Thermal Losses calculations

[a.nashed]

Hello

I was wondering why the abosrptance is included in the equation used to estimate the module temperature:

U · (Tcell - Tamb) = Alpha · Ginc · (1 - Effic)

I mean doesn't the module efficiency already account for the TAU-Alpha?

In other words shouldnt the equation be as follows:

U · (Tcell - Tamb) = Alpha · Ginc · (1 - Effic/Alpha) (assuming that the cover transmittance is 1)

Or the efficiency used here is actually the cell efficiency which is already divided by Alpha and is internally calculated?

Thanks

Regards,

Amir

[maia]

Hi Amir,

I have the same question. Have you already figured it out?

[André Mermoud]

Formally, you are quite right. I will correct this for a next version.

However the error is not very important: if we take Alpha = 0.9 and Effic = 15%, the error (1-Alpha) * Effic = 1.5%.

This is a correction on the U-value, which is usually not known with a better accuracy than perhaps 10 % or more.

The only cases when this U-value may be well known is when you fit it on your measured data. But in this case, if you fit it with this erroneous equation, the simulation result will be coherent with your measurement !

[lprosperi]

André,

Did you implemente the correction in PVsyst already?

I am using Version 6.24 (21.05.2014 - pre-fix), and I have set Uc=28.8

I am using the older formula to verify Uc=28.8

Uc = Alpha · GlobInc · (1 - EffArrR)/(TArray-TAmb)

But I am not getting 28.8

I used the PV module "internal model result tool" to calculate the efficiency/Module, but I still didn't get 28.8

Is Tcell the same as TArray?

I have TArrMes too, what is the right formula to calculate Uc?

Thank you.

[André Mermoud]

No I did not yet perform this correction. However its effect should be very little.

But I don't understand what you are trying to calculate: the U value is an input parameter. On which data are you calculating it (i.e. which Tarray are you using ? ).

PVsyst doesn't make a difference between Tcell and TArray: if there is any, it will be included in the U-value.

If you have TArray measurements, you can plot (TArray(meas) - Tamb) as a function of GlobInc.

Then you can obtain the U value by a linear fit on your data, and by identification of the slope to the energy balance equation:

Uc = Alpha * (1-EffArr) / Slope

[lprosperi]

André,

Thank you, I did a mistake in my calculations.

I was trying to calculate U using measured data.

But I tried to do a check using PVsyst output data, and back-calculating the input Uc.

[lonav]

Hello,

I've tried to check module efficiency in the balance for thermal loss U · (Tcell - Tamb) = Alpha · Ginc · (1 - Effic)

(using U=Uc=29;alpha=0.9, as for panfile) by back-calculating from hourly Tarray values and I see the outcome is greater than the module efficiency most of the time. Am I missing something? Is this a relative efficiency perhaps? Relative to what?

Thanks.

Regards,

Lola.

[André Mermoud]

You cannot do that.

The efficiency is a "little" correction to the Thermal energy balance.

The U-value is a rough estimation of the themal balance, with a high uncertainty.

If you try to estimate the efficiency from the temperatures, the uncertainties on the U-value - and the temperature instantaneous values - will give completely erratic efficiency values.

[lonav]

Thank you André, that was what I needed to know.