Solar Panel Efficiency

System

We live above latitude 61 degrees North in Alaska and rely on solar power for our system out here in the wilderness. We have a 580 watt system and another 340 watts waiting to be installed this spring. I’m a bit confused, however, about the solar efficiencies that I am getting up here.

We have a hydromet station on the other side of the Nizina River from us that is run by the Bureau of Land Management by way of solar power and satellite telemetry. The web site for the station is:

www.met.utah.edu/cgi-bin/droman/meso_base.cgi?stn=MCKA2#cooliris

It records hourly data, such as snow depth, temperature and solar radiation. Solar radiation during the months of December and January is not very high, but the station is now recording peak intensities in the 400 watt/m*m range depending on cloud cover. Historical records indicate incidence rates as high as 1000 w/m*m during the months of April and May.

However, even during the low incidence months, my solar panels will consistently put out 80% of their rated power. Temperatures during this time of year regularly range between minus 40 degrees Fahrenheit and sometimes 20 above.

My question is this: if total solar radiation is only a fraction of what it would be in the optimum months of April and May (say 20 % of maximum), why do my solar panels convert such a high percentage of this low incidence energy to electricity (80% of their rated capacity)? During April and May our system peaks out, as would be expected.

mike95490

Re: Solar Panel Efficiency

I guess that the brand of panels you have really likes the cold.

or

don't look a gift horse in the mouth

How do you keep batteries from freezing up solid

System2

Re: Solar Panel Efficiency

My batteries have never frozen as I keep them charged up. However, when we are away, the voltage in the batteries does go way down (11.9V). A bit of heat brings them back up to normal. I will be relocating my batteries to the root cellar this summer and that will help. The root cellar temperatures never go much below freezing even during the coldest part of winter.

crewzer

Re: Solar Panel Efficiency

Nizina,

I suspect there are at least four effects that collectively contribute to your PV array’s exceptional performance:

1)  Sunlight reflection off the snow.  The PV array is harvesting both direct sunlight as well as reflected sunlight. You might try placing a 4’ x 8’ sheet of plywood (matte black, if possible) on the ground in front of the array and checking the effect.
2)  Very cold ambient temperature. PV module voltage increases as ambient temperature decreases. The baseline STC spec is based on PV cell temperature of 25 C, which correlates to an ambient temperature of roughly -10 C, or ~14 F.
3)  Actual module specs. At least one manufacturer’s tolerance specs are +10% / -5%, so you might have a set of modules in the “+10%” category.
4)  New Modules. New PV modules tend to operate at or above spec when they’re new. After a short period of conditioning, the output decreases a bit.

So, what make and model PV modules are you using, and what kind of controller? Are you using a remote battery temp sensor to enable the controller to compensate the charge voltages for battery temp? Note that the freezing temperature for a fully charged lead-acid battery is ~-77F.

HTH,
Jim / crewzer

niel

Re: Solar Panel Efficiency

"Note that the freezing temperature for a fully charged lead-acid battery is ~-77F."
say what?

i also don't think a 4x8ft sheet of plywood will block the bulk of the reflected sunlight at a lat of 61 degrees in the colder months as it is coming in at nearly the horizon. i do also agree with the reasons crewzer gave for the increases. your solar horizon also rotates from about ne to the s and back to the nw at times making for collection times to be longer if it could track it.

crewzer

Re: Solar Panel Efficiency

say what?

Here's the link to my source: http://www.uuhome.de/william.darden/carfaq4.htm#freeze_points

Trojan says -92 F; see: http://www.trojan-battery.com/Tech-Support/FAQ/Maintenance.aspx

Do you have a different value?

Regards,
Jim / crewzer

System2

Re: Solar Panel Efficiency

I suspect that you are right about the reflected light being a major part of the answer. Our maximum solar month is actually April as opposed to July when we get the greatest amount of sunlight. This is because we are getting a lot of reflected light from the snow cover in April, but this snow is obviously gone by July even though the days are longer.

My solar array is a new string of Sharp 80s wired for 48 volts and a second string of old Solarex 64 watt panels also wired for 48 volts. I use an MX60 (and love it) controller for a nominal 12 volt system consisting of four 6-volt L-16 Trojan batteries. I do use a battery temperature sensor for the controller. I'll be adding four 85 watt Shell monosilicon panels this spring. This will max out the MX60, but I might rewire for 24 volts so that I can add more panels later.

I hope I never see the day that my batteries freeze. -77 degrees would be just too brutal to experience.

System2

Re: Solar Panel Efficiency

Thanks for the help and in particular for the link to the site regarding battery freeze temperatures. It shows the importance of having the batteries 100% charged when we let the fire go out and leave for a spell. 75% charge freezing at -35 degrees just wouldn't make it for my batteries.

niel

Re: Solar Panel Efficiency

thanks for that info jim as i didn't know it was that low.

