Finally, the start of my 25kw Ground Mount grid-tie system

[live4soccer7]

Bummer.... none of the panels I'm looking at are in the lists provided and I can't input my own values.

 

[MarkSolar]

Woah! That's pretty awesome. saves a boat load of time. I may end up creating a spreadsheet version so any inverter could be used.

The only shortcoming is that it won't do calculation for combining MPPT1 and MPPT2 together. You can get at it by fiddling with their separate tracker numbers.

 

[MarkSolar]

Bummer.... none of the panels I'm looking at are in the lists provided and I can't input my own values.

In the lower right hand corner they have a link to a newer international version of the calculator. Try using that, it's newer and might have your values. I found it more difficult to understand, but has more options.

 

[Hedges]

Do you have a preferred string calculator online to use? The last time I did calculations I did them by hand. I don't mind that, but it does take a bit of time.

I had used SMA's calculator. Over time it has been download, web, various implementations. It supported all their inverters and a huge number of panels.
Now I just use Excel.

 

[live4soccer7]

Yeah, i've began creating an excel sheet to help automate the calculations for different configurations. I'm following these guides online:

String Sizing Guide: How Many Solar Panels Can I String Into My Inverter?

Learn how to calculate string size to optimize your inverter's efficiency and get the most production out of your panels.

unboundsolar.com

Calculating Solar PV String Size - A Step-By-Step Guide - SolarDesignGuide

Calculating Solar PV String Size – A Step-By-Step Guide One aspect of designing a solar PV system that is often confusing, is calculating how many solar panels you can connect in series per string. This is referred to as string size. If you are unfamiliar with the terms “series” and “string”, it...

solardesignguide.com

 

[wattmatters]

Wow, had no idea such a thing existed.

There are a lot of grid tied Fronius inverters in homes in Australia which also have a grid tied AC coupled battery system, typically a Tesla Powerwall 2, but it can be any AC coupled battery system. The battery inverter controls the Fronius via frequency signal during grid outage operation. It has to because in any grid, be it the national grid or just one home operating off-grid, supply must be balanced with demand at every moment.

The issue with battery systems is they will have maximum charge/discharge rates and that can limit the size of the solar PV system which may be connected in this manner.

Consider this scenario:

PV system, say 13kW array and 10kW inverter is pumping out maximum power to the grid as household loads are low during middle of the day. Battery meanwhile is fully/near fully charged and it can only manage an inrush at 5kW, maybe 7kW for a brief surge. Then the grid connection gets cut. If the solar PV inverter is set up to remain online and form a microgrid with the battery's inverter during outages then the 10kW being pumped out by the PV system has to go somewhere. Without a load dump it will try to send it all to the battery. But the battery system can't cope with such a high in rush current.

Yes it can ultimately use frequency control to curtail production from the PV system but that initial brief period is very dangerous and can potentially kill a battery system very quickly. So when designing systems to be able to instantly form a local microgrid when the main grid goes offline, it's really important to limit the amount of PV connected so that the inrush current won't overwhelm the system before it has a chance to catch up and adjust the PV system's output. This is why you'll find many AC coupled battery systems with such microgrid capability will specify an upper limit on the size of the PV system which can be connected in this manner.

As a result, large PV systems often need to be split, with one smaller system set for microgrid duties while the rest of the PV system goes off-line during a grid outage.

 

[wattmatters]

I used the Fronius tool.

They also have this:

https://fronius.solarconfigurator.de/solar.configurator/Quick

 

[wattmatters]

The only shortcoming is that it won't do calculation for combining MPPT1 and MPPT2 together. You can get at it by fiddling with their separate tracker numbers.

Use the other tool I list, it does all that.

 

[live4soccer7]

It looks like both the Bluesun BSM455-72HPH (455W panel) and the Hyundai HiS-S450YI (450w on the front side) have near identical specs electrically. It puts a max string size (taking my coldest day of -20C in consideration) of 17.

