Farm workshop system

[Naptown]

Yep that's what he said

[Sunking]

So, I think what you're saying is in agreement with what Schneider was saying. It sounds like your objection to putting more wattage on a charge controller is that the power is wasted assuming that the system is within the voltage limits (including extreme temperature ranges).

Yep look at it like this. You are in the desert with a empty 8 oz glass, and a 1 gallon bucket full of water. You set the glass down on the sand and empty the bucket to fill it. You just lost 120 oz of precious water.

[kevinc_63366]

Do you think the engineers at Schneider Et al haven't taken that into account?
Almost every panel I have ever looked at has a PTC rating between 89 and 91% of STC
They also have a positive power tolerance so that 250W panel may very well be a 255.
You are over thinking this whole thing. And applying different parameters of your own choosing which may or may not be accurate.
So put some diet on your collectors pit them through the burn in process. Buy about 100K worth of testing equipment and then engineer your own system.
The standards are there for a purpose. It is to allow for consistent design parameters that work in real life.
Trying to tweak things for perhaps a 1% increase in harvest doesn't make sense.

Actually, just using the published parameters associated with the panel I'm currently considering. THEY tell me the PTC is 275W, STC is 305. Trying hard not to make up too much when the manufacturer will happily provide me with real numbers. They guarantee 80% panel function for years 10-25, so I use those as conservative values.

Using the MidNite Solar Inc String Sizing tool, for instance, along with the spec sheet from the Astronergy 305, I come up with the following for a Midnite Solar Classic 150:

Power (Watts): 305
VOC : 45.29
VMP : 35.77
ISC : 9.42
IMP : 8.53
VOC Tmp. Coe : -0.332%
VMP Tmp. Coe : -0.45%
Coldest amb. : -40F
Hottest amb: : 110F
Nominal V : 48
# in series : 3
# in parallel : 6

Results show 1.2 Classic 150s required, and that I'm pushing the MAX VOC, with it entering HyperVOC at 21.2F (common overnight, but not during our days), right at the limit of their suggested range for a single Classic 150.

Easily improved by going from a 3x6 array to a 2x9 array. Now down to 1.1 Classics, without a MAX VOC issue, and entering HyperVOC when hell freezes over (-279.4F). Interesting that they indicate 3 panels in a series is acceptable using HyperVOC though.

Take that same 2x9 array, age the panels 10ish years (80% output), and 305 becomes 244. Now my array is only providing 90% of the power a single Classic 150 could eat.

All of this is based on STC, which is fair, for an estimating tool.

It is my OPINION that STC is unlikely. Especially in winter, so I'm personally fine with overpowering my charge controller a bit.

What I really find of interest are other stats their calculator provides: They show that when using STC numbers, their calculations are based on that nameplate data. E.g. (18) 305's result in the array power used in their calculations 5490 watts. Lots of other numbers as well to set expectations, like based on 5490 rated watts, power output in the heat of the summer is expected go down to 5038 watts. This makes sense, hotter arrays are not as efficient. It also shows that they (likely) did not factor in a PTC correction, which is fair, since it varies by panel manufacturer and model. Lets not even go down using NOCT numbers, where the panel is rated at 209.5W.

Since I'm not particularly fond of the charge controllers shutting down at 21F, if I didn't want to go 2x9, I could install a pair of MidNite Classic 200s. Likewise, a pair of MidNite 250s would allow me to go into a 4x5 array and use all the panels I planned on buying. (BTW - I wouldn't be "wasting" 2 panels, I need a solar DC water pump setup or two and would simply use them for that a bit later.)

Even using STC numbers with (20) 305W panels (6100W) and panels tilted 15 degrees, my Decembers average only 3.4 hours/day, so with perfect STC production at 3.4 hours/day, no conversion or storage losses, I would have 20.7kw/day, some of which would need to be reserved to recharge drained batteries, and, I suspect, a non-trivial amount would be lost to reality and less than 100% efficiency in components. If we use the ballpark 1.5 factor recommended by some on this forum to account for all inefficiencies and other factors, that 20.7kw/day would become 13.8kw/day - pretty darn close to the 13.4kw I originally foretasted as my need (think it was 13.4, it was something similar to that).

In any case, all of the above makes me comfortable oversizing the array a bit. Factor in that the panels themselves are less than 30% of the total system installed cost, and it feels to me like an area where a little extra won't hurt, especially in years down the road.

Kevin

[kevinc_63366]

Not to rain on your parade but Lowes sell 4/0 AL three wire direct burial cable for $1.89 a foot. Add a # 2 for ground and you could have about a 100 A service ( enough for all you want and then some but I have not calculated voltage drop to keep within 3% ) for about $1200

Wow.. just checked Home Depot's website and they wanted $3.99/ft for 6/0 direct burial cable three wire. That ground wire appears to be a couple bucks a foot too. At $30/month service fee, might be cheaper just to have the electric company set another pole.

For sake of conversation, lets assume I did this.

I presume then that everything gets grounded back to the house over that #2 ground wire? Or do I put in a separate ground rod system for the PV Panels/chargers/inverter output/batteries and isolate the incoming AC from the house to just the inverter/charger AC input side (making it a grid fed system, but not grid tied)?

[Sunking]

Wow.. just checked Home Depot's website and they wanted $3.99/ft for 6/0 direct burial cable three wire.

That is what we have been trying to tell you from the start. Do not go off-grid unless there is no other choice.

[Naptown]

What I mean is the workshop will be easentially a sub panel off the house.
Therefore it will require a ground back to the house and the neutral and ground wires separated in the sun panel.
Do that and install a hybrid system.
All excess will be fed back to the grid or used in the house to reduce the grid purchased electricity. You can run a smaller battery bank.
Here is the beauty of this
The panels will supply the power to the shop you have the emergency battery back up when needed ( think smaller battery bank) and any excess like on a day when you aren't in there the solar feeds the house or grid.

Get it now?

[ChrisOlson]

Ok folks, please rip this apart. Do note his is off-grid, must be off-grid, not-going-to-do-it grid tied. Please do NOT argue that point, but most everything else is fair game.

FYI: targeting about 8000 wH per day, winter usage, another 2500 wH/day in the summer.

Kevin, I don't know what your shop is used for, how big it is, or where you live. But the subject states "farm shop". We live off-grid here for many years and I could never justify spending money on solar panels, batteries and all that, except for the house that has to stay powered up 24/7. You can't even power the Lincoln 225 with the system you described, and that's the main tool in a farm shop. It would be hard pressed to even run a decent sized air compressor.

We have a 7,200 sq ft machine shop on our farm here. I have a Monarch lathe, Bridgeport mill, CNC plaz table, stick and MIG welders, and all the other stuff that makes a shop a shop. I have a full 200A single phase service to the building provided by a Cummins 4BT 45kVA cogeneration set. The generator also supplies heat for the shop in the winter. I think it is by far the more economical and practical method for powering a large off-grid shop. Even the Amish people around here that have gone "modern" are powering their wood working shops with the same type of setup.

In the winter, when I work in the shop 8 hours a day, the genset is preheated in the morning with a small 12V pump and a cheap boat battery. The pump circulates warm coolant from the shop heat exchangers and storage tank thru the engine. It only takes about 30 minutes and a puff of ether and the Cummins will start at 30 below zero with no problem. It burns 18-20 gallons of fuel per day and operates at about 55% overall thermal efficiency.

I would strongly consider something along those lines for your shop. A shop with inadequate power is about useless. And if you put the pencil to it, the generator and diesel fuel is cheaper in the long run than using solar panels and batteries and inverter, etc..
--
Chris