System production in relation to heat

[Mercracing]

Hello,

Hoping to gain a better understanding of how a solar array will perform in cold vs hot weather. My system was installed at the first of this year and consists of a Sol-ark 15K with 8200 watts of Sunpower panels (20- 410 watt panels, aimed south, and set at 30* I believe) installed on a ground mount. The panels are in an area with zero shading and have been cleaned once since install (we live on gravel and there are fields under plow near the array) (really didn't make a difference). What I am trying to understand is the difference I am seeing in production from cooler weather to what we have today. On May 1 and May 2 of this year the temps were in the 40's up to about 60 degrees and today has been from 70*-90*. With the hotter weather the system is down a 1000-1100 watts once the day starts to warm up versus a month ago. I know there are other issues like sun angle, but I don't know they all "work" and or what is a normal behavior. On the two days I mentioned in May we generated a little bit over 61 kwh each day. Now the system struggles to generate over 55 kwh a day (even with the longer days). Is this normal? I have asked the company that installed the system and Sunpower these questions was was (at least I felt) brushed off with the "yeah, heat hurts panel production". Thank you in advance for any advice.

 

[sunshine_eggo]

Your panels have a temperature power coefficient. Look it up.

If your panels are tilted for your latitude, they were optimal back in March. Now they're suboptimal.

Colder panels = more power.
Hotter panels = less power.

Panels rarely produce rated power.

Panels are rated based on CELL temperatures @ 25°C/77°F. In 70-90°F, your CELL temperatures are likely in the 110+ range.

Higher ambient results in even higher cell temperatures because they can't cool themselves as effectively.

Based on AMBIENT temperature change alone and typical numbers, you should be down 500W. This does not consider the accelerated nature of cell heating or the change in solar angle.

In areas with good year-round solar, the decreased insolation during winter due to poor solar angle is partially offset by the improved production due to lower temperatures.

Here was my full profile yesterday:




I'm in ENE AZ @ 6500 ft elevation with peak temperatures around 80-90, but I also get about 10% more solar energy because of the thinner air. My 2970W rated array peaked at 2765W. It's not making MORE than rated because of the elevated cell temps and the suboptimal angle (set at 30° at 34° latitude, sun is about 10° off of perpendicular.

Lastly, 55kWh on a 8.2kWh array is 6.7 solar hours per day... that's pretty exceptional. My plot above... in Arizona was only 7.1 solar hours, so on a relative basis, sounds like everything is great at your place.

 

[Mercracing]

Thank you for the response. Where would I find the temperature power coefficient data? I have called Sunpower trying to get that data three different times and come away with nothing other than being frustrated.

 

[sunshine_eggo]

You should be able to locate a datasheet for your specific model of PV panel.

 

[BentleyJ]

Just for fun I looked up the datasheet actually found 2. The one attached lists -0.34%/ deg. C while the other one had a value of -0.36%/deg. C.
Example: STC panel rating is at 25 C. At 50C that's 25 degrees higher x -0.35% = -8.75%. This the expected power reduction. Keep in mind the panel temp will be higher than the air temp. so you can't use the weather report to do the calculation. You will need something like a hand held temperature meter with a thermocouple wire stuck to the back of the panel.

 

[Mercracing]

It's seems crazy there can be a 1000-1200 watt difference at the same time slot during the day with change in sun angle and temperature. The early May dates I referenced peaked at a bit over 8000 watts and stayed above 7000 watts for several hours versus the last couple of days were the system barely touches 6900 watts for 15 or so minutes. Thanks again for the information/education.

 

[sunshine_eggo]

It's seems crazy there can be a 1000-1200 watt difference at the same time slot during the day with change in sun angle and temperature. The early May dates I referenced peaked at a bit over 8000 watts and stayed above 7000 watts for several hours versus the last couple of days were the system barely touches 6900 watts for 15 or so minutes. Thanks again for the information/education.

Hotter cells produce less.
Hotter ambient drives cell temperatures even higher.
Sun is at a less favorable angle now than it was in early May.

It all adds up.

Check the cell temperature with an IR thermometer.

Something I didn't think about, but the Canadian wildfires might be impacting insolation by a % or two as well.