8 ms·

For more on the general problem of storing thermal (rather than electrical) energy on a seasonal basis see https://en.wikipedia.org/wiki/Seasonal_thermal_energ

For more on the general problem of storing thermal (rather than electrical) energy on a seasonal basis see https://en.wikipedia.org/wiki/Seasonal_thermal_energy_storage https://en.wikipedia.org/wiki/Seasonal_thermal_energy_storag... I was especially surprised to learn that serious attempts at storing heat in soil and rock have been made https://en.wikipedia.org/wiki/Seasonal_thermal_energy_storage#Underground_thermal_energy_storage https://en.wikipedia.org/wiki/Seasonal_thermal_energy_storag... https://en.wikipedia.org/wiki/Drake_Landing_Solar_Community https://en.wikipedia.org/wiki/Drake_Landing_Solar_Community which in the winter is able recover >40% of the energy stored in the ground through boreholes in the summer ("BTES Efficiency") https://www.dlsc.ca/reports/swc2017-0033-Mesquita.pdf https://www.dlsc.ca/reports/swc2017-0033-Mesquita.pdf

nikanj 5y ago
Furthermore, we have a real problem with the soil storing heat: https://citymonitor.ai/transport/londons-tube-has-been-running-so-long-its-literally-raising-temperature-earth-around-it https://citymonitor.ai/transport/londons-tube-has-been-runni...
- alexchamberlain 5y ago
  Stupid question: can we harness this 100 year old heat to power something today?
  - thescriptkiddie 5y ago
    Yes https://www.newcivilengineer.com/innovative-thinking/harnessing-heat-from-the-tube-11-05-2020/ https://www.newcivilengineer.com/innovative-thinking/harness...
  - vikingerik 5y ago
    You can't exactly harness heat; what can be harnessed is a temperature differential, to convert into some form of mechanical motion or electrical potential. You'd need some apparatus that connects the hot mass and some cold sink with thermocouples. Whether that's possible or feasible on the scale of the London Tube, I don't know. https://en.wikipedia.org/wiki/Thermoelectric_effect https://en.wikipedia.org/wiki/Thermoelectric_effect Instead of that, probably a more feasible way to make use of heat is just as heat, by some way of circulating it into buildings in the winter. One interesting angle of looking at this would be that it's really an artificially created geothermal source.
    - simulate-me 5y ago
      Heat is energy, so I find it surprising that it can't be harnessed. Is it really not possible to take some of the energy out of a system and use it for something?
      - willcipriano 5y ago
        Air conditioning is really silly when you think about it. It takes a ton of energy to move some energy outside. It feels like it could be self powered.
        BenjiWiebe 5y ago
        It's actually forcing the energy apart. The energy wants make this one big mess where every bit is the same temperature as every other bit. Your AC is working hard at pushing all the heat outdoors and making a pocket of cool inside.
        ouchjars 5y ago
        Self-powered evaporative air conditioning is an ancient technology. Air conditioning as we understand and use it today is a gold-plated band-aid over bad design. It's so entrenched you almost can't get a house built any other way.
      - ac29 5y ago
        Not all heat is useful. If there was a magic device that could take any heat and convert it into electricity, global warming could be trivially reversed.
        arcticbull 5y ago
        There is the Seebeck effect, where you run a Peltier junction in reverse. Efficiency is, to put it mildly, low. [1] [1] https://en.wikipedia.org/wiki/Thermoelectric_generator https://en.wikipedia.org/wiki/Thermoelectric_generator
  - stubish 5y ago
    Pump cold air down, suck warmer air up, heat buildings in winter. Same idea lots of passive houses use to regulate temperature.
pfdietz 5y ago
Storing heat in rock is very natural. It's how geothermal works, after all. The basic idea is that the thermal time constant of a uniform sphere (or other shape of fixed aspect) is proportional to the square of its radius. So, for a sufficiently large sphere, the time constant can become very long, >> 1 year. Rock is a good choice because it's very cheap. The other thing one can do is store cold. This is useful not only for cooling, but for increasing the efficiency of storage of work, since one can generate heat and cold via a heat pump, then reverse that cycle to recover the work. An efficiency of 65% or so may be reasonably achievable with the high temperature in the temperature range of ordinary steel.
- jessriedel 5y ago
  > The basic idea is that the thermal time constant of a uniform sphere (or other shape of fixed aspect) is proportional to the square of its radius. Shouldn't the thermal mass go like volume (radius cubed) while the power conducted away go like surface area (radius squared) so the time to cool should go like mass/power ~ volume/area, so linearly in radius?
  - pfdietz 5y ago
    The rate at which heat conducts out is also affected by the temperature gradient, which is inversely proportional to radius.
    - deleted 5y ago
      [deleted]

