UD ELEG 620 - Photovoltaic Modules (46 pages)

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Photovoltaic Modules



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Photovoltaic Modules

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Pages:
46
School:
University of Delaware
Course:
Eleg 620 - Solar Electric Systems
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Photovoltaic Modules ELEG620 Solar Electric Systems University of Delaware ECE Spring 2009 S Bremner Photovoltaic Modules We have seen previously seen the behaviour and design of solar cells in isolation In practice they are connected together and packaged as a module to provide specific power output and to protect the solar cells from the elements We will look in more detail at the following issues Connection of solar cells and mismatch between Packaging of modules Failure modes for modules ELEG620 Solar Electric Systems University of Delaware ECE Spring 2009 S Bremner Connecting solar cells We need to understand how the different connections between solar cells affect performance and most critically what happens when solar cell performance is mismatched We will look at whether the solar cells are connected in Series give greater voltage Parallel gives greater current Mismatch between solar cells must be taken into account when designing a module how is this done How do we construct a module that will be relatively cheap but also provides good reliable power ELEG620 Solar Electric Systems University of Delaware ECE Spring 2009 S Bremner Connecting Solar Cells Series connection increases voltage Parallel connection increases current This is for identical solar cells what happens when they are not identical depends on the connection ELEG620 Solar Electric Systems University of Delaware ECE Spring 2009 S Bremner Connecting Solar Cells What a solar cell does depends on it s bias condition ELEG620 Solar Electric Systems University of Delaware ECE Spring 2009 S Bremner Solar Cells in Series Simplest thing to consider is when we have two identical solar cells connected in series Since the cells are in series the currents will be matched not a problem as they are identical voltages will add Useful for when we want a specific voltage typical voltages for a single solar cell will be 0 6 V ELEG620 Solar Electric Systems University of Delaware ECE Spring 2009 S Bremner Solar Cells in Series Recall the I V characteristic for a solar cell Realistic I V curve tells us that a slightly higher current can be obtained when solar cell is reverse biased This is important when we consider solar cells that are not identical in performance ELEG620 Solar Electric Systems University of Delaware ECE Spring 2009 S Bremner Solar Cells in Series Since the voltages add when in series if the mismatch is in voltage there is no problem When the mismatch is in current then we have a much bigger problem since in series we want current constant through all of the solar cells So in series connected solar cells the current for the chain is set by the current of the worst performing cell this is bad but it gets worse when we have a short circuit condition We can get a situation where the worst performing solar cell is reverse biased and is dissipating power Major cause of cracking and all around destruction of solar cells in modules ELEG620 Solar Electric Systems University of Delaware ECE Spring 2009 S Bremner Series Mismatch Can get a serious mismatch for nominally identical cells when one or more is shaded What actually happens when this is the situation Need to consider the current match condition and the I V characteristics for the solar cells Current mismatch is worse than voltage mismatch ELEG620 Solar Electric Systems University of Delaware ECE Spring 2009 S Bremner Voltage Mismatch in Series Voltages add together at each value of current At maximum power point the overall power is reduced compared to identical cells as the bad cell is producing less power For current mismatch we see a more drastic effect ELEG620 Solar Electric Systems University of Delaware ECE Spring 2009 S Bremner Current Mismatch in Series At low currents no problem as all cells can produce the required current At higher currents output is pinned by the ISC of the bad cell therefore power reduction is severe Power is being dissipated in bad cell Situation is most severe if we have a short circuit over the chain of cells ELEG620 Solar Electric Systems University of Delaware ECE Spring 2009 S Bremner ISC Mismatch in Series In order to match the current from the good cells the bad cell is reverse biased since we are in short circuit Easy way to find the ISC of the chain is shown above where we simply set the V of the good cell to be V We see the ISC for the chain is a little above the ISC for the bad cell and the reverse voltage across the bad cell may be close to VOC of the good cells ELEG620 Solar Electric Systems University of Delaware ECE Spring 2009 S Bremner Solar Cells in Parallel Currents add voltage is the same across cells in parallel Obviously can use parallel connection to boost current output But what if the cells are non identical ELEG620 Solar Electric Systems University of Delaware ECE Spring 2009 S Bremner Current Mismatch in Parallel Currents add so no real problem as long as open circuit voltages are same Power is reduced slightly compared to independently biased cells but effect is minimal Mismatch in voltage is more drastic ELEG620 Solar Electric Systems University of Delaware ECE Spring 2009 S Bremner Voltage Mismatch in Parallel At low voltages there is no problem When voltage is higher than the VOC of the bad cell it stops generating power and now dissipates Overall VOC of the cells is reduced to something between the high and low values ELEG620 Solar Electric Systems University of Delaware ECE Spring 2009 S Bremner Voltage Mismatch in Parallel Can find the VOC for the parallel cells quite easily Simply reflect I V curve of good cell across Voltage axis i e put I into the equation ELEG620 Solar Electric Systems University of Delaware ECE Spring 2009 S Bremner Mismatch In practice we have nominally identical solar cells so why is there mismatch Shading degradation of cells etc Mean that in practice we can have mismatch Parallel connection is less sensitive to mismatch as it is a voltage mismatch that creates bigger problem and the VOC scales logarithmically In series the current which scales linearly is the bigger problem First conclusion is to connect mainly in parallel In reality most cells are connected in series remember we need to boost the voltage ELEG620 Solar Electric Systems University of Delaware ECE Spring 2009 S Bremner Connection for a Module Most often for a module we have 36 solar cells connected in series Reason is we will typically get 17 18 V output voltage which makes it compatible with 12 V


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