7kWh per day off grid design for Adirondacks - critique wanted

[Living Large]

I have a small year-round off-grid house at Lat 43.4N Long 75.2W. My usage will be 7kWh per day. The insolation in winter isn't great, maybe 1.8. There would be a 6kW Generac running on propane to make up for shortages. This would be a 120V/240V system. I figure the maximum AC load I would have at one time is 4000W.

The house has a roof that faces East and West - not suitable for an array. Thus, I am planning to build a detached garage with a saltbox style roof with a steep pitch for the solar array, facing due South. There is a good sized clearing for the house, and there is a dropoff to a stream, so I am hoping I will have clear illumination between at least 10am and 2pm on Dec 22 - I will verify this in a few days on a site visit using some measurement devices. The array is not in the shadow of the house, so that is a good start.

Solar panels and batteries
A reasonably sized two car garage roof as described will provide enough space for 16 panels. I chose 275W Mono. 16 x 275 = 4400W. The VOC is 39.4V and the Iss is 9.58A. I did a little research on batteries, and got drawn in by the Aquion AHI "on paper". I liked the safety aspect, not worrying about water, and being able to handle 100% DOD. They aren't cheap, so I settled on under-sizing and planning to use the generator to make up for lack of full capacity. I chose nine 48V stacks, which is 459Ah and 21.6kWh @ 20 hours.

MPPT and Inverter First design
I first had 4 strings of 4 panels, which has a VOC of 158V. The Midnight Solar Classic 150 MPPT can actually handle this (using a proprietary circuit), and puts out 80A max.

I chose the Xantrex XW4548, because it does 120V/240V, 48V batteries, and can handle 4500W to the load. It has an optional controller to start the Generac.

MPPT and Inverter Second design
I was concerned about intermittent shading from trees (that I still need to assess on site), and I noticed the Xantrex MPPT has a "shading algorithm." This was when I became aware of what happens if shading of only one panel occurs. I looked briefly at micro inverters, but decided against them. The Xantrex XW MPPT 80 600 can also put out 80A, and I thought I would lower the amperage (and wire size) by only using 2 strings of 8 panels. The open circuit voltage of 8 is 315V which is no problem.

I liked the idea of using Xantrex for both the MPPT and Inverter, and getting an SCP to control/monitor both. There is also box that interfaces to a LAN available. However, it isn't clear that much benefit exists from these two "working together" for my off grid application.

Comments?
It was just tonight that I discovered the AHI battery hasn't taken off like I assumed. Aquion did give me a quote including shipping, so I assume they are making them and someone is buying. Do you seasoned people have comments on my attempt at a design? I was thinking that worse case I would like the generator running maybe two hours a day in winter. What replacement battery type would you suggest, and what capacity in Ah? Any and all comments are welcome. I had a local installer who is certified that was supposed to help, but he bagged on me, saying "off grid is difficult - all the systems I have done are on grid." The shading issue is bothersome - is there any benefit of four series strings vs two, or the opposite, or are things equally as bad as far as power reduction due to the MPPT if one panel becomes shaded?

I attached a cartoon showing the design as I understand it.

Thanks for any advice,
Don

[Mike90250]

Aquion AHI Often, early adopters pay high prices, and have deficient products (Oh, it does that for you ? Gee, let us think about it)

Parallel strings of ANY battery - not a good plan for starting out. please read the writeup here:

http://www.smartgauge.co.uk/batt_con.html

My suggestion - try a "beginner set" of 6v, 200ah golf cart batteries. If one string will work, great, if you need 2 strings, you can wire on the diagonal, and that's pretty good. Often, while systems are getting set and tuned, the battery bank gets murdered. Cheap golf cart batteries are almost disposable. In 2 years, see if AHI is still in business. No water, no measuring S.G., no EQ is a strong selling point. Recharging below frost point might be a problem for AHI, but not lead acid.
Since you are planning on the back-up genset, you don't need 3 or 5 days of storage, just 2 days should do it. And have a backup genset plan too.

Inverter - check out the price difference between the 6048 vs 4548 and the internal idle consumption. For a few $ more, you get a lot more inverter. which helps when you are charging, you have more charger too (needs 240v generator).

Classic MPPT controller. Panel shade.
Xantrex controllers When you check the price of the XW-600 controller, you will need to be sitting down. Their 150V string controller is more reasonable. MPPT algorithm. The differences are well displayed in the lab, in real life, less so.

I've found in cloudy weather, my classic using the "Legacy" choice of algorithm works much better than the newer "Solar" algorithm. At least I can chose between 4 modes in the classic.

Shading, arrange your array so that 4 panels in 1 string get shade, not 1 panel in each of 4 strings. Shade in PV reduces the AMPS, so if one panel in a string is shaded, it chokes the whole string. Internal panel bypass diodes help, but they can and do burn out after years of abuse (shade) forcing a whole string of current through the diode.
Classic Controller HyperVOC - the controller senses it, and stays in standby UNTIL the panels heat up and the VOC drops. On a frosty morning, with thin sunlight, that can be a while.

