Sun hours

[J.P.M.]

Thanks for the input. I think I can do it all at the combiner box, which is next to the ground and directly under
(one end of) the array, minimum copper voltage loss. Some electronic sensors may be attached directly to
the underside of panels, perhaps 4 spread around and averaged as you suggest. I think comparing voltage
out of clipping will give calibration (turn off a couple strings). Then the higher voltage seen with clipping can
be compared to the cell output curve, to get a decent estimate of % clipped. That can be cranked into the
electronics to be automatic. Probably need to do this for seasonal temp shifts to cover a year, to improve on
V/degree calculations.

So I propose comparing Vclipping to Vmp should allow a decent estimate, allowing for the operating temp.
1% not needed, but would like to detect 5% to 15%. With all the rain, don't think dirt is a factor. Bruce

You're most welcome.

On sensors, temp. measurement and accuracy: I'd keep in mind that thermosensors are better for use in measuring temperature differences at a point under different conditions rather than as an accurate measurement of panel or cell temperature. (More ) Accurate temp. measurements require low or less thermal resistance between the sensor and the panel than between the sensor and the surrounding environment. Hence, the best readings are from instruments in intimate contact with what's being measured and lots of insulation between the sensor and the external environment. The result of all that is that a panel's local temp. around the well applied sensor is higher than the rest of the panel (if in daylight) because of the back insulation that prevents heat transfer to the environment from the back of the panel, raising the panel temp. and so probably a cause of an erroneous temp. that's not as representative of actual panel temps. Conversely, a poorly or loosely affixed and/or uninsulated sensor will give an inaccurately low reading. Relative error from fixation can be estimated as roughly in proportion to the ratio of thermal resistances between the sensor to panel and sensor to environment.

All that's why the best readings ( or at least not the worst) are usually done in the field w/ IR thermometers when other than relative temp. differences are sought, and they're probably not the best choice in those situations either.

Dirt, or the lack of it can be a complicating factor, but, more no brainer, a lot of rain will tend to keep things clean(er).

Regards,

[bcroe]

Bruce: FWIW, after measuring both panel instantaneous individual panel temps and quasi simultaneously monitoring array voltages many times, as is needed to estimate array fouling, I've found a pretty good 1st approx. to ave. array temp. change for any string can be found by reading the voltage off my monitor.

To get some measure of Voltage change per deg., I'd measure each of 16 panels w/an IR thermometer at 4 points per panel, then run to the monitor and record string voltages, then run back to the roof and repeat the measurements, all in ~ 12-16 min. around min. incidence angle on very clear days. I repeated this well over 200 times and chose 60 days, 34 in winter, and 26 days the following summer that were tightly grouped and consecutive as much as weather allowed, wishing and knowing that the N==26 was less than perfect, and ran the Voltages and a lot more variables including all weather variables including Horizontal Irradiance recorded at 1 min. intervals and other measurements (example - roof temps. under the array in 16 spots), and ran a normal distribution on all the variables. One result of all that was to get what seems a pretty good way to est. a representative array temp. from array or string voltages.

Bottom line: To SWAG ave. panel temp. from voltage: Note the voltage at any two times and divide that voltage diff. by (the published Vmpp change per deg. C. times the number of panels in the string). Long story how I got the measurement method right, and there's a lot more to it, particularly for extrapolating the voltage at an ave. array temp. of 25 C. to guesstimate the voltage drop up to the inverter, but that method is the basis for it and, after all the hoopla described above and some tweaking, it has shown to be quite reliable over many measurements.

More no -brainer: (Current per string)/(Impp) is a pretty good 1st approx. to P.O.A. irradiance. Just multiply that ratio (string current/Impp) by 1,000 to get Watts /m^2 P.O.A. irradiance on the array or string. The result compares quite favorably with measured GHI corrected to P.O.A. using the HDKR algorithm, and a pretty good way to treat your array as a pyranometer.

Take what you want of the above, scrap the rest.

Thanks for the input. I think I can do it all at the combiner box, which is next to the ground and directly under
(one end of) the array, minimum copper voltage loss. Some electronic sensors may be attached directly to
the underside of panels, perhaps 4 spread around and averaged as you suggest. I think comparing voltage
out of clipping will give calibration (turn off a couple strings). Then the higher voltage seen with clipping can
be compared to the cell output curve, to get a decent estimate of % clipped. That can be cranked into the
electronics to be automatic. Probably need to do this for seasonal temp shifts to cover a year, to improve on
V/degree calculations.

