My New 7.56KW Ground Mount.. First Light, First Watts..

[J.P.M.]

A confrontational statement that lead to a half page of banter that wasn't required reading for anyone.

Could have simply said "Great! Output seems to match predictions". No newbs following this thread would have been lead astray by Murby's "99.97%" comment.

Then why did you read it, or not stop reading it ? None of it's required reading in the first place.

If I'd written as you suggest, It would not have been my opinion, nor would it be a true representation of how I see the situation.

As usual, and as in the past, take what you want. Scrap the rest.

[Mike90250]

Just because sunspots align this week, does not mean they will align next week.
The point being that the OP was so happy about PV watts matching what he harvested, was a random stroke of luck. At any one day, PVwatts could be expected to be + - 10% or more, it's using a historical 20 year average. Cool day, haze, clear, all will affect the harvest, and if blind faith was used on a non-matching day, the OP was just as likely to head to the suppliers office with a ream of data in hand to make a fool of himself.

[J.P.M.]

Just because sunspots align this week, does not mean they will align next week.
The point being that the OP was so happy about PV watts matching what he harvested, was a random stroke of luck. At any one day, PVwatts could be expected to be + - 10% or more, it's using a historical 20 year average. Cool day, haze, clear, all will affect the harvest, and if blind faith was used on a non-matching day, the OP was just as likely to head to the suppliers office with a ream of data in hand to make a fool of himself.

Mike: thank you for the comment. Understood. However, on one point: About 90% or so of the TMY data is synthetic, not historical. While the data is more than stochastic, and probably fairly representative from a climatological standpoint, few of the data are actual recorded readings, particularly the irradiance data, and therefore not historical. See the best practices handbook, NREL/ TP-5D00-63112 of Feb., 2015 for details on that. Records are nowhere near that comprehensive. That's another reason for the uncertainty in the model's output. As the model's help/info screens state, monthly data can be off by +/- 30 %. as for daily variation ,common sense will tell you a daily variation for any one calendar day can be off as much as the difference between a clear, sunny day's output and a completely overcast day's output.

[DrLumen]

Murby, nice install. Well done!

bcroe, yours appears to be made of sub-arrays of 6 panels each. How many sub-arrays are there? Hard to count as they trail off into the distance.

[azdave]

Then why did you read it....?

I have not yet figured out a way to learn what a new post is about without reading it first (I'm working on it though).

[max2k]

... Hard to count as they trail off into the distance.

more like into infinity

[AzRoute66]

Murby, nice install. Well done!
bcroe, yours appears to be made of sub-arrays of 6 panels each. How many sub-arrays are there? Hard to count as they trail off into the distance.

They are both very nice, but every time I see a ground mount system without seasonal tilt it seems a bit wasteful. I can only assume that seasonal tilt is not nearly as impactful up north as it is down here at 31.5 degrees. Single axis tracking is a more complex matter of elbow room, mechanics, and extra wire, but seasonal tilt seems so easily achieved for most installations.

[J.P.M.]

They are both very nice, but every time I see a ground mount system without seasonal tilt it seems a bit wasteful. I can only assume that seasonal tilt is not nearly as impactful up north as it is down here at 31.5 degrees. Single axis tracking is a more complex matter of elbow room, mechanics, and extra wire, but seasonal tilt seems so easily achieved for most installations.

As it turns out, for several reasons, there is not much change in insolation enhancement as f(latitude), at least not solely based on latitude. For relatively sunny climates, the difference in yearly isolation between collecting surfaces fixed facing south and tilted at the local latitude is about 4 % less than the same collecting surface facing south, but adjusted twice/year to latitude + 15 deg. for winter and latitude -15 deg. for winter.

When talking about single axis tracking or tilt adjustments, it's important to be clear about which axis is being adjusted or rotated. Rotation about a N-S axis while fixing the tilt at latitude will result in a 35 % or so insolation enhancement over a fixed south facing tilted at latitude surface in Tucson, while the same at tilt at latitude surface in Muskegon, MI rotated about a N-S axis will result in ~ a 25 % enhancement - but a lot of the 25% @ Muskego vs. 35 % at Tucson difference may be due to the difference in cloudiness (the clearness index). For Sacramento - a sunnier climate but further north than Tucson, the enhancement of a fixed tilt, N-S rotation over a fixed south facing at latitude surface is ~ 35 %, or about the same as Tucson.

The combinations and permutations are many, so general statements are probably neither too valid or very helpful, but it seems that insolation enhancement based on seasonal tilt adjustments may need careful analysis as it may not gain a whole lot more in terms of annual P.O.A. insolation. Maybe for 2X/yr. manual adjustments, but tracking via motors and control mechanisms may cost more in $$ and hassle more than the relatively small gains in insolation are worth.

[Murby]

They are both very nice, but every time I see a ground mount system without seasonal tilt it seems a bit wasteful. I can only assume that seasonal tilt is not nearly as impactful up north as it is down here at 31.5 degrees. Single axis tracking is a more complex matter of elbow room, mechanics, and extra wire, but seasonal tilt seems so easily achieved for most installations.

