Environment & Energy

phantom power

(25,966 posts) Fri Mar 9, 2012, 05:08 PM Mar 2012

LED produces 69 picowatts of light using 30 picowatts of power

The LED produces 69 picowatts of light using 30 picowatts of power, giving it an efficiency of 230 percent. That means it operates above "unity efficiency" -- putting it into a category normally occupied by perpetual motion machines.

However, while MIT's diode puts out more than twice as much energy in photons as it's fed in electrons, it doesn't violate the conservation of energy because it appears to draw in heat energy from its surroundings instead. When it gets more than 100 percent electrically-efficient, it begins to cool down, stealing energy from its environment to convert into more photons.

In slightly more detail, the researchers chose an LED with a small band gap, and applied smaller and smaller voltages. Every time the voltage was halved, the electrical power was reduced by a factor of four, but the light power emitted only dropped by a factor of two. The extra energy came instead from lattice vibrations.

...

69 picowatts of light, of course, is a very small amount -- so you're not likely to be able to read in bed with one of these LEDs. However, it could have applications in low-power electronics, acting as a thermodynamic heat engine but with fast electrical control.

http://www.wired.co.uk/news/archive/2012-03/09/230-percent-efficient-leds

8 replies

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LED produces 69 picowatts of light using 30 picowatts of power (Original Post) phantom power Mar 2012 OP

Maxwell's Demon's bedtime reading lamp ? nt eppur_se_muova Mar 2012 #1

It could light my house and cool it at the same time. tinrobot Mar 2012 #2

Imagine a ceiling made of millions of tiny LEDs ArcticFox Mar 2012 #3

The freper types will still complain that it's not a light bulb..... wandy Mar 2012 #4

Quite… uh… cool! OKIsItJustMe Mar 2012 #5

If it works, make trillions of them, and reduce the input power to near zero saras Mar 2012 #6

A heat pump can move more joules than it takes as input: phantom power Mar 2012 #7

Can it move more heat than a heat engine can extract? saras Mar 2012 #8

eppur_se_muova

(36,289 posts)

1. Maxwell's Demon's bedtime reading lamp ? nt

Reply to phantom power (Original post)

Fri Mar 9, 2012, 05:39 PM

Mar 2012

tinrobot

(10,916 posts)

2. It could light my house and cool it at the same time.

Reply to phantom power (Original post)

Fri Mar 9, 2012, 05:41 PM

Mar 2012

Now, that's cool.

ArcticFox

(1,249 posts)

3. Imagine a ceiling made of millions of tiny LEDs

Reply to phantom power (Original post)

Fri Mar 9, 2012, 05:44 PM

Mar 2012

wandy

(3,539 posts)

4. The freper types will still complain that it's not a light bulb.....

Reply to ArcticFox (Reply #3)

Fri Mar 9, 2012, 06:01 PM

Mar 2012

Before too long they won't be able to complaine about CFLs. Somewhere between this tech and CFL's LED lamps will dominate.
I alredy use 2 low lumin LED lamps.
It won't be long.

OKIsItJustMe

(19,938 posts)

5. Quite… uh… cool!

Reply to phantom power (Original post)

Fri Mar 9, 2012, 06:31 PM

Mar 2012

http://dx.doi.org/10.1103/PhysRevLett.108.097403

saras

(6,670 posts)

6. If it works, make trillions of them, and reduce the input power to near zero

Reply to phantom power (Original post)

Fri Mar 9, 2012, 07:15 PM

Mar 2012

My guess is that they will eventually find constraints in the environment they work in that prevent them from being useful. Or problems in the math. Or problems in the physics. Or experimental error, like with the FTL particles.

BECAUSE

for a moment, never mind producing light, just look at it as a refrigerator. At 230% efficiency, you put in 30 picowatts of power, and it sucks MORE than 30 picowatts of heat from its environment and shoots it long distances away as photons. It is greater than 100% efficient as a heat pump. THAT can be exploited for infinite free energy as long as there's an expanding universe to shoot the photons into.

phantom power

(25,966 posts)

7. A heat pump can move more joules than it takes as input:

Reply to saras (Reply #6)

Fri Mar 9, 2012, 07:35 PM

Mar 2012

So there's nothing thermodynamically weird about their little photonic pico-refrigerator.

When comparing the performance of heat pumps, it is best to avoid the word "efficiency" which has a very specific thermodynamic definition. The term coefficient of performance (COP) is used to describe the ratio of useful heat movement to work input. Most vapor-compression heat pumps use electrically powered motors for their work input. However, in most vehicle applications, shaft work, via their internal combustion engines, provide the needed work.

When used for heating a building on a mild day of say 10 °C, a typical air-source heat pump has a COP of 3 to 4, whereas a typical electric resistance heater has a COP of 1.0. That is, one joule of electrical energy will cause a resistance heater to produce one joule of useful heat, while under ideal conditions, one joule of electrical energy can cause a heat pump to move much more than one joule of heat from a cooler place to a warmer place.

http://en.wikipedia.org/wiki/Heat_pump#Efficiency

saras

(6,670 posts)

8. Can it move more heat than a heat engine can extract?

Reply to phantom power (Reply #7)

Fri Mar 9, 2012, 08:58 PM

Mar 2012

It really just moves the "efficiency" measurement to a different location in the system. A heat pump loses "efficiency" as the difference between input and output increases, whereas a heat engine's "efficiency" increases. Unfortunately heat engines suck, so even at the optimum point, gathering all the photons AND using the energy available in the temperature difference STILL won't add up to what goes in in the form of electricity and environmental heat.

So what it has to beat is not 100% but the inverse of how close you can get to theoretical efficiency of a heat engine, and given that the hot side is room temperature, it's not going to be good. Even with the cold side at absolute zero, that's only 300K difference, and 20%. Not good enough.

If you made a big one, then, it would be sort of like a visible-light Peltier junction. One side gets cold and the other side radiates energy, in this case visible light instead of heat.

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