So I propose comparing Vclipping to Vmp should allow a decent estimate, allowing for the operating temp.
1% not needed, but would like to detect 5% to 15%. With all the rain, don't think dirt is a factor. Bruce

[bcroe]

Can't you eyeball that already by just eyeballing the power output curve, and
extrapolating where the curve would go if it weren't clipped?

Dan, that isn't the right approach. You may remember (or can go back over many curves here) that the array
doesn't run the classic SW desert curve at all. And the reason for pursuing this is to CHANGE the curve to
something where the potential power curve is just the minimum over the clipping level, at the maximum number
of hours. In any case plotting any of these curves is quite a time consuming all day project, highly dependent
on a mostly clear sky.

But I think a temp-MPPT voltage reading can give me an instantaneous answer. I may be out there cranking
the inclination and watching the meter. Bruce

[J.P.M.]

I would like to have another type of READOUT. What is the percentage of power available beyond the current
level of production (at clipping)? Is the clipping 1% or 25%? The first thought is to monitor panel output VOLTAGE,
and compare it to the Vmp voltage. Given that Vmp varies so much with temperature, the panel temp would need
to be known to figure the curve. Grabbing the MPPT voltage before clipping starts might be a starting point for the
day, but subject to a lot error as things heat up.

What use would this be? Checking it over the day could suggest a more optimum panel orientation, subject to
monthly or seasonable changes. This would be sensed and read out at the array location for direct feed back
on changes, with a remote readout to just keep an eye on things at the house 700' away. Lots of details.
Bruce Roe

Bruce: FWIW, after measuring both panel instantaneous individual panel temps and quasi simultaneously monitoring array voltages many times, as is needed to estimate array fouling, I've found a pretty good 1st approx. to ave. array temp. change for any string can be found by reading the voltage off my monitor.

To get some measure of Voltage change per deg., I'd measure each of 16 panels w/an IR thermometer at 4 points per panel, then run to the monitor and record string voltages, then run back to the roof and repeat the measurements, all in ~ 12-16 min. around min. incidence angle on very clear days. I repeated this well over 200 times and chose 60 days, 34 in winter, and 26 days the following summer that were tightly grouped and consecutive as much as weather allowed, wishing and knowing that the N==26 was less than perfect, and ran the Voltages and a lot more variables including all weather variables including Horizontal Irradiance recorded at 1 min. intervals and other measurements (example - roof temps. under the array in 16 spots), and ran a normal distribution on all the variables. One result of all that was to get what seems a pretty good way to est. a representative array temp. from array or string voltages.

Bottom line: To SWAG ave. panel temp. from voltage: Note the voltage at any two times and divide that voltage diff. by (the published Vmpp change per deg. C. times the number of panels in the string). Long story how I got the measurement method right, and there's a lot more to it, particularly for extrapolating the voltage at an ave. array temp. of 25 C. to guesstimate the voltage drop up to the inverter, but that method is the basis for it and, after all the hoopla described above and some tweaking, it has shown to be quite reliable over many measurements.

More no -brainer: (Current per string)/(Impp) is a pretty good 1st approx. to P.O.A. irradiance. Just multiply that ratio (string current/Impp) by 1,000 to get Watts /m^2 P.O.A. irradiance on the array or string. The result compares quite favorably with measured GHI corrected to P.O.A. using the HDKR algorithm, and a pretty good way to treat your array as a pyranometer.

Take what you want of the above, scrap the rest.

[DanKegel]

Can't you eyeball that already by just eyeballing the power output curve, and extrapolating where the curve would go if it weren't clipped?

[bcroe]

I would like to have another type of READOUT. What is the percentage of power available beyond the current
level of production (at clipping)? Is the clipping 1% or 25%? The first thought is to monitor panel output VOLTAGE,
and compare it to the Vmp voltage. Given that Vmp varies so much with temperature, the panel temp would need
to be known to figure the curve. Grabbing the MPPT voltage before clipping starts might be a starting point for the
day, but subject to a lot error as things heat up.

What use would this be? Checking it over the day could suggest a more optimum panel orientation, subject to
monthly or seasonable changes. This would be sensed and read out at the array location for direct feed back
on changes, with a remote readout to just keep an eye on things at the house 700' away. Lots of details.
Bruce Roe

[bcroe]

Just an observation about shadowing here under good sun. By beginning afternoon the WEST system had
collected 47 KWH, but the EAST system had collected only 40 KWH. The two systems show identical
production under good midday sun. The difference is the rising sun hitting the WEST system first, completely
clearing the shadows on EAST much later. So WEST is running some 18% ahead for this time. Toward the
end of the day EAST about catches up, which means the same effect is reversed for the setting sun.

