Slow start on an off-grid cabin

M1A1M1A1 Registered Users Posts: 6
Hi folks! Sorry for the novel ahead of time.

My father just purchased 40 acres of mostly timber about 2 hours North of the Twin Cities and would like to go solar for the sparse electricity needs. We will be using it mostly as a hunting & weekend property from ~April through December, with maybe biweekly trips during the other months. There is an elevated 17'x24' log cabin, roof facing East/West, with basic amenities including a wood fired stove and a 'Biolet' biological toilet (no complaints so far). There are no utilities of any kind connected and no plans to connect any due to the cost.

I do have a solid plan for the solar electric system, but am concerned now with how any battery setup will do in an unheated, unattended cabin. I have read the Battery FAQ and know we'll be seeing a 20 to 50% drop in capacity for our winter weather temperatures. I was initially going to use 2 Trojan T105 in series for the battery bank but it appears that an AGM battery will do better given the temperature, maintenance, and usage requirements. Is this a safe assumption?

My other question concerns a solar thermal system for heating the cabin, to help keep it reasonable for the batteries as well as for when we drop in. We have plenty of space available on the South side of the cabin for a few large thermosiphon panels, but my father would rather not put holes in the walls (yet). Will it be worth it to build one or two 3'x6' panels, even if I have to duct everything through half of a 24" square window? I am looking at adding some thermal mass to these panels and/or inside as well, otherwise I think the nighttime temps will negate any daytime gains?


Cheers for the great forum,
Marty

Comments

  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Slow start on an off-grid cabin

    most beginners go for the golf cart batteries even if they think agms may do better because of cost differentials and mistakes they may make. either will probably work as long as they can be kept up with a full charge as a not charged battery has more water in it and can freeze easier. a diy wooden battery box with some foam insulation could help keep it from freezing too, but give it a bit of room at the top and no metal foil facing the batteries that can short things out, even accidentally. no matter what you elect for the batteries the controller you pick must have a battery temperature sensor to compensate for the wild fluctuations. remember that centered on late december you will have the least amount of solar illumination time and intensity, but on the plus side the snow will reflect light to increase output and the cold temps add just a tad more power to be had from the pvs. also you will need to have the pvs face south at a good angle for both summer and winter. hope you cleared enough trees too.
    the thermal solar may help a bit to warm up the place, but it is a total waste if you don't have an appreciable amount of insulation in the walls and ceilings and even the floor will lose much of the interior heat to the ground beneath it(rugs?). passive solar through southern exposed windows will do good, but do you even have windows and are they at least double pane? this encompasses allot with many ins and outs as to what you can or should do and will it be worth it to you for a cabin?
  • icarusicarus Solar Expert Posts: 5,433 ✭✭✭✭
    Re: Slow start on an off-grid cabin

    Your batteries will be fine, (assuming that they can be kept at float via solar or generator). I have a set that sits ~10 months a year and they are essentially as good as new.

    While you are right that battery capacity goes down with temp (fully recovering as the temp rises) battery life goes UP with colder temps. As long as the electrolite is not allowed to freeze, and a fully charged battery won't freeze until way below -40f, and even a 50% charged battery won't freeze until ~-10f, you battery life will be great.


    If you enclose your battery(ies) in a stryo-foam box, or even a ice chest, they will generate considerable heat from charging/discharging, further preventing freezing. (Proper venting is essential once you use the batteries and bulk charge them, as they will off gas).

    I would not worry on whit about heating the cabin for the sake of the batteries. Assuming you properly drain your water, I would be way more inclined to shutter the place to keep out the bears (and the humans!) rather than leaving the sun to try to heat it. Even a shuttered building will be ~10-15F warmer inside over the course of the winter (depending on locale of course!), just by picking up and holding some solar heat through the walls. I have a Max/Min thermometer inside and outside. The outside one goes to -40--50f most every winter, but the inside on never goes much below -25. Doesn't seem like much, but believe me it is.

    I also would (if it were me!) [email protected]*t can the composting toilet in favour of a good outhouse. As I have pointed out here in a couple of threads, composting toilets NEED outside heat, especially in the fall, spring and winter to work at all well. This can be a BIG tax on any Pv system,,,5-1500 watts, 12 hours a day! I know that many are squeemish about outhouses, but if they are well designed AND well taken care of, they are perfect, especially in a place that gets only occasional use. (there are all kinds of cold weather tricks to make the trip to the outhouse less of an issue. Personally, I have,,, well I guess I shouldn't go there!)

    I also agree with Neil that Golf cart batteries are the way to go, both due to initial cost, and the fact that they will take some considerable abuse and still survive. Think of them as training wheels.

    A couple of other notes, as you design your system, keep these two things in mind. Most people over estimate their solar capability, and underestimate their loads. With this in mind, assume that your loads WILL grow with time, and design accordingly. Plan as much of an expandable system as you can, especially charge controllers. Inverters can be stacked, panels can be added, even batteries can be added (if they are cheap enough, see the battery faq) but charge controllers are the rub. Within reason, buy the biggest one you could potentially need in 5 years, rather than buy three in between.