nikanj 5y ago

Furthermore, we have a real problem with the soil storing heat: https://citymonitor.ai/transport/londons-tube-has-been-running-so-long-its-literally-raising-temperature-earth-around-it https://citymonitor.ai/transport/londons-tube-has-been-runni...

alexchamberlain 5y ago
Stupid question: can we harness this 100 year old heat to power something today?
- thescriptkiddie 5y ago
  Yes https://www.newcivilengineer.com/innovative-thinking/harnessing-heat-from-the-tube-11-05-2020/ https://www.newcivilengineer.com/innovative-thinking/harness...
- vikingerik 5y ago
  You can't exactly harness heat; what can be harnessed is a temperature differential, to convert into some form of mechanical motion or electrical potential. You'd need some apparatus that connects the hot mass and some cold sink with thermocouples. Whether that's possible or feasible on the scale of the London Tube, I don't know. https://en.wikipedia.org/wiki/Thermoelectric_effect https://en.wikipedia.org/wiki/Thermoelectric_effect Instead of that, probably a more feasible way to make use of heat is just as heat, by some way of circulating it into buildings in the winter. One interesting angle of looking at this would be that it's really an artificially created geothermal source.
  - simulate-me 5y ago
    Heat is energy, so I find it surprising that it can't be harnessed. Is it really not possible to take some of the energy out of a system and use it for something?
    - willcipriano 5y ago
      Air conditioning is really silly when you think about it. It takes a ton of energy to move some energy outside. It feels like it could be self powered.
      - BenjiWiebe 5y ago
        It's actually forcing the energy apart. The energy wants make this one big mess where every bit is the same temperature as every other bit. Your AC is working hard at pushing all the heat outdoors and making a pocket of cool inside.
      - ouchjars 5y ago
        Self-powered evaporative air conditioning is an ancient technology. Air conditioning as we understand and use it today is a gold-plated band-aid over bad design. It's so entrenched you almost can't get a house built any other way.
    - ac29 5y ago
      Not all heat is useful. If there was a magic device that could take any heat and convert it into electricity, global warming could be trivially reversed.
      - arcticbull 5y ago
        There is the Seebeck effect, where you run a Peltier junction in reverse. Efficiency is, to put it mildly, low. [1] [1] https://en.wikipedia.org/wiki/Thermoelectric_generator https://en.wikipedia.org/wiki/Thermoelectric_generator
- stubish 5y ago
  Pump cold air down, suck warmer air up, heat buildings in winter. Same idea lots of passive houses use to regulate temperature.

pfdietz 5y ago

Storing heat in rock is very natural. It's how geothermal works, after all. The basic idea is that the thermal time constant of a uniform sphere (or other shape of fixed aspect) is proportional to the square of its radius. So, for a sufficiently large sphere, the time constant can become very long, >> 1 year. Rock is a good choice because it's very cheap. The other thing one can do is store cold. This is useful not only for cooling, but for increasing the efficiency of storage of work, since one can generate heat and cold via a heat pump, then reverse that cycle to recover the work. An efficiency of 65% or so may be reasonably achievable with the high temperature in the temperature range of ordinary steel.

jessriedel 5y ago
> The basic idea is that the thermal time constant of a uniform sphere (or other shape of fixed aspect) is proportional to the square of its radius. Shouldn't the thermal mass go like volume (radius cubed) while the power conducted away go like surface area (radius squared) so the time to cool should go like mass/power ~ volume/area, so linearly in radius?
- pfdietz 5y ago
  The rate at which heat conducts out is also affected by the temperature gradient, which is inversely proportional to radius.
  - deleted 5y ago
    [deleted]