Space-Based Solar Power

a public discussion sponsored by the Space Frontier Foundation

Tech Pieces: Darkest Material May Boost Solar Conversion Efficiency

Posted by Coyote on February 22, 2008

Here’s one for our technical experts.

Staff writers for the Tech Space section of Space Mart reported back on 20 Feb 2007 in an online article titled, “Darkest Material Developed In Lab:”

The material, a thin coating comprised of low-density arrays of loosely vertically aligned carbon nanotubes that absorbs more than 99.9 percent of light, could one day be used to boost the efficiency and effectiveness of solar energy conversion, infrared sensors and other devices…(bolding by coyote)

Would a couple of our technical friends please explain why this could boost efficiency and effectiveness of solar energy conversion…better yet, apply this to space solar power?

Coyote

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13 Responses to “Tech Pieces: Darkest Material May Boost Solar Conversion Efficiency”

  1. Ed Minchau said

    If you are doing solar thermal power, then you want your heat collector to be as black as possible, absorbing all wavelengths of light. Similarly on your cold side you want it to radiate the heat out into space, rather than reflecting it back into the system, so you also want it to be black.

    The efficiency of a heat engine is determined by:

    efficiency% = 100*(1 – Temperature[cold_side]/Temperature[hot_side])

    With all temperatures in Kelvin.

    So if it absorbs as much as possible on the hot side and radiates as much as possible on the cold side, then it is as efficient as possible.

    For space solar power, if the cold side is 3 Kelvin (ambient temperature in the shadow of a reflective collector) and the hot side is 300 Kelvin then a heat engine is 99% efficient.

  2. Kasei said

    Somebody please correct me if i’m wrong somewhere!

    Blackness means the ratio how much energy(light) something absorb/reflect and how much energy is radiated away if it’s heated.

    The efficiency of energy conversion in some kind of heat engine (with you need if you choose the path of solar dynamic systems) depands theoreticaly only on to things: the temperatur of the hoter and of the cooler reservoir. (its 1-(T_cooler/T_hoter)) So if you can get the upper hoter because it absorps more energy(sunlight), and you can get the lower cooler, because it radiateds energy more easily, and radiation is the only way to cool things somewhere in space, your heat engine becomes more efficient (the same reason why power plants are mostly placed near rivers, easier and better cooling).

    The same thing makes solars dynamics more effinient on earth although, however not in the same degree. (On earth your lower temperature is somewhere around 280°K to 300°K; in space, I dont know)

    Does someone know what temperatures can be reached in space in heat engine and therefore the effectiveness?

    Some numbers: 1mW/cm^2 (FCC standart by #23) means 10W/m^2 where normal solar cells collects far more energy per square meter(around 50-100W/m^2).-> 1GW rectenna will need 100 km^2.(too huge IMHO, so we need higher intensities 10-100mW/cm^2 or 100 W/m^2 to 1000 W/m^2)

    Under a microwave rectenna there is of course a much smaller intensity as in the air above. if it converts 90% only 10% reach the space below, so if you radiate with 10mW, below will only be 1mW. So only birds will suffer some higher intensity. and because light shines through microwave rectennas, you can grow things below, some biomasses that you can use to make biofuels, so nobody has to fear to eat some radiated corn or something.

    So like Neil said, if frequency is determinded make some studies, how living beings react to different intensities of such radiation. Like does grass or corn or wheat grow under constant radiation etc.

    To what can the ISS do? the ISS is to date the largest sturcture in space with the most power. So get some mircowave transmitter up there and start messuring the atmosphere absorbtion of microwaves.

    (Are there some studies to find some numbers for 2.4 to 5.8 GHz range? I found that from 1968: http://handle.dtic.mil/100.2/AD686664 but it discuss 10-100GHz)
    .
    .

  3. Ed Minchau said

    Kasei, where are you getting 10W/m^2? If we assume 1AU from the sun, the incident light is 1340 W/m^2 or thereabouts.

    The background temperature of space is roughly 3 Kelvin, and if one can get the hot side of the heat engine to 300 Kelvin (roughly 80 Farenheit) then the heat engine efficiency would be 99%. Therefore the heat engine would produce roughly 1327 Watts of kinetic energy per square meter of collector. Note that this assumes a large parabolic reflective collector focusing sunlight on a black “hot” side of the heat engine, and the “cold” side radiating heat in the shadow of the large reflector. Even if only 50% of the kinetic energy produced by such a heat engine is converted to electricity and then microwaves, that is still over 650 Watts produced per square meter of the reflective concentrator. 1GW would thus only require a reflector about 1240m by 1240m, or a circular reflector with a radius of 700 meters.

  4. Robert said

    Well, I’ll go ahead and ask. Except for the discussion of the parabolic reflector, could the satellite logically be a large flat black rectangle floating in space? (Give me some slack, I’m a musician type.)