    Good luck and welcome to the forum.

    Getting cold here, fog on the lake during the day today, a sure sign of fall. Frost warnings for the next few nights.

    Tony
  • Ralph DayRalph Day Solar Expert Posts: 952 ✭✭✭✭
    Re: Slow start on an off-grid cabin

    I agree with Tony, can the composting toilet.

    I had one for 16 years, 11 on the grid and 5 off. On grid ran just the fan no heater, off grid i removed the squirell cage fan and put in a computer fan. It worked, but not well.

    It never really composted, ever. What I had was an outhouse in the basement. Recently I got infested with some kind of fly, bigger than a fruit fly, but not house fly sized. Could not get rid of them with any poison, organic or not. Climate change...for almost 15 years no problem, then these flies from another world arrived (South Carolina?)

    The composter sits outside in my wood shed until it's place as a flower planter is decided. Replaced it with a SaniCompact 48 macerater type toilet. Fingers crossed.

    Batteries in the cold...just remember to temperature compensate the sg readings if FLA batteries are used. A search on the forum will find you the algorithm.

    Ralph
  • M1A1M1A1 Registered Users Posts: 6
    Re: Slow start on an off-grid cabin

    Thanks for the replies. We went up on Friday, only the second time I've ever been up there, and things are a little different from what I was told.

    The Biolet is broken in a couple different ways, I'm pretty sure. The previous owner had installed a 5W solar battery maintainer but was using it to power the fan (25W), mixer (25W), and heater (225W) on the Biolet. He advised my Dad to only turn the inverter on when you're there, but even those shorts spurts were apparently enough to drain the battery down to a damaging level. So the battery is not holding a charge, which means the fan/mixer/heater aren't working at all... Wonderful aroma. :roll:

    There is only insulation under the floor boards and metal roof; the walls are just 6 inch thick laminated Pine, I believe. So heating would indeed be a waste of time/money/energy. But we do have a few large styrofoam boxes with lids, so we can keep the batteries insulated. I will attempt to have the Biolet removed and use that vent for the battery box since it is a nice straight 2" pipe.

    As for the solar electric system, since there isn't electrical service at the cabin, we don't know what our usages are. The largest reason for a PV system was to provide lighting 'at the flip of a switch' and this is about all I'm counting on right now. I know our demands will increase, but hopefully the equipment I've chosen will allow us to make additions easily:

    - Sanyo HIP200BA3 - I found a local company getting out of the solar business and picked up a panel for about 40% of retail. I've read the perceived problems with hybrid panels and would have rather bought a nice Sharp/Evergreen/etc panel, but this was a great deal and it'll be a good panel to start with.
    - Outback FM60 MPPT Charger - I might change this to a Xantrex XW60 having read the many comparisons and experiences on here. Either unit will allow us plenty of upgrade room in the future.
    - 2x Trojan T105 - Can buy them locally, appear to be good batteries to start off with.

    Thanks again for all the comments.


    Marty
  • help!help! Registered Users Posts: 9
    Re: Slow start on an off-grid cabin

    Marty,

    I can't help you with your solar questions, but I do have some suggestions for your off-grid cabin.

    Buy a few good solar lanterns for light (Harbor Freight puts them on sale, and has 20% off coupons every week. You can find them at http://forums.slickdeals.net/showthread.php?sduid=0&t=1276399 ). They also sell propane cooktops and unvented propane radiant space heaters with thermostat controls. Propane refrigerators, though expensive, are available online. A 40lb bottle of propane can last an amazingly long time.

    Or you might replace the wood-burning heater with one that doubles as a stovetop and oven (like they used 150 years ago), that also gives off heat.

    Does the cabin have a water well, or do you have to bring water each time you go?

    If you really do plan on spending so many weekends there, you need a water well and septic tank more than you need a solar system. Electricity can come from a generator and heat from propane or woodburning, but nothing replaces sanitation, as you've found with the compost toilet. (I believe Coleman still makes a port-a-potty, for sale at camping supplies or on Craigs List, but I've never used one. That might be a short-term solution for you.)

    You don't have to have complete plumbing -- just the septic. You can use camp shower bags, warmed in the sun, for hot water and hung in a shower stall to bathe, wash dishes, and brush your teeth.

    We've been doing something similar, but there was no cabin on the land my husband bought.

    He was gung-ho to build a cabin, but I convinced him that a used RV would be instant shelter, complete with a working kitchen and bathroom, once the well and septic were installed on the land, and he could live in it while building the cabin. He's been using a generator for the RV battery charging, pumping the well, and powering his tools.

    Now I'm here for information on putting a small solar system in. Good luck to you and your father, and I hope you enjoy the cabin.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Slow start on an off-grid cabin

    Let's see.

    Cost of the self-installed septic system I just put in last year: $8,000
    For occasional use it makes no sense whatsoever. There's this thing called an "outhouse" - look into it. I re-dug ours after 20 years just because I had the machine there. (The ones before that were dug by hand. This place has been in use for 50+ years - although not by us.)

