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Selecting a solar energy conversion method

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Mineyes
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re: Selecting a solar energy conversion method
Mineyes   7/6/2012 7:39:23 PM
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"Southern Exposure is Obvious" That is true only in the Northern Hemisphere. A better statement, might be "Equatorial Exposure is required"

Brakeshoe
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re: Selecting a solar energy conversion method
Brakeshoe   7/6/2012 8:14:28 PM
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Yes, the Thevenin model applies in figure 2, as you are looking back into the terminals of the source. What's more, if you apply a variable resistive load, you will get a linear graph of voltage vs current.

chrisrodgers
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re: Selecting a solar energy conversion method
chrisrodgers   7/7/2012 2:40:06 PM
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I see that my understanding is slowly beginning to clear. I found the found Fig. 3.7 on pg. 24 of the following article to be helpful: https://digital.library.txstate.edu/bitstream/handle/10877/3171/fulltext.pdf I think what was bothering me was that the boost converter cannot increase the load voltage to any arbitrarily high level. It is necessarily limited by the PV maximum power curve for a given set of conditions (illumination, temp., etc.) This Fig. (3.7) seems to bear that out that.

Robotics Developer
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re: Selecting a solar energy conversion method
Robotics Developer   7/7/2012 6:29:49 PM
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A very nice article with good technical details! Very much appreciated, thank you! I am wondering if this level of complexity is needed when using solar power both locally and not at normal line voltages (ie. 12V or 24V DC lighting/systems)?

HarrisMarcus
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re: Selecting a solar energy conversion method
HarrisMarcus   7/9/2012 8:33:35 PM
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RL will normally be much greater than Rs. Interposing the SPV1020 boost converter between the panel and RL decreases the load resistance seen by the panel. The load resistance seen by the panel Rin = RL x (1-du)^2 where du is the duty cycle internally set by SPV1020. The duty cycle is set by the SPV1020 so that RL x (1-du)^2 = Rs, the source resistance of the panel.

HarrisMarcus
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re: Selecting a solar energy conversion method
HarrisMarcus   7/9/2012 8:34:29 PM
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You are correct. The intent of using the Thevenin equivalent is to show that whatever the value Rs even though non-linear, RL must match it to provide maximum power transfer.

HarrisMarcus
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re: Selecting a solar energy conversion method
HarrisMarcus   7/9/2012 8:35:36 PM
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The maximum power available from the panel does not change but the power that is actually extracted from the panel does change and it is a function of the load resistance RL. This maximum power point occurs when RL = Rs. The boost converter decreases the value of RL by adjusting its duty cycle. The input resistance seen by the panel is RL x (1-du)^2 where du is the duty cycle internally set by the SPV1020. The SPV1020 duty cycle is internally adjusted so that RL x (1-du)^2 equals the panel output resistance Rs.

HarrisMarcus
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re: Selecting a solar energy conversion method
HarrisMarcus   7/9/2012 8:36:44 PM
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You have stated “As the cell voltage rises, the current in the internal diode rises, leaving less of the photo current for the load”. This is not so. If you follow the panel I-V curve shown in Figure 6 when the cell voltage increases the cell current decreases. The maximum cell voltage is the open circuit voltage, Voc, where the current is zero.

HarrisMarcus
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re: Selecting a solar energy conversion method
HarrisMarcus   7/9/2012 8:37:41 PM
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The article you provided is very thorough and detailed. Thank you. You cannot get more power out than you have coming in.

HarrisMarcus
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re: Selecting a solar energy conversion method
HarrisMarcus   7/9/2012 8:39:00 PM
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You would need to monitor the panel output voltage and input voltage.. This can be done through the SPI bus in the SPV1020. The SPI bus provides a direct measurement of input voltage, input current and duty cycle. Output voltage can be computed by Vout = Vin/(1-du).

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