Category: Lighting

Electrochromic Windows: We Need to Get the Cost Down

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?

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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.

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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.

Lighting: A Revolution In Progress

An energy revolution is underway before our very eyes – the replacement of traditional incandescent light bulbs with much more energy efficient and longer lasting light-emitting diodes (LEDs). It is a significant revolution because, according to NYSERDA, lighting accounts for 22% of electricity consumption in the U.S.. Other sources put this number at 19% on a global basis. It is estimated that LED use could cut the U.S. number in half by 2030.

At this point it may be fair to ask: What about CFLs (compact fluorescent lamps), which had been gaining market share for many years. A few words about lighting technology before we answer this question.

An incandescent light bulb, the most common type today in households and the least expensive to buy, produces visible light from a glowing filament wire (tungsten) heated to a high temperature (several thousand degrees) by an electric current passing through it. It was not invented by Thomas Edison, as is often stated (many earlier inventors had experimented with hot filament lamps), but he did invent the first commercially practical incandescent bulb. It was introduced into residential use more than 125 years ago. Its principal shortcoming is that more than 90% of the energy used by the traditional incandescent bulb escapes as heat and less than 10% goes into producing light. Filaments also burn out and are fragile, and a typical bulb lifetime is about 1,000 hours.

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Halogen lamps, also in common use today, are incandescent lamps with a bit of halogen gas (iodine or bromine) added to the bulb. The chemical reaction between the halogen and the tungsten wire allows the filament to operate at a higher temperature and increases the bulb’s efficiency and lifetime.

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A fluorescent lamp or fluorescent tube is a low pressure mercury-vapor gas-discharge lamp that uses UV-stimulated fluorescence of a phosphor to produce visible light. It is more energy efficient than an incandescent lamp but does require a ballast to regulate the current through the lamp, increasing its initial cost.

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Compact fluorescent lamps (CFLs) fold a fluorescent lamp tube into the space of an incandescent bulb with a ballast in the base. They use 3-5 times less energy than incandescent bulbs of the same light output and have much longer lifetimes. They do contain a small amount of mercury, creating a disposal problem.

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Light-emitting diodes (LEDs) are monochromatic, solid-state semiconductor point light sources. First appearing as practical electronic components in 1962, early LEDs emitted low-intensity red light, but modern versions are available at visible, ultraviolet, and infrared wavelengths with very high brightness. Today they are used in applications as diverse as aviation lighting, automotive lighting, advertising, general lighting and traffic signals. They are also used in the infrared remote control units of many commercial products including televisions, DVD players and other domestic appliances. Their high switching rates are useful in advanced communications technology.

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LEDs have many advantages over incandescent light sources including lower energy consumption, longer lifetime, improved physical robustness, smaller size, and faster switching. However, LEDs powerful enough for room lighting are still relatively expensive (but costs are coming down) and require more precise current and heat management than compact fluorescent lamp sources of comparable output. Their advantages over CFLs are greater efficacy (i.e., more light output in lumens per watt), longer lifetimes, smaller size, directionality of the light produced, and very importantly they contain no mercury which has to be disposed of. These factors will limit CFLs’ time in the ‘limelight’ (I know, bad pun).

(Note: LEDs are based on inorganic (non-carbon-based) materials. OLEDs are organic (carbon-based) solid-state light emitters which are made in sheets that provide a diffuse-area light source. They are still in an early stage of development and several years away from broad commercial application. Interesting potential applications include TVs, computer and cell phone screens, wall coverings that allow changes in color, and automobile skins that allow you to change the color of your car.)

It is useful to compare these different lighting technologies, as white light emitters, in terms of their current efficiencies (efficacies), lifetimes, and color temperatures (measured in degrees Kelvin, as an indicator of the warmth or coolness of the light emitted). Efficacies for monochromatic LEDs are higher but are not listed here.

Technology Efficacy Lifetime Color Temperature
(lumens/watt) (hours) (K)
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Incandescent 12-18 750-1,500 2,400-2,900
CFL 60-70 6,000-10,000 2,700-6,500
Fluorescent tube 80-100+ 20,000 2,700-6,500
Halogen 16-29 2,000-4,000 2,850-3,200
White light LED 20-50. Up to 100,000 2,700-6,500

A quick calculation will help to demonstrate the cost effectiveness of lighting sources that may be more costly to buy but save energy and money over extended lifetimes (and don’t forget that not replacing bulbs as often also saves money by reducing labor costs). I will use CFLs as my example.

Assume we buy a 15 watt CFL bulb that today costs $6 and replaces a 65 watt incandescent bulb that costs $1. We further assume that the CFL will last 6,000 hours, the incandescent 1,500 hours (clearly a worst case for CFLs and a best case for incandescents), and that electricity costs 10 cents per kilowatt-hour. Over 6,000 hours the CFL will consume (0.015 kW)x(6,000h)=90 kWh for a total cost (purchase + energy use) of $15. The incancandescent will have been replaced four times in 6,000 hours and consumed (0.065kW)x(6,000h)=390 kWh for a total cost of $43. You save lots of money ($43-$15=$28) despite the higher initial cost for the CFL, and this is per bulb. In addition to this reduced cost the reduced energy consumption will be reflected in fewer carbon emissions from power plants supplying the needed electricity.

Finally, a word about the claim that the U.S. Congress has outlawed use of the incandescent bulb. This is not true, although other countries have done so. What the U.S. Congress has done is pass the Energy Independence and Security Act of 2007, which set performance standards for all general service incandescent lamps producing 310-2,600 lumens of light. The efficiency standard will start with 100-watt bulbs and end with 40-watt bulbs. Light bulbs outside of this wattage range are not covered, along with several classes of specialty bulbs (e.g., stage lighting). Thus, if bulb manufacturers can develop an incandescent bulb that meets the specified performance standard it can be marketed and sold in the U.S. Some are even beginning to appear. This is the same approach that is taken with respect to reducing the electricity consumption of many other household appliances such as refrigerators and dish washers.