Xantrex Control System: SCP or Combox. I have both. I recommend the Combox. I recommend the Combox. I recommend the Combox. (did I say that enough)

Lastly - propane tank. You know propane is a pressurized gas, and that under pressure it becomes a liquid. And to re-vaporize, it has to "boil" from the liquid state to vapor state. Colder that Frost, there may not be enough heat in the tank to continue to supply vapor to the engine. Your gas stove will just have a little smaller flame, but the engine will not run. Check with your propane supplier about this in your area. You may have to warm up 20# cylinders on your wood stove, then carry them out to the genset ! YOW !

And then there is the balance of parts, DC breakers, boxes, wires, lugs, hydraulic crimper, fuses...... Midnight makes a GREAT ePanel for the XW inverters. And combiner boxes, all the goodies.

PS Use breakers in the combiner boxes, not fuses. You can switch off a string at a time for troubleshooting, but not with a fuse.

[Living Large]

Mike - I appreciate your reply. I learned in the past few days there are some key principals I don't understand and some design details I missed so I am backtracking and trying to learn each step. I was premature in posting this. However, you posted some great tips. The 4548 is discontinued, and replaced by the 5548, I believe. Also - the system's battery bank was undersized - I discovered that for my needs, I may truly need the typical 3 day backup capacity, whereas I was thinking I didn't. I don't have any real large loads like Chris had for his "peaking" setup described elsewhere here. My loads are spread out pretty evenly. The biggy would be the well pump coming on - but I don't want to start a generator just for that.

I appreciate the tip about shading - one string vs 1 panel in each string makes a lot of sense. I'm hoping to not have that situation - I checked the horizon at each azimuth for 9am to 3pm on Dec 21, and I am clear. This was the concern, since there are trees in the distance. However, I will keep your tip in mind as I get further along and actually determine the configuration.

That said, I gave up on AHI a few days ago, because they they can't be used to their capacity as pointed out by SunKing due to the voltage range not aligning with off the shelf equipment. They also are expensive, which I knew going in. Maybe this will change in the future. Currently I am planning for AGM, but there is a chance I will come around to FLA, which has been encouraged by members here. There is a lot to know about these systems and take into consideration.

I have seen the problems with paralleling battery strings. I believe the wisdom is not more than 3. One would be ideal, but right now I need two to create 830Ah with 6V batteries. I saw 2V batteries - but the thought of even more batteries gave me some pause. Maybe I should consider getting past that.

Thanks again. I posted this prematurely, but you gave me some good material.
Don

[Living Large]

Mike - I'm curious of whether the charge controller and inverter/charger have to be in sync. My understanding is the Xantrex controller and inverter/chargers let each other know which state of charge the batteries are in. What happens when you have a MS Classic (which I am also considering) controller and an XW inverter/charger? The ePanel seems to be designed to team the MS controllers with the Xantrex equipment, so I assume these play well together?

Thanks,
Don

[Sunking]

The Aquion batteries are not going to work. Their operating voltages are outside the operational limits of your equipment. In addition to their queer operational voltages they are very high resistance batteries which means they have to be over sized significantly compared to lead acid batteries.

[polestar]

I take it the shade trees are not on your property else you would just get out the chainsaw. North of I-90 is shade really desirable?
--

[Living Large]

Regarding the two previous replies - I am not using AHI batteries (see post #3 above), and no I can't cut down every tree in sight. As it is, I will be removing more than I am supposed to by local deed restrictions. Back to the topic -

Mike - I'm curious of whether the charge controller and inverter/charger have to be in sync or on the same page. My understanding is the Xantrex controller and inverter/chargers let each other know which state of charge the batteries are in. What happens when you have a MS Classic (which I am also considering) controller and an XW inverter/charger? The ePanel seems to be designed to team the MS controllers with the Xantrex equipment, so I assume these play well together?

In other words, to what extent do the controller and charger have to be on the same page, and does the MS and Xantrex setup permit anything more than if I just went and got one from Company A and the other from Company B?

Thanks,
Don

[Amy@altE]

Many people use mismatched charge controller and inverter/charger. For example, Magnum doesn't make a charge controller, so most people these days use the Midnite Classic with it. The advantage to using all the same brand is the reporting, using the Combox is nice to have 1 screen showing both the charge controller and the inverter, but the Midnite Classic also has a network interface, so worst case is you have to go to 2 screens to view the system. I do tend to design with an all XW system, but not always, depending on customer's needs.

The ePanel breaker box doesn't care who's equipment you use, just that you size the breakers right.

For a 150V charge controller in the Adirondacks, you want to do strings of 3, not 4. 4 is too high for the cold winter.

