Building an Off-Grid house in Alaska, now what?

aksalaaksala Registered Users Posts: 37 ✭✭
 So I'm trying to do my homework. I have no previous experience with solar outside of a solar powered gate opener. I'm here to learn, ask questions and hopefully figure some things out. I may very well utilize the expertise of a system installer but I still like to know what is being specified makes sense and I'll also likely be the one maintaining things later. So it pays to know what's going on.

 The home will be in Alaska. I have insolar data for the location is about 3.3 average for the year. I'm more interested in Mar - Sep, when I can get 4.1, 5.5, 5.2, 5.0, 4.8, 4.2 and 3.3 for an average 4.5. I can clear all the land I need to clear the panels from any shading. I intend to ground mount the panels. 

 In designing it's a given that in the winter, you're just going to be paying a fuel bill for diesel instead of paying the utility for power. So the goal is to get what I can from the solar array within a reasonable budget but to also size the battery bank so that I can run the house but only have to run the genset for a few hours a day, keeping my fuel bill to at least resemble the low end of what I'd be paying the utility if I could afford the $90,000.00 to hook up! 

 I started to tackle this by looking at my current usage, but quickly determined that using my current grid tied home is not a good comparison for what I'll be using in the new home. So started by plugging things into a watt meter and putting things into a spreadsheet. This is when I started to get worried because I'm not building a small cabin in the woods with the intent to live an austere off-grid lifestyle. No, I'm trying to live much as we do now on the grid and continue to support a kennel of dogs, which means I have a couple of extra refrigerators and a couple of large freezers to go along with everything else. 

 While I intend to heat the place primarily by wood, I'll still have a fuel oil fired on demand hot water heater and because the home will be 4 or 5 star energy efficient, an HRV system which tends to run fans on an almost continuous basis and adds another 1.5kWh/day to the mix. So my current calculations come out to requiring approx. 18 kWh/day or about 540 kWh/month. And this figure is likely more a winter time figure with lights burning longer than during the summer when it's light all the time and we're busy outdoors a lot more of the time. I haven't calculated the summer utilization but I'd guess maybe 15 kWh/day might be more like it from May - Aug. 

 So with roughly 18 kWh/day as my target, factoring in inverter loss I came out to roughly 547Ah needed per day off the battery bank at a system voltage of 48V. At only one day of autonomy and a target of 30% DOD and accounting for temperature offsets I came up with another rough calculation of 1824Ah needed in the bank. 

 For the battery bank I looked to Trojan SIND line, as I needed the capacity and I liked the projected cycle life of their industrial line. At this point I'm looking at the SIND 06 1225, 942aH @C20 with two strings of 8 for 1884aH and on paper, a projected lifespan well beyond 10 years if properly maintained and only run to 30%. 

 Next I looked at the associated CC's etc.. I was looking at the Conext gear, an XW 6848, an MPPT80 and the associated auto gen start and commbox and distribution panels. 

 This is where things start to get confusing for me. Like charging the battery bank. I see figures like .10 or .13 of C20, but if I'm wired as one large bank at 1884 even 10% is over the max the 6848 can deliver which is 140A. And if I need to run an equalization, the limit is 64V, but with temp offsets per Trojan I would need at least 67 to 69V for an equalization charge. And it looks as though even if I double up the CC's, you have to configure the max charge rate to 1/2 of target so they balance. So then I still get only 140A from two CC's and it doesn't help my target for equalization. These are the questions I'm starting to wrestle with. I think I understand that at if max charge rate is 100% and the CC delivers 140A at the recommended 59.3V +/- for temp, it would finish the bulk charge to replace say 570aH in about 4 hours. Then what? Then absorb charge for how long? I'm struggling to determine just how long the generator would have to run to get the battery bank back up to 100% SOC. 

 Then I get to PV's. What do I need to be able to charge up that bank every day? A pair of MPPT80's tied to two arrays so I can approach max current into the battery bank? 

 Well, this is the start of my questions and I'm sure I'll have more as I continue to learn the pieces of this puzzle. 


  • EstragonEstragon Registered Users Posts: 3,128 ✭✭✭✭✭
    Welcome to the forum. You're on the right track taking the time to understand what you're getting into, even if you end up getting a pro to install. The trade-offs are highly personal choices, and only you should be making them.

    18kw/day is on the large size for an off-grid power budget, but doable. Conservation is generally cheaper than generation, but those are the choices only you can make.

    At high latitudes, using a generator in winter is the only practical alternative. To make most efficient use of the generator, you would typically want to run the genny in the morning for some or all of a bulk (max current) phase charge, which will get the bank up to ~80-85% state of charge. Depending on the battery, you can charge at rate of maybe C/5, so ~400a in your case. 400a x 60v charging = 24kw genset, possibly a bit more to handle loads while charging. Many generators run best at around 75% or so of capacity, so buying too much capacity isn't a good idea. How often and how long you run the generator will obviously depend on winter weather. A couple of hours per day is a reasonable guess to bulk charge, and let solar do an absorb (constant voltage, declining current) charge to the extent possible. A full absorb cycle will take ~3hrs. You don't have to get a full absorb everday, but should at least a couple times per week.

