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solar panel characteristics etc.

Discussion in 'Mathematics and Physics' started by PG1995, Mar 22, 2014.

  1. PG1995

    PG1995 Active Member

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    Hi

    Q1: Silicon has barrier potential of 0.7 V and germanium has barrier potential of 0.3 V. I think barrier potential is dependent upon doping agents used. Please correct me if I'm wrong. The question is why silicon and germanium have different barrier potential.

    This is what I think. The amount of energy required to free an electron from the valence band of a silicon atom is called the band-gap energy and is 1.12 eV (electron volts) and for germanium it's almost 0.78 eV. The difference between the band-gap energies shows that in case of silicon electrons are more tightly bound to positively charged nucleus. So, when depletion region has been formed, positive ions on the side of n-region pull less vigorously on the electrons on the side of p-region in case of germanium than for the case of silicon. Do I make sense?

    Q2: A solar panel's voltage is negatively affected by temperature - voltage drops with increasing temperature. On the other hand, the panel's current increases with increasing temperature to a small degree. What I want to understand is that why the voltage decreases with increasing temperature.

    In answer #2 on this link provides a clue but I'm still struggling to grasp it.

    Suppose we have a silicon solar cell as show here but assume that initially there are no electron-hole pairs. There will be a barrier potential of, possibly, 0.7 V. When photons start hitting the solar cell, electron-hole pairs are generated. For the sake of simplicity, we will focus on only one electron-hole pair which is generated in the n-region. As it is mentioned in answer #2 that at elevated temperature it needs less energy to unbound an electron from an atom. How would this translate to less open circuit voltage for a solar panel at elevated temperatures?

    Thank you for your help.

    Regards
    PG
     

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  2. NorthGuy

    NorthGuy Well-Known Member

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  3. PG1995

    PG1995 Active Member

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    Hi NG

    I have been to that site. But you can see that it doesn't provide any explanation on why and how temperature affect the open circuit voltage. Surely, it does explain it using math equations. I hope I'm not missing something. Thank you.

    Regards
    PG
     
  4. dave

    Dave New Member

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  5. NorthGuy

    NorthGuy Well-Known Member

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    I think this is the explanation:

    "Increases in temperature reduce the band gap of a semiconductor, thereby effecting most of the semiconductor material parameters"

    As you said in your comments to Q1, smaller band-gap means lower voltage.
     
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  6. nsaspook

    nsaspook Well-Known Member

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    If you look at the circuit model for a solar cell you see a current source in parallel with a forward biased diode.

    RESIST.gif

    The value of current from the current source is based on the photon -> electron energy transfer changing the depletion zone charge separation equilibrium created when the junction was formed similar to what we see in ionic charge separation inside a battery. If you think of the solar semiconductor junction also as a diode shunt voltage regulator whose forward junction voltage (that varies with temperature) determines the output voltage from the current source, it's straight forward to see how this effects the open circuit voltage.

    http://org.ntnu.no/solarcells/pages/Chap.4.php
    http://org.ntnu.no/solarcells/pages/Chap5.php
     
    Last edited: Mar 23, 2014
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  7. PG1995

    PG1995 Active Member

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    Hi

    It was good thing that nsaspook mentioned that solar cell model because I was already trying to understand it without any success. I have been through many sources but I'm still confused. To me it looks like some sources were making the things more confused than they already were.

    Let's try again. The curves for a current source and solar panel are very much the same. Please have a look here. But this shouldn't come as a surprise because a solar panel is very much a current source whose current output capability is dependent on incident light. In a model of solar cell, the Rsh is ideally infinite and Rs is ideally zero. But the fundamental difference between the model of a solar cell and that of current source is the diode. What is diode doing there? Why is it an important and integral part of the model? By the way, notice that barrier potential of a pn diode varies with temperature as does the Voc of a solar cell.

    This is my own attempt at the answer. Please have a look here. I have ignored Rs because it's ideally taken to be zero. The intensity of incident light on the current source is constant. The forward bias voltage for the diode is taken to be 0.7V. We gradually starts increasing the resistance of variable load resistor. Notice that Rsh and R_L are in parallel therefore as the resistance of R_L is increased, the combined parallel resistance also increases. The increased overall resistance would mean more voltage around the nodes "A" and "B". As we keep on increasing the resistance, there would be come a point when ammeter reading starts falling and theoretically voltmeter's reading would be around 0.6V. As we increase the resistance, the current will fall to zero around 0.7V because the diode has started conducting and the current has not chosen a different path to flow. Do I make any sense?

