Finnish City Inaugurates 1 MW/100 MWh Sand Battery

Here's a summary of the themes from the Hacker News discussion, with direct quotes:

Advantages of Sand Over Water for Thermal Storage

A primary theme is the exploration of why sand is chosen over water for this type of thermal energy storage, centering on its ability to reach higher temperatures and its physical properties.

• Higher Temperature Capability: Sand can be heated to much higher temperatures than water, which is crucial for efficient thermal energy storage and certain industrial processes.

  • "The article mentions that they heat the sand to 500°C, which is not possible with water (well, at least not without turning into steam along the way)." - decimalenough
  • "For this specific use case, you need to heat to far above the boiling point of water to retain some thermal efficiency. Sand/rock is better suited for storing the thermal energy at ~500 celcius." - isoprophlex
  • "The Wikipedia article says: 'Rock, sand and concrete has a heat capacity about one third of water's. On the other hand, concrete can be heated to much higher temperatures (1200 °C) by for example electrical heating and therefore has a much higher overall volumetric capacity.'" - tarvaina
  • "The higher temperature output is a good point, you can't get 400C output for industrial processes from a 100C water based heat battery." - fulafel

• Volumetric Heat Capacity and Temperature Delta: While water has a higher specific heat capacity per unit mass, sand's ability to be heated to significantly higher temperatures allows for a much larger usable temperature range, leading to higher overall volumetric heat capacity.

  • "If I add some fixed amount heat to some fixed volume of water, it might rise by 1℃, while the same volume of concrete rises by 3℃. And by the same logic, on release, that fixed volume of water dropping by 1℃ releases 3x as much heat as when that fixed volume of concrete drops by 1℃." - internet_points
  • "So if you can max heat water to 100℃, and max heat concrete to 1200℃, and on release you let it go to 10℃ (probably the range is less in practice), then the water can drop 90℃ and the concrete 1190℃, so even if the water releases 3x the amount of heat per ℃, the water just releases 270 (per volume) while the concrete releases 1190 (per volume)" - internet_points

• Material Stability and Inertness: Sand (specifically silicon dioxide or soapstone like talc) is chemically stable at the high temperatures involved, unlike water at extreme temperatures or pressures.

  • "Pure, fine sand being mostly silicon dioxide, it melts at ~2000 and boils at ~3000 C, still without decomposing or reacting. It is really extremely chemically stable." - isoprophlex
  • "It's as inert as it gets" - LtdJorge
  • "This is crushed soapstone, so it's mostly talc. Talc is apparently more or less stable up to about 800C, where it starts to break down into enstatite and silica" - grues-dinner

• Ease of Handling and Safety: Sand is less problematic to handle at high temperatures compared to pressurized steam or superheated water, reducing risks and complexity.

  • "Sand also mostly stays where you put it. While obviously water can be put in tanks easily enough, there's still more maintenance and inspection required and a gigantic watertight tank that will last n decades is substantially more expensive then a steel sand box." - grues-dinner
  • "Unplanned release of that much water at 100C is also extremely dangerous. Whereas even 500C sand will mostly just sit there." - grues-dinner
  • "The hassle of handling hot water is also presumably why they use hot air rather than water as a working fluid for heating the sand in the first place. The worst case if you spring a leak in a heat-transfer tube inside the tank is that a bit of air escapes. Leaking super-heated high-pressure water or steam into the (unpressurised) tank would be a much larger problem..." - grues-dinner

• Insulation Properties: Sand itself can act as an insulator, and its low convection reduces heat loss.

  • "Insulation isn't such an issue with sand because sand itself is fairly good insulator and obviously doesn't convect. 1m of sand is about the same as 10cm of air." - grues-dinner

Efficiency and Economic Considerations

The discussion frequently touches upon the efficiency of sand batteries compared to other storage methods, particularly in the context of renewable energy economics.

• Round-trip Efficiency: While some users mention a high round-trip efficiency, others clarify that this often refers only to the heat transfer, not the overall electricity-to-heat-to-electricity cycle.

  • "It can store 100 MWh, with a round trip efficiency of 90%." - tarvaina (quoting the article)
  • "It's storing heat, so if you need electricity then you eat a lot of efficiency. I think Vernon said ~45% round trip efficiency. Batteries are 90%+." - jnsaff2
  • "Yeah the efficiency is much less than 40% if you compare to heat pumps. It'll be something like 15% compared to those." - HPsquared
  • "smokel: That's only in heat transfer, that does not include generating electricity from it." - smokel (responding to bjoli)

• Cost of Energy Input: A key argument for the viability of sand batteries is that the energy used to heat the sand, often surplus renewable electricity, can be very cheap or even negative in price.

