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Comment by retrac | Hacker News Reader

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.

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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.

A ground source heat pump (also geothermal heat pump) is a heating/cooling system for buildings that use a type of heat pump to transfer heat to or from the ground, taking advantage of the relative constancy of temperatures of the earth through the seasons.

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

You can also recharge your geothermal well or ground heat collection field by heating the outgoing thermal collection liquid with either cheap electricity (rooftop solar?) or direct solar heat collection. I think this will be a growing thing as the earliest mainstream ground source heat wells start to be a few decades old. Many of them are sized so that they don't fully recover during the summer, so the heat output slowly drops.

Heath energy required != electricity requirement.

A modern house in Finland needs around 15-24kWh a year of heat energy if it's well insulated. On the higher end for big + northern houses, and less if you're smaller and further south.

Some get this energy by burning wood, others with heat pumps, and some with direct electricity.

24kWh is 1kW drawn continuously for 24hrs.

That can’t possibly heat any home for an entire year.

I think they mean per square meter of living space.

I think MWh is meant, otherwise it makes no sense

My 90sqm bungalow in the U.K. uses about 15MWh a year for heating - 1500 litres of oil, almost all in winter. Peak load is about 2.5kW over a day (60kWh)

24k kWh

it does! With a heat pump and insulation.

I think you are off by about 3 orders of magnitude as my Austrian flat need about 7MWh a year for heating and 3MWh of electricity. I could generate 24kWh per year on an indoor bicycle.

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.

I remember that one, but also can't find a link. Wasn't the heat from the people living there, and from appliances etc. enough to keep it at a comfortable temperature?

I love ideas like this.

I'd like to see the stats on temperature levels over a lifetime.

I'd be interested to see the ventilation system.

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.

> 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.

Something like that was attempted south of Calgary, in Canada: https://en.wikipedia.org/wiki/Drake_Landing_Solar_Community

Sounds like it didn’t go so well: In 2020, the system started showing signs of deterioration resulting in significant maintenance issues. System components, knowledge, and technical expertise for repairs were becoming increasingly challenging to find. In response to system failures, the Drake Landing Solar Company added redundancies to the system to be sure that homes in the community were receiving heat… In 2024, a decommissioning process for the Drake Landing Solar Community began, where the majority of the 52 homes were converted to natural gas-fired furnaces.

Nowadays, solar thermal collectors are completely obsolete except in very niche applications. Solar PV is so cheap that it’s literally cheaper (not to mention much less maintenance) to wire a bunch of PV panels to a resistance hot water heater than it is to directly heat the water with thermal collectors!

And if you use resistance heaters, you can get to much higher temperature with practical systems.

> 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

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.

Some PassivHaus designs incorporate such systems. There is also district thermal storage which services multiple homes with a thermal storage reservoir.

Thorstein Chlupp, of Rienna LLC, built several net-zero homes in the Fairbanks, AK, area. He hasn't been active for a number of years now, but released several videos in the mid-2010s detailing his design, construction, thought, and results.

He's apparently been with NREL since 2023: <https://research-hub.nrel.gov/en/persons/thorsten-chlupp/>.

There's a nearly-decade-later review of one of Chlupp's Alaskan homes, the Sunrise House, written in 2020 (the house was constructed in 2011): <https://www.greenbuildingadvisor.com/article/the-sunrise-hou...> (paywall).

And the original video series and channel:

<https://www.youtube.com/watch?v=AtHkvpRI6fc>

<https://www.youtube.com/@REINALLC/videos>

At the core of Chlupp's homes was a thermally-stratified 5,000 gallon storage tank. This was ideally aligned vertically (to improve thermal stratification), though at least one home had a horizontal alignment. The tank was inside the core of the home, which means it occupies significant internal space. I don't know if there have been any mechanical / technical issues with the design over time, though at least initial results were strongly positive.

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.)

As I understand it, most of the heat that was deposited by radioactive decay in the crust three billion years is still in the crust, not having had time to conduct its way to the surface, and most of heat deposited in the crust even today is from radioactive decay in rocks in the crust itself. But https://en.wikipedia.org/wiki/Geothermal_gradient#Heat_sourc... says that's wrong: "An estimated 45 to 90 percent of the heat escaping from Earth originates from radioactive decay of elements, mainly located in the mantle."

https://en.wikipedia.org/wiki/Geothermal_energy#Resources says 20% of the Earth's internal heat content is residual heat from planetary accretion 4.5 billion years ago, but of course that's mostly not in the crust. It also says, "the conductive heat flux averages 0.1MW/km²."