    Cost of pumping water from well and related plumbing .... ???? Buckets. Used 'em for years. Got lake? Got bucket? Got water!

    Lights can be had any number of ways. 12V system w/LED bulbs is a good place to start. You can add an inverter later, and as long as you don't go over 2kW power requirement you can keep the 12V set-up.

    The Outback controller is a good choice for its flexibility. You can add panels and/or increase system voltage yet keep the same controller. I'm not familiar with the Xantrex, but I'll bet it's the same in that respect.

    The Trojans are a good choice: reliable and value for money. With good sun exposure that 200W panel should keep them up (you'd need a minimum charge rate of about 12 Amps as the 105's are 225 Amp/hrs).

    As for solar heating ... How much of a Southern exposure does it have? Get some second-hand double paned windows and let the sunshine in.
  • M1A1M1A1 Registered Users Posts: 6
    Re: Slow start on an off-grid cabin

    We haul up whatever water we plan to use plus some extra to store. Water is heated either via propane grill, Coleman stove, or on the wood burning stove when needed. The roof is guttered so we could potentially catch rain water, and there is a reasonably fast flowing river just a few hundred feet from the cabin. Both would require some storage tank, probably inside so it wouldn't freeze.

    Southern exposure: One 17' by 10' wall with a single little window. I would love to add windows (and a solar heating panel) but from what I've seen, the cabin won't retain heat very well with the lack of insulation. And that is one aspect we really can't do anything about given the construction. We'll see how bad it actually is in a few months when the temps drop.

    There are a few gas lanterns up there already, the LED lighting will be in addition to these not in place of them. There will be three 5W arrays in the main room (~300 lumens each), and one 5W array in the bedroom and utility room. The house has had wire run for 120VAC but no connections were ever made, so I'll be using this wiring for the LEDs since it is 12 gauge and I won't be using more than about 1 amp.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Slow start on an off-grid cabin

    Rigid insulation can be added inside or out and then covered over. The pink type is about R5 per inch, and if you can fin it there's polyurethane sheets @ R7 per inch.

    As for heat loss: roof first, always. Getting it above R19 is the first goal: the 'return on investment" tends to diminish after that, although it is much more profitable in colder climes (like, say, The Cariboo). :D

    Log cabins are built without wall insulation because the walls are 12" thick and present an R value nearly identical to a standard 2x4 insulated wall. So 6" of pine should be fairly close to R6.

    Floor insulation is least important. It really only comes into play when heating a place in severe cold (providing there's no loss as per below). Just make sure the foundation is 'skirted' so air can't draft underneath.

    Next to the roof, look for leaks and drafts. No matter how well insulated the rest of the place, one hole the wind can blow through will drop the temperature.
  • M1A1M1A1 Registered Users Posts: 6
    Re: Slow start on an off-grid cabin
    ...
    As for heat loss: roof first, always. Getting it above R19 is the first goal: the 'return on investment" tends to diminish after that, although it is much more profitable in colder climes (like, say, The Cariboo). :D

    Log cabins are built without wall insulation because the walls are 12" thick and present an R value nearly identical to a standard 2x4 insulated wall. So 6" of pine should be fairly close to R6.

    Floor insulation is least important. It really only comes into play when heating a place in severe cold (providing there's no loss as per below). Just make sure the foundation is 'skirted' so air can't draft underneath.

    Next to the roof, look for leaks and drafts. No matter how well insulated the rest of the place, one hole the wind can blow through will drop the temperature.

    We have to take care of some rather shoddy construction first before considering any more insulation. There are a number of joints through which you can see daylight, including a few roof joists I can easily get a finger in to. In addition, the cabin is raised five feet and is not skirted at all, so the challenges for retaining heat are stacking up quite nicely.

    In a different direction, I thought I'd post my crude wiring diagram and see if anyone has any recommendations. I think I covered all my bases in regards to NEC code, even though we're off grid and don't need inspection, but that is a long document and I have a short attention span. So, here it is:

    NMK-SolarWiringDiagram.jpg

    The only item purchased so far is the Sanyo panel, and I changed my mind on the Outback I had listed earlier after some of the more recent discussions here about that product. We'll probably order the Xantrex XW60 and pick up the batteries and enclosures some time this week. I hope to have a draft install done by next week so I can order or pick up electrical supplies for final installation.


    Cheers,
    Marty
  • mike95490mike95490 Solar Expert Posts: 9,359 ✭✭✭✭✭
    Re: Slow start on an off-grid cabin
    M1A1 wrote: »
    even though we're off grid and don't need inspection,

    Not always true, in Mendocino County, CA, you need permit if:
    over 24V
    or
    more than 50 Watts

    Of course, they have to catch you, that's the "rules".
    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

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • M1A1M1A1 Registered Users Posts: 6
    Re: Slow start on an off-grid cabin

    I'm sure that is true.
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