Standard Thermal: Energy Storage 500x Cheaper Than Batteries

Dirt keeps a constant temperature year round quite close to the surface that’s a ~60 degree difference between summer and winter in many areas. So 600c would just be a tradeoff between depth, heat loss, and thermal efficiency. However, what they aren’t saying is electricity > heat > electricity is quite lossy and even just using the heat directly is far less efficient than a winter heat pump.

A 10 ft pile of dirt (assuming 10 ft between heat exchanging pipes and the outside air) has an R value of 24 to 96, which is extremely significant.

I expect there would still be notable losses trying to keep it at 1100F indefinitely, but 10 ft of dirt will have insulation values approximating many feet of fiberglass insulation.

You’d want a very large mass to heat however, scaling matters a lot. You’d want the ratio of surface area to mass to be as small as possible, and that means as large a volume with as thermally dense a material as possible inside. Surface areas increases by the square, while volume increases by the cube.

Also, no matter what you do, you would eventually cook whatever was at the surface or underground, so don’t do this where you want trees - or where there are underground coal seams

Is this because of geothermal energy leaking upwards? If so, it's not the dirt, it's the geothermal energy.

That’s not entirely insulation. Some of the heat flows upward toward the surface during winter and some warmth flows downward during summer.

> If we could just create hot and cold piles or underground wells or something that we could tap into 4 months later when the temperature has changed, you would have completely solved heating and cooling.

Geothermal heating and cooling already exists. It’s semi-popular in some areas. It can be expensive to install depending on your geology and the energy savings might not compensate for that cost for many years. Modern heat pumps are very efficient even if the other side is exposed to normal outdoor air, so digging deep into the earth and risking leaks in the underground system isn’t an easy win.

Start getting into permafrost though where the cold is more constant and that cold layer gets deeper.

I can imagine that there's a lot of total energy in the dirt 10 feet down. But once you've tapped the energy near your well, how long does it take to replenish? How long until the immediate vicinity reaches equilibrium with the surface?

You put pvc pipes into a hill of dirt that is covered by a plastic sheet or other waterproof membrane; during hot summer months you use a small fan to put heat into the pile; during winter the heat moves from the dirt to the house.

But depending on your definition of this, it's been around for hundreds if not thousands of years. People used to cut ice out of frozen lakes and store it in underground basements for year-round cooling. And in arid climates they have windcatchers [1] and other techniques where they store the nighttime cool for usage during the day, or these [2] to store or even create ice, all without using electricity.

[0] https://en.wikipedia.org/wiki/Seasonal_thermal_energy_storag...

[1] https://en.wikipedia.org/wiki/Windcatcher

[2] https://en.wikipedia.org/wiki/Yakhch%C4%81l

I'm sorry, but you write this as if that's nothing. Making a 10 foot hole is a massive amount of energy being spent. It's a massive amount of weight as 1 cubic yard of dirt is roughly one ton. In 10 cubic feet, that's roughly 3.5 tons. I say this as someone that moved 6 cubic feet of dirt by myself with a shovel and a wheelbarrow.

So to think of 10 feet of dirt as a slow insulator would have to be one of the worst insulators out there.

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

We (USA) could have 80% of our Northern homes off fossil fuel and electric heat for less cost if we were a little more forward thinking and willing to work together.

But after nearly two decades they're decommissioning because the one-off components needed too much NRE to refurbish. If we all adopted this it'd be cheaper than what we pay today and zero greenhouse gas emissions. It'd finally make living in the temperate climates more climate-friendly than the warmer latitudes.

Surprisingly, that's only equivalent to about 10" of polyiso rigid foam.

What this project is really taking advantage of is the super cheap thermal mass. Dirt has about a quarter of the specific heat of water, but it is, literally, dirt cheap, and much easier to keep in place than a liquid.

Has anyone looked at the subsurface ground temperatures after days, weeks, months, even years of heat pump operation?

