Solar Module Temperature Coefficients

[iwire]

I need some help, I must learn how to calculate the expected output voltage of a solar module at a given temperature.

My module specification tells me that the open Circuit voltage @ 25?C is 32.6 volts

The Voltage (Voc) temperature coefficient they give me is Voc (%/?C) -0.34

So lets say my cell temperature is not 25?C but 15?C. How do I determine the expected voltage at this reduced temp? Treat me like a beginner and I will do fine.


Thanks in advance, Bob

 

[Besoeker]

The PV array has a negative temperature coefficient.
From your figures I would take it that the Voc goes down by 0.34% for every 1 degC rise.
Conversely, the voltage would go up as the temperature drops.
If you drop by 10C from 25C to 15C, the voltage increase would thus be 3.4%
Or from 32.6V to 33.7V.
At least that's how I see it.

 

[winnie]

I concur with Besoeker. This is the same sort of calculation that you will see for the change in resistance of conductors with temperature, or just about any parameter that is linearly related to temperature.

You multiply the _change_ in temperature by the temperature coefficient, and this gives you the change in the parameter being modeled.

Most diodes show a similar characteristic.

V_oc = V_oc_25 + V_oc_25* <tempco> *(T - T_25)
33.7 = 32.6 + 32.6 * (-0.0034) *(15-25)

-Jon

 

[iwire]

Guys I really appreciate the help but can you dumb it down even more?:smile:

 

[ELA]

Heres something that could help?
It does the math for you if you go to their "string calculator".

http://www.pvpowered.com/installation_support.php

 

[Besoeker]

Guys I really appreciate the help but can you dumb it down even more?:smile:

For every 1 degC above 25C the Voc will reduce by 0.111V
For every 1 degC below 25C the Voc will increase by 0.111V

25C = 77F
So, in degF:
For every 1 degF above 77F the Voc will reduce by 0.0616V
For every 1 degF below 77F the Voc will increase by 0.0616V

All based on the 32.6V at 25C.

 

[winnie]

Guys I really appreciate the help but can you dumb it down even more?:smile:

I don't think that the equation can be 'dumbed down' any more than it already is, and I know you can handle the math. This says to me that something about the equation just doesn't sit right with you, eg. you see the math, and see the numbers, but they don't fit your physical intuition.

You have some parameter that changes with temperature. Rather than OC voltage on a solar cell, think length of a conduit. The _percentage_ change depends upon the temperature change. In the case of your solar cells, the voltage changes by -0.34% for each C temperature change. In the case of PVC conduit, the length changes by about 0.006% for each C temperature change.

So for your solar cells, if the temperature drops by 10 degrees, then the open circuit voltage will change by 3.4%.

This _change_ comes off (or adds to) the original value. If you have a 100 foot length of conduit, and you calculate that its length changes by 0.12%, then you have to add (or subtract) about 1.5 inches from the length.

In the case of the solar cells, you start at 32.6V, and for a 10C temperature drop, you have to add 3.4%. 32.6V + 3.4%*32.6V = 33.7V

-Jon

 

[Cold Fusion]

bob -
Are you look at the size of the step the percentage is being applied to?

Example below is negative temp coeficient (voltage goes up with temp decrease)

For example, if the Delta(T) is applied in one step, 10C, the the change is 0.034000

But if applied in 10 each, 1C steps, the change is: 0.034525

If 100 each, .1C steps: 0.034579

If 1000 steps: 0.034584

The way the change is stated, the Delta(V) does not look linear to me.

To contrast, every time I have seen a junction voltage change with Temperature change, the coeficient is in mV/deg C, not %/deg C. This is linear.

If one of the others don't get to it, I'll figure it out tonight.

cf

 

[winnie]

To be clear, the formula given is intended to be a linear approximation to reality. The real physics will be more complex, and if you need to more exactly know how resistance changes with temperature, you will need to use a more complex formula.

Normally you take the _total_ change in temperature as a single step.

Your point about taking the same percentage change for each degree of temperature difference is a good one. It is not how the temperature change of Voc is calculated, but such a 'compounding' change equation is used elsewhere.

-Jon

 

[iwire]

To contrast, every time I have seen a junction voltage change with Temperature change, the coeficient is in mV/deg C, not %/deg C. This is linear.

It depends on the module manufacturer, some give it as mV/deg C and others as %/deg C.

Guys looks like I will have to embarrass myself and lay it on the line. I am intelligent but not educated. :smile:

Can someone lay it out step by step in words, not just formulas?

I have no idea what 'Delta T' or 'Delta V' means.

 

[wallyworld]

It depends on the module manufacturer, some give it as mV/deg C and others as %/deg C.

Guys looks like I will have to embarrass myself and lay it on the line. I am intelligent but not educated. :smile:

Can someone lay it out step by step in words, not just formulas?

I have no idea what 'Delta T' or 'Delta V' means.

Delta T means the change in temperature, being careful to use the right units, c or f, Delta V means the change in Volts

 

[Besoeker]

It depends on the module manufacturer, some give it as mV/deg C and others as %/deg C.

Guys looks like I will have to embarrass myself and lay it on the line. I am intelligent but not educated. :smile:

Can someone lay it out step by step in words, not just formulas?

I have no idea what 'Delta T' or 'Delta V' means.

Delta in this context just means the amount of change.
If you go from 15C to 25C the delta T is +10C.
If you go from 25C to 35C the delta T is +10C.
It disconnects change in temperature from actual temperature.