    I would also consider ways of using waste heat from the genny. About 1/3 of the fuel heat goes out the exhaust, and another ~1/3 is radiated. Some portion of that is recoverable.

    For batteries in this size, you may want to consider single 1900ah 2v cells instead of multiple strings of higher voltage multi-cell ones. More cells = higher probability of failure (albeit with a degree of redundancy), and means more cells to check SG and water. A bank of 2v is 24 cells to check, a bank of 6v is 3x that.

    For the charge controllers, I don't understand the need to limit current of multiple units in parallel. Running controllers flat out for hours will shorten their life, but planning for 50% of capacity seems a bit extreme to me, whether it's one controller, or dozens. Using a charge controller rated at 100a, for example, you could run two at 75% for ~150a, or three for 225a. If at altitude, you may need to derate. The controller should be capable of outputting temp compensated EQ voltage. Before ruling out an otherwise suitable controller, you could check with the manufacturer to see if the voltage limit is software or hardware limited. If software, it may be possible to override the limit. My MN classics, for example, handle up to 72v nominal battery bank, so any EQ limit would be in software.

    In sizing the array, figure on getting about 75% of STC rated output most of the year unless you're at high altitude. In winter, it may be 50% or less with the low sun. 200a ÷.75 x 60v = 16kw.

    You may want to consider locating the bank in a way that limits extreme temp variation (eg underground vault or insulated crawlspace). Doing so would allow lower voltage charging in extreme cold, and reduce the loss of capacity. In my climate it can get to -40 and stay there for weeks at a time. In an insulated but unheated crawlspace, the bank doesn't get much colder than ~ -15C.

    Anyway, a few thoughts to get started.
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • mike95490mike95490 Solar Expert Posts: 8,073 ✭✭✭✭✭
    Invest in one 10kw genset, as your backup, and in winter, use a 1 or 2kw inverter genset running 24/7 (except for oil changes)
    you will kill the little inverter genset - just wearing it out,  but it can save size on your giant battery bank, and reduce the cycle count

    Re-think the HRV, they need lots of TLC, to keep them clean and sanitary.  Airflow, moisture, dust = mold
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    gen: ,

  • mcgivormcgivor Solar Expert Posts: 2,512 ✭✭✭✭✭
    edited March 2018 #4
    A large challenge for a first time installation, given environmental conditions and geographic location, the winters will be particularly difficult. The small generator suggestion would be well worth considering, spending on fuel to conserve the battery, a calculation of fuel costs, generator replacement, oil changes every week?, versus battery replacement would need to be done to justify such a move. Keeping batteries at least as close to the optimum 25°C to maintain capacity would be preferable to operating in freezing conditions, this would allow somewhat normal charge voltage requirements, so location would be critical. Diesel and cold are not good bedfellows, there's that to consider, all part of the master plan I'm sure. Hats off to you for thinking and planning that's the best move, putting faith in an installer who knows you have no knowledge is a dangerous thing, unless of course they are honest and have your best interests in mind. 
    1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery bank 
    900W  3 × 300W No name brand Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah FLA 24V nominal as a backup system. 
    5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergencies and welding.
  • Dave AngeliniDave Angelini Solar Expert Posts: 4,199 ✭✭✭✭✭
    edited March 2018 #5
    When I looked, that battery EQ's at 65V. Why did they tell you 69V? There is firmware to run the Schneider MPPT-80-600V up to 69V.

    I am with Mike on the HRV. For offgrid you want simple. Less is more.

    Using Blue Ion for my clients on big systems like this. Take a look.

    If I were you I would use the same brand for the whole power system, In doing this you will be able to monitor easily from anywhere and especially at home. The screen shot above does not have the generator in it but it can be added.

    A client up your way has huge ground mount solar array 15KW that he shuts down 3/4 for summer. There are lots of ways to minimize the generator run times. I think your  KWH number is high for offgrid and I know a 6848+ will easily do this unless you have a machine shop.

    Good luck!

    "we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
    E-mail [email protected]

  • aksalaaksala Registered Users Posts: 37 ✭✭
    @Dave Angelini - 64.8 for EQ according to Trojan on that batt, but nominal winter temp compensation even in an insulated moderately heated space is likely to be 20C cooler than the baseline 25C. In looking at a 2V 24 cell bank I'd still be adding 2.4VDC to both Absorb and EQ charging wouldn't I? I haven't talked to a tech rep but nothing in their tables indicate the EQ voltage is exempt from temperature. 

    I also see that Trojan recommends the Bulk setting on the CC should be set to the same voltage as the Absorb value. I'm not sure why that is but I'm fine with that. 

    I am considering changing to 2V batteries, using the SIND 02 2450, it's not much of a price delta and while it's more batteries it does at least simplify the bank with only one string. 

    I realize the KWh is high but it is what it is. One large fridge, two smaller fridge/freezers, two large chest freezers etc.. My calculations are also on the high end, some items are not used every day and so 18kWh is more the worst case scenario but that's how I like to engineer things. Still, 15kWh is quite likely for daily use in the winter. 

    I don't find the HRV system that big of a deal and certainly can help with mold and moisture in the home when things are buttoned up tight in the winter. It's not a huge power drain and I'd likely put it on a controller to time its function. 
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