Temperature Considerations in PV Design

[tallgirl]

690.7(A)(1) and (2) refer to methods employing the "lowest expected ambient temperature" without further defining. The ASHRAE suggestion is an informational note which is not strictly binding. I suppose if you want to be a stickler, it doesn't say "lowest expected ambient temperature between dawn and dusk", but I think if we are debating how extreme a minimum temperature we want to design for, that could be taken into account if one is conservative with other factors. The code is not otherwise specific on how you choose the temperature.

I don't know if there are any industry standard methods (see 690.7(A)(3)) for doing this for systems over 100kW that use my suggestion, but that is potentially another option.

You don’t want to screw around with “but it was dark!”. I’ve seen equipment ruined by weird combinations of cold and wind direction and then the sun came up.

 

[tallgirl]

Right, but how many photons are enough?

I used to know all those things.

I had a client 10+ years ago with an array which faced a street light. I forget what Voc was at night, but it wasn’t as low as other clients.

 

[ggunn]

I used to know all those things.

I had a client 10+ years ago with an array which faced a street light. I forget what Voc was at night, but it wasn’t as low as other clients.

I once got painfully shocked by a small indoor training array on a mockup roof when the only light source was a few fluorescent tubes up on the 25 or 30 foot ceiling. When I checked the open circuit voltage it was at Voc for the string.

 

[ggunn]

690.7(A)(1) and (2) refer to methods employing the "lowest expected ambient temperature" without further defining. The ASHRAE suggestion is an informational note which is not strictly binding. I suppose if you want to be a stickler, it doesn't say "lowest expected ambient temperature between dawn and dusk", but I think if we are debating how extreme a minimum temperature we want to design for, that could be taken into account if one is conservative with other factors. The code is not otherwise specific on how you choose the temperature.

That isn't the same as finagling Voc for lower irradiance.

 

[tallgirl]

I once got painfully shocked by a small indoor training array on a mockup roof when the only light source was a few fluorescent tubes up on the 25 or 30 foot ceiling. When I checked the open circuit voltage it was at Voc for the string.

Yup. (Near) constant voltage variable current power supply.

You can make an irradiance sensor using a very solar cell and a small burden resistor so long as the input impedance of the measuring device is sufficiently high.

 

[ggunn]

Yup. (Near) constant voltage variable current power supply.

I did a stoopid thing and the shock knocked me off the "roof". I was only a couple of feet off the floor; if I had been up on a real roof in bright sunlight it might have killed me.

There's one good thing about getting painfully shocked; if you survive it it teaches you a lesson that you are not likely to forget.

 

[winnie]

Do you know the mathematical correlation between irradiance and Voc? It isn't a number published on module data sheets and I don't know of any publications that discuss it. It's a non-issue for the design process, though. I would not, for example, fudge my maximum Voc numbers to enable longer strings if I expect the maximum irradiance to be less than 1000W/m^2.

While it might not be a number explicitly published on data sheets, the information is often available in the curves on the datasheets.

For example snip from a datasheet shows the open circuit voltage at 200, 400, 600, 800, and 1000 W/m^2:

(source https://www.altestore.com/static/datafiles/Others/Trina DE09C.07 395W Datasheet.pdf for a Trina Vertex bifacial panel)

-Jonathan

 

[winnie]

Once enough photons with enough energy - proper wavelength start to strike any of the junctions, electrons begin to move and the voltage is always somewhere around Voc.

The equivalent circuit of a solar cell is a doide, in parallel with a resistor, and a series resistor in the output. Since V = I * R, a low irradiance and low resultant current, Vdrop is very low, so Voc will be close to whatever the name of that voltage drop is across the junction.

I’m sure someone will now tell me I’m wrong. I do not care. My cheese soup was delicious and so was the carrot cake.

IMHO your description quite well matches the what is seen in the datasheets. VOC is almost (but not quite) constant, just as the VF of a diode is almost (but not quite) constant. Just eyeballing the chart there is less than a 10% change from low irradiance to standard irradiance.

-Jonathan

 

[ggunn]

There's one good thing about getting painfully shocked; if you survive it it teaches you a lesson that you are not likely to forget.

A memorable experience for me was when I leaned up against the body of a car with the engine running, and I reached in and pulled off a spark plug wire with my bare hand. I would imagine that many have done that once but few have done it twice.

