Standard Thermal: Energy Storage 500x Cheaper Than Batteries

An AA battery contains approximately the same as 1 ton raised 1m. (About 3Wh)

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.

Hoisting 100 tons of stuff high into the air, and then efficiently converting that into the high RPM needed to drive a generator seems like it would take a truly staggering construction effort. Suspending that amount of weight high above your house also has some... interesting potential failure modes.

Every time I see again the idea of moving big concrete blocks for storing energy, I remember the time I made the calculations, and estimated around USD100K of infrastructure to store the same amount of energy as a nissan leaf.

Some guy on Youtube tried this. I think the conclusion was, not worth it.

https://www.youtube.com/watch?v=l19AMYd0oks

Gravity based with weights is generally considered not cost effective; others already did the math that your 100 ton proposal can still only store a fraction of what a consumer grade battery pack can do, but on top of inefficiency there's space and maintenance requirements. It works in situations where e.g. trains go uphill empty and full downhill, but generally it doesn't work.

Water based systems work better because water is easy to move, plentiful, and there's natural basins to pump into / flow out of that can contain billions of liters.

EnergyVault are building these GESS (Gravity Energy Storage System) arrays right now.

https://www.energyvault.com/projects/cn-rudong

> an array of ten 10-ton concrete blocks lifted 10m in the air could power a house for a day

No, that's only 2.7 kWh. Most homes use 10-20 kWh/day. A battery of that size is easily under $1k. Good luck building your ridiculous concrete block system for that.

Batteries are really good. Gravity, not so much. It only works when you can lift & store a tremendous amount of stuff "for free" because nature has done most of the work, e.g. in valleys, mountains, aquifers, caves, etc. If you have to build the whole thing it will never be viable.

Pumping water up is a super old idea, but as far as I know you'll need some natural terrain to build it efficiently.

Someone has already been thinking along the same lines: https://www.swissinfo.ch/eng/business/energy-vault_revolutio...

This type of plant is generally used for emergency power to balance the grid, whilst other plant come online.

You need a lot of weight. IMO for home use the risks heavily outweigh the benefits for anything outside of a hobbyist project... that's just a lot of potential energy in a system that can go wrong. Weights falling quickly, pullies and cables under tension.

The same is true for batteries of course, but at the very least there are protections and checks for failures in most consumer accessible home solutions (and decades of engineering at this point). Worst case you at least have smoke detectors... not sure if there's a "cable is wearing thin and might snap and decapitate you" warning system.

Stones are typically not very dense. Iron or lead or 2-4x denser than typical stones, and so will net you better energy density.

You already have some decent amount of weight in the house itself, so I'd look into raising it all or parts of it

Of course it's probably not the simplest engineering effort...

There are some videos on Youtube discussing the hypotheticals of this. They're never really very positive about the feasibility. Neither on a small scale nor on a country-wide scale.

If you'd want to store 1kWh at 10m height, assuming no loss at all from heat, friction, etc, you'd need about 4 of those blocks block weighing 10 tons (according to ChatGPT). So you'd need a lot of those blocks to power a house for a day, unless you're very efficient.

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.

> 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.

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.

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?

> 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.

the reason that thermal storage is becoming very atractive is that electrical power from PV, wind, and other sources has become increadably cheap, and there is litteraly no where to put it, so prices go negative now, which is a new thing. so 500x cheaper may be an understatement, considering the nature of how cheap, mature and availible the technology to build a thermal storage battery is, any municicipal civil engineering team can build one from off the shelf and localy sourced materials, and the basic battery "housing" could be a re purposed industrial building, cheap does not begin to describe it.

Solar prices are coming down quite fast, I don't think a factor of two is going to be a killer here if the storage is cheap and long-lasting enough. Some people are already considering over-provisioning solar panels relative to available transformers/grid connections so that they can maintain output on cloudier days. "What do we do with all the extra power when the belly of the Duck Curve [1] hits the ground" is a problem lots of people are thinking about.

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

One thing they also mention is how incredibly cheap storage of natural gas is. https://en.wikipedia.org/wiki/Power-to-gas#Efficiency The efficiency of power to gas is not great, but it's about the same as this thermal storage method, with probably much longer lifetimes,easier transportation and more general utility (the natural gas could for example be converted to methanol using the holy grail catalyst that was in the news recently).

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).

They mention it:

> 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.

More efficient, but much more expensive. I'm sick of people handwaving $100 per kWh. That is two orders of magnitude off where it needs to be to do anything more than virtue signal.

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.

I didn't have time to read the whole thing so I don't know if they mention it, but another article about about using sand as heat storage pointed out one of the advantages is that the material isn't toxic, unlike chemical batteries.

They do mention it, but it’s downplayed relative to how much of a problem it can be.

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.

It's been done, but not without controversy. https://en.wikipedia.org/wiki/Ivanpah_Solar_Power_Facility

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.

How would you move the solar energy into the piles of dirt? You’d need something like an array of mirrors focusing the rays, which has definitely been done already but has drawbacks. Electricity can easily be moved to where it’s needed.

They’re using the stored heat energy as heat, not converting it back to electricity.

And if it’s very cheap, does it matter if the conversion is wasteful?

I think there have been about 5 different contexts over the years where I've been surprised by how good an insulator a pile of dirt is.

> Pipes run through the pile, and fluid flowing through them removes heat to supply the customer.

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.

Apparently the insulation value (R value) of dirt and soil is between .25 and .8 per inch (depending on moisture content). That wouldn’t be great if it was a material like fiberglass, but since it’s dirt cheap (ba-dum-tssh) and easy to pile up in large quantities with little to no ongoing maintenance in this kind of context, it matters.

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

they mention that demand source should be close by to reduce losses in transportation

Exowatt requires rethinking the solar energy system entirely. This is meant to be bolted onto an existing solar panel field that is mostly selling energy to the grid, but gets curtailed sometimes

There are simpler solutions using thermal solar panels which don't require converting solar energy into electricity first:

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

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

I don't think you ever get enough of a temperature difference just by having a passive black pipe in the sun, to do any useful work besides potentially keeping someone warm in the winter. You could do useful work if you concentrate the energy from the sun somehow, like with mirrors.

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.

It's done a bunch, all over the place: https://en.wikipedia.org/wiki/Solar_thermal_energy. Most commonly people use them either for batch pre-heating of hot water or for full heating. I looked in to getting something like that for the hydronic heat in my house because currently it's propane-fueled and the deliveries are quite expensive in the winter.