Operating point resistance

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PVresistif
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Operating point resistance




by PVresistif » 26/07/18, 18:34

My PV in summer with the heat does not work anymore like winter, this PV works with a resistance of 4.3 ohm (hot) is a power of 234W under 31.5 V
But if in winter it works pretty well - I fall on the values ​​of the resistance around 4.5 ohm at the moment the readings give 5.3 Amp for 28V at the height of the day (13h) is power 150W. But this corresponds to a resistance of U2 / P is 28 ^ 2 / 150 = 5.23 Ohm what is very different anyway from 4.23 value of my resistance that can not have changed?
Do you have an explanation? my ammeter error? it is an ammeter clamp that I do not seem very easy to use (see manual), a package bought Bricot Depot (50 € I think!)
The curves of the panel indicate a power which should be at 6.8 A or 190 W, in this case R would be 4.12 ohm and we would be in phase; then measurement error of my "pourave" device?
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Gaston
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Re: Operating point resistance




by Gaston » 27/07/18, 10:49

For me, the probable cause is the modification of the characteristics of the panel because of the increase of its temperature.

It is commonly a voltage drop of 2mV per cell per degree.
For a panel with 72 cells in series, the voltage drop is 144 mV per degree.
If the panel is 25 ° warmer than in winter, the fall is 3,6V.

Without modification of the load resistance, this drop in voltage of the panel therefore causes a decrease in the power produced.
To compensate, the resistance of the load should be reduced in order to keep it adapted to the new characteristics of the panel.

The problem is that the characteristics of the panel also change throughout the day (it is less hot in the morning than at noon, it is brighter at noon than in the evening, ...). It is the job of an MPPT regulator to make this permanent adaptation to the temperature and lighting conditions of the panel.
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Re: Operating point resistance




by Grelinette » 27/07/18, 12:07

This information of optimal operation of a PV according to its temperature is interesting. Although we suspect this because the temperature changes affect all the electrical and thermal systems, we do not talk about them for the PV and we assume that the more sun there is, the more the PV produces, which does not is not the case.

It seems to me that we had discussed the subject a little on econology, especially to talk about self-refrigerated PV or simply cooled in the summer (or warmed up in the winter) to keep it running at its best. To my knowledge, there is no (or very little) PV on the market that regulate their temperature to optimize their production. (It can also be a calculation of the angle of the surface of the PV with the sun's rays to reduce the solar energy transmitted to the PV in the extreme temperatures, as for example at this moment in certain regions where there is heat wave).

What is the ideal temperature for a PV to work at best?
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Re: Operating point resistance




by Gaston » 27/07/18, 13:50

Grelinette wrote:What is the ideal temperature for a PV to work at best?
The colder the cell, the better - in "normal" temperatures -.

Unfortunately, the energy source (solar radiation) tends to warm up the cell.

So we are stuck between the desire to have a maximum of solar radiation (to have more power to operate) and the desire to keep the cells as cold as possible (to make the most of the power).

In the whole, one compensates for the other, provided that the change in operating point of the panel is taken into account.
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Re: Operating point resistance




by Grelinette » 27/07/18, 14:58

Gaston wrote:
Grelinette wrote:What is the ideal temperature for a PV to work at best?
The colder the cell, the better - in "normal" temperatures -.
Unfortunately, the energy source (solar radiation) tends to warm up the cell.
...

There must be a curve of the efficiency of a PV according to its temperature?

This chart could be supplemented by a graph showing the overall performance of a PV if some of the energy produced was used to cool the PV: I consume some of the energy produced by the PV to enable it to produce better!

Likewise, we are seeing more and more tracking systems to orient the PV towards the sun to optimize the yield, but by shifting a little the orientation to not receive the sun's rays perpendicularly when the temperature is high, the efficiency PVs should be better?

To return to the problem of PVresistif if the fall in performance of its HP is caused by heat, it is a detail that is important and would be fairly easy to adjust.
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Re: Operating point resistance




by Gaston » 27/07/18, 16:30

Grelinette wrote:There must be a curve of the efficiency of a PV according to its temperature?
[...]
To return to the problem of PVresistif if the fall in performance of its HP is caused by heat, it is a detail that is important and would be fairly easy to adjust.
When the temperature increases, there is indeed a drop in the maximum power that can provide the panel (we can call it the yield), but there is also and above all a shift of the optimal operating point to a new point with less tension and (a little) more intensity.

Image

To obtain the maximum power of a hot cell, it must therefore be asked to provide more power, even though it produces less voltage, it is necessary that the device to power reduces its impedance in the circuit (exactly what the MPPT circuits do).

A fixed resistance can not do it, the only palliative is then to approach the previous operating point by cooling the PV, but, as you noted, it requires to consume some of the energy produced and that greatly complicates the system.

Grelinette wrote:Likewise, we are seeing more and more tracking systems to orient the PV towards the sun to optimize the yield, but by shifting a little the orientation to not receive the sun's rays perpendicularly when the temperature is high, the efficiency PVs should be better?
If we "shift" the panel relative to the perpendicular, it will be "a little" less hot (not much, because the ambient air remains warmer than in winter) but it will also receive less light power. It is likely that the result will rather be to decrease the maximum electric power.
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Re: Operating point resistance




by izentrop » 28/07/18, 01:58

Have you measured the temperature of the panel to your 2 summer / winter measurements?

In this documentit can be seen that on a variation of 30 °, between 15 and 45 °, at equivalent illumination, the power at the optimized operating point varies by a little less than 12%, whereas the efficiency of only 2%
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Re: Operating point resistance




by izentrop » 29/07/18, 08:02

PVresistif wrote:The curves of the panel indicate a power which should be at 6.8 A or 190 W, in this case R would be 4.12 ohm and we would be in phase; then measurement error of my "pourave" device?
The lowest gauge of your device being 400 A, you can not have an accurate measurement unless 10 A.
I prefer to measure the voltage across a portion of wire, knowing its resistance deducedNo need to insert the device into the circuit, but you need a device that goes lower than yours.
A multimeter first prize under 10 € has a first caliber at 20 mV
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Re: Operating point resistance




by izentrop » 29/07/18, 21:18

Erratum: it is 200 mV the lowest caliber and not 20 Image
It's still better than the 4 V of your clamp.
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PVresistif
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Re: Operating point resistance




by PVresistif » 29/08/18, 18:31

thank you for your msg
I replaced the batteries of my device and I could fall on my paws, but the intensity measurements incidentally it is less simple t less reliable
Moreover the loss of power at high temperature is more significant than I thought: from 220 W in winter I fall to 190 in summer at 35 ° C.
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