Are these super efficient panels?

[roamwood]

As someone with a keen interest in solar I am a long term loiterer on this forum, so thought it was time I joined. I also have a puzzle I can't work out.

A year or two back I purchased some sunpower spr-90 90w panels for installation on my motor-home, and on my fathers. So far I have only installed my fathers but its performance is so much exceeding expectations that I am wondering what is going on!

A bit of background information: For a 90w panel they are very small at 1038mmx527mm and the blurb claims higher efficiencies as all the electrical pickups are on the rear of the cells. Although the installation manual said they could be used in strings, an extra note in the box said they should only be used individually which I thought was odd. Checking on the internet I found out that their was an issue with the panels being used in series that caused their performance to drop due to a build up of -ve charge over time. This effect was apparently fully reversible by making the aluminium frame of the panels positively charged. I decided to link the +ve panel output to the frame of the panel, which was the recommended course of action. To cover the risk that aluminium frame might come in contact with the -ve earth of the vehicle (only a slight risk as my dads camper has a fibreglass roof) I put a 5 amp fuse in the line between panels and the charge controller. I sized this on the claimed output of the panels 90w at 18v. I never expected to hit those sorts of figures as the panel is horizontal, so hardly optimal. That was all done late last year.

Jump forward to mid May and I'm on holiday with my Dad in the north of Scotland. The weather was great. He has quite a quite a sophisticated (but non MPPT) controller that monitors and records panel voltage and current, as well as battery voltages, so we were regularly checking it and getting high figures. One particularly sunny day we checked it to find zero output from the panels. The 5 amp fuse had blown. The solar controller recorded 6.4 amps and 21.5v at the time of failure, so 137w by my maths! I replaced the fuse with a 7.5 amp one and watched the output increase further during the day to 7.1amps at 22v, so around 156w. How can this be possible, in the North of Scotland with such a small panel, or am I missing something obvious??

Any thoughts would be much appreciated.

Mike

 

[Gavin A]

were you driving at the time?

the main thing would be that the panels might have been cooler than the 25 deg they're rated at, which would give higher than rated performance.

Also sunlight levels can be higher than 1000W/m2 rating.

and sunpower do postiive tolerances.

 

[SolarCity]

I was thinking the same, but 156w from a 90w panel? Have you seen anything like that before?

 

[roamwood]

were you driving at the time?

Good point, we were certainly driving when the fuse blew, but got the 7.1amp reading parked in a lay-by after replacing the fuse during a lunch stop. It was cold though and windy, with snow on the ground - we were quite high up, so that could be a factor too I guess.

 

[Vegelen]

Solar panels generate optimum instantaneous generation in Spring when the weather alternates between, wind, rain and intense sun.

Driving in cold windy conditions at altitude in the North Of Scotland would significantly lower the surface temperature of the Sunpower solar panels. Intermittent cold Scottish rain would serve to further reduce panel temperature. Light intensity at altitude can also be higher.
Your panels also have an excellent positive tolerance.
Inadvertently excellent conditions for generation were created and your dad's sunpower solar panels performed extremely well.

 

[roamwood]

Now I'm in another quandry. I was going to fit two of the SPR-90's to my camper, which has an Electrobloc EBL-99 in it for power distribution. My Electobloc has a 10amp fused input for solar, later models of the EBL-99 have a 15amp fuse. According to the Electrobloc expert I spoke to the components weren't uprated in the later model so I can safely increase my fuse to 15amps as well. The only reason I wanted to do that was so there was no chance of the Electrobloc fuse blowing and disconnecting my solar regulator from the batteries (which apparently does the regulator no good!)

I was only ever expecting a total of 10 amps from the two panels, but possible 14amps is getting rather close to 15amps, when you further consider that my dads panel was horizontal, so not inclined towards the sun. Looks like I had better fit a switch to disconnect one panel on cold sunny days, or a resettable 10amp fuse on the panel side of the solar regulator to be on the safe side!

Thank you very much for your answers, much appreciated. Glad it wasn't my maths!

Mike

 

[Energetic]

If you wire the panels in series (connected to each other) the voltage will double but the current will be the same (7A) leaving you within your fuse rating.