Comment by retrac
19 hours ago
There's an interesting property to thermal storage, as a consequence of simple geometry. Consider a cube. volume = n³ and surface area = 6*n². Surface area increases more slowly than volume. The ratio of surface to volume decreases with more size. Thus: a sufficiently large thermal reservoir becomes self-insulating with its own mass.
It's even better than that. In addition to the factor of n from ratio of volume to surface area, there's also a factor of n from the increased thermal resistance of the mass of the storage volume (the temperature gradient from the surface to the center goes as 1/n). So, the thermal time constant of the object scales as n^2.
This very favorable scaling is why natural geothermal retains heat even though the input energy was delivered gradually over as much as millions of years.
I've always wondered why we don't build homes with a buried tank of water used as heat storage. In the summer it can be heated with solar thermal to around 90c, and in the winter heat can be drawn out and go through radiators or underfloor heating, with a mixer valve. You just need a few pumps and valves, not even a heat pump is needed.
If you assume a modern house with a heat load of 1800kWh per year (fairly standard for a new build medium sized home where I live, in Northern Europe) that means you'd need a tank roughly 50m3, or 10,000 gallons for Americans. In terms of insulation you'd need around 50cm of XPS foam, and it would be buried a meter below ground.
It's nothing terribly complicated in terms of construction or engineering. Of course you'd pay more upfront, but then your heating bills would be practically zero. In warmer climates it would be much simpler, you could probably get away without burying it.
This is essentially what a ground source heat pump system is. Except instead of a sealed water tank you just make a tall hole that fills with water and the sun will warm it for you during the summer automatically.
1800 kWh is very little. We use around 12000 kWh and our neighbours' new house uses around 8000 kWh annually and most of that is heating. I'm not sure how many houses can hit 1800.
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I can't find the link now, but there was an episode of Grand Designs here in the UK (a show detailing private individuals developing interesting or unusual homes) where the owner was building a passively heated house based on an idea by his architect father.
The ground beneath the footprint of the house was insulated around the sides to a depth of about 2m, effectively extending the thermal mass of the house into the ground. After construction, it took about 2 years (IIRC) to warm to a stable level, but thereafter required little to no energy to stay at a comfortable temperature year round.
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> I've always wondered why we don't build homes with a buried tank of water used as heat storage
Skip the tank completely. Use the ground directly. This is what geothermal heating does.
Drill a deep hole and drop tubes into it. Use a heat pump to pump heat into or out of the ground. There is so much easily accessible thermal mass in a borehole that you don’t need to deal with a giant underground water tank
> I've always wondered why we don't build homes with a buried tank of water used as heat storage. In the summer it can be heated with solar thermal to around 90c, and in the winter heat can be drawn out and go through radiators or underfloor heating, with a mixer valve. You just need a few pumps and valves, not even a heat pump is needed.
Because building houses is already expensive, and that would add significant amount, pushing it into "can't afford it in the first place". And zero ability to realistically service it means anything going wrong might make whole investment moot.
On top of that, any investment like that competes with "why not just put the money into low risk fund"
> 1800kWh per year
now factor in losses for months now factor the fact the energy you're using for heating is one you're not using for... energy or selling
also is that heat or energy ? Because if that's "what power heat pump used", multiply that by 3-4
It's just... expensive to do it like this. Expensive enough that most people that could did the math and it wasn't mathing
I'm not sure if the 1800kWh is correct here. I'm guessing it's one of these two:
- You're talking about what heat pumps use in electricity. However, the system would store heat. If a heat pump uses 1 kWh to get 3 kWh of heat into the house, a heat based storage system needs to store the 3 kWh.
- You're confusing gas & electricity. 1800 m3 in gas would be about correct. However, that's about 9,5 kwh per m3 in heat.
There are interesting heat storage methods though, there is a long term basalt heat storage system in 'Ecodorp Boekel' in The Netherlands. It uses solar to heat during the summer and heats the homes with that in winter.
Due to size though, it only really works in 'collective' communities. The bigger the size, the more heat it can store per size.
50m³ is huge. IMO that would be an engineering challenge that would probably impact the sability of the foundation if not done right.
Ground source heat pumps are expensive because of the buried piping, I imagine this would be even more costly.