System2

Re: Solar Panel Efficiency

i had no idea the stc ratings were at 14f thats ridiculous. no wonder everybody's disappointed at first (well, except the OP). i guess thats about the only good thing about weather that extreme!

btw - hardly EVER see anything better than 70% of my sharp rated output even in winter here (say 10-20F)
today i just happened to calculate it: 67%

crewzer

Re: Solar Panel Efficiency

Matt,

I probably ought to explain the “14 F / -10 C” numbers, as they’re not “official”. Instead, they’re derived (reasonably so, I hope) from other specs.

PV module specs sheets include all manner of numbers, terms, and miscellaneous gobbledy-gook. One subset is those numbers is associated with NOCT, or Normal Operating Cell Temperature. NOCT, usually ~47 degrees C, is the temperature at which PV cells operate when exposed to 80% insolation (800 mW/m2) in an ambient temperature of 20 C (68 F) with a light breeze.

Extrapolating from this data, it would appear that 80% insolation causes cell temperature to rise by 47 C – 20 C = 27 C above ambient. Assuming a linear relationship, 100 % insolation would cause the cell temperature to rise by 27 C x 100/80 = ~34 C.

“Advertised” PV module power is based on STC, or Standard Test Conditions, which specifies the PV cell temp of 25 C under 100% insolation. Borrowing from the assumption and calculation above, the ambient temperature would have to be 25 C – ~34 C = ~-9 C, or ~16 F, and with a slight breeze, for a clean and correctly aligned PV module exposed to 100% insolation to generate full name-plate power.

Wow… Now that’s not as bad as I thought! :evil: And, it seems to correlate fairly closely to work done at the NREL. See: http://www.nrel.gov/docs/fy03osti/35645.pdf

btw - hardly EVER see anything better than 70% of my sharp rated output even in winter here (say 10-20F) today i just happened to calculate it: 67%

Assuming your modules are clean and well aligned (both azimuth and tilt), and that your MX60 is operating in B-MPPT (bulk) mode, then something’s just not right… When was the last time you cleaned your modules? I cleaned mine with Windex and paper towels a few weeks ago for the first time in ~1-1/2 years, and I saw an immediate 15% improvement in power generation. Can you tell us more about how your modules are mounted?

HTH,
Jim / crewzer

System2

Re: Solar Panel Efficiency

ah.. well truth is the tilt, cleanliness and weather conditions were NOT perfect that day. pers ome reading on this forum i setup my 24v panels in parallel when i bought a third so as to get something when one is shaded. anyhow the sort answer is tilt is not ideal for this time of year, yes they are probably dirty, and almost definitely theyre getting some overcast / dilluted shade at that time. 2 panels are roughly 45 deg angle, one is far flatter (maybe only 15 deg) , so since this time of year this all probably makes sense. im excited that we're probably going to be able to have a roof put over these stairs this year and ill put the panels up there so i should do a LOT better re: shading, sun hours and tilt.

http://lamplight.no-ip.org/gallery/main.php?g2_itemId=165
(the bottom panel ive angle flatter so that while the tilt is far from optimum especially in winter, at least there no shade from the fence, which is worse
as i say, an ugly temp solution, hope to have that stairwell covered that will be excellent

System2

Re: Solar Panel Efficiency

i saw 545w briefly out of my 525w (sharp 175's parallel/24v) setup today, about 35 deg outside. i turned on small space heater on the inverter to see how high i could turn it before it overloaded the inverter , i found that it runs ok (under 600w) on the low setting. go figure!!

note: 2 of te panels are in use 3 yrs, 1 about a year.

crewzer

Re: Solar Panel Efficiency

I was just thinking about this topic on Saturday... We had a bit of sleet late Friday and I ended up witha layer of sleet/ice on the roof o my house. My array faces south and "looks" out over the roof... I saw sustained periods of >700 W from my 725 W (STC) array on Saturday afternoon, and the peak power, no doubt boosted by the sleet/ice and an edge-of-cloud event, was 814 W.

This was an excellent example of why the NEC requires some operating headroom while designing a system.

Regards,
Jim / crewzer

System2

Re: Solar Panel Efficiency

crewzer

More than once i've seen "edge of cloud event" mentioned, but never have read about it in any r/e literature. Could you, as potentate, give us a thumbnail sketch of it, its effects, what we'd see reflected on our MX's and such? I'm sure a lot of us wonder about some of the stuff we read on the forums and our eyes glaze sometimes. Not this time...since clouds and their edges are always around, inquiring minds want to know.

thanks

ralph

crewzer

Re: Solar Panel Efficiency

Ralph,

Edge of cloud event: A brief power surge (V x A) from a PV module (or array), as the Sun reappears from behind a passing cloud. Intense sunlight, focused by water vapor, causes unusually high PV module output current, and the relatively cool PV cells develop relatively high Voc and Vmp. The event passes quickly as the insolation returns to “normal” and the PV cells, exposed to direct sunlight, heat back up.

HTH,
Jim / crewzer

mike95490

Re: Solar Panel Efficiency

moved to http://www.wind-sun.com/smf/index.php?topic=2115.0
Mike