings of about 44%, compared to a reference case with no daylighting controls. film layer containing suspended small particles of carefully designed chemical composition is sandwiched between the two panes. The normal distribution of the particles is such that they are randomly oriented and block up to 99.75% of incident light from passing through the glass. When ac voltage is applied to the conductive layers, the suspended particles orient in the electrical field and allow light to pass. By adjusting the applied voltage, the glass can be infinitely controlled from more transparent than typical building windows to almost totally opaque. The range of light transmission for any given product can be varied according to the amount of suspended particles in the film. The range can vary from less than 1% to 50%, but other ranges with a higher high end can be achieved. The switching time of SPD glass is fast and independent of the size of the window and temperature. Typically, a window changes from fully opaque to its greatest transparency in about one second and in the reverse direction in less than three seconds. The developer considers the fast switching speed a major competitive advantage for smart windows using SPD technology. This characteristic not only allows users to precisely adjust the level of light entering a room at any moment, but permits design of control systems that can adjust to changing lighting conditions and room occupancy in real time. Windows can be powered up or down rapidly in response to a range of sensors and signals, and can be programmed to optimize lighting and energy use from minute to minute throughout the day as occupants enter and leave rooms and as external environmental conditions fluctuate. Systems also can be programmed to adjust to changing seasons throughout the year. Because SPD technology is based on lamination of an SPD film onto glass or plastic surfaces, it is amenable to forming curved surfaces or irregular shapes. SPD end products have been tested in accelerated environmental chambers and show excellent durability. The number of switching cycles is said to be unlimited. Electricity consumption is low. A room full of large SPD windows can be operated in the fully clear state for about the cost of powering a typical night light. Of course, SPD windows consume no electricity during times when they are in the darkened state. SPD technology is used in a broad range of end uses. Installations have been reported in a number of airplanes, several types of buses and specialty transportation vehicles such as ambulances and armored cars. All of the major automobile manufacturers are said to be evaluating SPD technology for cars. SPD windows have been installed in a number of commercial and residential buildings. Recently, the largest SPD project in the world was completed at the Indiana Health Information and Transitional Sciences Building at the Indiana University School of Medicine.4 106 ASHRAE Journal Energy Saving Potential According to the 2008 Building Energy Databook, in 2006 buildings were responsible for 39% of all energy use and 73% of electricity consumption in the U.S.5,6 A 2006 California Energy Commission study on electrochromic windows estimated a lighting energy savings of about 44%, compared to a reference case with no daylighting controls. Also, peak demand reduction of 19% to 26% related to window cooling loads occurred on clear sunny days.6 Market Factors The ultimate market for smart windows based on these two technologies (and others yet to be introduced) appears to be large. The two technologies differ in many ways, including the fact that one is dark in the powered state and the other is transparent in the powered state. This difference alone may dictate a preference for one or the other in a given application. Other differences include switching time and relative energy savings. Studies directly comparing these two technologies with respect to cost/benefit and payback time are lacking (performance data for the smart windows were taken from company promotional materials). The ultimate determining factor in capturing market share may well be the extent to which each can bring prices down as commercialization grows and how soon they are able to make this happen. There ought to be many opportunities for each of these technologies and plenty of room for these and others as markets expand. References 1. 2009 ASHRAE Handbook—Fundamentals, Chap. 15. 2. www.sage-ec.com. 3. Sullivan, R., M. Rubin, and S. Selkowitz. 1997. “Energy performance analysis of prototype electrochromic windows.” ASHRAE Transactions, 103(2):149 – 156. 4. “Innovative Glass completes world’s largest state-of-the-art SPDsmartglass project at Indiana University.” Innovative Glass Corp. http:// tinyurl.com/SPDglass. 5. “2008 Buildings Energy Data Book.” 2008. Prepared by D&R International for the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Building Technologies Program. http://buildingsdatabook.eere.energy.gov/. 6. California Energy Commission. 2006. “Monitored energy performance of electrochromic windows controlled for daylight and visual comfort.” CEC-500-2006-052-AT8. http://tinyurl.com/CEC2006. Stephen E. Rudolph, Ph.D., is a director in the Applied Chemistry Group and John Dieckmann is a director in the Mechanical Systems Group of TIAX LLC, Cambridge, Mass. James Brodrick, Ph.D., is a project manager with the Building Technologies Program, U.S. Department of Energy, Washington, D.C. ashrae.org July 2009

ASHRAE Journal - July 2009

Table of Contents for the Digital Edition of ASHRAE Journal - July 2009