At the end of the day, a chemical battery is a reusable bomb that is designed to go off very, very slowly.
> Wood placed in an oven at 700°F. catches fire almost immediately. At oven temperatures of 450°-500°F., the wood gradually chars and usually ignites after several hours.
> “Pyrophoric carbon,” formed when wood slowly chars, absorbs and combines rapidly with oxygen. This produces heat which under certain conditions causes the charred wood eventually to catch fire at temperatures well below those required to ignite the original wood. Cases are recorded where wooden flooring in contact with steam pipes at 250°-300°F. has caught fire after years of EXPOSURE.-FACTORY MUTUAL RECORD.[1]
I disagree with most of the naysaying in this thread, but pretty much all energy storage that is easily accessed can also have a failure state that is difficult to handle. It would probably be impossible to save a structure with a 600°C blob resting against it.
[1] https://www.fireengineering.com/leadership/ignition-temperat...
Flywheel batteries are usually placed in small bunkers, because the failure mode of a giant rapidly-spinning concrete wheel jumping its bearings is not exactly pretty. Dams are usually designed with diversion channels and mechanisms to limit the damage if they burst. Etc.
Really, it's strange that we are so cavalier about lithium battery failures compared to the other sorts.
https://en.wikipedia.org/wiki/Energy_storage: “A device that stores energy is generally called an accumulator or battery”
“a group of two or more cells (see CELL sense 5) connected together to furnish electric current”
Battery is a generic term for collection of things working together, guns/generators/medical tests etc. This why batteries where initially called such, they are a collection of smaller cells operating together which doesn’t apply to say pumped storage.
Sometime battery is used descriptively Ex: “A Carnot battery is a type of energy storage system that stores electricity in thermal energy storage.” Note they said it’s a type of energy storage system rather than type of battery. https://en.wikipedia.org/wiki/Carnot_battery
Instead people try to chose words so their audience understand what was said because people aren’t telepathic.
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.
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.
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.
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.
Sounds like the battery is charged via electricity.
Ultimately its about energy transfer, efficiency, and storage.
Traditionally district heating systems use waste heat from thermal generation of electricity, which heat is free.