Sun hours

[sensij]

In other words if you have East and West facing strings combined then the MPPT for each string is vastly different over the course of a day, but the strings are combined.

The mpp voltage is not vastly different for the array orientation more directly receiving the sun and the one that is not, so combining the two orientations in parallel does not cause unacceptable loss. A lot of the effect is thermal... the array getting direct sun will be hotter, and have a lower mpp voltage than the cooler parallel string, but I think the effect won't be as bad for ground mounted panels as it might be on the roof. The mppt will seek whatever voltage maximizes power, so it will tend to follow the mpp voltage of the array in direct sun.

[DanS26]

Bruce, Just wondering how your strings are attached to your Fronius inverters? Specifically.....do you combine strings at the arrays then send a single DC home run to each inverter? I'm just curious how the inverters handle strings of different powers. In other words if you have East and West facing strings combined then the MPPT for each string is vastly different over the course of a day, but the strings are combined. Just curious how the Fronius inverters handle that from an MPPT standpoint?

I know that the Fronius inverters that you have can each handle 6 individual strings each with a different MPPT....but when you combine strings and use a buss bar in the inverter it appears to me that differential capability gets lost. If that analysis is correct......how much is lost if any?

[bcroe]

So, this is all East facing right?

Correct; will double my East facing panels. With the tilting top and below ground done,
am working on the middle (above ground bearing supports).

If this array proves out, will do a West facing mate, start removing the worst shadowed
South facing panels. Bruce Roe

[SunEagle]

Looking good bruce

[AzRoute66]

So, this is all East facing right?

[bcroe]

As the good weather ends, I finally got the new foundation concrete finished. 6 supports for 6 bearings, for a
variable tilt array. The 12 L bolts are in place for above ground attachment. Since I don't want it to emulate
the HOLLYWOOD sign (like the original), a full size 66' X 5' fixture was built to hold the L bolts at exactly the
right location and orientation: straight and square, but sloped to my 10% grade.

They certainly are in a straight line; will be easier to do the final shimming for bearing alignment. That is my
yellow laser on the side. Bruce Roe


Hollywood.JPG




StrButSlope.JPG
LboltFx.JPG

[bcroe]

At this point my net metering KWH reserve for winter is exactly the same as the best of the previous
4 years. Should be plenty, but it always depends on how severe the winter is. Meantime there has
been very slow progress on my southern "solar acre", looks like this today. The input on building a
"percentage above or below MPPT" meter will also be used when this is finished, in theory to set tilt.

Bruce Roe


PVSep17prgr.JPG

[bcroe]

I noticed the panels the other day when I went to check where the bad weather and
tornado was predicted in your area.

Yes, we sent a bunch more rain water down the river this week, brief local flooding. My lot is not rectangular, and the
map shows my installer just lined up on the south fence. Since then I have found the survey pins and plotted out a
pretty accurate E-W line for future work. Its about a 10% grade downhill NE, and everything will be mounted that way.
A before and after reveals a lot of rough brush and shading is now gone. Bruce Roe

[SunEagle]

After 4 years google earth has updated. After 4 years can finally see all my solar panels. Even the short
timed test panels; apparently taken around the end of winter. Bruce Roe

I noticed the panels the other day when I went to check where the bad weather and tornado was predicted in your area.

[bcroe]

After 4 years google earth has updated. After 4 years can finally see all my solar panels. Even the short
timed test panels; apparently taken around the end of winter. Bruce Roe

[bcroe]

By beginning afternoon the WEST system had collected 47 KWH, but the EAST
system had collected only 40 KWH. 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.
Bruce Roe

Getting that extra 18% isn't an urgent project, but the sun is out and things are beginning to happen. Last
night I went out in the dark and got the (12 fuse) combiner box ready. Getting close to ordering enough
racking to support 24 panels facing the rising sun, and out of the shade. Some other day, the setting sun.
Then there is the matter of digging a foundation...

Anyway nice to see sun again in this oft cloudy area, been running about 140 KWH for 7 of the last 8 days.
The DC wiring was cool, the AC wiring warms quite a bit after running maxed 8 hours. Another upgrade?
Bruce Roe

[J.P.M.]

Vclipping/Vmp is a tough number to use. The IV curve is really non-linear near the maximum power point, so the voltage change per watt of power "clipped" is probably not constant, nor solely a function of temperature.

Dan's suggestion helps ballpark the maximum energy that could have been produced on a clear day, but I understand that isn't quite what you want. I'm not sure there is a satisfactory answer without taking enough data to build a single diode model of your specific array across enough conditions to be useful. The CEC model implemented in SAM is a good starting point.

FWIW, I'd agree with that. Depending on how the inverter(s) root around looking for an Mpp, V at clipping can be galloping around quite a bit at/if clipping. Most all measurements any backyard fanatic such as Bruce, you, I, etc., can pull off with any reasonable meaning are best done when things are at least semi or quasi stable. FWIW, all my fouling data/stuff is done under as clear conditions as possible and at the time of min. daily incidence angle on the array. That's usually about 19 min. or so after solar noon for my stuff. I apologize if I was less than clear on that point.

[sensij]

Vclipping/Vmp is a tough number to use. The IV curve is really non-linear near the maximum power point, so the voltage change per watt of power "clipped" is probably not constant, nor solely a function of temperature.

Dan's suggestion helps ballpark the maximum energy that could have been produced on a clear day, but I understand that isn't quite what you want. I'm not sure there is a satisfactory answer without taking enough data to build a single diode model of your specific array across enough conditions to be useful. The CEC model implemented in SAM is a good starting point.

[J.P.M.]

Thanks, there will always be some temp error; if its pretty consistent, most will cancel out. Sensors here are pretty
small, TO92 transistor package can be stuck on and insulated on one square inch. If I want to have a simple
read out, I won't be using super accurate IR readings. Bruce Roe

Understood. After being around thermal stuff for 40+yrs., I've learned to not take what I'm told by instruments as gospel. I'd not be surprised if you've found something similar and behave accordingly

Regards,

[bcroe]

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. Regards,

Thanks, there will always be some temp error; if its pretty consistent, most will cancel out. Sensors here are pretty
small, TO92 transistor package can be stuck on and insulated on one square inch. If I want to have a simple
read out, I won't be using super accurate IR readings. Bruce Roe