Sun hours

[DanS26]

Bad way for me to formulate the question.

One more try:

What's the design wind speed in your area ?
Any calculations for wind forces and loads required ?
Is AHJ check/approval of the calcs/design required ?

Just curious.

No one required me to do perform or submit wind calcs. But as you can see from the pics the arrays are over built from a purely structural point of view. The only approvals I received was to comply with the POCO interconnection agreement. They were really only concerned about nameplate capacity and anti-islanding. The engineer did take a look at the array and his only comment was "sweet".

I live in a small valley surrounded by a hardwood forest. High speed winds usually pass overhead. A few years ago a tornado passed right over my house.....I could tell from satellite views of the tornado destruction path. Sitting in the valley allowed the winds to jump over my structures. But tornadoes still are a concern in this part of the country anywhere including low lying areas.

[DanS26]

Yes I sourced the panels late last year at a very good price......less than $0.40 per watt but I had to drive 100 miles to pick them up. They are Trina 245's a Tier I manufacturer.

With these 24 panels I will now combine 4 strings of 12 panels each for each inverter. Short Circuit Isc for each home run will be no more than 35.4 amps per inverter....the Fronius max allowed is 44 amps. Maximum Voc @ STC is 450 volts which should be no problem.

The Imp @ nominal temp is 20.7 amps for the Fronius 7.5.....I will probably exceed this by around 5 to 6 amps. Here is where the robustness and active cooling will play a role. I have six good operating years on these inverters and have them located on a north wall in very shady cool conditions.

[bcroe]

Well I made a little bit more progress today........this pic show the bracing/support bracket with a couple of bottom rails laid.....
Here's a pic with all of the bottom rails attached....
Finally this pic is a close up of another repurposing of the IronRidge 3" top cap assembly. I used the pipe clamp assy to hold in place the bracing/support bracket........

Looks good, do you have the panels to be put up? I wonder if it is better working in the recent heat
wave, compared to my erecting 24 panels in the middle of last winter?

No doubt some wires will be trenched in soon. I just got a very small trencher for that here, waiting
for cooler weather to use it. Bruce Roe

[bcroe]

Actually, my question was somewhat rhetorical.

But in the case of array orientation, and with due respect and no snideness or rancor intended, that if you think a split E-W array will produce more annual output than an equally sized south facing array in the same location and duty, you both have missed the boat with regard to resource assessment as it relates to array orientation and output. Respectfully,

I was too busy with a zoning meeting last night to write. A proposal for a 20MW solar installation was not well
received, in this rural, zoned AG area. My schedule shows 10 meetings to attend, I am part of the opposition.

Rhetorical, yes I actually got that. If I missed the boat, it was in trying to generate all home energy from solar
at this location. The arithmetic has proved out, and it is actually working. There have been many incremental
improvements over the duration of this experiment, more to come. I met a lifetime goal of reducing the
number of utilities I must connect to, with their ever increasing ways of taking your money.

J.P.M. we all agree with your clean sheet design figures. But they primarily consider the panels, not the entire
plant. My own guess was the KWH cost might increase 5% since the rest of the plant is smaller. Fronius
estimates the cost is actually less. The truth is that cost is much more influenced by things like ground vs
roof, tilt vs fixed, snow immunity, available area for 2 sided vs individual facing arrays, amount of clouding,
and of course existing equipment. Don't forget that a KWH will always cost more under levels of clouds
than under clear skies. Yesterday I had the pleasure of seeing the inverters running above 90% capacity
under mild clouding, instead of being knocked down to 50% with a conventional design. Later it stormed.

Everyone has a different situation to evaluate. Some factors here:

The PO did me a favor of burying a 600' loop of 4 gauge out to the shed with the inverters, instead of the
usual 6 gauge. Running 58A at 260VAC puts losses over 3%, wiring is warm but not overstressed. The
9V loss added to the PoCo high voltage means approaching the limit of inverter monitor tripping. Lower
line voltage would increase my current, and losses by the square which I can't really afford, its a balancing
act.

The PoCo contract lists 15KW of inverters. With the shifts in voltage seen to date, larger inverters could
require a new permit, a bigger 200' buried run to the road, and a larger line transformer.

The 2 loops of 480' of 6 gauge in the DC run at 21 ADC manage a good efficiency of just over 1% loss,
this also would increase with more peak power.

My 200A box has a 125A PoCo feed breaker at the top and an 80A solar inverter feed breaker at the
bottom, about ideal but no room for growth.

There were already 4 dozen extra panels on the property (previous experiments) when the 15KW PV
solar system was installed. The installer suggested I donate them, but I soon realized they could be
added E-W for just the cost of installing some treated wood supports.

