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