Arianit,

welocme

There are several factors that makes the field measured open circuit voltage of the string of modules smaller than the standard open circuit voltage.

- The first factor is the lower incident solar radiation than the in the standard test conditions

- The other important factor is that the temperature of the modules may be higher than the standard test conditions of 25 degree centigarde.

- There are also the tolerances in the open circuit voltage such that not all modules have the standard open circuit voltage. These modules in the sting may have lower open circuit voltage.

Generally the open circuit voltage can be expressed by:

Voc= nVt ln (Iph/ Is),

with Vt the thermal voltage, Iph is the short circuit current and Is is the reverse saturation current.

The saturation current is sensitive to temperature and increases appreciably with temperature. Iph is proportional to the incident to solar radiation.

Best wishes

The Voc for each of your panels will be specified at 1 kW/m2 not the figures you quote (~700 W/m2), so the photocurrent will be less and the drop across the shunt diode in each cell will be less - so the Voc of each cell will be less and the overall string voltage will be less. Prof Zekry has given all the technical details you require to estimate the reduction in voltage expected - I haven't done the calculation but I suspect the voltage loss will be largely due to this.

I doubt your cell temperature of 16C when the ambient is 24C and the irradiance is ~0.8suns, this is almost NOCT so the module temperature would be more like 50C. What's the beta coefficient? It's usually around -0.4%/K. At 50C that would be -10% from Voc.stc just from the temperature which would make it about 710V - nearly what you got.

"Ta (C)=24.08; Tpv (C)=15.79;"

The surface temperature for OC-mode must be more higher relative to ambient temperature.

Dear Mr Zekiri,

I agree with steve and Vladimir, the PV cell temp. must too much higher that ambient temp. that is 24°. This temperature can be estimated using NOCT of the module. Use for instance the following equation:

Tcell=Tamb+G.(NOCT-20)/0.8)

The increase in temperature deceases the Voc and hence Vmpp. This decrease can be evaluated also using the Temperature Coefficient of Voc available is the datasheet ( -0.32 %/°C ).

Voc=Voc,stc*(1-0.32*(T-25)/100)

I took a look at the different measurements shown in 1.pdf and I did some calculations to check whether there is agreement between the decrease in Voc and that of Vmpp.

The percentage of Voc decrease is about 10.264%. Using this value, I can estimate the voltage drop of Vmpp that I have found to be 582,78V. This value is close to the measurement given in bottom of uploaded file (589.4V).

My opinion is that this measurement makes sense.

Hope my answer is useful.

Dear Dr. Arianit Zequiri,

I have just saw the question today as it is forwarded to me only few days back by RG.

My comments are the following:

1. In the PV module data you have not provided number of solar cells in a module. Generally it will be 60, 72 and so on. dependent on site conditions.

2. If these solar cells are made of Mono or multi crystalline silicon, the Voc of the solar cell decrease by -2mV/Deg C.

3. I also agree with other scientists that the internal cell temperature ( in-fact it is junction temperature of solar cell and we assume that Jn temperature and cell temperature is the same) can not be less than the ambient temperature.

4. I not sure where is your temperature sensor attached. I suggest to attach the sensor to the back side of the module. Ideally you should remove the tedlar & EVA and attach the thermal sensor to exactly to the back of solar cell.

5. Make sure the sensor is actually in contact with the back of the solar cell. Let us say now, the measured solar cell temperature is =K deg C

6. Now, the number of solar cell in series in a string will be Sc= number of cells in module(pleasure ensure all the cells in module are connected in series) X number of modules in a string.

7. Now, with a voltage drop of -2mV/ cell/ Deg C, you can calculate the voltage drop in a string of 17, Z= Sc X -2mV

8. As explained above the rise in the temperature is with reference to STC, i.e. 25Deg C. i.e= K-25 Deg C

9. The actual drop in the Voc from STC value will be = Z X(k-25) in mV,

this co be converted in to Volts.

10. This way you can calculate actual drop in voltage.

11. If you can provide the number of cell in module and actual back temperature of cell during the measurements, it is possible to accurately estimate the Voc.

Thanks

Regards

O. S. Sastry