All the above was in place before the disappointing performance under clouds was revealed. There are
even spares for my hard pressed inverters. All could agree with a continuous cloud cover extra panels
should be used. No clouds, a J.P.M. favored design would be good. A compromise, middle ground design
might be good in this area. Developing that compromise has been going on here, the constraints listed
above are observed but not terribly limiting to the goal. Every time I walk out under some clouds and see
the inverters putting out serious power, I feel good, not like I missed the boat. Bruce Roe

[J.P.M.]

My calcs said around 75 mph for 48 inch holes using schedule 40 pipe. The arrays behind are rated for a 90 mph wind load.

Considered using supplemental duck bill anchors but have not decided yet.

Bad way for me to formulate the question.

One more try:

What's the design wind speed in your area ?
Any calculations for wind forces and loads required ?
Is AHJ check/approval of the calcs/design required ?

Just curious.

[DanS26]

You're welcome. R.U.A.

Did you need to do any wind calcs on the structure ?

My calcs said around 75 mph for 48 inch holes using schedule 40 pipe. The arrays behind are rated for a 90 mph wind load.

Considered using supplemental duck bill anchors but have not decided yet.

[J.P.M.]

JPM...thanks for your comments...all valid points. Yes I could have augmented the south facing structure...but that would have required replacing inverters, asking permission from POCO to exceed 15kW nameplate capacity plus a total rewire to install a line side tap. I did not want to do all those things.

It's as simple as that.

You're welcome. R.U.A.

Did you need to do any wind calcs on the structure ?

[DanS26]

JPM...thanks for your comments...all valid points. Yes I could have augmented the south facing structure...but that would have required replacing inverters, asking permission from POCO to exceed 15kW nameplate capacity plus a total rewire to install a line side tap. I did not want to do all those things.

It's as simple as that.

[DanS26]

Well I made a little bit more progress today........this pic show the bracing/support bracket with a couple of bottom rails laid.....

IMG-2594.JPG

Here's a pic with all of the bottom rails attached....

IMG-2595.JPG

Finally this pic is a close up of another repurposing of the IronRidge 3" top cap assembly. I used the pipe clamp assy to hold in place the bracing/support bracket........

IMG-2596.JPG

[J.P.M.]

Well I never really explained it but Bruce has a few times. The concept is to expand the productive capacity of the DC side of the system without having to expand the AC side of things. Since I already clip a few days of the year at mid-day it is better to increase production in the mornings and evenings without aggravating the mid-day clipping too much by using an East/West oriented array.

It just so happens that my Fronius string inverters will accept a fourth string without exceeding the max current limitation. The Fronius units are sturdy with an active cooling system.

Finally I'm on a net billing system which allows me to sell all my excess at wholesale rates, currently around $0.075 per kWh. So by doing the work myself and getting an especially good deal on Trina 245's my ROI is relatively short.

Thank you.

Actually, my question was somewhat rhetorical. That is, as long as either maximizing annual production per STC kW or offsetting a specific amount of an electric bill, in the most cost effective way, or both using a grid tied, net metered PV system are goals.

So, to those ends, if they are goals, the idea of placing panels in less that max. annual production orientations without intervening constraints makes little sense to me.

If the area under the curve of the combined output combined of a dual direction, E-W oriented array is compared to the area under the curve of the output of an equally sized equator facing array, the dual oriented array will indeed have a flatter curve, but not enough to make up for the larger area under the south facing curve, the area under either curve being array output.

Reality is, and as we all know, some of those intervening physical constraints might be shading, roof orientation or terrain snow removal, etc., and then there are the financial constraints such as specific POCO policies or pricing that would make some orientations more profitable than the max. production orientation. Lots of reasons.

But, assuming an open, unobstructed ground mount location that's buildable, like a field, as your photos seem to show, splitting arrays to less than optimal orientations for only such reasons as to avoid clipping, or saving $$ on inverter costs alone makes no sense in most any scenario I can conger up.

For starters, in maximizing annual production, in most any location or site there is one array orientation that maximizes annual output per installed STC kW. If so, logically, any other orientation will therefore produce less output per installed STC kW. I suppose there are exceptions, but they're called exceptions because they're rare, and while the off optimum penalty to annual output is often relatively small for a surprisingly wide tolerance of off optimum orientations, a +/- 90 or more deg. off optimum orientation will be outside of such a range.

Other orientations than the optimum will simply produce less annual output per installed STC kW. So, if the goal is to meet a defined annual output, off optimum orientation will increase the required size of an array to meet that desired/design annual load. That size increase will increase the array cost, and so lower the cost effectiveness of such split arrays that have at least 1 less than optimal orientation regardless of who does it, DIY or anyone else.