Amy
Your Solar Queen

[Living Large]

Many people use mismatched charge controller and inverter/charger. For example, Magnum doesn't make a charge controller, so most people these days use the Midnite Classic with it. The advantage to using all the same brand is the reporting, using the Combox is nice to have 1 screen showing both the charge controller and the inverter, but the Midnite Classic also has a network interface, so worst case is you have to go to 2 screens to view the system. I do tend to design with an all XW system, but not always, depending on customer's needs.

The ePanel breaker box doesn't care who's equipment you use, just that you size the breakers right.

For a 150V charge controller in the Adirondacks, you want to do strings of 3, not 4. 4 is too high for the cold winter.

Amy
Your Solar Queen

Thanks. The Classic has what they call "HyperVOC", which advertises that it allows up to somewhere around 180 or 190V (can't remember). The MS string sizing tool told me my cold OC voltage for 4 in a string was "MARGINAL", but acceptable. Seems like it makes sense, now that you brought this up, to not design around a single manufacturer, and anticipate 10 years down the line when the controller needs replacement but the panels do not. On the other hand, one could always step up to a higher voltage controller. Or would you as an installer simply advise against trusting the headroom that the MS advertises?

On the compatibility matter, would it not be handy for both the controller and inverter know, in a system with charging from both PV and the inverter, what mode of charge the batteries are it? I guess they might often agree, but there would be cases when they don't. Maybe it isn't that big a deal. A Xantrex CC and inverter share parameter settings with each other, and a Schneider rep told me they do coordinate to an extent on charge mode. Not sure if he was right about the second part because he wasn't very descriptive.

[Amy@altE]

The Classic HyperVOC feature is designed to turn the charge controller off to protect itself when the voltage gets too high. Why would you want to design the system to turn off every sunny winter day when you need the power the most? Trust me on this one, do strings of 3. If you wanted to do higher, then yes, you can get the Classic 200 or 250, but then the surrounding BoS equipment also has to be rated for higher than 150V, so, while doable, it does add complication to the design. Not saying not to do it, but I prefer simple designs, as long as the designs do the trick. If you have a long run from the panels to the charge controller, and therefore the size of the wire is a concern, absolutely use a higher voltage charge controller.

I don't do many installs myself, just for friends and family, we design and sell the equipment here, not install. Most of the designs I do for XW systems have the XW charge controller for the reasons listed.

Amy

[Living Large]

The Classic HyperVOC feature is designed to turn the charge controller off to protect itself when the voltage gets too high. Why would you want to design the system to turn off every sunny winter day when you need the power the most? Trust me on this one, do strings of 3. {snip}

Amy

I trust your experience, but I don't understand why on this sunny day it would shut down, since voltage decreases with increasing temperature, and I checked the voltage at -20 deg C to make sure it isn't higher that the 198V that is the limit. 4 panels is 179V at -20C. Did I miss something?

[Sunking]

I trust your experience, but I don't understand why on this sunny day it would shut down, since voltage decreases with increasing temperature, and I checked the voltage at -20 deg C to make sure it isn't higher that the 198V that is the limit. 4 panels is 179V at -20C. Did I miss something?

Yep you are missing something. Cold Winter morning when the sun rises. VOC will be at their highest voltages of the year.

[Amy@altE]

I think you entered something wrong in the Midnite string sizer. I just did 4 string of 4, and the Classic 150 charge controller will always be in HyperVOC, regardless of the temperature. The Classic 200 can handle the voltage, but it will be overpowered and will likely clip the output. So again, why design with limitations? If you look at 5 strings of 3, the 150 will be happy all the time. Yes, you are 1 panel less than you want, but with a system that will work at all temperatures.

Amy

[Living Large]

I think you entered something wrong in the Midnite string sizer. I just did 4 string of 4, and the Classic 150 charge controller will always be in HyperVOC, regardless of the temperature. The Classic 200 can handle the voltage, but it will be overpowered and will likely clip the output. So again, why design with limitations? If you look at 5 strings of 3, the 150 will be happy all the time. Yes, you are 1 panel less than you want, but with a system that will work at all temperatures.

Amy

I was misinterpreting HyperVOC, and also the operation of going between open circuit and with load as charging occurs and stops, and thus voltage jumping up and down. I get it now. I also didn't understand the significance of being in HyperVOC "Always", which you are correct is the case. I got it now - and the 3 vs 4 discussion is academic. 4 simply won't work. Thanks for helping me understand.
.

[Living Large]

Yep you are missing something. Cold Winter morning when the sun rises. VOC will be at their highest voltages of the year.

I didn't miss that the VOC will be the highest on that cold winter morning.

I checked the voltage at -20 deg C to make sure it isn't higher that the 198V that is the limit. 4 panels is 179V at -20C

I got that part. But I didn't understand that evidently I won't get any power until the temperature rises enough that VOC goes under 150V - which if true is obviously an unacceptable situation - and I now assume is the case based on Amy's reply. I'm a beginner here with no experience with solar hardware.