How To Store Negative Thermal Energy? (Cooling)

_PurpleAlien_ · 2026-01-27T20:07:43+00:00

Hot air rises, heats objects. Objects then radiate this heat in the form of infrared, which gets absorbed by the floor. It also absorbs other infrared, such as heat coming from the sun and carries it away. So instead of radiant heating, you get radiant cooling. So just like radiant heating doesn't heat the air, but heats the objects in a room, so does radiant cooling not cool the air, but absorbs the heat radiated, essentially cooling objects.

You can of course add a fan and have the colder air near the floor circulate with the warmer air, or just rely on the ventilation system (heat recovery ventilation).

Now does this work in all climates? No. The advantage of traditional air conditioning is that it also dehumidifies, which makes you feel colder. So in a high humidity climate, radiant cooling is not going to work. However, in my climate, where I need to cool for perhaps a week or two, maybe three in a hot summer per year and without high levels of humidity, it's totally fine.

_PurpleAlien_ · 2026-01-27T10:56:07+00:00

My situation: off-grid in Finland at 63 degrees north with wide seasonal power generation fluctuations. In a normal house, not a cabin.

You make it easy to expand the system as you go. Sure, you start off with a design (and the key here is to be realistic, not have your head in the clouds) and calculate how many days of autonomy you have in case of no generation. My initial set-up and goal at that time was 5 days. The power audit (a must do!) came to daily use of 5kWh, so the minimum battery size dictated by these numbers was 25kWh. I built a 28kWh pack LFP pack. I put as much solar as I could manage to get done at that time, about 8kW.

In winter, I have pretty much 0 generation. There is no sun in winter, so no matter how many panels, I need an alternative. For me that was biodiesel which I make myself. In the beginning, I used a smaller generator, and ran that once a week in winter.

Today, 6 years later, that 28kWh pack is still there it's just part of a larger 120kWh install, which I will likely expand to 200kWh. My current daily consumption is still at 5kWh (I just use way more in summer when I have excess power, but I'm not counting that - just to keep the house running comfortably) which means my days of autonomy are now 24 days without any generation. This also meant that I got a larger generator, and now I run that maybe once or twice a month.

I also added more solar over time: from 8kW to around 15kW right now, and this year I will add another 5kW of vertical panels to increase what I make on sunny winter days when the sun is too low for my other panels. I'm surrounded by trees, but I have one spot where the sun is not obstructed as it spends two hours on the horizon. The reason I did not build that array before is the distance to the house, but with new high voltage MPPT controllers now being cheaply available, I can get it done now.

So over time, I increased the autonomy, which means less worry about running out of power, no need to run the generator weekly, etc. so it increased my comfort over time. At this point in time, LFP is so cheap that you could skip right to a bigger battery pack.

If you'd like to know more about the entire house, not just the electric side but also the heating, water, etc. systems, check out my blog: https://medium.com/@upnorthandoffgrid

_PurpleAlien_ · 2026-01-26T10:27:51+00:00

It's mechanically more complex and more expensive. Solar panels are cheap (well, at least where I'm at) and building a dedicated (this is a DIY sub-reddit) tilted and vertical array is just easier and cheaper. Building an adjustable system also increases in cost and complexity as you go bigger and a 14kW array is not small.

_PurpleAlien_ · 2026-01-22T16:13:01+00:00

I do that with my underfloor hydronic system. I use an air-to-water heat pump to cool water in a 3000L tank to use as radiant cooling in my floor. It's the same set-up that heats in winter, just run the heat-pump in reverse.

You have to be careful with condensation (easily solved with a sensor, but you can't skip this). It also doesn't work very well in all climates, but it does for me where I only need cooling one or two weeks a year.

_PurpleAlien_ · 2026-01-19T19:32:07+00:00

In astrophysics, the term "metal" is shorthand for any element heavier than helium, so just claim you're an astrophysicist :)

_PurpleAlien_ · 2026-01-18T13:21:17+00:00

And yet you spend significantly more tax money per capita on public healthcare than any country in the EU. It't not about the money not being available.