  5. Neil Cox said

    Yes Robert, a large black rectangle, except we need narrow access strips to carry the energy in and out of the rectangle and to make repairs.
    My idea is several satellites connected by power cords and/or flexable steam pipes. Each several kilometers long. This allows each unit to aimed (and positioned) separately, provide redundency and helps keep the size within reason. One or more parabolic mirrors to concentrate the sun’s energy. One or more heat disposal platforms which can be in the shade of the other platforms. One or more receivers which make electricity and/or steam. One or more platform to send the energy from space to Earth.
    Hybred dynamic, sun pumped laser and photo voltaic might be best, with the photovoltaic using the outer edge of the illuminated spots. The center of the illuminated spots would be too hot for photovoltaic.
    The concentrating mirrors can possibly illuminate part of the night side of Earth when not needed to make power. A concentrator mirror would look like Venus over several million square kilometers of Earth, when aimed at Earth. Aimed elsewhere the concentrator would look like a dim asteroid. Several concentrators would light up the night usefully. Neil

  6. Robert said

    Arthur Clarke has just died. Thank you for your reply, Neil. And thank you Mr. Clarke and Mr. Kubrick, on behalf of more of us than we know here today. I’m alright with the idea that those 40 ft. deep lunar pits in the film “2001: A Space Odyssey” might in fact yield large flat black rectangular objects which, when placed purposefully in Clarke orbit (and cutting very much larger profiles thanks to the magic of stortelling and solar-fired space-based processing plants), would indeed represent “the first evidence of intelligent life off the earth”, namely our own children, living in O’Neill habitats, who will make a fine living delivering clean energy to the earth and to ships and homes beyond the moon, via those magnificent black rectangular objects planted mysteriously in our situation by uncompromising Uncles Arthur and Stanley.

  7. Edawg said

    Robert #6~solar-fired space-based processing plants

    could this uber dark material be used in solar fired smelting ovens?You could have a all in one SSP plant/ISRU facility!

  8. Dan Lantz said

    All:

    I suspect that this darkest material is already in the realm of vanishing returns, in that true radiators(collecting and dissipating) will work well at the high temps needed for industrial purposes, as long as they are fairly dark. Going from 99 to 99.9% is great if you are doing sensitive dectection of faint cold signals, as it is a 10-fold improvement. Doing the same when 80-90% works somewhat well is only a small improvement, relatively. I doubt it is a major breakthru, from our point of view, esp if expensive to produce.

  9. Robert said

    I’ll get off my movie kick. I’m sure the shape and appearance of an SPS will follow its function well enough. Always enjoy the progress on these pages.

  10. Akhenaten said

    Many amateur astronomers might well fear that solar power satellites will cause more light pollution in the night sky; I imagine the ultra-low albedo material spoken of here would obviate this concern?

  11. Robert said

    OK. I vote for dark sats as a tribute to Kubrick and Clarke, if for no other reason. However, I’ll never get over the swinging food tray as it drifted off of Dr. Floyd’s lap.

  12. Neil Cox said

    It is unlikely the black can help photo voltaic, but it is helpful for thermal solar. Using concentrating mirrors the upper temperature is the temperature of the Sun’s photosphere = about 8000 degrees f, but all material is a gas, more corectly plasma at 8000 degrees f, so the practical upper limit, for solar thermal is about 4000 degrees f, a bit hotter than is practical on Earth’s surface.
    My guess is the light polution by SSP is minor for astronomers. Venus is brightest when it is about 80 million miles from Earth, and has a diameter of almost 8000 miles. An 8 mile diameter SSP 80,000 miles from Earth should appear the same size as Venus. A two mile diameter SSP, 20,000 miles from Earth should appear the same size as Venus. In most cases dimmer than Venus. Neil

  13. Rob Mahan said

    Lonnie Johnson, an Atlanta-based scientist and inventor, has worked for SAC and JPL, holds about 100 patents … and he invented The Super Soaker squirt gun! The revenue from squirt gun sales has allowed him to continue to be creative and he was recently honored for a new technology deemed the “Johnson Thermo-Electrochemical Converter System” (JTEC). It is a solid state, closed system thermodynamic engine that uses a temperature differential to generate electrical energy by pushing hydrogen ions through two membranes. The closed JTEC system does not burn oxygen and heat is the only fuel required. It is claimed to be highly scalable and suitable for space-based applications.

    When I read about JTEC, I thought it might be a possible alternative to PV cells for collecting and converting space-based solar power. The naturally occurring temperature differential in the vacuum of space would fuel the system and it seems like it might be much easier to harden it against damage from micrometeorites and space debris.

    Here is the website where I read about JTEC: Johnson Electro Mechanical Systems : JTEC. I hope some of the contributors here will take a look at JTEC and see if it merits further evaluation for a space-based solar power application.

    Rob Mahan
    Citizens for Space Based Solar Power

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