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Forget electric cars, what about try to design photonic cars?

  1. Dec 8, 2009 #1
    Okay, I know that solar radiation from the sun is already a source of energy to be used as an alternative energy source for light, but solar energy is only one source of light. Scientists have already thought of applying photons as a source of current that would be used to carry out a myriad of digital computations on an optical computer, rather than electrons because electrons generate soo much waste heat and photons essentially do not. I suspect that batteries needed to power up electrical cars generate unnecessary heat as well. Well , why not create a voltage supply that carries optical current rather than electrical current to power a voltage supply for an electrical car, or rather in this case it would be a photonic car. Would you even need to recharge a battery where the source of current is light. Electricity is soo 19th century and 20th century . Why does are source of energy for light have to come from the sun? Why couldn't we used the same source of light that used to produced lasers a certain frequencies on the electromagnetic wave spectrum?
     
    Last edited: Dec 8, 2009
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  3. Dec 8, 2009 #2

    BobG

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    How will you generate your own light?

    It might be hard to design a device that can store light. It will probably be more realistically to store electricity in a battery and use the electricity to power the device that generates your light.

    Which brings you round to the reason solar radiation is the favorite source of light. The Sun's generating it like crazy and a lot of it happens to pass our way. May as well tap into it as it's passing by - just as we may as well tap into the power of water as it passes our hydroelectric dam.

    The heat benefit you speak of is very important when it comes to computers. The closer the components are to each other, the faster the computer is. The speed of electricity (and the speed of light) is fast, but it still takes more time to go long distances than short distances. The problem with moving components very close together is there's nowhere to dissipate the heat. Using light instead of electricity is a solution for the heat problem, not the power problem. It allows the components to be moved very, very close together.
     
  4. Dec 8, 2009 #3
    Well of course we get all of our light from the sun, but why couldn't light be used as a power source as well as a source for storing information?. But why not used the same storage mechanisms that are used to store the light in lasers and liquid crystal displays, why does the source of light in order for a solar vehicle to function at its full potential have to come directly from solar energy? I know there is not a heavy investment in solar cars because it is more expensive to produce the photovoltaic cells than it would be to produce conventional electricity. I cannot why it would be more expensive because as you said, we have that gigantic ball of fire in the sky called the sun that produces most of our light for us.
     
    Last edited: Dec 8, 2009
  5. Dec 8, 2009 #4

    BobG

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    What mechanisms in lasers and LCDs store light? Light is used, but it's not stored.

    You could store the energy from photons of light. When a photon of light is absorbed by an atom, an electron jumps to a orbital with a higher energy level. When the electron falls back to the lower energy level orbit, a new photon of light is emitted, carrying the energy away with the new photon. Keeping the electron in the higher orbital as long as possible is a way to store the energy.

    Or, having a system of perfect mirrors so the light never escapes - it just bounces around with no loss of energy, forever, until tapped. Designing a perfect mirror would definitely do the trick. Anything less than perfection? Total failure. With light moving at 3x10^8 meters/sec, interactions with a usable size mirror system will be so frequent and numerous that any energy loss at all will quickly suck all the energy from your system.

    Humans don't generate energy. We find a way to tap into existing energy. We can tap into energy as it passes by (solar radiation, hydro power), or we can release energy stored in some object (releasing the energy stored in gasoline by igniting it, for example). A solar vehicle doesn't have to use the Sun for its source of light. You could use a gasoline powered generator to turn on a big light that provides the light necessary to run your solar panels. What's the advantage in doing that, though?

    The cost is in production. And while the Sun emits a huge amount of electromagnetic radiation across the entire electromagnetic spectrum, solar panels only convert a small bandwith of frequencies into electrical energy, and only a small portion of the energy in the bandwidths it does convert.

    Do you know how solar panels work? A photon of light pushes an electron to a higher energy level and, instead of falling back to its original energy level in the same electron, it jumps across to a different material. The only way back to the original atom it started from is to jump back across (not going to happen) or to travel through your electrical circuit. When you think about how solar cells work, you're looking at some pretty big challenges to get very many of those electrons to jump across instead of just fall back down to a lower energy level in the same atom.

    You spend a lot of money to make a solar cell and you don't get much energy out of it. It's just a challenge, though. The efficiency of solar cells keep improving and the cost of other fuels keeps increasing.
     