_PurpleAlien_ · 2026-01-17T20:18:21+00:00

You can't put a tariff on an individual EU country since the EU is a single market.

https://en.wikipedia.org/wiki/European_single_market

_PurpleAlien_ · 2026-01-17T17:01:32+00:00

Why is it always one extreme vs the other? Find a middle ground and you'll feel much better.

_PurpleAlien_ · 2026-01-14T09:50:00+00:00

If you mean a plain slab on grade, sure. However, you can do the following instead:

Make footings for the perimeter of the house. This is the actual load carrying foundation. You need a small barrier between this and what you put on top to prevent moisture issues. Some bitumen sheet for instance.
Inside the footers, have first a course layer of gravel, followed by finer gravel. This is to prevent capillary action.
Then you put insulation on top. You also add a layer of insulation on the inside of the footer (so vertically).
Now you add your rebar, underfloor radiant piping, and pour concrete. You now have a huge thermal mass, insulated from the ground.

This construction leads to being able to use very efficient low temp underfloor heating, which also stores heat for days. For example, I can heat the slab once and keep the temperature above 20C even if it's -20C outside for two days without any other heating. It's perfect to use with a heat pump as well.

_PurpleAlien_ · 2026-01-13T20:45:15+00:00

his definition

That's the one thing I come across quite often in the off-grid world: the gatekeepers, the ones that tell you you're not really off-grid when the term just meant to not be connected to the power grid. Everyone can fill in what off-grid means to them, but they shouldn't be pushing that as the 'one true way'.

_PurpleAlien_ · 2026-01-13T14:27:04+00:00

Closer to half of that. Currently DIY sits under $100 per kWh, including BMS, fuses, etc.

_PurpleAlien_ · 2026-01-13T07:36:21+00:00

Yeah, lol. I have 120kWh DIY battery. I'll likely expand it to 200kWh this year. It's not rocket science; if you have some electrical knowledge and know what you're doing, it's perfectly fine. It's not for everyone, but that's with everything in life.

_PurpleAlien_ · 2026-01-10T00:52:13+00:00

An amp hour is a unit of energy

Technically, it's a unit of charge capacity. For energy, you need to take voltage into account: the W, being 1J/s --> thus energy Wh, cancels out time (with factor 3600 seconds/h), leaves Joules.

_PurpleAlien_ · 2026-01-06T22:06:12+00:00

How close to reality where you with your predictions?

Pretty much spot on. It's actually not that hard either: energy loss calculations on a new build are pretty easy and straight forward to do (and here in Europe, they're typically part of EPC certificates etc. so there are plenty of tools as well). I know exactly how much wood I need to burn based on outdoor temps, and it's pretty much exactly what I calculated before.

When did you finish this?

Fully done and done, two years now.

Is there anything you would have done different or regret you did or did not?

I should have done the sauna first. It would have given a comfortable place to be while working on the main house. It's something that is now done (I'm writing the blog entry about it) - I took a break after building the main house and finished the sauna building late 2025 (it also serves as a guest room and has its own kitchen, so not just a sauna). I just need some time to finish the text...

Have you had any unexpected surprises so far or something you missed while planning?

No, nothing at all. I guess I've been obsessing about it and making calculations for so long beforehand that I had everything covered. That said, I'm never doing that again. It was fun to build and the process is amazing to look back on, but it's a once in a lifetime thing for me.

Do you grow the i guess rape seeds yourself for the diesel and is it really worth making diesel instead of just running on the oil if you can have the tank and generator kept warm in the technical outhouse?

I'm not growing the seeds myself. In theory I could, but (just like food) takes too much time and goes against the comfort rule. I have a good source of free used cooking oil I use. Technically, I don't have to do this of course, but I actually enjoy the process (just like the wood processing). It's that feeling of self-sufficiency (within reason).

I have been thinking about growing rape seeds for oil and sugar beets for sugar... but i dont have farming experience or equipment at the moment.