Something like that was attempted south of Calgary, in Canada: https://en.wikipedia.org/wiki/Drake_Landing_Solar_Community
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Its kind of done. Active heating systems often have the intake air go through the foundation so it heats up in summer and cools down in winter reducing both heating and cooling costs.
Billions, mostly.
Not in the rock that geothermal can access. That rock is recharged by heat flow from below on a shorter timescale than that (but much longer than geothermal would extract the heat.)
Just as important here: The higher the temperature of the storage medium, the higher the fundamental limit to how much electric energy you can recover.
Put differently: If you used the same amount of energy to heat one bucket of sand by 200C (A) or two bucket of sands by 100C (B), you would be able to recover more electric energy from case A because of the fundamental Carnot Limit. This is why sand is a good storage medium (as opposed to e.g. water), and why some solar power systems work with molten salts. Also why steam-based power plants need to operate at high pressure to be able to obtain high-temperature steam.
I'm pretty sure this is intended to store and produce heat anyway. They aren't going to be using this for generating electricity.
Yes and a freezer that is only a bit larger, taller or deeper has a lot more liters of storage. Not just because it's in 3D but also because a smaller freezer still needs big sides. So, a lot more liters but only a tiny bit more energy consumption.
ps: living in an area with a high price per square meter goes against this strategy unless you manage to share a freezer
Yeah but if you transfer the energy as heat then you will end up with elongated structures (pipes).
You speak theoretically but metropolitan areas in these countries all have those pipes in place and in use for the better part of a century.
Using heat for heating has many redeeming qualities. Heat is high entropy and it is not a good idea to "waste" low entropy energy to create high entropy energy. Many industrial processes run on heat and waste heat is generated everywhere. The systems are also cheap to run once in place.
That's a real issue, but this is for a district heating system which already exists and already faces this issue. And yet the district heating system is presumably practical.
Changing to a different central source of heating (i.e. storage) seems orthogonal.
Is that a problem? Pipes are not technically complicated. Is there something else I'm missing?
Larger storage structures are easier to (thermally) insulate. Because geometry.
But going with larger structures probably means aggregation (fewer of them are built, and further apart). Assuming homes to be heated are staying where they are, that requires longer pipes. Which are harder to insulate. Because geometry.
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Pipes are competing with wires, which are much less technically complicated than pipes.
From the article:
> [250MWh] held in a container 14m high and 15m wide
According to Gemini 3.0 Pro, lifepo4 is 1.5-3.5x more dense than this, which isn't bad. 250MWh is a lot of capacity for such a small land footprint. At 2MW it can power ~2000 homes for ~5 days while taking up the land footprint of ~1 home.
What's the price? And how does the price scale with capacity?
The problem seems to be heat quality - they don’t get electricity back, it’s only good for heating. (Which admittedly makes perfect sense in the winter near the North Pole.)
The issue we have in Finland is the assymetric electricity usage between winter and summer. This is driven by the need for heating.
In the past, district heating systems burned coal. Now that's out the window we haven't got enough to burn. We do burn waste products from forestry, trash and the like but there's not enough to go around before you start felling trees en-mass just to heat a city.
A lot of municipalities in Finland are now starting to play with thermal storage. There's this sand battery, but there's even more hot water storage being built and has been built.
In the medium term, winter electricity production and consumption is starting to become a bit of a risk for us.
It doesn't just make sense in the very far north, it makes sense just about anywhere that you'd have many people living close together (i.e. even a village).
Most homes don't need to have their own electricity generators, their own sewage treatment systems, or their own water wells, they hook into utility infrastructure.
In a lot of european towns and cities, heat is also a utility you can hook into, e.g. my apartment has no heating infrastructure in it, we just get all of our heat through a pipe connected to a nearby heat reservoir that's primarily loaded with waste heat from a gas power turbine. Within the next couple years though, the heat from gas power will be supplemented with the biggest heat pump in the world though [1]
It's not just a city thing though, I have friends who live in a village of 300 people in the Alps and they also have a utility district heating system in the village.
[1] https://www.man-es.com/company/press-releases/press-details/...
> near the North Pole.
Finland is not near the North Pole. Lahti is at 61°, right in the middle between Greece and the North Pole.
But yes, heating needs are higher than in most European or North American populated areas.