LEDs vs. Lightbulbs

zymano writes "www.technologyreview.com has this nice article on LEDs vs lightbulbs" Follows the exploits of one Shuji Nakamura, the same man who brought you the practical violet laser.

Comments

efficacy versus efficiency

[Michael.Forman]

This article, like most articles that tout LEDs as a replacement for light bulbs, confuses efficacy and efficiency.

Luminous efficacy is defined as the power of radiated visible light (visible luminous flux) divided by the dissipated electrical power. Given in lumens per watt, efficacy provides a useful means of comparing which source provides the most useful illumination for a given power.

Efficiency on the other hand is defined as the power of radiated visible light (visible luminous flux) divided by the power of all radiated light (total luminous flux). Ideally given as a unit-less ratio, efficiency provides the ratio of useful light to wasted light and does not necessarily correlate with efficacy. It is possible to have simultaneously high efficiency and low efficacy.

The question of why LEDs haven't already replaced all lights can be answered quickly by comparing the efficacy of different sources of light. All numbers below are approximate.

Filament light: 10 lm/W
White LED: 20 lm/W
Halogen light: 25 lm/W
Red LED: 40 lm/W
Florescent light: 100 lm/W
Sodium light: 150 lm/W
It can be seen why red LEDs have replaced halogen bulbs with red filters in stop lights -- red LEDs have a much higher efficacy. Note that the efficacy of LEDs are still well below that of florescent lights. If you feel frustration in seeing how far LED technology still has to go to compete with the boring lighting technologies of yesteryear, assuage your sorrows in the knowledge that you can save billions in energy costs right now, simply by switching to florescent lighting.

Michael.

Re:efficacy versus efficiency

[Jason1729]

but most do not use any more or any less energy than the equivalent incandescent or incandescent halogen at the same wattage.

That's like saying "what weights more, a pound of feathers or a pound of nails?". Wattage is a measuere of the energy you are using per unit time.

LEDs give off more light for the same power than incandescant bulbs; super-bright LED takes about 15mA at 1.7 volts. That's 25 miliwatts of power. For the same power a 100 watt lightbulb takes, you can power 4,000 super-bright LEDs.

The difference is even more extreme with flashlight bulbs. To get more brightness, the bulbs are run at about 25% higher than the rated voltage. This does make it a lot brighter, but at a bigger penalty for power draw, and cuts the life expectancy down from 1000 to 10 hours.

Jason
ProfQuotes

A long way to go...

[Eivind]

We hear that leds will replace ligthbulbs every 2 years or so, and yet, they are still not even in the ballpark.

A white led today has about twice the efficiency of a normal ligthbulb. That sounds great -- until you consider:

• The brigthest white led existing (in a lab environment, not on the shelves) is a 5 watt led, equivalent to a 10W ligthbulb, yay !
• Twice the efficiency ain't that good, this still means only about 10% of the energy-input gets turned to ligth, even halogen can do better than that and fluorescent has it beat into the ground with like 20-30%.
• The prices are out of this world, no, each LED ain't that expensive, but it also has a tiny ligth-output, try calculating the price for reasonably ligthing a single room.
• The color-spectra suck. Seriously, led is inherently monochromatic. Yes they can remedy this with various phosphors and the like, but those reduce efficiency (which was supposed to be the advantage of leds, remember?) and even with those it's hard getting a natural full ligth-spectrum.

In the meantime pluorescents are developing at a breakneck pace. Today you can buy pluorescents compact-bulbs that fit in a normal bulb-socket, are 5 times as efficient as a standard bulb, cost around 2$ a piece, are available in wattages up to 25 W (equivalent to 125W standardbulb), and last for around 10000 hours.

This is a no-brainer people. Replace a single 100W ligthbulb with a 20W energy-saver and the math looks like this over the 10000hour lifetime:

• Cost of bulb: 2$ instead of 5*0.20$, extra cost 1$
• Energy comsumed: 10000*0.02=200Kwh instead of 1000Kwh for the normal bulbs.

You pay 1$ extra for the bulb, and you save 800Kwh over the lifetime of the bulb. With an energy-price of 13 cent (most pay more!) you will save over 100 dollars over the lifetime of that single bulb.