It appears that relocating panels can solve most of the discrepancy, without cutting another series of trees.
That and solve some other issues previously mentioned. Early work saw one system under producing while
the other was in clipping, which was corrected by rearranging strings between inverters. It looks like a
complete checkerboard pattern of the 2 systems is the ultimate arrangement. The skies often start clear
here, then cloud later, so the first change will be to maximize the rising sun. If it ever stops raining, I'd like
to start digging new foundations. Bruce Roe

[bcroe]

Watching my latest "smart meter", apparently it has a running display of my KWH consumed. Its up
to 329 KWH in a month and counting. For this season it only goes up at night, while the PV solar is
down. With the house now coasting through the night without the heat pump, it won't advance much
till another season. There is no detectable indication of all the KWH being pushed out days, though
that did show up on the billing. Bruce Roe

[SunEagle]

The test panels from last year are back in storage, pulled up the stakes and ran the mower over those spots.

The swapped out SMART METER (useless to me) has been in service a month, and there may
actually be something on the display I can read. The first billing, presumably by radio report
instead of meter man, came today only 3 days into the month. I received a net metering credit
comparable to past Aprils, so I presume the NET METERING is operating correctly as before.
While announcements indicated everyone would be paying $3 a month extra for a smart meter,
carful examination of billing shows no increase. Guess us net metering customers are
exempted by our original $100 contract fee, the current bill was actually $0.24 less. Now to
try to find online access.

In the mean time I have seen ads and discussions about panel racking. Things that are universally
absent are any seasonal means of changing tilt, or anything to reduce the labor of snow removal
(PV solar isn't confined to rooftops in the SW desert). My own array used to take 90 minutes to
clear of heavy snow, but with modifications that was reduced to 40 minutes, and taking a
lot less muscle and snow pile relocating. Plans are forming here to deal with the tilt issue, not
much history to go on. The foundation will track the slope of the ground as usual.

Also advertised was a grid tie with some battery backup. The KWH ability of the battery plant
wasn't mentioned, of course. Best to see how the phone Co has done it for over a century. The
first mission of the room sized battery plant is to smooth the ripple of the AC powered rectifiers.
In this gentle service batteries can last decades. The other mission is to maintain continuous
service with an AC outage, JUST LONG ENOUGH to get the backup diesels running (gas turbine
in Chicago). Not much battery strain there either, no reduction in available power. The batteries
discharge rarely and very little, this is the way to go. Bruce Roe

Based on my hands on history of battery systems at multiple Industrial facilities for either large UPS or Medium voltage switchgear the batteries were rarely used because the normal power source did not go down.

We did perform monthly testing on the UPS and yearly on the switchgear battery system so they did get some exercise.

To be safe the UPS batteries were replace every 5 years (or sooner if they failed). The Switchgear seemed to last a lot longer but IMO the reasoning not to replace them periodically like the UPS system was a gamble.

Based on my 40 years experience, most people forget about battery systems until they are needed. Some times the end result is that the battery fails and the equipment goes down. Then there is a fire drill getting in a new set to replace the ones that have sat there and slowly died.

I truly shy away from suggesting a UPS of any kind to be installed for systems that are not checked or maintained on a monthly basis.

[bcroe]

The test panels from last year are back in storage, pulled up the stakes and ran the mower over those spots.

The swapped out SMART METER (useless to me) has been in service a month, and there may
actually be something on the display I can read. The first billing, presumably by radio report
instead of meter man, came today only 3 days into the month. I received a net metering credit
comparable to past Aprils, so I presume the NET METERING is operating correctly as before.
While announcements indicated everyone would be paying $3 a month extra for a smart meter,
careful examination of billing shows no increase. Guess us net metering customers are
exempted by our original $100 contract fee, the current bill was actually $0.24 less. Now to
try to find online access.

In the mean time I have seen ads and discussions about panel racking. Things that are universally
absent are any seasonal means of changing tilt, or anything to reduce the labor of snow removal
(PV solar isn't confined to rooftops in the SW desert). My own array used to take 90 minutes to
clear of heavy snow, but with modifications that was reduced to 40 minutes, and taking a
lot less muscle and snow pile relocating. Plans are forming here to deal with the tilt issue, not
much history to go on. The foundation will track the slope of the ground as usual.