I looked into this when I did the design work... Just two years ago, it would have been worth the extra hardware to do a seasonal tilt.. But the darn solar panels are so cheap now that it just doesn't pay.. I paid 60 cents per watt for my SolarWorld SW270 panels..

It just seems easier and more cost effective to put in more panels rather than more hardware. But again, just two years ago, I probably wouldn't have said that.. You know, when I started considering solar back in 2009, the darn things were still up at $4 per watt..

Heck, you can even buy a 5000 watt SMA inverter now for only $500.. It might not be the fancy transformerless versions they have now, but it will do the job..

The other consideration I think most people might have is physical space.. which for me isn't really a consideration since I have more acres than fingers to count them on..

I'm already considering putting up another 4kw worth.. As I said elsewhere, I want to be able to waste electricity like we waste air and water..

[AzRoute66]

[...] but adjusted twice/year to latitude + 15 deg. for winter and latitude -15 deg. for winter.

Got me on that one. I tend to compare the winter tilt (L+15ish) to my fixed pitch roof of 22 degrees (essentially 'summer tilt') so get pretty significant changes. Comparing a winter tilt to a latitude tilt would be much less difference. Thanx. Next time I am in SAM or PVWatts I'll try to remember to run one against straight latitude instead of the roof.

[AzRoute66]

I'm already considering putting up another 4kw worth.. As I said elsewhere, I want to be able to waste electricity like we waste air and water..

Now that you are familiar with PVWatts, and have your mid-day solar needs well considered, you might want to waste a few hours running some PVWatts scenarios with easterly and westerly facing arrays. I have been doing that in SAM recently and am very appreciative of the extra time in the day (up to two hours) that meaningful power can get generated. To me, extending the productive day hours from May through August has more 'oomph' (for off grid) than just piling on watts at noon. Linking that with the 'movable' array concept perhaps some 'wings' that can face south in the winter and swing some to the east/west in the summer is just the deal for an enthusiast with a lot of dirt.

A while ago I started a thread titled 'Slowing down the summer sun' (or some such nonsense) where Bruce presented a graph of his E-W array production that might be of interest to you - it blew me away. At any rate, the concept is called 'virtual tracking' and can, of course, be searched for both in this forum and the web in general.

[Murby]

Yes, I can imagine how grid-tie and off grid would be after two different goals.. and its an interesting perspective that most would not consider..

For us grid-tie people, we want to generate as much power as possible so we aim out arrays where it will generate the most watts... But you off grid folks have a problem we don't have, and that's a finite amount of storage capacity... once you exceed that capacity, any excess power is wasted.. So with that in mind, you off-grid'ers aren't interested in generating maximum power, you're interested in generating power for as many hours as possible because once you start drawing off your batteries, you're basically on a rope and you do not want to reach its end.

Interesting challenge to deal with. Man I learn so much from this forum...

[AzRoute66]

[...] But you off grid folks have a problem we don't have [...]

Just to be clear, I am not an off-grid person (yet). It's just that I find the challenges of off grid design and perspective much more fun than the grid-tie.

Your summary of it was spot on.

But it is a bit more than just getting off the batteries early and back on late. If the power generation can be leveled over a longer period, for the same watts, the peak amps can be reduced, thus reducing the controller and the battery bank. Combine that with longer charging hours and you won't find yourself wandering around in a Partial State of Charge (PSOC) quite so much. It's less 'days autonomy', but you are going to have a generator (or other alternate supply) anyway, so is the ability to go for multiple cloudy days on only solar really a huge issue? If you design for 'n' days of autonomy, what happens on day n+1...

[J.P.M.]

Got me on that one. I tend to compare the winter tilt (L+15ish) to my fixed pitch roof of 22 degrees (essentially 'summer tilt') so get pretty significant changes. Comparing a winter tilt to a latitude tilt would be much less difference. Thanx. Next time I am in SAM or PVWatts I'll try to remember to run one against straight latitude instead of the roof.

Not a gotcha. I don't operate that way. Just an example of many. Back on the day, before PC's and spreadsheets made per degree and per day changes a cakewalk for most all, tables, charts and examples/go bys to zero in or point in a direction were the norm. My choices here were examples to show global possibilities in perhaps a semi symmetric way. More extreme seasonal adjustments will yield a slightly higher yield, particularly for winter in places with a lot of snow cover.

[DrLumen]

They are both very nice, but every time I see a ground mount system without seasonal tilt it seems a bit wasteful. I can only assume that seasonal tilt is not nearly as impactful up north as it is down here at 31.5 degrees. Single axis tracking is a more complex matter of elbow room, mechanics, and extra wire, but seasonal tilt seems so easily achieved for most installations.

FWIW, I understood what you meant in regards to seasonal tilt (ie elevation). I'm not really sure what that says about either of us...

If I were to build something like that I would put in an elevation tilt. Not that anyone else would or should - just that I would. It would also be very likely that after about the 2nd or 3rd year I would probably not bother changing them though.

Might be handy if the world goes off its current tilt for whatever reason. If that were the case, we would have a lot more pressing things to worry about.