I do seem to remember seeing one article on the subject showing that after one winter the subsurface temperature had declined enough to materially affect the heat pump's COP. But the timescale didn't extend to multiple years.

edit: found this one: https://pangea.stanford.edu/ERE/db/GeoConf/papers/SGW/2024/M...

In one of eight cases studied where heat flow is unidirectional (cooling load only) over a 20 year timescale the authors find:

  "the mean ground temperature ... increased from 21.87 °C to 26.18 °C, ... . This significant rise could have a potential impact on the performance of the system in the later years of its operational life."

An additional graph shows COP variation over 15 years and the worst case shows a decline of perhaps 10% (just eyeballing it).

Surprisingly, some cases showed a long term improvement in heating COP - presumably the injection of summer heat into the soil made for warmer soil than just sunshine and natural diffusion?

So my takeaway: "it depends." :-)

When 5 of the 6 semiaxes of possible heat flow have no temperature gradient, the temperature becomes much more stable.

Insulation would be if a 10 ft radius dirt ball maintained a stable temperature all year round - which would surprise me, although dirt does have some insulative value. However, wet dirt is really not very insulative - try sleeping on the ground sometime.

We already do, in a way: septic tanks

This is literally what ground-loop heatpumps are doing. The ground loop is used as an energy source in winter, and since water is always at 0C, the heat pump efficiency can always be around 500%. And vice versa in summer.

In a situation where you have a lot of energy generation that would go to waste, storing it in a system with low round trip efficiency could be better than losing it.

For planned installations where the generation cost is nontrivial (like a solar install) then increasing the generation to compensate for poor battery efficiency isn’t as easy of a decision.

The power to gas is also carbon neutral, even negative depending on what you decide to do with the natural gas (if you don't burn it for power but use it for industrial chemistry, you get some sequestration out of it).

Meanwhile multiple grids are now paying renewable to curtail, because guess what, the variability is correlated (it's the exact same damn mathematics we used to fuck up the entire global economy in 2008, which is why I'm so surprised people are handwaving that too, but whatever). If you want to minimise cost without relying on gas to save you on dark still days, you want a cheap use for the surplus, round-trip be damned.

[1]https://en.wikipedia.org/wiki/Duck_curve

And when electricity is in essence too cheap like with solar and wind it can be, losing half in efficiency actually doesn't matter too much.

> There is an efficiency penalty converting back to electricity; round-trip efficiency is 40%-45%, but sometimes the steady supply of electricity is worth it.

..or into a giant sand-filled barge which transports the thermal energy up north for the winter.

Heat pumps do magic by changing the pressure at which a working fluid changes phase, so you can boil the fluid over here, have it absorb an enormous amount of energy then compress it back to a fluid elsewhere and push that heat back out -- this works pretty well because you're just moving the heat and only pushing the temperature on the "hot" side up a relatively small amount. I don't think, for instance, you could make an oven with heat pumps.

To do useful work you need a _substantial_ energy gradient -- it's hard to live in the sun even though its got lots of free energy floating around. The sun is very useful to the earth because the energy it provides is so much more energetic than the ambient environment.

Edited to add:

There are discussions of using exotic working fluids like compressed CO2 -- that'd allow you to manage the phase change maybe to a region where you could concentrate the energy in the fluid then expand it elsewhere at "room temperature" temperatures -- but I think things like compressed (to a _fluid_) CO2 are really hard to work with.

https://www.sciencedirect.com/science/article/pii/S266711312...

https://youtu.be/OdyrF96q2TQ?si=GT7ar0yoS6jR0mZe

At home, it's suitable in warm climates but is more challenging in snowy / very cold regions. Generally speaking, converting to electricity then using an electric water heater is more efficient because there's much less insulating, heat loss, and piping that can leak and cause water damage.

Why not building it under already wasted dead space like parking lot and have snow-free parking lot as extra bonus.

A Tesla Powerwall contains about 13.5kwH (about 4,000 times as much)

So you can either raise 100 tons 10m above your house, or you can have 1/13 of a Tesla Powerwall.