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  1. 27 Apr '12 08:19 / 10 edits
    http://physicsworld.com/cws/article/news/2012/apr/26/quirky-solar-cell-sets-new-efficiency-record


    Quirky solar cell sets new efficiency record
    Apr 26, 2012 8 comments

    Increased efficiency
    Researchers in the US have built a new type of solar cell that emits light as well as absorbs it, making it the most efficient solar cell ever developed. The efficiency of their prototype cell allows it to convert 28.6% of the Sun's energy into electricity. This is a considerable increase from the previously recorded highest efficiency of 26.4%, which was achieved in 2010.
    Scientists have known since 1961 that the absolute limit for the amount of energy that can be harvested from sunlight hitting a typical solar cell is about 33.5%. However, for almost five decades researchers have been unable to come close to achieving this theoretical efficiency. But now, Eli Yablonovitch and his graduate student Owen Miller from the University of California, Berkeley have designed and built a new type of solar cell that gets closer to that limit by mimicking the behaviour of a light-emitting diode. That is to the say the solar cell is highly capable of absorbing light as well as emitting it. In fact, it is the controlled emission of light that has boosted the efficiency.
    The researchers have shown that the better a solar cell is at emitting photons, the higher its voltage is and the greater its efficiency. "[The result] is almost paradoxical and counterintuitive. It can be quite confusing to grasp at first," says Yablonovitch, as he tells physicsworld.com that he and his colleagues discovered the connection while trying to resolve the large gap between the theoretical and achieved limits for solar-cell efficiency.
    Managing photons
    The solution lay in a mathematical connection between absorption and emission of light – a phenomenon better understood as "photon management". Conventionally, photon management involves controlling the photons incident on a solar cell so that a photon ejects as many electrons as possible, thereby generating the maximum amount of electric current. "But there is another aspect to photon management, in that we manage not only the incident light, but also the emitted light. Emitted photons sometimes get 'lost' within the cell, so what we do is make sure those photons are emitted," explains Yablonovitch. In a conventional solar cell, photons from the Sun hit a semiconductor material, knocking electrons loose and allowing them to flow freely. But this process can also generate new photons, in a process known as "luminescent emission". As there is a fundamental thermodynamic link between absorption and emission, designing solar cells to emit light causes an increase in the voltage produced by the device.
    The researchers' novel concept has been put into practice by a company called Alta Devices, which was co-founded by Yablonovitch and California Institute of Technology physicist Harry Atwater in 2007. The firm was set up specifically to produce economic and high-energy solar cells. The new prototype solar cell is made of gallium arsenide, a material often used to make solar cells for satellites. The result is a device that operates at 28.6% efficiency.
    First to put into practice

    The prototype solar cell
    While the theory of luminescent emission causing an increase in voltage has been known for a while, it has never been put into practice. "It is somewhat puzzling why it has never been used in the field of solar-cell development until now. But a lack of certain requirements might explain that," says Yablonovitch. He goes on to say that solar cells are "grown" on substrates that are generally of poor quality and act as "sinks" for the emitted luminescent photons, which are then lost. The new cell made by Alta Devices is separated from the substrate, which delivers a much better performance. "In fact, we separate the substrates on which the cells are grown and then re-use them. This not only helps with efficiency, but it also brings the cost of producing our cells down, and so it is a key factor," says Yablonovitch. He explains that the cells are still as thin (1 µm) as traditional cells and so people are genuinely shocked to know the devices have been developed cheaply using gallium arsenide. Alta Devices is already producing the cells on an industrial scale, with samples being shipped to customers.
    Yablonovitch says he hopes researchers will be able to use this technique to achieve efficiencies close to 30% in the coming years. And given that the work applies to all types of solar cells, the findings have implications throughout the field.
    The team will present its findings at the Conference on Lasers and Electro Optics to be held in early May in California in the US.
    The research is to be published in Journal of Photovoltaics.