  • "I like these technologies. They may not be as energy efficient as using more exotic materials, but what they do use is simple, cheap and often sourced locally. Such economic factors are often as important to the ROI as the purely scientific ones." - kitd
  • "yes and given that the energy you put in is practically free, it doesn't matter if it's not as efficient." - pilif
  • "I think with enough renewable in the grid, there will always be times when the costs are 0 or negative, so you can help stabilize the grid by consuming." - looofooo0

• Scalability and Insulation Costs: As the systems scale up, the cost of insulation becomes less significant relative to the total capacity.

  • "Square cube scaling means that insulation becomes trivial in total costs as you scale the installation up. Something that's convenient for a single household would probably be too hard to insulate, but this thing holds 2000t of sand." - Tuna-Fish

• Potential for Profitability: The long lifespan of these systems suggests they could be profitable over time, even with lower efficiencies, due to low energy costs.

  • "Given the supposed 50+ year lifespan of such a battery, I find it hard to believe it doesn't turn a profit at some point." - arowthway

Comparison to Water-Based Thermal Storage and Other Systems

The discussion also highlights existing and alternative thermal storage methods, particularly those using water.

• Existing District Heating Infrastructure: Finland, where this technology is being deployed, already has established district heating networks, and water-based thermal batteries are also in use.

  • "Several cities in Finland have water based thermal batteries already, connected to local district heating networks." - anttisalmela
  • "Puts: Helsinki did the same thing but storing the energy in water instead of sand: [link]" - Puts

• High-Pressure Water Systems: While technically possible to heat water above its boiling point, it requires extreme pressures, introducing significant safety and engineering challenges.

  • "To be pedantic, yes you can but you'd need to pressurize it to uuhh... According to this calculator [0], you can get water to 370 degrees C if the pressure is 207 atmospheres, which is about the pressure of the ocean two kilometers deep." - Cthulhu_

• Rocket Mass Heaters: Some users draw a parallel between the sand battery and smaller-scale concepts like rocket mass heaters.

  • "Interestingly this looks like the same principle as a rocket mass heater or masonry heater, but on a larger scale and powered by renewable energy." - Fwirt

• Underground vs. Surface Storage: There's curiosity about why a surface silo is used instead of underground storage.

  • "So why a surface sand silo instead of going down and using the soil/clay/bedrock/whatever is there? Ease of installation and maintenance?" - Cthulhu_
  • "presumably you need the insulation from the air, otherwise it would just sort of dissipate through the ground" - whimsicalism
  • "A housing complex near mine got a massive tank like this installed thirty years ago, and I think they put it underground to be able to build a house on top." - Findecanor

Energy Equivalence and Scale

A few comments offer perspective on the energy stored by comparing it to more familiar concepts like TNT.

• TNT Equivalence: The energy stored in the sand battery is framed in terms of explosive potential.
  • "How many kiloton of TNT equivalent?" - nick49488171
  • "Well, if you say the energy stored is the 100MWh from the headline figure, and say you can arrange release every joule of all at once by flashing high-pressure water to steam at 1 atm that's about 0.1kT." - grues-dinner

Skepticism and ROI Concerns

Despite the apparent advantages, some users express skepticism, particularly regarding the return on investment (ROI) and overall practicality.

• ROI Calculation: A recurring question is about the economic feasibility and payback period.

  • "Very interesting. Did anyone find a ROI calculation?" - slumpvaldperson
  • "A comment on the YouTube video below complained, “Not a word about return on investment in the presentation. That means it’ll never pay off”" - arowthway

• "Too Good to Be True" Sentiment: Some find the concept almost unbelievable and welcome further real-world data.

  • "Really interested in seeing how it fares in reality, almost sounds too good to be true." - napoleoncomplex
  • "jnsaff2: Why too good to be true?" - jnsaff2

• Efficiency Drawbacks: The lower electricity-to-heat efficiency, especially when compared to heat pumps, is a point of concern for some.

  • "The storage is at a high temperature (500-600C) which means that you can't use heat-pumps to produce the heat to be stored. This means that you miss out on ~400% energy gains possible from converting electricity to heat. So the efficiency is pretty low." - jnsaff2

Other Observations

• On-Brand for Finnish Companies: The concept of a "gigantic sauna heater" is noted as being characteristic of Finnish engineering.

  • "So, a gigantic sauna heater? Very on brand Tommi and Markku!" - jonespen

• District Cooling Potential: A user from a desert climate wonders if a similar system could work for cooling.

  • "I live in a desert where we have district cooling (and no shortage of sand or solar power), instead of district heating. Wonder if they can pull off the same trick." - petesergeant
  • "In theory, yeah, cooling the sand would work, and it wouldn't freeze / expand. You'd need to use a coolant that doesn't freeze though, and of course keep any liquid out of it." - Cthulhu_