Green Energy

Adding little bit extra context here.

In Finland large number of homes are heated by district heating. This means there is heat plant or combined heat and power (CHP) plant, which heats water to around 70C - 110C (158F - 230F) and that is distributed to homes.

In this system heat batteries are useful, and near all CHP plants in Finland have done heat battery in last few years. These heat batteries are just 7-store high insulated water storage. Usefulness to store just water is that system is relatively simple. Water in - water out. This makes it that the turbine in CHP can be run more freely towards electricity price, not the network heat demand.

Plain heat plants generally don't yet have these, because the buffer created by heat battery is not that much needed. But if these sand batteries can store heat longer, and because they might be cheaper to build, it can make sense.

Source: I work in company that owns 19 district heating networks.

it’s hard to get any cheaper than the crushed soapstone now housed inside an insulated silo in the small town of Pornainen. The soapstone was basically trash — discarded from a Finnish fireplace maker.

Hell yeah. I'm glad that this works with that kind of sand, and is being done without the kind we're running out of.

I heard that the electrolytic medium used in this battery is coarse and gets everywhere

They report about 10/15% loss when doing heat -> heat. Cost per Kilowatt of storage about 25 euro, compared to 115 if lithium batteries were used.

It's a pretty neat system:

• can be set up anywhere
• can supply high grade heat (process heat, not mere space heating heat)

However, heat stores are subject to scaling laws which don't favour sand on the large scale, at least unless it's underground (and then you have to keep groundwater out to avoid vaporizing it). Large thermal stores benefit from storing heat in water, and placing the water deep underground, so the boiling point rises. If local rock has low thermal conductivity, even better.

For comparison Helsinki (.fi) has a 10 GWh underground thermal store. Where I live, Tallinn (.ee) will soon get a 1 GWh surface thermal store. And Vantaa (.fi) will soon complete a whopping 90 GWh thermal store that's located 100 m underground, so their water will boil at 140 C instead of the usual 100 C. Boiling points up to 300 C are attainable in practise, then the curve starts leveling out.

What a stupidly simple yet clever idea.

100MWh from that one little silo, that's incredible.

Here's the operator's website https://polarnightenergy.com/

this shit is so cool. it’s like those potential-energy-battery ideas with the stacked blocks, but ACTUALLY efficient!!

Pornainen’s battery is charged using electricity from the grid

How so, do they use heater resistors or a heat pump?

Electricity may be cheap sometimes but still, heater resistors aren't the most efficient.