Now, for the other consideration of maximizing annual bill offset, and with respect to TOU and optimum orientation, POCO rates like TOU may and probably will shift the max. bill offset orientation as f(POCO tariff schedule, tariff rules). But with respect to TOU and array splitting, unless the POCO tariff structure has a really big off-peak/super off peak rate both in terms of time and rate reduction stuck in the middle of the day or so, the max revenue orientation will also be unique to that application and location. That is, just like the uniqueness of the max. output per STC kW orientation, there will, in all likelihood, only be one max. revenue or bill offset orientation per installed STC kW. That means that all other orientations will be less cost effective. That means at least one orientation of a split array, and probably both, or all, will be less productive, at least in terms of revenue generation and probably annual generation as well.

On decreasing inverter cost by decreasing inverter size, I just looked around on the net and it seems that increasing the size of most string inverters doesn't cost that much. for sizes of, say, 5-10 kW for example, the cost goes up a few hundred $$ or less per. Example: Not because I like them necessarily, but a 5 kW string inverter from Fronius is ~ $1,800. an 8 kW Fronius is ~$2K. a 10kW Fronius is ~$2,600 From that, it looks to me like increasing an inverter size isn't the biggest concern in array design.

Still, and using the goal of meeting an annual load, of say, 10,000 kWh out of an inverter, and say in Rockford , IL, I did a couple of simple PVWatts runs and got model outputs for 1 kW arrays.

A 180 az., 45 tilt puts out ~ 1442 kWh/yr. with an hourly max. D.C of 1,049 W. the 90 az, 45 tilt puts out 1095 kWh/yr., with an hourly max. D.C. of 903 W. The 270 az., 45 tilt puts out 1,103 kWh/yr. with an hourly max. D.C. of 885 W.

Using a 60 deg. tilt, which, while not going into particulars here, is probably more practical in a snowy climate, and also what seems to be about what you have in mind from the photos:
180 az. puts out 1,360 kWh/yr. per STC kW, hourly max. of 1083 W.
The 90 deg. az. 60 deg. tilt puts out 1,004 kWh/yr., hourly max. of 915 W.
The 270 deg. az., 60 deg. tilt puts out 1013 kWh/yr., hourly max. 889 W.

All of the above are in D.C. watts input to the inverter.

Sticking with the 60 deg. tilt, and still assuming the goal is a 10,000 kWh/yr. generation out of the inverter(s), and 95 % inverter eff., the approx. size of a 60 deg. tilt, 180 az. array will need to be 10,000 kWh/yr/((1,360 kW/yr. per STC kW)*.95) = ~ 7.74 kW. Max. hourly output is ~ 1,083 W per STC kW, or 7.74 * 1083 = ~ 8.38 kW.

With the same 10,000 kWh/yr. goal w/ 60 deg. tilt but equally split 90-270 az.:

90 az. output is ~ 1004 kWh/yr., max. hourly is ~ 915 W.
270 az. output is ~ 1013 kWh/yr., max hourly is ~ 889 W.

Averaging the two annual outputs per STC kw and sizing the combined array to meet a 10,000 kWh/yr. goal:

10,000 kWh/yr/((1,004+1,013)/2 kWh/yr. per STC kW)*(.95) ~ = 10.44 kW.

Combining the two PVWatts hourly outputs for each orientation to get a rough idea of max. combined total hourly output of a 10.44 kW array in two orientations gives a max. hourly output of 997 W/STC kW for a max. hourly output of 997W*10.44 kW = 10.4 kW.

So, it looks like the inverter will need to get bigger to meet the bigger output of the larger array that's required to meet the 10,000 kWh/yr. goal when having 90-180 az. split.

It also seems that to meet a 10,000 kWh/yr. load in Rockford IL, if the choice is either an ~ 7.74 STC kW south facing array with about an 8Kw inverter or an ~ 10.44 STC kW array split in two with 90-270 azimuths with about a 10 kW inverter that probably has 2 MPP inputs, I'd bet the single array south facing will cost something like 35 % less and produce about equal annual output.

I'd also bet that if I took the average hourly output over each hour for 365 days for each array and graphed them, the area under the split array would be a somewhat flatter, meaning the inverter would run closer to clipping for more hours, but the south facing array, while having a more pointed distribution, will have a greater area under the curve, indicating greater annual output.

A thought experiment: Imagine 2 separate, equal sized arrays of 1 kW STC kW each in the same location, one facing east, one facing west. Compare the combined annual output to that of a single 2 STC kW array in the same location and the same tilt as the other two arrays. I'd bet the 2 kW south facing array will have greater annual output.

