Solar panel Temperature Coefficients 1

[BigInch]

Been trying to figure out what these things mean.

On a BP solar panel model SX 120S,

http://www.bp.com/liveassets/bp_int...oduct_data_sheet_bp_sx_120s_01_4016_v1_en.pdf

I see,
Temp Coefficient of Isc, or [α]= 0.065+- 0.015 %/ºC
Temp coefficient of Voc, or [β] = -80+-10 mV/ºC
Approximate effect of temperature on power -0.5+-0.05 %/ºC

As I understand these things, they are given at 25ºC and,
from an MIT paper I have, supposedly, when reaching a new temp, they should change to,
Isc = Isc_25 + Isc_25 * [α] * dT
Voc = Voc_25 + Voc_25 * [β] * dT
However when I try this, I don't seem to get curves anywhere similar to BP's curves. The MIT paper doesn't give units, so I just used consistant units there. The BP and other PV suppliers are using those units given above. I'm wondering if there is some secret code, since I haven't seen these %/ºC units before. What is an X %/ºC unit? Assuming there is an amp in there somewhere. A mV/ºC, appears that it should be millivolts/ºC, but the %/ºC appears again in the Power coefficient. What gives?

I also would think that the power temp coefficient can be calculated from [α] and [β] to get W/ºC or W%/ºC, but it doesn't seem so.

How can I calculate what the new Isc, new Voc and new Power Output is at a new temperature? And are they inter-related or not? I'm stumped.

Thanks,

http://virtualpipeline.spaces.msn.com

 

[xnuke]

The formulas from the MIT paper work if the coefficients are in %/C, which you have for current and power. If you have the units given instead of percent as in the voltage the formula you would use is simply Vnew = Vold + ?dT.

I appear to get the same Isc and Voc values that BP has plotted on their curves when I try the calculation for the SX 120S:

Isc(T) = Isc(25 C) + Isc(25 C)*.00065*(T - 25 C)
Isc(75 C) = 7.74 A + 7.74*.00065*(50 C) = 7.99 A

Voc(T) = Voc(25 C) + (-.080)*(T - 25 C)
Voc(75 C) = Voc(25 C) + (-.080)*(50 C) = 17 V

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[BigInch]

xnuke, Thanks very much. I had an extra Voc buried in the spreadsheet. I should take a walk sometimes rather than continue to blankly stare x-eyed at this stuff. Appreciate you checking into it.

http://virtualpipeline.spaces.msn.com

 

[BigInch]

One last question. ... ya sorry.

I converted the Vmp and Imp values from 25 to 47ºC, to get 7.22A and 15.04V. Multiplied together = 108.6 W output. I think I'm happy with that part.

Now, trying to get that same Wattage using only BP's power coefficient (noting the unit designation is %/ºC) and dividing that numeric value by 100, as was done for the current coefficient, doesn't seem to work here. If I use the -0.5 and call the unit W/Cº, I do get the same value for W output at MP as above, = 108.6 W.

So, is that a bust in BP's unit designation for the power coefficient? Should that have been written as W/Cº similar to the mV/Cº, ...or should I just throw in the towel at this point?

http://virtualpipeline.spaces.msn.com