The world's largest sand battery just went live in Finland

This isn't a battery, it's a Thermal Energy Storage (TMS). Just because it stores energy doesn't mean it is a battery.

The sand is heated up electrically, and energy is stored thermally. They don't even yet have a system for converting that stored thermal energy into electricity again, it's just used to heat water.

‘Battery’ is the name for any device that stores any energy. It doesn’t have to be electric energy.
Seebeck generators exist.
- Language is funny sometimes. Battery used to refer to a collection of individual electric cells, 'a battery of cells' that worked together to supply the required voltage. Old cell tech was such low voltage, that several cells were required to juice itself to practical voltages for working purposes. Here the word was a mot emprunté - borrowed, from the military where battery was a term to describe a functional group of artillery that worked together for improved function. The military stole the old french term battre, 'to beat'.
  Eventually "battery" evolved to mean electric power storage device regardless of cell construction. Now apparently it also includes thermal charging, storage and discharging, even when only a single cell.
  Language is weird. The proper term would be accumulator, but weirdness reigns supreme.
- Last I heard Seebeck generators weren't that good/practical for large scale use; did things change?
- Sure, and windmills generate electricity and high voltage is anything above 30V.
  People say a lot of nonsense, particularly when they're adjacent but not really a part of a technical field.
  A "battery" is a multiple of some kind of module. You can have a "battery" of Anti-Aircraft guns. In electrics, a battery is made up of multiple cells, and these cells are made of two metals inside an electrolyte.
  This is not multiple modules of anything, and this is not made up of metals and electrolytes. This is not a battery.
  Edit: Also, your wiki link literally starts with (my emphasis):
  An electric battery is a source of electric power consisting of one or more electrochemical cells with external connections[1] for powering electrical devices.
Yes, and the basic principle of thermal batteries has been quite common here in Finland for some time.
All Finnish cities have district heating networks, so there is some heating plant that generates heat, which is distributed to homes using water as medium. It is closed system where hot water goes in, colder water comes out, which is heated back up. This energy is used to heat the home and heat the cold usage water (faucet/shower etc)
Because the network is lots of water, there is already quite a lot of energy storage in the heat grid itself, so itself works as a battery. In last five years almost all big networks have created water based thermal batteries. Those are 7-8 stories high insulated water containers. These make sense because you just start taking the already heated water from the container to the grid when ever you need.
So the tech itself is quite standard here, just the medium of using sand is new. Sand brings you bit longer storage time, but adds bit of complexity to the process.
Source: I work in a company that owns these kind of networks
- I'm not knocking the tech, I'm criticising the article for getting the terminology completely wrong. Moreso, the manufacturer has even fewer excuses.
  Maybe there's a language thing here, but in English battery is not the appropriate term for this. "Battery" really refers to just a bank of some multiple of something. Originally it was naval weapons and then in electrics it was multiple cells of electrolyte. An electric battery is a specific type of energy storage, this is a different type: thermal energy storage or TES.
  Hell, on the grid you don't even refer to battery electric storage as batteries that much, the common term is BESS - Battery Energy Storage System.

don’t tell anakin

Why build it so high above the ground, where it needs these strong walls of steel? Wouldn't it be cheaper to dig a hole in the ground and put the sand in there?

Then you'd end up making strong walls to keep the surrounding soil out when the bin is empty
The ground is.... 4C or something? Would it lose more heat more easily to the solid ground, than to the air?
Its probably easier to keep it dry above ground too.
I got interested in this tech a while back myself (thinking I should build one myself, how hard can it be?) and I made some simulations (because who wouldn't). The thermal gradient in sand is actually quite steep, even with very high temps around the core where the resistors are, the temps fall off pretty quickly. Then when you are at a point where the surface temp is reasonable, you switch to a conventional thermal insulator. Putting it underground would cause all sorts of issues in eg. maintaining the resistors, insulation etc. so it's probably just easier to do it like this.
Maintenance? Might be easier above ground, but I know nothing of the tech, so I could be completely wrong 😞
- More likely construction cost. Deep excavations are expensive, it's much cheaper to just install a concrete pad and land things on top of it.

