A technology that has fascinated me since I first saw it demonstrated nearly forty years ago is the electrochromic window. It is part of the family of smart glass technologies that control the amount of light and heat that the glass transmits. This control can be activated by temperature (thermochromic), by light (photochromic), or voltage (electrochromic). This blog post will focus on the latter, which offers significant potential for reducing the energy consumed in buildings. Electrochromic windows have other useful applications as well.
How do electrochromic windows work?
When a voltage is applied between the transparent electrical conductors (usually less than 5 volts) an electric field is set up in the window material. This field moves ions reversibly through the ion storage film through the electrolyte and into the electrochromic film. Different ions (typically lithium or hydrogen) produce different colorations, and the window can be switched between a clear, highly transparent state and a transparent blue-gray tinted state with no degradation in view (similar to that achieved in photochromic sunglasses) by reversing voltage polarities. Critical aspects of electrochromic windows include material and manufacturing costs, installation costs, electricity costs, and durability, as well as functional features such as degree of transparency, possibilities for dimming, and speed of transmission control (complete switching can take several minutes). Many different electrochromic window options at different price points for buildings are now available, and active R&D efforts are underway. One recent advance is the development of reflective, rather than absorptive, windows which switch between transparent and mirror-like.
Electrochromic windows are an attractive energy efficiency measure because they can block heat (infrared radiation) in the summer, reducing air conditioning loads, and allow infrared wavelengths to pass into buildings in the winter and reduce heating loads (windows account for about 30% of building energy load). This also reduces utility peak load demands. Tunable electrochromic windows also serve to reduce lighting loads when adequate natural light is available, reduce glare, provide privacy without the need for blinds and curtains, and reduce fabric and art fading by blocking ultraviolet radiation.
Important applications, in addition to reducing energy demand and increasing human comfort, include use in automobile windows, sunroofs and rear view mirrors, in aircraft (e.g., the Boeing 787 Dreamliner uses electrochromic windows in place of pull down window shades), and as internal partitions in buildings with the ability to switch screens and doors from clear to private.
Given that electrochromic (EC) windows have been under development for many decades, their obvious ability to block or transmit wavelengths of light as needed, and their many applications, why hasn’t greater use of such windows become a standard part of building construction. The simple answer is cost. NREL looked at this issue in its December 2009 report entitled ‘Preliminary Assessment of the Energy-Saving Potential of Electrochromic Windows in Residential Buildings’ and compared the cost of low-e argon-filled windows with that of EC windows and concluded that “..EC windows would have to reach a price point of approximately $20/square foot before they would be competitive..” At that time EC windows were in the range $50-100/square foot, with commercial buildings on the lower end and residential applications on the higher end. Another approach bring taken by a few EC window companies is to add an EC film to existing windows, which reduces costs considerably.
How much energy can EC windows save? The NREL study, using a model to evaluate the performance of EC windows in a single-family traditional new home in Atlanta, predicted that whole-house energy demand could be reduced by 9.1% and whole-house electricity demand by 13.5%.
Looking globally, the U.S. and China have joined in a $150 million consortium called the U.S. China Clean Energy Research Center aimed at facilitating “joint research and development on clean energy technology. The consortium estimates that in the next 20 years China will build more square footage of floor space than the current total in the United States. The goal is to make those buildings as energy efficient as possible.”
Several new factories have been or are being built to produce EC windows or EC films and reduce costs significantly through economies of large-scale production. My intuition says this will happen soon, and will serve as an important step toward zero-energy buildings – i.e., buildings that use no more energy in a year than they produce through PV generation. A future blog will discuss zero-energy buildings in more detail.