17 * 450 = 7650W

I may need to tweak the count to a little less. I think I'd like to look at the MPP range on the inverter and the average temp over winter to make sure I will still fall within the MPP range at pretty much all times. I would say our average temp is -3 to -4C. That shouldn't be too hard to calculate.

Two strings per fronius 15kw inverter and one string for the sunny boy 7.7kw (string size will have to be smaller due to lower DC voltage on the SMA) if I decide to go with two fronius 15kw and one sunny boy.

Please let me know if my thinking is off on this.

 

[Hedges]

I may need to tweak the count to a little less. I think I'd like to look at the MPP range on the inverter and the average temp over winter to make sure I will still fall within the MPP range at pretty much all times. I would say our average temp is -3 to -4C. That shouldn't be too hard to calculate.

Do you think that Vmp of the array will sometimes be above top of MPPT voltage range for an inverter?
I wouldn't expect that to be common, with Voc within limits. And even if it happens, inverter draws down array voltage from Voc past Vmp to something a bit lower, the power vs. voltage curve doesn't roll off fast at reduced voltage.

Max voltage 600V
MPPT rated up to 480V
MPPT operating up to 550V (whatever the distinction between "operating" and "rated"?)

https://files.sma.de/downloads/SBxx-US-DS-en-39.pdf

One SunPower 327W panel: 64.9 Voc 54.7 Vmp

Assuming an array that just hit 600 Voc at some temperature,
54.7/64.9 x 600 = 505 Vmp estimated. A bit over 480V, a bit under 550V
(Haven't worked out whether Vmp temperature coefficient is same as Voc)

https://us.sunpower.com/sites/default/files/media-library/data-sheets/ds-e20-series-327-residential-solar-panels.pdf

 

[live4soccer7]

With 17 panels, on an average winter day 25-32F I would still be hovering just under 800vDC with 17 panels on the Fronius 15kw inverter.

 

[live4soccer7]

Do you think that Vmp of the array will sometimes be above top of MPPT voltage range for an inverter?
I wouldn't expect that to be common, with Voc within limits. And even if it happens, inverter draws down array voltage from Voc past Vmp to something a bit lower, the power vs. voltage curve doesn't roll off fast at reduced voltage.

Max voltage 600V
MPPT rated up to 480V
MPPT operating up to 550V (whatever the distinction between "operating" and "rated"?)

https://files.sma.de/downloads/SBxx-US-DS-en-39.pdf

One SunPower 327W panel: 64.9 Voc 54.7 Vmp

Assuming an array that just hit 600 Voc at some temperature,
54.7/64.9 x 600 = 505 Vmp estimated. A bit over 480V, a bit under 550V
(Haven't worked out whether Vmp temperature coefficient is same as Voc)

https://us.sunpower.com/sites/default/files/media-library/data-sheets/ds-e20-series-327-residential-solar-panels.pdf

It could mean that the optimal range is up to 480, it will still work at 550 and then after that it cuts back and then shuts down at 600? Just a guess.

The fronius is a much better option for me, I think. I have a MPP range of 320-800vDC and a max of 1000vDC. This allows me to make larger strings and have less runs back to the inverters because of that. The wire will be a decent expense since it is a 300 foot or so run for the wire. I'll need to look at the specs and perhaps you guys can let me know if I'm wrong or not, but I'll prob need to go with 10gauge/6mm2 wire to carry the current and keep the voltage loss under control on that run. I would also rather over rate the wire because if I put in new panels way down the road it is very likely they will have a higher output and I'll need the larger wire to carry the additional current.

I'll probably lay a separate conduit and leave it open with the possibility of pulling fiber optic and 120vAC.

I'm beginning to think that I'll simply go with 3 fronius units and 5 strings of 17 panels each. This will leave me expansion room of another string if I ever wanted to do it.

17 * 5 * 450 = 38,250W. = Way more than enough power. Guess I'll be able to plug more stuff in and keep the house warmer in the winter. HAHAHA

 

[live4soccer7]

I think the next step would be to figure out the orientation for mounting. Any tips or suggestions on this? The panels are both very similar in size.