Also advertised was a grid tie with some battery backup. The KWH ability of the battery plant
wasn't mentioned, of course. Best to see how the phone Co has done it for over a century. The
first mission of the room sized battery plant is to smooth the ripple of the AC powered rectifiers.
In this gentle service batteries can last decades. The other mission is to maintain continuous
service with an AC outage, JUST LONG ENOUGH to get the backup diesels running (gas turbine
in Chicago). Not much battery strain there either, no reduction in available power. The batteries
discharge rarely and very little, this is the way to go. Bruce Roe

[bcroe]

A nice sunny day Tues, did a post winter array inspection. The clamp on DC ammeter verified that each string
was delivering current nearly identical to similar strings. Structures looked good. Need to pick up the test panels,
remove snow blower and install mower. Bruce Roe

[bcroe]

Sounds like you had a Smart Meter all along. Did the POCO not swap
out your meter when you signed the interconnect agreement?

I paid them $100 for the 20 year agreement, which included swapping out my old mechanical meter for an
electronic meter. But its been swapped again, I don't know when, probably to a "smart" meter. Perhaps
they took off the cover and put jumpers around the meter for the swap, so service wasn't disrupted. Because
I haven't had to reset anything in quite a while. I will probably complain to CUB that the $3 a month charge
is way too high to pay for a meter, which is only there to make more money for the PoCo anyway. Bruce Roe

[DanS26]

Sounds like you had a Smart Meter all along. Did the POCO not swap out your meter when you signed the interconnect agreement?

[bcroe]

New PV adventure this month. True up time came; by not running the propane furnace at all the past winter, I
only gave the PoCo 5000 left over KWH, instead of the 8000 a year ago. The 2017-18 plan is to put a larger
mini split in the car shop, and try to keep this building above freezing all winter. And at the same time investigate
the possibility of feeding some excess PV solar power directly into the main rectifier section of the mini; a chance
to do more PV without going through the PoCo. If it works well, I may try to upgrade to entire house to mini splits,
which my MN cousin likes so well.

Friday I get a robot call from the PoCo that they are doing an area wide conversion to SMART METERS in the
next 10 days. A previous letter had mentioned this. What does it do? Eliminate the meter reader via its cell
phone antenna. They say gives me internet access, no details. Charge me time of day rates, add $3 to my
monthly bill. Save me more on electricity overall, except I don't buy any electricity.

This is entirely in line with what other utilities have done; while you are trying to be more and more efficient to
reduce energy use, they keep raising the fixed monthly connect charges. $2 to $28 in a relatively short time
for the gas co, no end in sight. That was the final straw causing me to PV grid connect, have as few different
utilities as possible. But I need the PoCo.

I called and was assured the smart meter would be net metering compatible. An appointment is needed, so
the inverters can be shut off the night before. Not good to just pull a meter with 60A of current flowing, hard
to say what damage might occur. No appointment yet, I'm in the garage today and hear a noise. Running
around to the front I startle the bejesus out of the guy carrying a new smart meter. We look at my meter and
he says "Its already been changed, I'm not putting in this one.". Good because the inverters are running flat
out. I haven't seen any blinking clocks, inverters not missing a beat, no apparent power interruption,
HOW DID THEY DO THAT? Anyway now I can go to the FERMILAB EMC meeting Wed instead of staying home.

The next monthly billing ought to show that my Grid Tie PV is still functioning (from a billing perspective) as
before. I just might find some early April KWH got lost, but that period is close to break even so I don't care.
The next few months are a different story. Bruce Roe

[bcroe]

It looks like it will take some experimentation to get the interior lighting right; was
hoping for a ready made solution on this front.

Winters here are extremely variable from year to year. A couple years ago we were getting record setting
snowfall, besides a relentless extreme cold. I wasn't about to lose KWH under the cold clear skies; was out
close to 2 dozen times getting panels cleared before the sun was up. The past month we were trying to set
a record minimum snowfall, and did see some rain. The AWD snowblower has been sitting so many weeks,
I should go out and connect a battery maintainer to it.

Bragging rights to not having bought a KWH in years are OK, but that means that I am turning over some
production to the PoCo for free every year. After last year, the new theme is "don't use ANY propane", and
don't worry so much about having a true up time surplus.

The worst cloud weeks are about past, and some serious sun sometimes shows. The last couple days each
set a KWH production record for 2017, 87KWH today isn't too shabby for the beginning of Feb. The PoCo got
the bill out very promptly this month, so here is the the running energy reserve curve. I have not spared the
KWH to keep things comfortable here, and the propane furnace backup hasn't been run at all this winter.

With the usual increased production and decreased consumption in March, it looks like there will be a good
surplus at 1 April true up anyway. More electric appliances are being seriously considered. I think she wants
a pantry maintained at 50F. Bruce Roe

PVresFeb17.jpg