Could Earth's Infrared Emissions Be a New Renewable Energy Source?

Zothecula (1870348) writes "Could it one day be possible to generate electricity from the loss of heat from Earth to outer space? A group of Harvard engineers believe so and have theorized something of a reverse photovoltaic cell to do just this. The key is using the flow of energy away from our planet to generate voltage, rather than using incoming energy as in existing solar technologies."

Comments

Power density?

[gregor-e]

Just how many watts per square meter are capturable this way? Enough to power a small LED?

Re:Power density?

[busstop]

The diurnal mean of the energy emitted is equal to the energy received (otherwise the oceans would quickly boil away).

The difference is that the energy emitted has a much higher entropy than the energy received: solar energy comes from a source with a temperature around 6000 K, i.e. low entropy, Earth emits the same amount of energy at a temperature of around 300 K, i.e. high entropy.

Hence, it is much harder to get any useful work from the emitted than from the received energy.

Re:Power density?

[BlackPignouf]

Score 5: Interesting/Insightful. WTF?

*) Diurnal. Does it mean what you think it means?
*) Energy received and energy emitted by the Earth aren't equal. You might have heard of global warming.
*) The energy emitted by the Earth isn't all infrared radiation.( http://en.wikipedia.org/wiki/F... and http://www.eoearth.org/view/ar... )
*) Temperature doesn't have color, pressure doesn't have speed and energy doesn't have entropy. You can only define entropy for a thermodynamic system (i.e. Earth, or Earth + atmosphere).
*) Entropy more or less describes the disorder of a system. All oher things being equal, the entropy goes up with the temperature (0 at 0K, higher at 6000K than at 300K)
*) You're probably talking about exergy : http://en.wikipedia.org/wiki/E...

Thermodynamics is hard. You have to define everything and understand the underlying mathematical concepts.

Re:Power density?

[blueg3]

Energy received and energy emitted by the Earth aren't equal. You might have heard of global warming.

True, they're not equal. To a reasonable approximation, they are equal: the heat picked up via global warming is tiny compared to the amount of heat added by the Sun each day (and subsequently lost to space by radiation).

The energy emitted by the Earth isn't all infrared radiation.( http://en.wikipedia.org/wiki/F... [wikipedia.org] and http://www.eoearth.org/view/ar... [eoearth.org] )

True, though it's mostly infrared and albedo.

Temperature doesn't have color

No, but a distribution of radiation does. When, in physics, someone says that radiation is "X Kelvin", it's shorthand for "a distribution of radiation very close to the ideal black-body radiation at X Kelvin". The great bulk of the Sun's and Earth's radiation is black-body radiation.

You can only define entropy for a thermodynamic system (i.e. Earth, or Earth + atmosphere).

Radiation certainly does have entropy. See, for example, Planck's "the Theory of Heat Radiation" or some more modern text.

All oher things being equal, the entropy goes up with the temperature (0 at 0K, higher at 6000K than at 300K)

This is just a misunderstanding of the meaning of 6000K vs. 300K light. Though it's incorrect to just assume zero entropy at 0K.

Entropy more or less describes the disorder of a system.

It's enormously more complicated than that. That's a Brian-Greene-level description.

You're probably talking about exergy

... Are you an engineer?

Re:Dyson Sphere?

[Nethead]

Yes, we become a world of programmers and gamers. The whole is is our mothers' basement.

By rule

[WinstonWolfIT]

No.

Not New

[urgelt2]

This is not a new idea. http://web.mit.edu/newsoffice/...

Re:Dyson Sphere?

[michelcolman]

We might use some system that lets most of the sun's rays pass through but that blocks the infrared from getting back out. You know, like a greenhouse. Maybe we could produce some kind of gas that has these properties?

Re:Seems like a bad idea, intuitively

[Hognoxious]

We'll just use some of the energy to run giant refrigerators.

Sheesh, it's tiring doing all the thinking.

Re:Very useful ON THE MOON

[Overzeetop]

I'm not sure this is actually going to get you that far. The difference in radiant flux (moon vs sun) is T^4, so you're talking about a 200-300K source vs a 6000K source, or (if I inverted correctly) about 8.7mW per m^2 power at 100% efficiency, compared to 1400W/m^2 of solar.

To help Jade Rabbit "not freeze" would have required (?) an acre of array, I'm guessing. It would have been better to use a deployable MLI canopy as a secondary shield against the radiative losses to space and capture the heat directly from the ground under the rover. Again, not that it would help much as you're then prevented from moving except within the canopy.

A stupid idea.

[Geoffrey.landis]

Correct: it's a mostly useless idea.

The problem really is in the laws of thermodynamics.

The total energy radiated is indeed equal to the sunlight energy (although the power density is less by a factor of 4: the Earth absorbs sunlight on an area pi r^2, but radiates heat over an area 4 pi r^2)-- but usable energy is produced not by a heat source, but by the transfer of energy from a heat source to a heat sink-- the Carnot efficiency. The difficulty is that in intercepting the outgoing radiation, you necessarily put a thermal resistor into the circuit. Basically, they end up converting at efficiency characterized by the difference in temperatures of daytime and nighttime. The efficiency is terrible.

Re:Dyson Sphere?

[Immerman]

>Glass (of a greenhouse) does pass solar radiation but reflects thermal radiation

Actually, mostly not. notably IR-reflective glass is a very recent invention that is still generally considerably more expensive that normal glass, much less plastic, etc. Most greenhouses primarily rely on retaining a mass of heated air separate from the outside environment, a job done fairly efficiently by gravity on Earth. Basically as an analogy "greenhouse gasses" is an unfortunate misnomer created by someone apparently ignorant of how greenhouses actually work.

A more accurate analogy:
* Gravity retention of atmosphere Greenhouse glass : retains air heated by contact with soil, drastically slowing the rate of heat loss by convection - the primary thermal channel.
* "greenhouse gasses" IR glazing applied to glass : partially reflects infrared radiation, slowing heat losses due to radiation, as well as gains from IR-band sunlight.

And there's no meaningful analogy at all to the primary mechanism of heat loss in a greenhouse: conduction through the glass. You can't conduct into vacuum.