    Last edited: Dec 8, 2009
  6. Dec 8, 2009 #5
     
  7. Dec 8, 2009 #6

    mgb_phys

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    In a car you don't want a source of energy - you want a source of motion, how are you going to get photons to turn a wheel?

    Converting them into electricity. Gathering photons is easy - a brick wall can do that, it's makign a few square meters of semiconductor that's expensive.

    No - you can get very good mirrors. If you are only bouncing the laser off them once then 99.9% is good enough. If you wanted to store light between two mirrors 1m apart then the light would make 300,000,000 reflections/second off each mirror. If the mirrors were 99.9999997% reflective (ie only absorbed 1/300,000,000 of the light) then the light would be gone in 1 second.
     
  8. Dec 8, 2009 #7
    I don't see why you wouldn't be able to used photons rather than electrons as a source of kinetic energy. You would think that photons would be easier to manipulate than electron since photons themselves massless particles and therefore you can generate kinetic energy since photons could themselves travel at the speed of light. Isn't that the reason why we choose electrons rather than protons as a source of current to produce electricity because protons are a lot heavier than electrons and therefore more difficult to pushed forward in motion?
     
  9. Dec 8, 2009 #8

    BobG

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    Just to expound on the difficulty involved, you have to develop two types of semiconductor material that will bond together and work together to convert the solar radiation into electricity. Your solar cells consist of an N-type material and a P-type material. Phosphorous doped silicon and boron doped silicon is the easiest and cheapest combination to create, but it's also the least efficient (look at your Periodic Table of elements and you should be able to figure out why those two elements are used for doping - at least if you happen to know a little chemistry and know 8 electrons in the outer shell is an atom's optimum configuration).

    This is a good point. If you look at visions for a space elevator, this is the idea that's proposed. The "cars" that move up the space elevator are propelled by lasers - i.e. the momentum from photons of light are transferred to the cars, propelling them up the space elevator. From a cost perspective, you're ignoring the cost to generate the laser beam, though. It would be pure fantasy to think you could have some cable system, chain drive, etc that work over such a long distance (as if a space elevator doesn't at least border on being pure fantasy). Beams of light start to look like the most efficient means of transferring energy over a long distance for things like that.

    Likewise, there was a spacecraft that was designed to use light from the Sun as its means of propulsion. Obviously, the spacecraft could only move one direction - away from the Sun. With the intensity of light inversely proportional to the square of the distance, the effectiveness of that system drops off pretty rapidly, but the necessary acceleration would have already been accomplished in this particular instance. (At least it would have if the rocket getting the spacecraft out the Earth's atmosphere had worked. The rocket didn't have quite enough boost and the spacecraft wound up in an orbit with a wet perigee - i.e. it wound up in the ocean).
     
  10. Dec 9, 2009 #9
    How many watts of power would be needed to generate a pulse of laser light? Why do you think generating laser light is way too expensive? They already created a laser at Lawrence Livermore's laboratory that generates 10^12 watts of power and 10^15 watts of power. But in an electrical car, it only the amount of 10^6 watts is needed to generate power for the car. Why do you considered generating laser light to make a car move forward to be infeasible when it has been demonstrated that laser lights generate power up to 10^15 watts? I suspect that you would need to generate 10^6 watts as well to propel a vehicle forward .

    I am afraid that I don't understand. Yes, I know that the intensity of light decreases as you move away from the sun, but why should that be a hinderance ? Photons are already used to bombard electrons to a higher level of energy, why should there be a big concern if photons were used as a direct source of energy?
     
    Last edited: Dec 9, 2009
  11. Dec 9, 2009 #10

    BobG

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    I didn't say laser light was way too expensive, nor did I say it was an unfeasible idea. I said the choice of laser light for a space elevator was made for reasons other than cost. I never said the idea of using photons to do work was a dumb idea.

    Consider this. Electric companies like to use coal as their fuel source to generate electricity. Automakers like to use gasoline as their fuel. Semis like to use deisel. If one of these fuel sources is the best, then why don't all three use the best fuel source?

