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Homework Help: Questions on solar cell technology?

  1. Dec 9, 2011 #1
    1. The problem statement, all variables and given/known data

    I'm a sort of a newbie student studying solar cell for 2 months and having my test next week, so i've in need of help here!

    1) Identify the majority and minority carriers in p-type silicon with n-type silicon solar cell?

    2) Explain the term diffusion and drift current?

    3) List and explain two reasons why photovoltaic cell has low efficiency?

    4) A ____??____ PV array is connected to a ____??______ battery bank through a charge controller. The PV array uses 24V DC 10A PV modules and the battery bank uses ________ batteries respectively. Draw the electrical diagram of the PV array, charge controller and battery bank.

    2. The attempt at a solution

    1) p-type --> majority: holes minority: electrons
    n-type --> majority: electrons minority: holes

    2) Searched online yet find to find an answer that suit the context of solar cell. Or just i don't understand it

    3) Is it shunt and series resistance causing the solar cell to have low efficiency?
    If it is, "series resistance represents the ohmic losses in the front surface of the cell and shunt resistance represents the power loss due to diode leakage currents".

    4) I know this might be hard but the above was clue given by my teacher on the question coming out, but i don't really know what should the variable be in order to tackle the question!!
    Last edited: Dec 9, 2011
  2. jcsd
  3. Dec 9, 2011 #2
    I'm not sure about answer number 2, I'd look on wikipedia for that kind of stuff. As for question 3, it's a little vague. Series resistance and shunt resistance will lower the efficiency of a specific cell, but in terms of the semi conductor material (which is what these tasks seem to be about) that isn't the case. Solar cells are more limited by their efficiency vs. wavelength relationship. The power coming from the sun is spread out across infrared, to visible, to ultraviolet. Each material will respond differently to different wavelengths (energies of light), and no one material can respond well to all wavelengths. The best solar cells nowadays are triple junction cells, usually one junction for infrared, one junction for visible, and one junction for UV, and these cells are most commonly used in CPV (concentrated photovoltaics). The problem with doing this is that 2 of the junctions will be limited by a third, lesser performing junction.

    Another intrinsic property of a solar cell is reflection. Specifically a reflection (%) vs angle of incidence property. This is a single step loss in efficiency that cannot be circumvented.

    Solar cells will also heat up in the sun. If you have a high end solar cell, it is only economically effective to use them in concentration or high concentration sunlight. These cells will heat up, and lose efficiency. As the cells heat up, I'm pretty sure the open circuit voltage drops (you might want to double check that)

    Finally, all solar cells will have carrier lines. I'm not sure what percentage of the cell is covered in these lines, but I know that it represents space on the solar cell that isn't a semiconducting material, which therefore isn't absorbing sunlight. .
  4. Dec 9, 2011 #3
    So is my question 1 correct? And the above explanation is targeted at my question 3 only right?
  5. Dec 9, 2011 #4
    I didn't answer questions 1 for you because the answer is well within your grasp.
    And yes, I only responded to question 3.
  6. Dec 9, 2011 #5
    By the way can i ask you something a bit off topic to the question i post above? If a 36 solar cell (series connected) have the following open-circuit voltage of 22.2V.
    What makes the same 54 solar cell (series connected) open-circuit voltage = 33.3V?

    Does solar cell have the same principle of adding up multiple open circuit voltages when they are connected in series? Sorry for for my poor questioning...
  7. Dec 9, 2011 #6
    I understand your question so no apology necessary. In series connections, current has to be constant, and voltage gets added. So if 36 solar cells connected in series have 22.2V Voc that means that each cell has roughly 0.6V Voc. If you have 54 of them, then its just 0.6 * 54, which will give you somewhere around 33.3.

    The key is that voltages add. It's the same with batteries, two 1.5V batteries in series will give 3V. If you had three of them in series, you would get 4.5V.

    Did I answer your question appropriately?
  8. Dec 9, 2011 #7
    Yes! Thank you very much for answering just want to confirm my doubts... Given the pdf file found in this link --> https://www.physicsforums.com/attachment.php?attachmentid=41767&d=1323446524 i'm having trouble on how to solve question b)? If you do know how to solve it, do care to share with me.. I've no idea what formula or steps i should perform to get it solve?

    Thanks a lot!
  9. Dec 9, 2011 #8
    I would know how to solve it but I'm missing a set of information on the "SQ85-P" solar cell.

    If you know how much power is hitting a certain area, and you know the area of your cells, then you can find out how much power is hitting the cells. If you know how much power is hitting the cells, and you know the efficiency of the cells, then you can find the power produced. Open Circuit Voltage is generally not very dependant on how much light is hitting the cell. That is to say that a small amount of light will yield a similar Voc as a large amount of light. So the Voc of the whole panel will be the Voc of an individual cell * number of cells. The Isc will be the current produced by any one of the individual cells at 0 voltage.

    I see now that the little pamphlet at the bottom there says that 85 Watts is produced at 17.2 Volts. My issue with this is that it is the maximum power prodcued, which does not necessarily give me the Isc and the Voc. I don't think that you can infer from this information the Voc and Isc. I could be mistaken...but I don't think so. A piece of information that I have but am not using is the temperature. Since you've taken the course, do you know of any key characteristics that depend on temperature?

    And this also seems to be a peculiar set-up, they are telling me that there is about 5 Amps of current running through these badboys. That means some big expensive wires are necessary to carry the current.

    Another problem is that in the question it tells me that 400W/m^2 irradiance, but in the pamphlet it doesn't tell me what solar irradiance their wattage is based on.
  10. Dec 10, 2011 #9
    From what i know my lecturer just told me to assume the Ultra 85-P as the solar cell we are looking for in the datasheet. But given only open-circuit voltage as 22.2V and short-circuit current = 5.45A are usable.

    Then how i should calculate question b)? Is it open-circuit voltage will be around 22.2V and short-circuit current drops to about 2.18A? This is my guess.. Correct me if i'm wrong..
    Last edited: Dec 10, 2011
  11. Dec 12, 2011 #10
    I'm sorry I don't follow what you're saying.
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