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It's probably not even that good a heat storage system. And that's for two reasons. 1. Sand has a lower heat capacity than water. Water is much cheaper than sa

It's probably not even that good a heat storage system. And that's for two reasons. 1. Sand has a lower heat capacity than water. Water is much cheaper than sand. Yes, you can't heat water to 600C, but that's not a problem if you're just wanting to heat houses with it. The only reason I can see that they want to use sand is so that they can deliver higher temperatures, which are necessary for some fairly rare demands, or so they can generate electricity, which their system is going to suck at anyway. 2. There are more efficient ways to get hold of heat. They're using electricity to heat it, using a resistive element. A heat pump (for example) can give you 3-5 times as much heat as the electricity you put in, so why not just heat the houses directly with much less electricity used? They are using solar electricity to heat the sand, but PV panels are typically only 21% efficient, and they're expensive. A concentrating solar water heater is very cheap and simple, and will give you your heat with a much greater efficiency.

amelius 4y ago
> Yes, you can't heat water to 600C, but that's not a problem if you're just wanting to heat houses with it. You can heat a home with 600C and some circuitry that steps it down to a comfy 22C. If that's more efficient than heating water, then I can see why they do it.
- mnw21cam 4y ago
  It's not more efficient, for the reasons stated earlier.
  - amelius 4y ago
    It can be more efficient in the sense that you need less space, for example.
audunw 4y ago
> 1. Sand has a lower heat capacity than water. Water is much cheaper than sand. Yes, you can't heat water to 600C The price of the storage material is probably almost completely irrelevant in the total cost of this solution, when comparing sand and water. They even mention using sand nobody else wants. But using water will probably increase the cost of everything else. If you can heat sand to a much higher degree, the fact that it has lower heat capacity is irrelevant on its own. The fact that they explicitly mention that they were setting out to find if a solid material was better than water makes me think they've considered all these factors and found sand to be better in total. "The fan is the only moving part and it's easy to replace if necessary" Heat pump is a good idea, but it will add quite a lot of up front and maintainance cost. I've looked at heat pump water heaters myself, and it's hard to get the economics to add up in the short term. With a water heater or a system like this, the heat pump isn't going to be running constantly, and when you're running it you need a lot of power, so you need a large unit. Ideally a heat pump should be running almost constantly at a low rate to make sure it's operating at high efficiency and to pay for itself in a reasonable time. I don't know if there even are heat pumps that can effectively heat things up to several hundred degrees? I think it makes more sense to put the heat pump between the "battery" and the building it's heating, so you draw less heat from the battery to supply a large amount of heat to the building. Then you can use the same heat pump to extract heat from the air when the "battery" is empty. You'll get much better utilization of the heat pump. Btw, I think significantly cheaper and better heat pumps would be a hail mary for solving the energy / climate change crisis. So many things like this would suddenly become more economically viable. > A concentrating solar water heater is very cheap and simple Sure, but that's kind of a different solution. This could take energy from the grid drawing power from many sources spread out over a wider area. A concentrating solar heater would require a lot of area at the exact place where you want the storage unit. That's not always viable. Using water would probably increase the total volume significantly too.
- Xylakant 4y ago
  > I think it makes more sense to put the heat pump between the "battery" and the building it's heating, so you draw less heat from the battery to supply a large amount of heat to the building. That’s not how heat pumps work. They move heat from one medium to another - so using a heat pump to move one Joule from the battery to the house will lower the amount of heat in the batter by one Joule, same as if you use any other means. The efficiency gain that heat pumps provide when heating houses is that they essentially cool some medium (air, ground, water…) outside the building and move the heat into the building. Moving the heat uses less electricity than turning the electricity into heat via a resistive heating element. The outside medium gets warmed up by sun, or retains heat from a warmer season.

amelius 4y ago

> Yes, you can't heat water to 600C, but that's not a problem if you're just wanting to heat houses with it. You can heat a home with 600C and some circuitry that steps it down to a comfy 22C. If that's more efficient than heating water, then I can see why they do it.

mnw21cam 4y ago
It's not more efficient, for the reasons stated earlier.
- amelius 4y ago
  It can be more efficient in the sense that you need less space, for example.

audunw 4y ago

> 1. Sand has a lower heat capacity than water. Water is much cheaper than sand. Yes, you can't heat water to 600C The price of the storage material is probably almost completely irrelevant in the total cost of this solution, when comparing sand and water. They even mention using sand nobody else wants. But using water will probably increase the cost of everything else. If you can heat sand to a much higher degree, the fact that it has lower heat capacity is irrelevant on its own. The fact that they explicitly mention that they were setting out to find if a solid material was better than water makes me think they've considered all these factors and found sand to be better in total. "The fan is the only moving part and it's easy to replace if necessary" Heat pump is a good idea, but it will add quite a lot of up front and maintainance cost. I've looked at heat pump water heaters myself, and it's hard to get the economics to add up in the short term. With a water heater or a system like this, the heat pump isn't going to be running constantly, and when you're running it you need a lot of power, so you need a large unit. Ideally a heat pump should be running almost constantly at a low rate to make sure it's operating at high efficiency and to pay for itself in a reasonable time. I don't know if there even are heat pumps that can effectively heat things up to several hundred degrees? I think it makes more sense to put the heat pump between the "battery" and the building it's heating, so you draw less heat from the battery to supply a large amount of heat to the building. Then you can use the same heat pump to extract heat from the air when the "battery" is empty. You'll get much better utilization of the heat pump. Btw, I think significantly cheaper and better heat pumps would be a hail mary for solving the energy / climate change crisis. So many things like this would suddenly become more economically viable. > A concentrating solar water heater is very cheap and simple Sure, but that's kind of a different solution. This could take energy from the grid drawing power from many sources spread out over a wider area. A concentrating solar heater would require a lot of area at the exact place where you want the storage unit. That's not always viable. Using water would probably increase the total volume significantly too.

Xylakant 4y ago
> I think it makes more sense to put the heat pump between the "battery" and the building it's heating, so you draw less heat from the battery to supply a large amount of heat to the building. That’s not how heat pumps work. They move heat from one medium to another - so using a heat pump to move one Joule from the battery to the house will lower the amount of heat in the batter by one Joule, same as if you use any other means. The efficiency gain that heat pumps provide when heating houses is that they essentially cool some medium (air, ground, water…) outside the building and move the heat into the building. Moving the heat uses less electricity than turning the electricity into heat via a resistive heating element. The outside medium gets warmed up by sun, or retains heat from a warmer season.