    Would it make a large difference if Rsh is also ignored just like Rs? From my attempt at the answer, it's clear that diode is an important and not an ignorable part of the mode.

    Thank you for the help.

    Regards
    PG
     

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    Last edited: Mar 28, 2014
  8. steveB

    steveB Well-Known Member Most Helpful Member

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    The diode is important, of course.

    The solar cell does not operate as a perfect current source, or even a non-deal current source with parallel resistance. It is really a combination of voltage and current source. That is why the diode is there, to bring in the voltage source effects. An current source would provide more and more voltage as needed to maintain the current, but the solar cell voltage eventually gets clamped. The fact that the solar cell is like a pn-junction is the reason that the diode works as an approximation, but you should realize the model is not perfect, so don't get too worried about it. It's basically a simple model that displays some of the main important features.
     
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  9. PG1995

    PG1995 Active Member

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    Thank you.

    But did my interpretation or explanation make any sense to you?
     
  10. steveB

    steveB Well-Known Member Most Helpful Member

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    Sure, it seems to make sense to me.
     
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  11. PG1995

    PG1995 Active Member

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    Hi

    Could you please help me with the queries below? Thank you.

    We will use this attachment.

    Q1: What is this transmission investment and why is high for solar thermal energy source compared to conventional coal? I have been to the following pages but didn't get it.

    1: https://www.ofgem.gov.uk/electricit.../transmission-investment-renewable-generation
    2: http://www.elp.com/articles/2014/03/electric-transmission-investment-on-the-rise.html

    Q2: Why is "Fixed O&M" so much for solar photovoltaic and solar thermal?

    Please note that hydroelectric generation could be classified as dispatchable generation if there is a reservoir and so is true of solar thermal with storage. You can refer to the Wikipedia article which says, "In general the only types of renewable energy which are dispatchable are biofuel, biomass, hydropower with a reservoir, and concentrated solar power with thermal storage".

    Regards
    PG
     

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    Last edited: May 15, 2014
  12. PG1995

    PG1995 Active Member

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    Would someone please help me? Thanks.
     
  13. steveB

    steveB Well-Known Member Most Helpful Member

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    Hi PG,

    I had read these questions when you first posted them. I was not able to answer them, and was hoping someone else had some knowledge about this to help you.
     
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  14. steveB

    steveB Well-Known Member Most Helpful Member

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    Well, let me say a few general things that you already know.

    First, I'll mention that I'm not familiar with all of the terminology in the articles you referenced and I'm not someone who looks at the economics of large scale systems, or even the design of large scale systems. Terms like "transmission investment" are not all that clear to me, although I can kind of guess at their meanings.

    Some general things we know.

    1. Newer technology requires research investment to understand and design new systems. This means that anyone who invests money to target this market is taking risk, as investors always do. However, someone who is thinking of investing in this area, might choose to wait because the first one's to invest are taking the higher risk and paying more money to learn what needs to be learned. Those who wait can take advantage of what others learn, or just buy up the companies that spent all their money and are not able to attack the market with the technology they developed. This creates a kind of "inertia" to getting the technology developed and ruggedized for practical use. This means that incentives (e.g. government grants) can help move things forward by making risk smaller and encouraging innovation.

    2. Newer technology has more uncertainty in maintenance costs, replacement costs, upgrade costs. People don't always know exactly what problems will be encountered over the long term use. This also adds to the risk of the technology usage. In my state, one of the first wind mills that went up, eventually broke and was not used for a long time because the costs to repair were too high. Here is a quote from a local newspaper (Providence Journal)

    "PORTSMOUTH, R.I.-- In March 2009, officials heralded the installation of a 336-foot-tall wind turbine at the high school. It was the largest in Rhode Island, and they believed it would generate dollars as well as watts, selling power into the electric grid.

    For three years, the $3-million windmill fulfilled that promise, making the town about $400,000 after maintenance and debt payments.

    But in February, technical problems started shutting down the turbine for weeks at a time. In June, it stopped again, and it hasn't started since.

