Long-term thermal storage is something I've been fascinated with the last year or so.

Heat loss inside of dirt is so incredibly slow it's hard to wrap your head around. One fact that I find helps is the fact that after an entire winter of extremely cold temperatures, you only need to go down 10 ft or so before you hit the average annual temperature. 4 months of winter buffered by 10 ft of ground!

Obviously there is incredible potential to this even if you just keep the energy as heat. The amount of electricity we use on heating and air conditioning is huge. 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.

Really excited by companies looking into this and wish them the best of luck!

> Heat loss inside of dirt is so incredibly slow it's hard to wrap your head around. One fact that I find helps is the fact that after an entire winter of extremely cold temperatures, you only need to go down 10 ft or so before you hit the average annual temperature. 4 months of winter buffered by 10 ft of ground!

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.

Underground heat storage isn't new nor anything startuppy though, we're well beyond the "companies looking into it" stage. This page [0] mentions it's been around commercially since the 90's and experimentally since the (19)30's, and interest started in the 70's.

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

10ft below ground is enough to take advantage of geothermal heat. You don't have to go "very far" to reach warmer soil in winter because the soil PLUS the snow on top is pretty much just insulating the deeper ground from the cold air.

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

With regards to wrapping your head around heat loss- this winter some work was done on our property while there was snow on the ground. A bunch of snow got covered in dirt. In the spring, maybe three weeks after all the snow on the ground had melted, I moved the pile and was surprised to find all the snow still frozen. It had been under maybe 18 inches of dirt. I was pretty surprised to see it.

It's already done and proven to work further North than anywhere in the USA:

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.

This is called PAHS, passive annual heat storage and has been tried in some alternative energy housing.

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.

This reminds me of the interesting articles over the last few years about the heat from the London metro system over the last century saturating the ground so the tubes have become extremely hot.

Does the slow heat transfer interfere with attempting to use that heat?

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?

I really can't help but think there might be a fundamental problem here. If heat loss in dirt is so incredibly slow, and we use a heat pump to extract that heat, then wouldn't it be equally slow to replace the heat extracted? (absent heat injection, i.e. reversing the heat pump's operation in summer)

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." :-)

Dirt has an r-value of between 0.25 and 1 per inch according to some quick googling, so 10ft would be maybe R-60.

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.

You are confusing the concepts of insulation and reservoirs.

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.

Tech Ingredients did a video on this last week. https://youtu.be/s-41UF02vrU?feature=shared

> you only need to go down 10 ft or so before you hit the average annual temperature

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

> 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.

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.

Wouldn't the thermal conductivity/insolution that makes this so appealing be a liability when you want to extract useful heat to use?

> If we could just create hot and cold piles or underground wells or something that we could tap into 4 months later

We already do, in a way: septic tanks

> 4 months of winter buffered by 10 ft of ground!

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.