How do they keep sand from cooling down though - keeping it at up to 600C sounds like it'd lose quite a lot of energy due to temperature difference with outside? It'll be cool when they add electricity generator, otherwise this battery makes sense only in colder climates

this battery makes sense only in colder climates
Almost as if there's a reason it's happening in Finland...
- Batteries like this have been developed for home use in Northern Alberta.
Anywhere with municipal heating really.
I can see the middle being 600C, the edges are probably cooler.
- Anywhere with municipal heating really.
  Yes, but in milder climates one needs heating for like four or five months only. Still great to reduce emissions and whatnot, but it'd sit idle the rest of the year.
  I can see the middle being 600C, the edges are probably cooler.
  Probably. But it'd be still nice to have specs on that.
Insulation and the thermal properties of sand ie it is good at storing heat. Think of it like a rock, it can store tons of heat but it takes a long time to transfer heat.
THIS IS NOT A BATTERY.
- While it's not a battery it mentions that they are working at heat conversion to electricity.
Its a thermal storage battery, no electricity is generated.

Maybe I'm dumb, but why are we still using steam turbines to turn heat energy into electricity?

There really isn't a more efficient process? Going from a heat source, transfer to water, change of state to gas, use hot gas + pressure to turn a mechanical generator/dynamo and THEN you get electricity.

There are so many failure points, maintenance points, and efficiency losses in that path.

We really have no means to convert heat energy to electricity? We do it with solar, we dont use sunlight to boil water.

What is missing here?

There are other methods, but they're far less efficient.
Solar panels convert light, not heat, into electricity. Specifically, photons (light) excite electrons in the solar cell, and these excited electrons then move through the solar cell and form a current.
This isn't really being used to generate electricity. They're developing a generator from it, but currently it's used for purely thermal energy transfer. Basically, the towns have big pipes running through them for communal heating, and these pipes are heated by this thermal energy storage.
I imagine they're only talking about electricity generation as an extra revenue stream for their customers who buy these, rather than it being a good solution for storing and generating electricity. The 90% efficiency is much better than combustion generators, but far worse than true electric batteries.
It is incredibly efficient at huge scale.
Large scale three phase energy generation is always something rotating in sync with the grid. Easiest way to do that is to spin turbine+generator.
All nuclear, coal, biomass power plants just heat water to 300-800°C and push it through turbine.
The thing is that it is really quite robust, and there isn't any other good solutions to it. They do have quite a lot of loss, but the cooled water after process (still over 100°C) can be used in other industries or district heating improving the efficiency.
Hydropower just spin the turbine with water flow. Wind directly spins the turbine, which is good for efficiency. Solar panels are still quite inefficient, but because they just use space, they make lot of sense even with poorer efficiency.
- Solar panels are still quite inefficient, but because they just use space, they make lot of sense even with poorer efficiency.
  You can't really compare those efficiencies with each other, between different technologies.
  With fuel, you're talking about how much energy per kg.
  With wind, you're talking about how much energy per m/s wind.
  With solar, you're talking about how much energy over the whole solar spectrum that gets through the atmosphere. However, a single junction p-n diode made of silicon is only meant to work at a specific wavelength, and will only get energy from around this wavelength, and as such could only ever get a maximum theoretical efficiency of ~36% of the total solar spectrum of light wavelengths. In the lab I think some have achieved ~33%.
  You can get higher efficiency solar cells, but you have to use novel materials and have multiple layers of different p-n junctions. Short wavelengths first, these materials are transparent to longer wavelengths, which are absorbed by lower layers. With a theoretical infinitely layered solar cell you could achieve ~88% of the solar spectrum energy. In reality it's really hard to make a semiconductor diodes that fit certain wavelengths, leaving gaps in the spectrum even with multiple diode layers.
  ~30% for solar cells sounds ridiculously low compared to like, maybe, 70% for fuels. But it's a completely different measurement. Grid scale battery systems are mayb 98-99% - but that's just electrical energy in and electrical energy out over a short time.
  There is no common denominator, but the solar energy is free.
- the cooled water after process (still over 100°C)
  Steam, right? Describing steam as cooled water seems somewhat odd to me, but it is indeed still water its gaseous form. So, cooled steam of water?