At the given thermal gradient of about 28°/km, a nominal thermal conductivity of 3.3W/m/K for granite (https://www.sciencedirect.com/science/article/abs/pii/S00137...), we can derive a heat flow rate, which works out to 0.09 MW/km², in good agreement with the Wikipedia number. By dividing by a nominal specific heat of 0.7J/g/K and some density estimate like 2.4g/cc, can we get a speed in meters per second? The units don't quite work out; the specific heat times the density gives us 1.7MJ/m³/K.

So I guess I don't know how I came to my conclusion, so maybe it's wrong. How did you come to yours?

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.

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.

I can't help but wonder how the efficiency compares to generating electricity, running that over wires, and having that run heat pumps.

The conversion to electricity loses energy, but I assume the loss is negligible in transmission, and then modern heat pumps themselves are much more efficient.

And the average high and low in February in 26°F and 14°F according to Google, while modern heat pumps are more energy-efficient than resistive heating above around 0°F. So even around 14–26°F, the coefficient of performance should still be 2–3.

> heat pumps themselves are much more efficient.

For electricity-to-heat conversion, heap pumps are indeed much more efficient relative to resistive heating, yes. About 4 times more efficient.

In absolute terms, though - that is still only 50% of "Carnot cycle" efficiency.

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

Similarly, heat-to-electricity conversion is about 50% efficient in best case:

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

So, in your scenario (heat->electricity conversion, then transmission, then electricity->heat conversion), overall efficiency is going to be 50% * 50% = 25%, assuming no transmission losses and state-of-art conversion on both ends.

25% efficiency (a.k.a. 75% losses) is pretty generous budget to work with. I guess one can cover a small town or a city's district with heat pipes and come on top in terms of efficiency.

We've got lots of heating districts around the world to use as examples. They only make sense in really dense areas. The thermal losses and expense of maintaining them make them economically impractical for most areas other than a few core districts in urban centers... Unless you have an excess of energy that you can't sell on the grid.

Geothermal heat is also not that functional in cities, you'd need so many wells so close together that you'd most likely cool down the ground enough in winter so your efficiency tanks.

I don't understand, what am I missing? The heat pump increases efficiency by having COP 2-4 right? Assuming air to air and being in, say, Denmark.

Heat (above 100C, say, burning garbage) to electricity: 50% (theoretical best case)

Electricity to heat (around 40C): 200%-400%

Net win?

The surplus energy comes from air or ground temperatures..

Yes you cannot heat back to the temperature you started with but for underfloor heating 40C is plenty. And you can get COP 2 up to shower water of 60C as well.

Yes, this is exactly why I asked. You need to include the COP in the calculations.

If the heat is stored at high temperature, but the demand (for heating buildings, say) is at lower temperature, it could make sense to generate power, then use that power to drive heat pumps. You could end up with more useful heat energy than you started with, possibly even if you didn't use the waste heat from the initial power generation cycle.

Alternately, if you are going to deliver the heat at low temperature to a district heating system, you might as use a topping cycle to extract some of the stored energy as work and use the waste heat, rather than taking the second law loss of just directly downgrading the high temperature heat to lower temperature.

High temperature storage increases the energy stored per unit of storage mass. If the heating is resistive, you might as well store at as high a temperature as is practical.

Gas-fired heat pumps have been investigated for heating buildings; they'd have a COP > 1.

I am interested if there are any cheap small scale external combustion engines available (steam? stirling? ORC?)

It can be anything between easy and impossible depending on the temperature difference. 200 C steam is easy with a commercially available turbine, but 50 C is really hard. There are things like Sterling engines that can capture waste heat but they've never really been commercially viable.

There's no way around it: We have to respect entropy.

I think the big cost difference is the geothermal generators to convert the heat back into electricity. More of a cost issue versus efficiency.

Existing district heating systems can be large.

I live in Denmark the powerplant that heats my home is about 30km away. There are old powerplants in between that can be powered in an emergency.

Yes, building district heating systems that large is difficult and expensive, it wasn't built yesterday, more like 50 years of policies.

Pipes are competing with wires, which are much less technically complicated than pipes.

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

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.

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