Would you even try a system like this?

JonF · November 2011

As I mentioned in my previous post, I operate an off-grid/cabin resort operation that is located at the bottom of a canyon with large trees surrounding it. The only open space is by a lake that has variable water levels, making a lake shore array a difficult option and various roof/ground mount sites being 50% shaded.

The best solar site, from both an aesthetic and solar exposure is on a ridge line above the steep canyon, approximately 1750-1800 feet away from the battery bank. Until recently, I thought that a solar system was not at all feasible but the Xantrex XW MPPT 80/600 might allow for an economical system.

I know all good system designs start with the loads, so here are my loads from a KWH meter monitoring inverter output:

Annual average:
14.8 KW/day

November - April: 8.5 KW/day
May, September, October: 17.8 KW/day
June-August: 23 KW/day

I've attached a sheet which shows readouts of the system from 2.5 years of monitoring with HR/KWH meters. My 3 year annual fuel cost is 5K per year, so I figure that I'm paying .45/KWH for generator power and about .90/KWH for inverter power (slight discount because generator also runs a well pump when on).

So the question is, what size array is best, what is the minimum acceptable voltage drop for a system of this distance away and does it make sense from a cost perspective to even think about this.

My baseline assumption is that I should max out the production of the charge controller and size a 80 amp system at the max open circuit voltage possible.

I have a few more things figured out, but I would rather get some opinions from some more seasoned pros if this is crazy to even think about. . .

Thanks in advance.

mike95490 · November 2011

Re: Would you even try a system like this?

Q's:
Does the lake ice up in the winter ?
Could you place a "floating array" on it ? Would that be less distance than the ridge?
Would swimmers in the lake, mind a "tingle" if a wire goes leaky?

Cariboocoot · November 2011

Re: Would you even try a system like this?

Let's try some basic numbers and see what happens.
We'll say 16 kW hours per day as a starting point, and use 4 hours equivalent good sun (could be better) and an over-all system efficiency of 50% (the long wire run may murder that). That would be an 8 kW array.

Now about that 1800 feet of distance. That is a lot of wire resistance even at high Voltage (such as might be used with the XW 80MPPT-600). Using 277 Volts @ 28 Amps as a minimum it looks like you end up with 2/0 wire. And that's just theoretical.

Maybe you could locate the whole solar/battery/inverter array at the remote site and then up the AC to something spectacular before transmitting it to the usage site and dropping it down. That one is beyond my ken for determining.

JonF · November 2011

Re: Would you even try a system like this?

No ice in the winter, and while a floating array is possible, I'm not sure the cost of the dock structure would be less than the wire to run the array on the ridge. Could be wrong on this, but I figure that the dock structure would be around 10K or more, which is about the same as the cost of less efficiency+wire run. Plus the aesthetic consideration.

mike95490 · November 2011

Re: Would you even try a system like this?

Cariboocoot wrote: »

.. Using 277 Volts @ 28 Amps as a minimum it looks like you end up with 2/0 wire. And that's just theoretical.....

If you go up to 500VDC, should only be about 8 amps, and I'd run aluminun wire for it, buy 2000' unspliced cable.

BB. · November 2011

Re: Would you even try a system like this?

Is this somewhere in the Chicago IL area?

-Bill

JonF · November 2011

Re: Would you even try a system like this?

Now about that 1800 feet of distance. That is a lot of wire resistance even at high Voltage (such as might be used with the XW 80MPPT-600). Using 277 Volts @ 28 Amps as a minimum it looks like you end up with 2/0 wire. And that's just theoretical.

This is what I figured the system would look like if I max out one charge controller, see attached. Is it at all reasonable to consider moving max voltage drop to something like 5% for the times when the MPPT voltage drops to 280 or so? If that is the case I could get away with #3 wire.

Also, does the seasonality affect the sizing consideration?

JonF · November 2011

Re: Would you even try a system like this?

Maybe you could locate the whole solar/battery/inverter array at the remote site and then up the AC to something spectacular before transmitting it to the usage site and dropping it down. That one is beyond my ken for determining.

Anybody here have experience with this on a smaller scale? I'm wondering if the price points on this stuff are reasonable.

