3 ms·

Why sand? - water has 5x the specific heat capacity of sand - water can be pumped around easily - water can be used directly for district heating

woodruffw 4y ago
Sand is non-corrosive and doesn't expand significantly at 500C (it takes temperatures ~3x that to get it to melt). That doesn't answer the "why" completely, but those are probably some of the factors. Edit: the article doesn't explain how the "battery" works, but my intuition is that the sand doesn't move. It's likely merely heated and then passively heats whatever actual transportation medium is used (likely water).
vkou 4y ago
Presumably, because sand can be heated to above 100C. Given Newton's law of cooling, it's not entirely clear to me why you want your thermal battery to be heated to above 100C... But I'm not an engineer - I only play one on TV.
- 8bitsrule 4y ago
  Sand (much denser = sinks in water) has low thermal conductivity, and isn't much more expensive. Simply put, in two bodies of equal volume and 'heat capacity', the one that's hotter stores more heat. Sand properties: [https://material-properties.org/sand-density-heat-capacity-thermal-conductivity/ https://material-properties.org/sand-density-heat-capacity-t...]
  - tommiegannert 4y ago
    From your link, sand has much worse heat capacity than water at 800 J/kgK vs 4000 for water.
    - lazide 4y ago
      Water also boils at STP at 100C, which limits upper energy storage.
- datadata 4y ago
  Newton's law of cooling just says that your loss is going to be proportional to the temperature difference. But 1) the capacity of the battery is also proportional to the temperature difference, so the loss as a percentage of total energy stored should be roughly a constant. 2) If the application is home heating, you can't heat the home to a higher temperature than the battery itself, and you also can't easily group together the energy into a higher temperature. You would on the other hand want a high temperature for the heat to transfer quickly from the convection medium to the actual houses. So starting hot would make a lot of sense despite the downsides?
droopyEyelids 4y ago
My guesses: - Water would either require a tremendous sized reservoir, or infrastructure capable of withstanding the pressure of superheated steam - superheated steam, and even water to an extent, has corrosive chemistry. Sand is basically inert. Those are just guesses though. I'd love an engineer who works on this to break down the real reasons.
fulafel 4y ago
It seems to be about buffering heat energy for district heating in a CHP (combined heat and power) generation station, in presence of fast electricty market swings (think wind power). So would allow the CHP plant to ramp up generation quickly to supply electricity at a high hourly price, while buffering the produced heat peak to be more slowly drained to the district heating network. Against this background it seems relatively cheap to build for the small size of plant in the picture, vs water tanks, and less risky (water entering the DH network has to be >100 C which means pressurization which in a big tank means a risk of a big boom). It's true that water is pretty great for this also due to high heat capacity and that's why most existing heat batteries in cities are using water, that's why this is "novel" and "patented"...

woodruffw 4y ago

Sand is non-corrosive and doesn't expand significantly at 500C (it takes temperatures ~3x that to get it to melt). That doesn't answer the "why" completely, but those are probably some of the factors. Edit: the article doesn't explain how the "battery" works, but my intuition is that the sand doesn't move. It's likely merely heated and then passively heats whatever actual transportation medium is used (likely water).

vkou 4y ago

Presumably, because sand can be heated to above 100C. Given Newton's law of cooling, it's not entirely clear to me why you want your thermal battery to be heated to above 100C... But I'm not an engineer - I only play one on TV.

8bitsrule 4y ago
Sand (much denser = sinks in water) has low thermal conductivity, and isn't much more expensive. Simply put, in two bodies of equal volume and 'heat capacity', the one that's hotter stores more heat. Sand properties: [https://material-properties.org/sand-density-heat-capacity-thermal-conductivity/ https://material-properties.org/sand-density-heat-capacity-t...]
- tommiegannert 4y ago
  From your link, sand has much worse heat capacity than water at 800 J/kgK vs 4000 for water.
  - lazide 4y ago
    Water also boils at STP at 100C, which limits upper energy storage.
datadata 4y ago
Newton's law of cooling just says that your loss is going to be proportional to the temperature difference. But 1) the capacity of the battery is also proportional to the temperature difference, so the loss as a percentage of total energy stored should be roughly a constant. 2) If the application is home heating, you can't heat the home to a higher temperature than the battery itself, and you also can't easily group together the energy into a higher temperature. You would on the other hand want a high temperature for the heat to transfer quickly from the convection medium to the actual houses. So starting hot would make a lot of sense despite the downsides?

droopyEyelids 4y ago

My guesses: - Water would either require a tremendous sized reservoir, or infrastructure capable of withstanding the pressure of superheated steam - superheated steam, and even water to an extent, has corrosive chemistry. Sand is basically inert. Those are just guesses though. I'd love an engineer who works on this to break down the real reasons.

fulafel 4y ago

It seems to be about buffering heat energy for district heating in a CHP (combined heat and power) generation station, in presence of fast electricty market swings (think wind power). So would allow the CHP plant to ramp up generation quickly to supply electricity at a high hourly price, while buffering the produced heat peak to be more slowly drained to the district heating network. Against this background it seems relatively cheap to build for the small size of plant in the picture, vs water tanks, and less risky (water entering the DH network has to be >100 C which means pressurization which in a big tank means a risk of a big boom). It's true that water is pretty great for this also due to high heat capacity and that's why most existing heat batteries in cities are using water, that's why this is "novel" and "patented"...