    Last week, the Town Council learned that the turbine's gearbox had failed and needed to be replaced for at least $460,000 -- completely erasing those three years of income.

    The situation has left council members angry and exasperated."

    3. Newer technology has some additional known costs for integration into the grid. For example, wind and solar have varying power generation in time which can require additional load leveling and storage systems. This problem gets worse if a higher percentage of the total power generation comes from these varying sources.

    I don't know if any of this can be applied to answering your question, but maybe it can help get the conversation going.
     
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  15. steveB

    steveB Well-Known Member Most Helpful Member

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    Here is a later (last year) update on the windmill problem I mentioned above. http://www.renewableenergyworld.com...fix-for-broken-wind-turbine-at-us-high-school

    Portsmouth, RI -- It's status quo for the Portsmouth High School wind turbine. On Monday night, town planner Gary Crosby informed the Town Council that negotiations ceased with two possible developers interested in fixing or replacing the 336-foot-high turbine, which has been broken since June 2012.

    The council allowed Crosby to issue a formal request to seek other bidders for the project.

    The town is considering three options: replace the turbine’s broken gearbox; replace the entire turbine with a new, more reliable model; and take down the turbine and sell if for scrap.

    “We’re asking to review the process and start again,” Crosby said.

    Without the revenue from the sale of electricity, the town is still paying two loans for construction of the turbine. Quarterly interest payments of $4,983 are paid toward the $2.6 million Clean Renewable Energy bond. A principal payment of $173,333 is also due Dec. 15 for the bond.

    A $400,000 loan from the state Economic Development Corpration is due a principal payment of $26,380 and interest payment of $3,079 on June 15.

    When running, the wind turbine earned the town about $160,000.

    Removing and replacing the gearbox costs some $780,000. It would likely require the town to maintain ownership of the turbine and make the debt payments.

    Selling the turbine to a developer would likely mean the new owner would make the interest payments, with little to no revenue heading to the town.

    Scrapping the turbine, which Crosby has called the “nuclear option,” would only generate a small, one-time payment from the sale of the scrap metal, and likely leave the town with a large portion of the debt.

    There was little discussion by the Town Council or from the public at the May 13 meeting. “I think we’ve got to look over every stone in the landscape before we get to it,” council President James Seveney said.

    The 1.5-megawatt turbine was commissioned in March 2009. The turbine had a 20-year life expectancy but was shut down June 18, 2012, after the gearbox showed significant wear. An independent investigation blamed the damage on a faulty gearbox. The gearbox, however, was no longer covered by warranty, and the manufacturer of the turbine, AAER Wind Energy of Quebec, is no longer in business. Three of five turbines of the same make and model erected in California and Templeton, Mass., also suffered gearbox failure.

    The manufacturer of the gearboxes, Jahnel Kestermann of Germany, had offered to sell two replacement gearboxes to the town for $203,000. It would cost the town an additional $407,000 to remove the old gearbox and install a new one. The two-for-one deal, however, no longer exists.

    Crosby said at least one developer is interested in responding to the new request for proposal.
     
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  16. steveB

    steveB Well-Known Member Most Helpful Member

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    duplicate post - crappy hotel internet connection
     
    Last edited: May 16, 2014
  17. cowboybob

    cowboybob Well-Known Member Most Helpful Member

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    Transmission investment is the cost of installing the wires, posts and transformers (simply stated) to get the power generated to the load(s).
    Conventional power plants (i.e., coal) can be built where there is the need. Solar plants must be built where there is abundant insolation, frequently NOT near the power need, thus requiring considerably higher transmission costs.
    For "dispatchable" solar generation sources, primarily energy storage costs (battery and/or thermal).
     
    Last edited: May 16, 2014
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  18. nsaspook

    nsaspook Well-Known Member

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    The vacation police called and said get back to the pool and bar!
     
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  19. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    Q2: I think you picked the wrong two highest.

    Any inverter based technology can be used for local power factor correction. That. puts a wrench into those numbers, I think.
     
  20. PG1995

    PG1995 Active Member

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    Thank you, Steve, cowboybob.

    steveB: Those two posts were very informative. Thanks.

    Sorry, I don't get what you are saying. Perhaps, you are saying that Wind-Offshore has higher "Fixed O&M" than Solar Photovoltaic.

    Regards
    PG
     

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