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Earth Power Technology

Could Earth's Infrared Emissions Be a New Renewable Energy Source? 78

Posted by samzenpus
from the power-up dept.
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."
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Could Earth's Infrared Emissions Be a New Renewable Energy Source?

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  • Power density? (Score:5, Insightful)

    by gregor-e (136142) on Friday March 21, 2014 @02:07AM (#46541203) Homepage
    Just how many watts per square meter are capturable this way? Enough to power a small LED?
    • Is this even remotely comparable to raw sunlight?
      • Re:Power density? (Score:4, Insightful)

        by busstop (36269) on Friday March 21, 2014 @03:31AM (#46541389)

        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: (Score:3, Informative)

          by BlackPignouf (1017012)

          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... [wikipedia.org] and http://www.eoearth.org/view/ar... [eoearth.org] )
          *) 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 + atmosphe

          • Re:Power density? (Score:5, Informative)

            by blueg3 (192743) on Friday March 21, 2014 @07:38AM (#46542209)

            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]... [wikipedia.org] and http://www.eoearth.org/view/ar [eoearth.org]... [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?

      • by mdsolar (1045926)
        Sunlight is captured over pi*r^2 (the crossection of the Earth) but it is re-emitted over 4*pi*r^2, the surface area of the Earth, so there is a factor of four down. There is also a question of how well you can make use of the emission. The Carnot limit gives about 85% for concentrated solar power (T(Sun)-T(receiver))/T(Sun) http://en.wikipedia.org/wiki/T... [wikipedia.org] The effective temperature of the Earth at the top of the atmosphere is about 250 K while the effective temperature of a space based passively cooled
    • by Framboise (521772)

      The total heat produced by radioactivity in Earth is 44.2 TW (Wikipedia).
      The total solar power received by Earth by the upper atmosphere is 174 PW (Wikipedia).
      This means 3937 more solar energy is received by Earth than produced by radioactivity in its interior.
      Furthermore geothermal energy is high entropy energy in regard of solar energy since the temperature difference between
      ground (~287K) and nearby space (>10K) (DT=277K) is much less than the temperature difference between
      sunlight (5778K) and ground

      • The total heat produced by radioactivity in Earth is 44.2 TW (Wikipedia).

        Which means if we can capture 5% of it, that would be enough energy to power the entire world's energy needs.

        • by Immerman (2627577)

          Check your numbers - 2008 global energy usage, 142,300TWh(wikipedia) /365/24 = 16.2TW average instantaneous energy consumption.
          We'd need to capture 37% to satisfy 2008s energy requirements, and today's are even higher.

          Then we need to worry about efficiency - assuming we could manage to get up to 10% efficiency, we'd need to blanket 370% of the planet with thermal energy collectors to satisfy current energy demands from georadiothermic sources.

      • I don't think they are talking about geothermal, they are talking about infrared that is radiated back into space at night from ground/water heated by the sun during the day.
    • by Anonymous Coward

      If nothing else, it's surely enough to power a small TED.

    • Hmm, well, I have a hand held sterling engine that runs a dynamo and lights up several LEDs. It runs off the thermal difference between my hand an the room temperature air.

    • Just how many watts per square meter are capturable this way?

      It seems that if you're after the earth's heat energy, you drill a deep enough hole and get it.

      No space lift required.

  • A mini Dyson Sphere around the earth?

    • by Nethead (1563)

      For this to work we'd may need to have a "hindmost" central world that was almost all arcologies. Nothing we need to worry about soon. I mean, I've been to Montana.

  • I've got a better way, lets release some greenhouse gases, trap the heat until the oceans boil, reuse old steam engines. WIN!

    • by Nethead (1563)

      Then we could put steam vents at 90 degree angles and spin the earth faster. The increased spin could then be used to generate power off of the magnetic field of the Van Allen belt.

      Profit!

      • by Anonymous Coward

        If you're capturing the earth's radiant energy...
        Wouldn't you already have needed to solve the climate change problem?
        Because this seems guaranteed to cause manmade global warming - that thermal radiation never escapes, we're harvesting the energy which creates heat...