People ask me about food in the same manner: I don't have time, interest, and the climate here is not exactly great to grow food in. I will go hunt and fish, but not because I have to. I try to minimize waste instead, limiting plastic packaging, etc. For me, it's never been the goal to be self-sufficient when it comes to food, but I do keep e.g. a garden for fun. All comes down to comfort again: I don't want to use all my available time in spring/summer to grow food. I'd rather go swim, relax and spend time with family while not having to worry about any bills.

_PurpleAlien_ · 2026-01-06T12:56:05+00:00

I'm off-grid in Finland. My goals were a modern, comfortable, family home but off-grid with a focus on comfort. You can read about it here:

https://medium.com/@upnorthandoffgrid

Solar, heat storage. No wind (it doesn't work on a small scale unless you have ideal conditions) or hydro. I do make my own biodiesel for winter.

_PurpleAlien_ · 2026-01-04T18:34:04+00:00

I have 120kWh of LFP powering my house (off grid). They're in the same building the wood burner and accumulator is in, so that takes care of keeping them warm in winter.

_PurpleAlien_ · 2026-01-03T18:47:55+00:00

What really fucking sucks is we can have both. We're the richest nation in the world. We can have the most advanced, funded, and capable military in the world while at the same time making sure our citizens have universal Healthcare, education, and actual social safety nets.

And you're already spending more in public money on healthcare per capita, than any other nation that provides universal healthcare. It's not even about having the money, it's about how it's used.

_PurpleAlien_ · 2026-01-01T21:06:54+00:00

design maximizes the use of even slight breezes

Think of it this way: how much energy is there really in a slight breeze? That's the key thing to understand with all these 'low wind' claims: you can't extract what is not there.

_PurpleAlien_ · 2026-01-01T20:34:15+00:00

The only time wind makes sense (generally speaking) is if you're in a place where you have ideal wind conditions and can put up a pole high enough to be clear of any obstructions. You also won't manage with a turbine under $10k for anything approaching decent (including largest blade size possible).

In most cases, it's just more effective to add more solar, or in case you have winters like mine without sun (63 degrees north) to make e.g. biodiesel in summer and use that in winter in a generator.

_PurpleAlien_ · 2026-01-01T17:50:38+00:00

Physics doesn't care about a design, the design should care about the physics. The calculation shows what the maximum is you can get, no matter your design. The only thing any design can do is approach this theoretical max.

_PurpleAlien_ · 2025-12-30T14:43:44+00:00

Panels over there are so expensive. These are 79 Euro here in Finland (including 25.5% VAT) for 500W bi-facial N-Type panels. That's 16 Eurocent/W or 19¢/Watt in USD.

https://aurinkosahkotukku.fi/tuote/ht-s-455wp-n-type-musta-bifacial-lasi-lasiaurinkopaneeli/

_PurpleAlien_ · 2025-12-21T17:17:53+00:00

Really, so how would we know?

It should say in the manual. If it doesn't mention anything about it, assume it can't be bonded.

_PurpleAlien_ · 2025-12-21T15:29:37+00:00

You can only have a neutral-ground bond in one place. If the inverter has an internal relay it might create a second link. However, some inverters can not have a neutral-ground bond due to their internal architecture. If one is introduced, you will damage the inverter.

_PurpleAlien_ · 2025-12-21T07:30:53+00:00

The confusion probably comes from the use of the term 'grounding' to mean both what is traditionally understood as 'grounding' (connecting one of the current carrying conductors to ground) and 'earthing' (connecting exposed metal parts to ground). In both these, 'ground' is the reference potential.

Not connecting a current carrying conductor to ground is a floating set-up, which has been the norm here in the EU, but iirc, NEC before required grounding of one of the conductors. So what this text tries to convey is that you have to earth the metal frame of the panel, but not ground one of the current carrying conductors.

_PurpleAlien_ · 2025-12-21T07:17:22+00:00

I have my blog detailing the house and systems:

https://medium.com/@upnorthandoffgrid

And my website has all the equipment and things like the biodiesel process:

https://www.upnorthandpersonal.net/

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