 

[tallgirl]

While it might not be a number explicitly published on data sheets, the information is often available in the curves on the datasheets.

For example snip from a datasheet shows the open circuit voltage at 200, 400, 600, 800, and 1000 W/m^2:
View attachment 2572946
(source https://www.altestore.com/static/datafiles/Others/Trina DE09C.07 395W Datasheet.pdf for a Trina Vertex bifacial panel)

-Jonathan

That doesn't mean what I suspect you think it means.
It means that the current is linearly dependent on the irradiance, regardless of impedance. Because of that, as the impedance changes, so does the voltage.

 

[tallgirl]

IMHO your description quite well matches the what is seen in the datasheets. VOC is almost (but not quite) constant, just as the VF of a diode is almost (but not quite) constant. Just eyeballing the chart there is less than a 10% change from low irradiance to standard irradiance.

-Jonathan

I wrote a PV cell simulator when I was at SolarBridge (they were bought out by SunPower before they spun the microinverters off to Enphase) and the model is just insanely simple and boring.

It's a diode, a parallel resistor for eddy currents or whatever in the silicon and a series resistor for the collection wiring. And that is it, and it produces very accurate models.

 

[wwhitney]

That doesn't mean what I suspect you think it means.

You can just read the Voc off the graph posted; they are the x-intercepts.

Cheers, Wayne

 

[winnie]

That doesn't mean what I suspect you think it means.
It means that the current is linearly dependent on the irradiance, regardless of impedance. Because of that, as the impedance changes, so does the voltage.

I did a bad job of being specific.

The Y intercept of those curves says exactly what you describe, and what you describe also pretty much applies to the maximum power point.

But the X intercept of the curves is the open circuit voltage, and even with no external current flowing that voltage changes with irradiance.

Jonathan

 

[ggunn]

But the X intercept of the curves is the open circuit voltage, and even with no external current flowing that voltage changes with irradiance.

It does, but without numbers on the intercepts it isn't possible to quantify how much it changes. But that really is beside the point; for design considerations we are bound by the NEC to assume STC with Voc adjusted for temperature.

 

[jaggedben]

It does, but without numbers on the intercepts it isn't possible to quantify how much it changes. But that really is beside the point; for design considerations we are bound by the NEC to assume STC with Voc adjusted for temperature.

Well maybe for over 100kW you have other options if you are an engineer.

 

[winnie]

It does, but without numbers on the intercepts it isn't possible to quantify how much it changes. But that really is beside the point; for design considerations we are bound by the NEC to assume STC with Voc adjusted for temperature.

Fair enough re: NEC requirements.

Re the plot lacking numbers for the intercepts, see: https://automeris.io/ (also see https://xkcd.com/2341/ )

-Jonathan

 

[ggunn]

Well maybe for over 100kW you have other options if you are an engineer.

I am a PE, but 690.5(A)(1)(2) for 100kW AC or more only provides alternative means of calculating DC current, not DC voltage.

 

[wwhitney]

690.5(A)(1)(2)

No such section in the 2023 NEC.

690.7(A)(3) says "For PV systems with an inverter generating capacity of 100 kW or greater, a documented and stamped PV system design, using an industry standard method maximum voltage calculation provided by a licensed professional electrical engineer."

Cheers, Wayne

 

[ggunn]

No such section in the 2023 NEC.

690.7(A)(3) says "For PV systems with an inverter generating capacity of 100 kW or greater, a documented and stamped PV system design, using an industry standard method maximum voltage calculation provided by a licensed professional electrical engineer."

Cheers, Wayne

Sorry, I meant 690.8(A)(1)(2) in the 2023 NEC which is for current; I fatfingered the number and did not catch it. As you say, 690.7(A)((3) does indeed say what you quoted for voltage. I do not know what the required industry standard methods would be, but Informational Note No. 1 appears to be related to coming up with a different way to calculate minimum voltage. I am not familiar with the publication listed in Informational Note No. 2.

I have never had occasion to invoke either of these provisions in the code.

 

[tallgirl]

I did a bad job of being specific.

The Y intercept of those curves says exactly what you describe, and what you describe also pretty much applies to the maximum power point.

But the X intercept of the curves is the open circuit voltage, and even with no external current flowing that voltage changes with irradiance.

Jonathan

No it doesn’t.

Honest.

We can sit down with some PV silicon and do the experiments.