    Your idea is good. You haven't put much thought or research into figuring out how you go from the the potential to do work (energy) to actually doing the work when you want it. Each solution carries its own overhead (you have to carry the fuel for your gasoline engine around with you in your car, for example). Each solution has its advantages and disadvantages (having to carry around all that fuel is a very major disadvantage to standard rocket engines, for example, which is one situation where using laser light to propel a vehicle could be the best solution when the vehicle is traveling a restricted path over and over).

    For solar sails, the intensity of light is as important as the wind for a sailboat. If the wind is blowing 20 mph, each particle of air carries more momentum when it hits your sail, plus more air particles hit your sail per second. At 1 mph, you have few air particles hitting your sail and each has very little momentum. Solar radiation will always go the same speed, so each photon carries the same amount of momentum, but the number of photons hitting your sail has an effect on how much momentum will be transferred to your spacecraft. The Sun puts out so much light. The greater the surface area that that light is spread out over, the less light there is per square meter (this applies to a lot of things, which is why you see the inverse square law so often in physics).

    By the way. Check out some articles on solar cell technology. I remember reading an article a while back that suggested we might be half way to developing much more efficient solar cells. Half way because you need two materials that will bond and work together. Or, maybe there is no material that will bond and work with the material the article mentioned and we're still sitting at the starting point. Improving solar cell technology (or any other technology) isn't impossible - it's just hard - which is why engineering is a good paying profession.
     
    Last edited: Dec 9, 2009
  12. Dec 9, 2009 #11

    mgb_phys

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    There is a difference between power and energy. A large laser can generate 10^15 watts for a fraction of a billionth of a second - the actual energy produced is tiny.
    And it takes a building full of components and a huge amount of electrical power to generate this pulse.
     
  13. Dec 9, 2009 #12
    Why can't you generate 10^15 watts of laser and sustain it for a long period of time? Would it be too dangerous to produced that amount of power for longer periods of time beyond a fraction of a second?
     
  14. Dec 10, 2009 #13

    mheslep

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    About 2x10^4 Watts are needed to keep a car moving at 60mph.

    Because the entire continuous electric power capacity of the US is only about 10^12 Watts.
     
    Last edited: Dec 11, 2009
  15. Dec 11, 2009 #14

    russ_watters

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    I'm pretty sure that's not the mechanism: just because a beam of light carries 1000 watts of power (for example), that doesn't mean that the radiation pressure can generate 1000 watts of mechanical power. Think about it: if that were true, if you held up a mirror toward the sun, it would knock you over!

    edit: yeah, the laser is used to generate heat, not to use its radiation pressure to move the object: http://en.wikipedia.org/wiki/Laser_propulsion
     
  16. Dec 11, 2009 #15
    In terms of joules, NIF produces a 4 MJ pulse. 4 MJ is about 4 candy bars worth of energy. I believe they hope to be able to do this up to 7 times a day. Well that would keep a couple of people in candy bars but it aint going to get you home in your car (unless you eat the candy bars and push). Gas (Petrol) is 2.2 x 10^7 J per pound....
     
  17. Dec 11, 2009 #16

    Chronos

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    Something is fundamentally wrong. Earth would drift away from the sun much more rapidly than presently observed if radiation pressure were enough to overpower gravity. I can't accept this idea.
     
  18. Dec 11, 2009 #17

    BobG

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    Increasing your cross sectional area increases force from solar pressure. Increasing mass increases the force due to gravity. A dense sphere doesn't make a very good solar sail.

    Ah, that makes sense. That would be silly to use that much energy to transfer such a small amount of momentum.

    Actually, if you were in a near vacuum (such as outer space), holding up a mirror toward the Sun would knock you over, even if your angular acceleration was a bit slow.

    A working model of a solar sail (even if it's propulsion is used for attitude control instead of to change it's orbit)
    http://en.wikipedia.org/wiki/File:GOES_8_Spac0255.jpg
    smgoes.gif

    The gold cone shaped device at the end of the boom is the solar sail. It has very little mass. Putting it at the end of a boom increases the torque. A trim tab on the end of the solar arrays changes the area on the solar array side of the satellite.
     
    Last edited: Dec 11, 2009
  19. Dec 11, 2009 #18

    CRGreathouse

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    Right, capturing light with mirrors isn't effectively possible. (I feel inclined to point out that that reflectivity implies a half-life of 0.764s rather than complete disappearance in 1s.)
     
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