Of all the posters here, you and Bruce are, IMO, among the better informed and I am of the opinion that I ignore what you write at my peril, particularly with respect to the finer points of electrical technology. But in the case of array orientation, and with due respect and no snideness or rancor intended, that if you think a split E-W array will produce more annual output than an equally sized south facing array in the same location and duty, you both have missed the boat with regard to resource assessment as it relates to array orientation and output.

Respectfully,

[bcroe]

Maybe cover one array with a tarp?

Attempts to make measurements on very early experiments, revealed that things here changed too
fast (moving clouds) for any time consuming setup process. Actually built some stuff to counter that,
to allow instantaneous comparisons not possible with a simple meter.

Besides time, a tarp will likely allow some light passing anyway, and there will a significant output
from light on the back of my free standing panels as well. So I wrote off covering panels. But
shorting one panel with a MOSFET is not very difficult electronically, and on/off action should not be
much of a shock to an inverter. Then after the inverter has adjusted (a second) do a couple more
panels and make the measurement. Recover in reverse, all in less than a minute. Bruce Roe

[ButchDeal]

Remember the effort to keep a flat output curve over a sunny day. Panels facing away from
the sun are only delivering like 10% output. So essentially they are derated to 10% capacity
as far as the inverter is concerned, my EFFECTIVE instantaneous DC/AC ratio is not based
on full panel rating.

not really derating as that implies that it its peak is decreased. It is more moving the peak of one array (east) to to earlier in the day and moving the peak of the other (west) to latter in the day. If you move them far enough the combined part in the middle at noon is within spec as well. (Bcroe knows all this of course as he has other posts on showing his production, just trying to explain it better for DanS26).
Think if it as if you just had one array pointing East. you would get a nice production curve but by afternoon the inverter would be doing almost nothing.
You would install a West facing array of the same size and put it on a different inverter which would do almost nothing in the morning but work all afternoon.

If the arrays are arranged such, you would get ALMOST the same production connecting both to a single inverter instead of two and keep that one inverter working all day long.

I did think about momentarily shutting down part of the array so that peak power could be read
directly at a level below clipping. If each orientation had 2 strings, perhaps shut down one. A
way to do this gracefully might be short one panel, then another, perhaps 3 total to take a string
out. I hope not to need to resort to this method.

Maybe cover one array with a tarp?

[bcroe]

Bruce, I know you have 2.0 DC/AC ratio. How do you keep from over
current situations? Do your Fronius inverters just ignore over current?

Or did you just increase string size and up voltage to keep current within specs?

If so those transformer machines are much more robust than I thought.

Remember the effort to keep a flat output curve over a sunny day. Panels facing away from
the sun are only delivering like 10% output. So essentially they are derated to 10% capacity
as far as the inverter is concerned, my EFFECTIVE instantaneous DC/AC ratio is not based
on full panel rating.

With 720 cells in a string, they run 360VDC in summer, 420 in winter, pretty much centered in
the inverter range. An inverter just draws less current to keep power limited, letting panel voltage
rise. My earlier thoughts were to create a readout indicating what the effective number is at any
time, based probably on panel temp and operating voltage. This might be used for fine tuning
the tilt of array sections (arranged by string). Since I tend towards clipping most of the day, I
can't just note the power level. Other crises around here in some of my other hobbies have
prevented any actual development. After last nights zoning meeting, came home and worked
on rebuilding brakes, the antenna preamp was taken out by lightning, etc, etc.

I did think about momentarily shutting down part of the array so that peak power could be read
directly at a level below clipping. If each orientation had 2 strings, perhaps shut down one. A
way to do this gracefully might be short one panel, then another, perhaps 3 total to take a string
out. I hope not to need to resort to this method.

So although my Fronius inverters may in fact be running within spec, I am still beating them
pretty hard with 8 straight hours of clipping on many a day. They have managed 5 years of this,
but just in case I have spares. Bruce Roe

[DanS26]

Bruce, I know you have 2.0 DC/AC ratio. How do you keep from over current situations? Do your Fronius inverters just ignore over current?

Or did you just increase string size and up voltage to keep current within specs?

If so those transformer machines are much more robust than I thought.

[DanS26]

Certainly a 2 sided array will be the cheapest. But I don't see a way to make seasonable tilt changes,
and it requires twice as large an unshadowed area. To minimize snow clearing efforts, and trying to
avoid a lot more tree cutting, I am moving toward single orientation arrays facing varied directions
across a common clearing, and easy tilt changes. Bruce Roe

My original design included the ability to make seasonable tilt changes......but I decided against it......fixed tilt is less parts, less cost and less labor.

In my life I need a lot less reminders to do things every so often.


ps....Fronius published that technical paper in 2012 and revised it in Aug 2013.