6 ms·
> Still not sure how I’ll ultimately get around this. If it's portable, your option works well. For year round off grid use, there's something to be said for
by Syonyk 5y ago
> Still not sure how I’ll ultimately get around this.
If it's portable, your option works well.
For year round off grid use, there's something to be said for sealed lead acid, or flooded lead acid - they don't care about being cycled while cold (capacity is down and the voltage sags a good bit), and as long as they don't get too deeply discharged, they don't freeze.
A single panel, facing south, absolutely vertical, will shed snow and if you have no other loads, is usually enough to offset controller loads and self discharge over time to keep the batteries fully charged (and therefore not freezing) over winter.
There's no way you'll generate enough heat on electricity to keep a battery bank (or cabin) warm in the winter most places. I've got 5kW of panel on my office, and still can't manage that for a small, well insulated shed.
- chana_masala 5y agoThanks for your post. I'm reading through some of your blog posts now - really great!
- devwastaken 5y agoHypothetically, for the lithium batteries, what if you made a big stainless steel vacuum wall insulated enclosure? Or just a metal box with r-60 value of foam around it? The amount of heat needing to be generated can then be calculated based on the average temp over a longer period of time, and those batteries I don't imagine have to be "warm" exactly.
- diamondlovesyou 5y agoWhat about summer? Those batteries also need to stay cool...
- londons_explore 5y agoA $2 circulation pump and an outdoor radiator... It's good practice to snake a water pipe around every lithium cell anyway in big packs, because otherwise one bad cell can self-heat to the point of catching fire, even while other nearby cells are cold. The flowing water means the bad cell can't get to the point of fire.
- theshrike79 5y agoYou can just dig a big hole for them, the earth stays cool enough when you go a meter or two down. Unless you live in the desert, having the batteries too hot isn't as big of an issue as freezing them.
- Solar2 5y agoThey do not need to stay that cool at all. When used for solar, the C rate for charging usually never exceeds .2C for overpaneled systems. And usually never goes beyond .5C for discharge. So the heat generated is very small. Now when I am pushing 30-100C rates with a lithium polymer, cooling is an issue. this is why nissan leaf cells were recalled. The battery size was too small for the application, and they overheated which caused significant degradation rates. This is not an issue for solar. Just keep them at room temperature and they are fine.
- lazide 5y agoLithium batteries don’t need to be warm for discharge (generally and with an asterisk), it’s charging. Discharge while freezing causes slower chemical reactions (aka lower discharge current), but that is rarely a huge problem compared to the charging problem. Any charging around or below freezing causes pretty quick destruction of the cells as the chemistry switches to a more favorable destructive one way electroplating process instead of the desirable (and reversible) ion exchange process. A common engineering solution to this has started to be temperature dependent shunts that route charging energy to heating pads around the battery until the battery temp gets above a certain safe point. It adds complexity, cost, and adds several undesirable failure mode however, and Lithium battery cost is already a major factor in these systems, so if you’re in a situation where you can’t reliably temperature control your batteries (cold climate, non-conditioned battery space), lead acid is still a very viable and often desirable chemistry instead. Round trip efficiency isn’t great with them, and you have to way over provision to get good cell lifespan, but it works.
- WalterBright 5y agoSeems to me that if you want to solar heat a cabin, a solar panel is going around the horn. Design the cabin to be a solar collector, like a greenhouse.
- lazide 5y agoYou’d still need backup heat production (heat pump, or wood burning, or fossil fuel, or electrical off batteries) for the inevitable month long ‘gray and cold’ stretches, or you’re gonna freeze. Single purpose but not reliably there heat capture methods add cost and maintenance. It’s often more cost effective to have a general purpose method that while not as efficient in one case is generally more overall (cost) efficient.
- WalterBright 5y agoPassive solar heating should be very maintenance free. It's the point of it. Sure, a backup plan is a good idea. But it doesn't have to be very good or very efficient, as it will be rarely used.
- lazide 5y agoPretty sure you don’t live in Central Europe or areas like northern Michigan? There are weeks with zero direct sun, just low cloud cover, scattered snow, and cold cold cold. I was in Michigan once when Lake Superior froze over, shore to shore. Unless the place is so well insulated you can keep it warm purely with body heat, you need supplemental heating in climates like this. The sun is not always shining.
- WalterBright 5y agohttps://permies.com/t/42722/Passive-Solar-House-design-Cloudy https://permies.com/t/42722/Passive-Solar-House-design-Cloud...
- lazide 5y ago
- Epa095 5y agoIs 5kW not enough to heat the cabin? I guess it must be really cold where you are. Modern heat pumps should unproblematic handle -25°C (air to air), and provide 4-5kWh of heat for each kWh of power used.
- adrianN 5y agoWhich heat pump reaches that kind of efficiency when the temperature differential is so high?
- kwhitefoot 5y agoNone as far as I can tell. Some people I know here in Norway turn off the heat pump when the outside temperature is below -25°C, but those pumps were installed some years ago. New ones do have a significant gain at that temperature. For instance this Samsung Nordic Home: https://www.elkjop.no/product/hjem-og-husholdning/oppvarming/varmepumpe/215947/samsung-nordic-home-25-varmepumpe-nordichome25 https://www.elkjop.no/product/hjem-og-husholdning/oppvarming...
- Epa095 5y agoGood point, I did not think about the fact that the effeciency went down. But it seems to me (as a noob) that modern heat pumps specialising in low temperature conditions still can get around 2.5 at -25°, which gives you more than 10kWh of heating at 5kW, which is quite a lot.
- CorrectHorseBat 5y ago5kW of panels don't produce 5kW in winter, that's the issue I guess.
- i_am_proteus 5y agoYep. Unless you've got running water or a high cliffside for wind, the best off-the-grid heat source for cold climates is an acre of woods and an axe. Insulation, too. A lot of sheds and cabins (and single-family homes) could do with more. Saves on cooling bills in the summer, too.
- Symmetry 5y agoAn acre of trees should be enough to give you an average of 4 kW of thermal power year round, way more than you probably need. Though at 1 W/m^2 how much industry an area of trees could support was the binding constraint on industry for a lot of recorded history. Especially there was always pressure to turn forests into farmland.
- i_am_proteus 5y agoCould get by on much less than an acre if it were managed, but better to have more land and be able to keep it as unmanaged forest. That'll also produce fresh game and give some buffer on the lumber supply for lean years. How much rain falls is strictly out of my hands.
- bregma 5y agoI live on 25 ha of forest, have an axe and saw, and heat with wood. Let me tell you: you want to use the saw for everything but splitting. Axes are fine for burly cavemen. Anyway, in a cold climates like mine an acre would not be enough to keep you warm because trees grow slow when it's winter six months out of twelve. It takes a lot of wood to keep you warm, and one tree gives surprisingly little wood. After a handful of years you'll have clearcut your measly acre and still have another couple of decades to go before it becomes harvestable again. I'd say you want at least 10 acres if you manage it well and the soil and drainage are at least half decent.
- jacquesm 5y agoBy the time you get to 100 acres you can basically heat your house with the stuff that blows over in storms and the remainder can yield a nice cash crop every couple of years, giving space for the other trees to grow. Personally I'd prefer a chainsaw for anything but the most incidental cutting, and a hydraulic woodsplitter (rented) to get the job of preparing firewood for a while year done in a short time.