#10 The Coldest Winter in 20 Years: How Did We Manage?
This winter in Finland was (still is at the time of writing this) the coldest in 20 years. Temperatures down to and below -40C (-40F, yeah they’re the same) in some places, and all the way to -33C (-27F) where I am. Even here in Finland, these cold temperatures can be challenging, especially when they are sustained for weeks at a time… How do we keep warm off-grid with this kind of weather?
Obviously since we’re in the middle of winter, I can’t rely on solar power for heating. That means the heat-pump isn’t even in the picture this time of year. I run the generator once a week to charge the batteries, to augment what little I get from the sun, but for a heat-pump I would have to run it every day. The only viable energy source for heating off-grid in this climate is wood as I said before — and I’ve got plenty of that. So let’s look at how much wood I use on these coldest days. I’ll use a few consecutive days of around -30C (-22F) as an example.
You might think I go through enormous amounts of wood, both in the wood gasification boiler as well as the masonry stove — but the actual numbers might surprise you. Keep in mind that I keep the house around 24C (75F) for a total floor space of about 135m² and that this also includes the domestic hot water for e.g., showers. Why so warm inside? Yeah, ask my wife :)
I burn at most 30kg per day in both systems combined: one fire in each per day.
At 30kg per day, that represents a daily energy need of around 132kWh when looking at just the energy content in the wood (at about 4.4kWh/kg). With an overall efficiency of around 80% (a conservative estimate — it’s likely better), that makes it a 105kWh true daily energy need at these low temperatures. This matches pretty much perfectly with a tool I made to calculate the theoretical heat losses of the building (and thus energy requirements) at different temperature differences between inside and outside.
How does this stack up (pun intended) with the amount of wood we need per year? While planning this building, I calculated that I don’t need more than 10m³ of wood per year. Each m³ weighs about 500kg. At 30kg/day we are looking at a total of 160 days. That is, it could be 160 days of -30C outside and I’d be able to keep the house at 24C and provide all the hot water. For reference, that 10m³(for our American friends) is about 3 cords — and that’s at the absolute maximum burned each day. Once February comes along, the heat-pump will be also be able to offset wood requirements again. My 10m³ estimate is a bit on the high side, since these temperatures are not sustained for that long after all.
If you compare these energy requirements to that of a typical house here in similar conditions, we’re doing pretty well! From what I’ve read over the past weeks from people sharing their energy bills and usages (and costs!), we’re doing very well indeed. Not bad at all for a log house with tons of huge windows!
As a final note, suppose I would have to buy this amount of wood if we weren’t self-sufficient for some reason. An amount of 1m³, cut, split, dried, and delivered, costs about 60 Euro here. If I were to buy all 10m³, that would cost me 600 Euro. That’s a much lower cost than people typically spend yearly on heating here, even taking heat-pumps into account.
Comments
Post a Comment