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.
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.
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.
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.
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.
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)
…………………………………………………….
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.