    I wondered where the 33.5% figure for the maximum possible efficiency came from in the: “the absolute limit for the amount of energy that can be harvested from sunlight hitting a typical solar cell is about 33.5%. “ bit and I found this:

    http://solarcellcentral.com/limits_page.html

    -so it is possible to go over that limit.

    I have a question:

    Given unlimited advances in nano/molecular technology, is it possible to design a solar cell that can convert 99% of solar energy into electrical energy even from indirect light ( 'indirect light' so that you cannot concentrate it to increase efficiency that way ) ? Or would that break the law of physics? ( esp the laws of thermodynamics )
  2. Standard member sonhouse
    Fast and Curious
    27 Apr '12 12:05 / 1 edit
    Originally posted by humy
    http://physicsworld.com/cws/article/news/2012/apr/26/quirky-solar-cell-sets-new-efficiency-record

    [quote]
    Quirky solar cell sets new efficiency record
    Apr 26, 2012 8 comments

    Increased efficiency
    Researchers in the US have built a new type of solar cell that emits light as well as absorbs it, making it the most efficient solar cell ever developed. The ould that break the law of physics? ( esp the laws of thermodynamics )
    [/b]
    Conversion efficiency is related to how many band gaps you can employ. There are cells that approach 40 % but they are triple cells with different band gaps in each one so they produce power from narrow slices of the optical spectrum. When they say you can only convert 1/3, they are talking about a single bandgap cell, a normal PV cell.

    If they can have say 10 different bangap materials that would be transparent to optical bands outside their own, stacked up with the bottom layer absorbing its own band and so forth a much higher efficiency rating can be achieved but they would cost more to make also and so would probably be relegated to space probes or something where cost doesn't matter.

    Overall efficiency can be increased by combining heating and PV cells. Collecting heat generated by the cells in special containers to hold the heat and transfer it to collection tubes filled with water or some other heat transfer liquid, you can get hot water for showers or winter heat at the same time as generating electricity. There are cells already made that do that. Here is one link to such a device:

    http://phys.org/news/2012-04-naked-energy-touts-hybrid-solar.html
  3. 27 Apr '12 13:53 / 3 edits
    Originally posted by sonhouse
    Conversion efficiency is related to how many band gaps you can employ. There are cells that approach 40 % but they are triple cells with different band gaps in each one so they produce power from narrow slices of the optical spectrum. When they say you can only convert 1/3, they are talking about a single bandgap cell, a normal PV cell.

    If they can have one link to such a device:

    http://phys.org/news/2012-04-naked-energy-touts-hybrid-solar.html
    great link!
    I noticed that the solar cells can turn towards the direction of the sun within each cylinder -didn't think of designing it that way but, by coincidence, I had independently came up with this concept of 'hybrid' solar panel but didn’t know it was called a “hybrid” panel nor knew that somebody had beet me to it.

    But none of this quite answers my question although it does give a few subtle clues but not sure what max energy efficiency figure they point to -anyone?
    Anyone; ignoring cost of production and the technology needed and any waste thermal energy that could be used, what is the absolute limit to how efficient a solar panel can convert indirect sunlight to electricity given the known laws of physics and the constraint that the solar panel must be made of ordinary matter?
  4. Standard member sonhouse
    Fast and Curious
    27 Apr '12 14:06
    Originally posted by humy
    great link!
    I noticed that the solar cells can turn towards the direction of the sun within each cylinder -didn't think of designing it that way but, by coincidence, I had independently came up with this concept of 'hybrid' solar panel but didn’t know it was called a “hybrid” panel nor knew that somebody had beet me to it.

    But none of this quite answers my ...[text shortened]... nown laws of physics and the constraint that the solar panel must be made of ordinary matter?
    I don't think anyone on the planet can answer that question with any degree of certainty. I know the three bandgap triple cell ones are too expensive for any kind of commercial use, probably over 20 bucks a watt. Might even be too expensive for space craft, which want's the creme de la creme cells, they use the best gallium arsenide ones that can take high heat and have high conversion efficiency, like 25 percent or so, but extremely expensive but if it saves weight by being robust for space and relatively high efficiency rating, the overall project would be cheaper.