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

          Sheesh, it's tiring doing all the thinking.

  • The answer: (Score:1, Interesting)

    by Anonymous Coward

    No!

    • by JWSmythe (446288)

      Actually, the answer is "yes", but isn't very efficient. It sounds like he's stumbled upon the idea of peltier coolers (or TEC). It could convert the released heat energy into electricity. It would work great, if we could build a wall roughly the size of the planet. :)

  • Just get Congress to abolish the Laws of Thermodynamics

  • Not New (Score:4, Informative)

    by Urgelt . (3586487) on Friday March 21, 2014 @03:16AM (#46541349)
    This is not a new idea. http://web.mit.edu/newsoffice/... [mit.edu]
  • a reverse photovoltaic cell converts electrical current into light, i.e. a light bulb!

    Is this a case of stupid journalism?

  • It'd be amazing if there were a hollywood blockbuster that theorises that doing this will make the Earth run out of rotational energy and fall into the Sun.

    • That's kind of what happened to my machine-planet powered by the tidal forces of its moon. It eventually just slung the moon away.

      In a simulation of course. There's no sentient machine race monitoring this planet's technical progress. Nope, this isn't a violation of any galactic statutes (not like I wouldn't need a vacation if it were though).

    • by blueg3 (192743)

      Orbital energy. Or, you know, just forward motion.

      Rotational energy maintains the night/day cycle.

  • Ok, even if this is just marginal on earth (because of the low temperature gradient due to the atmosphere), it should be GREAT on the moon.

    During the long lunar night when temperatures drop hundreds of degrees, it should be much easier to generate significant power from the still warm lunar soil. Coupled with the solar power from the long lunar day, it should make long term lunar exploration much more feasible (and prevent problems like the shutdown of "Jade Rabbit" due to freezing).

    • 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

  • So basically they're proposing an extremely inefficient, extremely expensive, tech heavy, and impractical solution to recapturing a tiny fraction of that energy.

    Or you know, we could just use the basics of passive solar heating. Capture the sun's energy with a large thermal mass and then use the concentrated energy.

    I don't know, covering a quarter of an acre of land to heat my place, or some nice statues inside the greenhouse half of my dome with passive piping to a large thermal mass underground. One of

  • Bedouins used to make ice by leaving shallow pans of water open to the desert night air, with a blanket under the pan to keep the heat of the ground from soaking into the water. This idea is essentially using a thermocouple in place of the blanket and exploiting the temperature differential to generate electricity.

    Can we dream of suburban rooftops that harvest photoelectric power by day plus whatever small amount of back-seepage of heat into the air can be reclaimed at night?

  • So, the earth is too hot. It absorbs energy, and then radiates it away. It reaches an equilibrium so it maintains a cyclical average temperature.

    So you want to capture the radiating energy and release it on the earth?

    Do you see a problem here?

  • Earth is emitting energy in the 300K temp range. Sun is emitting at 6000K range. Classical thermodynamics defines the maximum possible efficiency of conversion purely based on source and sink temperatures. With earth emitting at 300K which is nearly the ambient, where are we going to find a sink? The sun/earth ratio is 6000/300 = 20. To get the same efficiency as the present solar collectors, you need a sink at 15K.

    For all this theoretical work, we could think of putting a huge thermocouple with one end i

  • My understanding (based on a youtube video I saw once...) is the photovoltaic cells don't capture all the energy of the photons that strike them.

    They absorb some energy, converting the visible wavelength photons into infrared wavelength photons.

    My question is, does this basically create heat pollution, which is trapped by CO2; while if the same area were covered by white/mirror surface, more energy would exit the atmosphere into space?

    How about photocells that convert UV light into energy, and give off visi

  • If this could be applied to my checking account I might be able to pay for my kids education before they graduate!

    If it was applied to my weight I would loose the weight of the snack before it hit my stomach!

    Hair loss could be a thing of the past?

  • So the idea is to trap infrared for energy. But infrared is effectively heat ( crudely spoken ). So you are trapping heat. Doesn't that add to global warming?

    This is why people look down on environmentalists.

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