5 ms·

HVAC engineer here. At the residential level this is already achievable with hot water storage from either solar thermal and/or heat pumps. Plus you get to use

by tjmc 4y ago

HVAC engineer here. At the residential level this is already achievable with hot water storage from either solar thermal and/or heat pumps. Plus you get to use the hot water (assuming you've sized the tank correctly!) Storing heat in rocks or sand doesn't make sense from a thermal efficiency standpoint unless the temperature is high and that isn't economically viable unless you have large volume storage - typically district or precinct level.

WalterBright 4y ago
How can it not be efficient? The heat to heat a rock is 100% recovered when the rock cools back down. A tank of water works, too, but tanks rust, mildew, and leak.
- olau 4y ago
  I'm not an engineer, but I've been following the various rock storage projects, like this one https://www.stiesdal.com/storage/ https://www.stiesdal.com/storage/ for some time, and water tanks are simpler. When you set up a water tank, you just pump water into it to get the storage medium, and when you need to extract heat, you can pump out actual storage medium and do the heat exchange directly. With a rock storage, you either need pipes in them or move air through the rocks, which requires a much beefier overall system since it's harder to extract the heat. Does that make sense? Rusting tanks sounds like a problem with either badly engineered mismatched piping, or badly engineered tanks. Ideally, you'd have a well-engineered tank as a closed system with a heat exchange unit next to it. By the way, I live in a town with a wide district heating system, and there's no storage at all in the individual houses here, only a small heat exchange unit. They are building a 200.000 cubic meter water storage system in the other end, though. I don't know about this particular project, but there are other projects where the water storage is large enough and insulated well enough that they actually do store cheap solar heat from the summer through the winter - and it's cheaper than using gas, even before the current price spikes.
  - WalterBright 4y ago
    Hot water heaters are always rusting and then leaking. Corrosion is always a problem with water and metal, along with sedimentation and the growth of slime. With the box of rocks, yes you'll need to duct hot air in to heat them, and duct cool air in to extract the heat and blow it through the house. I don't know that this needs a beefy system, if you've got central air there is already most of the duct work in place.
- UkrainianJew 4y ago
  Heat keeps leaking out. The amount of stored heat is proportional to the cube of the linear size, the leak rate is proportional to the surface area (square of the linear size). Hence, the bigger the storage, the higher the efficiency. Same why a flea can jump >10x its height, and a human cannot (square growth of strength, cubic growth of weight).
  - WalterBright 4y ago
    Thanks for the answer. I suppose the answer is a decent insulation on the box. It only has to last for a few hours. Of course, some engineering would have to go into optimizing the size of the rocks and the surface area / mass for the expected use.
    - mtrycz2 4y ago
      So like, sand?
    - lazide 4y ago
      The other issue is ‘heat quality’ (or temperature deltas). It’s easier to recover a given amount of heat from very hot sources than from only luke warm sources, and smaller heat stores will have a very pronounced difference very quickly, resulting in control issues. Basically, if you only have a small amount of sand, it’s harder to keep it warm for longer (more surface area for the mass, so more insulation required), and harder to deal with effectively (fan needs to blow much harder when it’s only warm, much less when it’s hot), etc.
  - GistNoesis 4y ago
    I'm not sure if the leak is really proportional to the surface area. (It's probably even less due to how the heat equation works). The equation for the thermal resistance of a cylindrical system is explained in https://youtu.be/6x-jdCGWuHI?t=801 https://youtu.be/6x-jdCGWuHI?t=801 The thermal resistance is the sum of 3 (or more) terms, and while the outer terms are indeed proportional to surface area, I'm not sure if the physical constants of the system aren't designed so that the middle heat conductive terms of resistance R=ln(r2/r1)/(2pikL) is the dominant factor.
riku_iki 4y ago
Article says they heat sand up to 500C, which is not very possible with water.
- wikfwikf 4y ago
  They have to heat to 500C, because recovering the energy is slow unless you have those high temperatures. The advantage of water is that it's easy to extract the heat energy and transmit it to where you want it.

by tjmc 4y ago

WalterBright 4y ago

How can it not be efficient? The heat to heat a rock is 100% recovered when the rock cools back down. A tank of water works, too, but tanks rust, mildew, and leak.

olau 4y ago
I'm not an engineer, but I've been following the various rock storage projects, like this one https://www.stiesdal.com/storage/ https://www.stiesdal.com/storage/ for some time, and water tanks are simpler. When you set up a water tank, you just pump water into it to get the storage medium, and when you need to extract heat, you can pump out actual storage medium and do the heat exchange directly. With a rock storage, you either need pipes in them or move air through the rocks, which requires a much beefier overall system since it's harder to extract the heat. Does that make sense? Rusting tanks sounds like a problem with either badly engineered mismatched piping, or badly engineered tanks. Ideally, you'd have a well-engineered tank as a closed system with a heat exchange unit next to it. By the way, I live in a town with a wide district heating system, and there's no storage at all in the individual houses here, only a small heat exchange unit. They are building a 200.000 cubic meter water storage system in the other end, though. I don't know about this particular project, but there are other projects where the water storage is large enough and insulated well enough that they actually do store cheap solar heat from the summer through the winter - and it's cheaper than using gas, even before the current price spikes.
- WalterBright 4y ago
  Hot water heaters are always rusting and then leaking. Corrosion is always a problem with water and metal, along with sedimentation and the growth of slime. With the box of rocks, yes you'll need to duct hot air in to heat them, and duct cool air in to extract the heat and blow it through the house. I don't know that this needs a beefy system, if you've got central air there is already most of the duct work in place.
UkrainianJew 4y ago
Heat keeps leaking out. The amount of stored heat is proportional to the cube of the linear size, the leak rate is proportional to the surface area (square of the linear size). Hence, the bigger the storage, the higher the efficiency. Same why a flea can jump >10x its height, and a human cannot (square growth of strength, cubic growth of weight).
- WalterBright 4y ago
  Thanks for the answer. I suppose the answer is a decent insulation on the box. It only has to last for a few hours. Of course, some engineering would have to go into optimizing the size of the rocks and the surface area / mass for the expected use.
  - mtrycz2 4y ago
    So like, sand?
  - lazide 4y ago
    The other issue is ‘heat quality’ (or temperature deltas). It’s easier to recover a given amount of heat from very hot sources than from only luke warm sources, and smaller heat stores will have a very pronounced difference very quickly, resulting in control issues. Basically, if you only have a small amount of sand, it’s harder to keep it warm for longer (more surface area for the mass, so more insulation required), and harder to deal with effectively (fan needs to blow much harder when it’s only warm, much less when it’s hot), etc.
- GistNoesis 4y ago
  I'm not sure if the leak is really proportional to the surface area. (It's probably even less due to how the heat equation works). The equation for the thermal resistance of a cylindrical system is explained in https://youtu.be/6x-jdCGWuHI?t=801 https://youtu.be/6x-jdCGWuHI?t=801 The thermal resistance is the sum of 3 (or more) terms, and while the outer terms are indeed proportional to surface area, I'm not sure if the physical constants of the system aren't designed so that the middle heat conductive terms of resistance R=ln(r2/r1)/(2pikL) is the dominant factor.

riku_iki 4y ago

Article says they heat sand up to 500C, which is not very possible with water.

wikfwikf 4y ago
They have to heat to 500C, because recovering the energy is slow unless you have those high temperatures. The advantage of water is that it's easy to extract the heat energy and transmit it to where you want it.