82.44" x 40.87"

 

[wattmatters]

I think the next step would be to figure out the orientation for mounting. Any tips or suggestions on this?

If you expect the array to be two or three rows lined next to each other, then you'll want multiples of 2 or 3 for your array panel count. Prime numbers suck unless they are all in one long line. Or you don't care about having one panel sitting out on its own. Will look a bit suckerific though.

 

[Hedges]

Hot spare?
Extra panels that don't fit your series/parallel arrangement can also be used for DC fans, or charging a different battery.
I didn't have array space, so mounted two 100W panels on Unistrut rails on an easel.

 

[live4soccer7]

Any recommendation for height off the ground on the front/lower side of the array?

I thought about dropping the panel count down to 16 a string so that I could have even numbers. That would likely work much better.

 

[live4soccer7]

@MarkSolar @SteinVT

I'm just curious, what is the height of the panels on the back side at the tallest? How about the distance off the ground from the lowest point?

If I were to stack two of these panel in "portrait" it would be about 10ft vertically (not counting the distance off the ground of the array) if I put the angle at 47 degrees (my latitude).

If I were to put three of them in "landscape", it would be about 8ft vertically (not counting the distance off the ground).

This week's goals are to get the mounting system settled on and get an application ready for the utility company and get the necessary information for permitting from the county. Once the utility company approves my system then I will submit a permit with the county and after that is provided then it will be time to start making some purchases and to get out the elbow grease.

 

[MarkSolar]

Any recommendation for height off the ground on the front/lower side of the array?

I thought about dropping the panel count down to 16 a string so that I could have even numbers. That would likely work much better.

Mine are 30" at front, 8' at the back. There's a bunch of variables you have to play with depending on your mounting structure. For example there is a maximum height allowed on the front edge for certain post depth and length. Some of it has to do with more uplift the higher you put the array. If you use Sunmodo's calculator it will generate all that for you, I'm sure all manufacturers have something similar. There is also a limit on distance between front and back row of posts before you have to add more posts. The rails come in modular lengths, I think mine were 10'. So if you compare portrait and landscape you get more or less wasted rail depending on how many rows you have and that affects how many clamps you need. There is a maximum amount the East/West pipes can overhang the last posts, and all kinds of other little variables that affect total cost and how many post holes you need. I'd play around with one of the mounting calculators to get a feel for what configurations are optimum for your array size and config.

 

[live4soccer7]

@MarkSolar Great point. That's probably how I will go about tackling that.

Taking a look at the bluesolar ground mount system, it is very similar to the sunmodo. I have attached their "brochure" on their mounting systems. It is the last page, IIRC.

For mounting of a 25kw system (original quote) it was about $3500 for their mounting system.

 

[Hedges]

Height of things like weeds or other shade would be the main concern.

My original arrays used 18' Unirac rails, which I think were custom cut to fit 8, 120W panels. Standard sizes seem to include 17' and 20'.
They come within about 18" or 2' of the ground.

What is your panel electrical (series/parallel) configuration?
By making two physical arrays of different orientations (e.g. 10:00 AM and 4:00 PM sun would be 90 degrees apart), parallel strings peak at different times, delivering more watt hours for a given peak wattage. Total available watt-hours/day would be less than with ideal orientation, but utilization of inverter could be about 40% higher because peak is 70% as high.
Depending on ratings of MPPT inputs, you may be able to have each string on its own input. If they have to combine on a single MPPT input, multiple strings of different orientation wired in parallel will capture about 2% less due to not quite operating at Vmp (unless shadows fall on a string, making loss greater.)

With mounts costing $0.15/watt, panels costing $0.20/watt, and inverters $0.15/watt (approximate), this should deliver more energy per dollar. Also, more energy within maximum power allowed to backfeed, or maximum charge rate of a battery backup system. Battery backup would be kept fully charged later into the evening, reducing capacity used at night.