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Comment by Syonyk | Hacker News Reader

> 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.

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Yep. 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.

I 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.

You could try coppiced trees: https://www.lowtechmagazine.com/2020/09/how-to-make-biomass-...

They grow faster since they already have a root structure, the size is consistent and the wood is generally straigter (the japanese have a similar technique for extremely straight cedar logs)

By 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.

An 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.

Could 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.

Seems 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.

You’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.

Passive 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.

Pretty 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.

https://permies.com/t/42722/Passive-Solar-House-design-Cloud...

As that thread confirms, you do need backup in a cold climate, even with a ton of insulation. Amounts of insulating including R50 and R70, which is pretty mind blowing (and are difficult and expensive to accomplish in practice due to thermal bridging/‘leaks’).

Some folks in less severe climates (like New Hampshire) seemed to get by with baking and other managed appliance heat. That does require you spend time doing, and orient your activities around, things to produce heat to stay warm. Some will find that palatable, others won’t.

Posters in Colder climates (like Ontario) noted propane, or thermal mass heaters (such as the rocket heater in the thread).

‘Then the propane could make up the modest difference on long cold cloudy stretches.’

Thanks for linking it!

Hypothetically, 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.

Lithium 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.

What about summer? Those batteries also need to stay cool...

They 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.

You 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.

A $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.

Is 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.

Which heat pump reaches that kind of efficiency when the temperature differential is so high?

None 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...

Good 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.

5kW of panels don't produce 5kW in winter, that's the issue I guess.

Correct, they will produce more.

Solar panels produce the most power on cold winter days. The 'rated power' is for 20 degrees ambient at full (90 degrees, no cloud) incidence at sea level.

In practice it's kWh you care about, and you get significantly fewer hours of sun in winter. No cloud days are rare in some regions in winter too.

Sure, but the time factor wasn't present in the above comment, it was just instantaneous power.

It was about how that could heat (or not heat) a cabin, time is definitely a factor in that.

"5kW of panels don't produce 5kW in winter" -> there is no 'time' factor in there.

What you could say is that solar panels in the winter do not produce the same amount of power in an average day as they do in the summer due to the shorter day and the bigger chance of occlusion as well as the longer path the suns rays have to travel through the atmosphere.

That really depends on how far north you live. Where I live a good winter day's production is ~10% of a good summer day's. (Also depends on the orientation, but it's never even close, let alone more)

That's total production over a day.

Problem is, the sun doesen't necessarily rise above the horizon for weeks in northern countries

Yes, in locations like that you will have to use some other solution for those periods. Maybe wind power. Battery storage will not last long enough unless you have an absolutely massive setup that would not be cost effective the rest of the year.

I had 48KWh worth of storage, 2500 W worth of wind and another 5000 W worth of solar. This was in Northern Ontario. Until we added the windmill we fairly regularly needed the generator, after we added the windmill the generator just sat there gathering dust.

Could you say something about the analysis you did to decide that wind was going to be useful for you? I'm interested but I don't really don't have a good sense for how cost effective a wind turbine is or how to evaluate a site.

The Canadian meteorology department puts out pretty useful stats, and I dropped in on a couple of people in the neighborhood that had windmills up to see what kind of windspeeds they were seeing and how long on average their machines stood still.

This was a major factor in designing the machine the way I did, variable pitch is a lot more work but a machine like that will start up far earlier than a fixed pitch machine. It also had its effect on that stator, which I made with the laminates at a slant over 1/3rd magnet width, which reduced cogging to nearly nothing. This is a huge factor in starting up a direct drive machine.

What does the 5kWh of energy turn in to, of not heat?

It says '5KW', not '5KWh'.

I just now realised that "panels" was solar panels, not the ovens (we call them "panel ovens" in my language). My fault!

Np, it is an easy mistake to make.

Thanks for your post. I'm reading through some of your blog posts now - really great!

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