Cariboocoot · November 2011

Re: Would you even try a system like this?

mike90045 wrote: »

If you go up to 500VDC, should only be about 8 amps, and I'd run aluminun wire for it, buy 2000' unspliced cable.

That's a good idea. Probably would not be much danger of hitting the controller's input max with all that wire eating up Voltage. :roll: Or you could "add one more panel" and go up to, say, 4 AWG and let the V-drop take its toll. With an 8 kW array, what's one 'extra' panel per string?

mike95490 · November 2011

Re: Would you even try a system like this?

Cariboocoot wrote: »

... Probably would not be much danger of hitting the controller's input max with all that wire eating up Voltage. ....

Actually, quite a Danger, because just before a cold start, there is ZERO current flow (no IR drop). Might have to put a Midnight Clipper on it, to prevent +600V cold AM startups. Maybe even a simple 100W, 590V zener could do it !

Cariboocoot · November 2011

Re: Would you even try a system like this?

mike90045 wrote: »

Actually, quite a Danger, because just before a cold start, there is ZERO current flow (no IR drop). Might have to put a Midnight Clipper on it, to prevent +600V cold AM startups. Maybe even a simple 100W, 590V zener could do it !

Yes indeed. But it all depends on how close to Vmax the Voc is.
Really you'd need to do separate arrays/controllers, so let's rerun the numbers with 1/2 current:
277 Volts @ 14 Amps over 1800 feet looks like 2 AWG with only a 2.9% drop (about 8 Volts). Multiply by 2 systems - 4kW on each.

If we keep poking at this it could work. :cool:

JonF · November 2011

Re: Would you even try a system like this?

277 Volts @ 14 Amps over 1800 feet looks like 2 AWG with only a 2.9% drop (about 8 Volts). Multiply by 2 systems - 4kW on each.

This is assuming perfect condtions, right? Considering the high cost of wire, would it be reasonable, at all to consider allowing for a 5% voltage drop in perfect conditions with the MPPT at the lowest voltage.

Here is what I get, for drop in ideal and 80% efficient conditions when I hold my voltage drop at VDI<20.(see attached)

Also, could I run a common ground, so I'm talking about 5 wires * 1800 ft?

Cariboocoot · November 2011

Re: Would you even try a system like this?

Here guys; pick this one apart:

Using two strings of eleven Suntech 190 Watt panels http://www.solar-electric.com/stp190s.html
Voc 45.2 * 11 = 497.2
Vmp 39.6 * 11 = 435.6
Imp 5.2 * 2 = 10.4
Isc 5.62 * 2 = 11.24
Total Watts 190 * 22 = 4180 Output from controller (XW 80MPPT-600) 67 Amps (room to up the panels a bit).
V-drop looks like it could be handled with 2 AWG.

Just a sample. The key here seems to be choosing a panel with higher Voltage and lower current. Going for 24 panels instead of 22 is not unrealistic.

JonF · November 2011

Re: Would you even try a system like this?

I was hoping to revisit system sizing here, I used a site analysis tool to gauge the % of full sun for my site and came up with the attached readings. I then took the sun rating for my area code and then came up with average production per month and cross referenced that to actual use numbers.

It seems that 7 KW might be more appropriate than 8 KW?

We'll say 16 kW hours per day as a starting point, and use 4 hours equivalent good sun (could be better) and an over-all system efficiency of 50% (the long wire run may murder that). That would be an 8 kW array.

Cariboocoot · November 2011

Re: Would you even try a system like this?

JonF wrote: »

I was hoping to revisit system sizing here, I used a site analysis tool to gauge the % of full sun for my site and came up with the attached readings. I then took the sun rating for my area code and then came up with average production per month and cross referenced that to actual use numbers.

It seems that 7 KW might be more appropriate than 8 KW?

That could well be. I picked on 8kW based on your average usage figures (raised to 16) and "typical" deratings with low-end equivalent sun hours. Erring on the side of caution, as it were.

JonF · November 2011

Re: Would you even try a system like this?

Voc 45.2 * 11 = 497.2
Vmp 39.6 * 11 = 435.6
Imp 5.2 * 2 = 10.4
Isc 5.62 * 2 = 11.24

I'm not really sure how the MPPT works on a string like this one, how often would the actual panel MPPT be down at, say 270 and the amps be producing full power.

