Naked Science Forum
Non Life Sciences => Technology => Topic started by: Expectant_Philosopher on 16/01/2013 10:23:30
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Looking over the research I see a bunch of techs used in isolation to improve Solar efficiency. It seems to me we should be combining these techs into one integrated product. Techs like:
Anti-reflective coatings, Carbon Black Coatings, nanospheres, Photovoltaic Nanoshells, Quantum Dots, Light waveguides, Heat Phonons, Lasers, Surface Plasmons, graphene, nanorods, nanowires, structural batteries, metamaterials, thermoelectrics. Imagine if we coat the photovoltaic nanoshells with quantum dots and when we make the nanospheres by subtraction the structure left behind could be made into a waveguide for light to feed concentrated light lasers of certain colors to activate the quantum dots, and surfaces of this structure carved with metamaterial properties to direct heat phonons to thermoelectric generators, all feeding electricity through nanorods and nanowires to a containing structural battery, with a shell of carbon black, and a shell of anti-reflective coating. Light is harvested by this solar battery to a 95% efficiency, with both direct photovoltaic conversion and with thermoelectric conversion, and electric power stored in its own structure available for use. With slight modifications to this structure it could be used with catalytic materials to produce hydrogen from water. A solar panel on your roof produces 230 watts at 11% conversion efficiency, imagine 2,000 watts at 95%. For 20 panels you go from around 6 Megawatts a year to 50 Megawatts a year. Imagine what kinds of products we will use in the future to take advantage of the surplus.
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In order for a solar cell to be more efficient it needs to have more exposure to light. Hence to harness more light we find ourselves angling the panels to maximize the amount of light or we use light concentration to focus more light onto a smaller surface area. With our present abilities to manipulate light, to slow it down (physicsworld.com/cws/article/news/2009/dec/.../slowed-light-breaks-record) or redirect it with Metamaterials (phys.org/tags/metamaterials/), couldn't we let light into a matrix, sort of a "Hotel California" from which it never leaves? The matrix could be a device where we capture light, redirect it, slow it down, repeatedly redirecting the same light to illuminate the conversion surface over and over, tapping more of the energy as electricity and less as waste heat?
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The military is very concerned about how it can recharge its portable electronics.
There are a variety of solar cell materials that can extract energy from different parts of the electromagnetic spectrum - to improve efficiency you could consider:
- Splitting the light into different wavelength bands, and directing each band to a separate solar cell optimised for that band
- Layering these different materials on top of each other, so that each extracts its own band (but then it would need to be transparent to other bands)
- Keeping the solar cells cool, even though they are exposed to full sun. Leakage currents increase at higher temperatures.
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With techniques like this, 50% solar cell efficiency has been demonstrated, but 95% seems a bit too optimistic for the near future. After all, the atmosphere absorbs more than 5% of the Sun's radiation...
See: http://spectrum.ieee.org/green-tech/solar/tapping-the-power-of-100-suns
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I like your multi-junction idea with harnessing different color bands. I actually conducted a science experiment a few years back, where I generated different color bands and directed the individual spectra onto a standard solar panel and registered output of the panel. I found the panel responded best to red spectra, much less so for other bands. And of course white light as the most energetic.
The transparency issue I think could be solved by the Metamaterials, where we make certain materials transparent to specific bands of light. As for cooling, slowing down light implies cooling effects.
With a device that could manipulate Metamaterials and manipulate how long light stays in a system, I envision a device that would absorb light, then create sort of a cold laser where the light continually bounces back and forth through the system and the different spectra only react with materials designed to best harvest energy from a specific band of light, the various spectra passing through materials not intended for their bandwidths. The solar reactive materials designed as a quantum dots on microspheres in a lattice, with the structure of the lattice held together by ultra thin graphene wires, all to avoid totally blocking the light. The device would also harvest generated heat by redirecting heat phonons through the graphene wires to induce additional electron production.
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Currently they find with their limited knowledge of Metamaterials that they can just make the materials invisible to certain frequencies of light, but don't you see that's perfect for our needs. The Metamaterials could be made of hybrid semiconductor/metals tunable to a specific frequency of light with light of a specific frequency passing through to a tuned solar cell and the rest of the energy be reflected back to continue to bounce around inside the laser cavity.
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Keep in mind that once a photon is absorbed, and converted to energy, then it no longer is "bouncing around".
The advanced triple junction cells are reaching an extraordinary 40% or so efficiency. They tend to function best with concentrated sunlight on Earth, but direct sunlight in space.
The idea is that each layer absorbs a frequency specific photon, and is transparent to other frequencies, so the light that isn't absorbed would pass to a second and third layer.
One may expand the frequencies being absorbed, but the strongest light reaching through Earth's atmosphere is the middle of the visible light spectrum.
Solar tracking panels provide advantages of maximizing the sunlight exposure per given area which may improve cell efficiency, like is found with the concentrated light used with the multi-junction cells. Without the tracking, the effective cross section of the panels would be less than maximum.
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The original question alludes to direct storage of the photons arriving at a collector, and then getting 'trapped'. But practically speaking there's no everyday means by which photons can be held in stasis - That is, it is possible to 'trap' a few photons like this.... in a lab, but I doubt it will ever be possible to hold streams of billions of photons as part of a mass produced device. And even if it were, there is no reason to believe this is an effective way of storing the energy from sunlight.
There is of course also the question of cost that, beyond the world of pure research, quickly becomes as if not more important than aiming for outright efficiency. If the end product is going up into space on a satellite say, then yes even tiny advances in efficiency may be worth millions but back on Earth the situation will be quite different.
For example, imagine if a (purely fictional) devise could be built that (somehow) could float in the air above the Sahara desert that cost only one penny a kW installed (costing nothing at all to maintain or clean) and could also store the electricity produced until needed - even if it returned only a couple of percent efficiency if would transform the world, practically overnight.
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Phosphorescent materials (http://en.wikipedia.org/wiki/Phosphorescence) give one absorption and delayed emission of photons in the visible spectrum. Although, I don't think the efficiency is very high.
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If this were possible , then we could satisfy our energy needs . Sterling engine powered by a reflector may not satisfy our needs , Sterling engines are not efficient.
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Found this article, that addresses Peppercorn's concerns about the ability to hold light in stasis.
Scientists from the MESA+ Institute for Nanotechnology at the University of Twente in the Netherlands have designed a novel type of resonant cavity that serves as a prison for photons. The cavity confines light in all three dimensions in space inside a photonic crystal. The crystals have a structure similar to how atoms are arranged in diamond gems. Confining photons has many applications in optics (efficient miniature lasers and LEDs), communication technology (on-chip storage bits of information), and even in life sciences (tiny, yet sensitive sensors of pharmaceutical materials). The results appear in the leading journal Physical Review B that is published by the American Physical Society (APS).