I guess what I'm getting at, is if, in my case because of the panel distance, that you accept a higher voltage drop in rare conditions to save money on wire?

techntrek · November 2011

Re: Would you even try a system like this?

What about sizing the array smaller so it just handles absorb/float (summer and winter)? Maybe you are and I missed it.

Cariboocoot · November 2011

Re: Would you even try a system like this?

JonF wrote: »

I'm not really sure how the MPPT works on a string like this one, how often would the actual panel MPPT be down at, say 270 and the amps be producing full power.

I don't have a proper V-drop calculator available at the moment so I went with what I had which is designed for AC systems. You can't tell it 400+ Volts, which is where you'd want to be. But you are right that the max current would not occur at the lower Voltage. On the other hand lower Voltage also affects V-drop, so it's within a number or two.

I guess what I'm getting at, is if, in my case because of the panel distance, that you accept a higher voltage drop in rare conditions to save money on wire?

Yes, if needed. As Mike said you have to watch out for the Voc because there is essentially no V-drop on cold, unloaded panels. As long as that doesn't exceed the controller's V-in max it will work.

Some of the other guys need to poke at this a bit: don't just go on my opinion; get seconds, thirds, fourths ..

JonF · November 2011

Re: Would you even try a system like this?

What about sizing the array smaller so it just handles absorb/float (summer and winter)? Maybe you are and I missed it.

Definitely a consideration, especially considering going above the 4.8 KW mark means another pair of wires and another charge controller.

I am considering just fixing the system at one maxed out charge controller, about 4.8 KW and plan on running the generator for the surplus.

Dave Angelini · November 2011

Re: Would you even try a system like this?

JonF wrote: »

I'm not really sure how the MPPT works on a string like this one, how often would the actual panel MPPT be down at, say 270 and the amps be producing full power.

I guess what I'm getting at, is if, in my case because of the panel distance, that you accept a higher voltage drop in rare conditions to save money on wire?

I'll add a little to your plate. The loss is often not as bad as you think. I do have experience with this controller and unless you are trying to sell power you really are not at maximum current long when charging batteries, if the design is balanced. You just need to balance the overall design.

Dave Angelini · November 2011

Re: Would you even try a system like this?

JonF wrote: »

Definitely a consideration, especially considering going above the 4.8 KW mark means another pair of wires and another charge controller.

I am considering just fixing the system at one maxed out charge controller, about 4.8 KW and plan on running the generator for the surplus.

Keep in mind that unless you are having to meet a local code, the controller just current limits above 23 amps on the input.

mike95490 · November 2011

Re: Would you even try a system like this?

batteries are 1,000AH

http://www.solar-electric.com/xaxwmp80amp6.html

Claims to take 550VDC at input

80A @ 50V = 4,000 w output 96% efficient

http://lib.store.yahoo.net/lib/wind-sun/XW-MPPT80-600-operation.pdf page 56
( manual claims it can deliver 4800w to 48v bank )

5,000 W @ 550V = 9A DC feed

The wire loss spreadsheet (assuming 1800' EACH WAY (3600')) @ 550V & 10 A, shows 4ga copper gives 2% loss (11V). 4 ga Cu = 2ga Alum wire.

JonF · November 2011

Re: Would you even try a system like this?

Mike,

From my understanding the charge controller's MPPT will shut down at 510 VDC, so I am trying to stay below that in my design. Also, since its an MPPT controller, I am looking to fix my wattage, say 4000 at PTC and then look at the amps that will be flowing through the system at the range of MPPT voltages, usually from around 280 V up to max Vmp for the string size.

If I'm trying to max out the system I will size it a little above the 80 Amp at 50V size for max power point, assuming that I'm going to be wasting a bit when the conditions are perfect.

TheBackRoads · November 2011

Re: Would you even try a system like this?

JonF wrote: »

Definitely a consideration, especially considering going above the 4.8 KW mark means another pair of wires and another charge controller.

I am considering just fixing the system at one maxed out charge controller, about 4.8 KW and plan on running the generator for the surplus.

Our host has a great deal on these Kyoceras, $1.60/watt for 20+.... 4,700W.....
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Would you even try a system like this?

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