It's more effective volume wise. I did the math, some time ago here are the rough numbers.
Sand has way less heat capacity then water per kg (about half).
Water can be heated to 95C with standard unpressurized vessel. Sand in this application is heated to 600C.
Sand is denser then water (kg/m3).
For the same heat energy stored this comes out to about 2.5x more volume of water(95C) compared to sand(600C).
Water and Sand are both dirt-cheap.
Hot water can be managed with standard plumbing equipment.
Sand needs some high temperature piping (hot air to water heat-exchanger, resistive heat tho heat up the sand).
How well both contain the heat is primarily dependent on the isolation. Which favors the smaller footprint of sand, but needs to isolate a higher temperature difference...
One advantage of heating water over sand is that you can heat it up with high temperature heat pumps which currently have CoPs ranging between 2.4 to 5.8 [1]. So for every kW of electrical energy you put in you get at least 2.4kW of thermal energy out.
So yes, the volume of 95C water would be much greater than that of 600C sand, but if volume wasn't an issue you could do it much more efficiently. Alternatively, you could use battery storage for just the electrical capacity required and not the (much higher) thermal capacity which may be more cost effective when you look at the conversion.
The temperature also matters. If you need at least 50 C water to run district heating, about half of the energy stored in near boiling water cannot be utilized. This is much less with 600 C sand.
And 50C is too low. That is minimum temperature of the heated tap water(Legionella and other diseases). And preferably you want some higher. And then as distance increases there are losses and other people using the heat. So temperature you need is actually quite high and in very cold days can be over 100C...
Probably not relevant to the specific problem at hand because sand that's getting heated to 600 degrees is going to quickly boil off any residual moisture. As a warning, though, so that people don't experience the pain I got to experience, a pile of sand is really good at holding onto water internally and freezing when it gets cold. We had to replace our sewer line a while back and for various reasons were taking care of filling the hole and redoing the concrete ourselves. The sand guys left a big pile in the driveway for us and as soon as winter came the whole pile turned into a single giant rock despite having been out in the hot sun previously.
Exactly to your point, though, one of the great things about using sand for this application instead of water is that you can probably just shut the thing off for maintenance without having to worry about draining all of the sand out. If it freezes up due a bit of residual moisture content it's not going to expand nearly the same way that a silo full of water would, and it should be easy enough to thaw out just by putting some heat into it.
A quick caveat/clarification: It's only true if you're pushing the system over the 100°C mark. Otherwise a volume of liquid water--with its greater latent heat-capacity--will outclass the same volume of sand.
Water's heat-capacity is 4.186 J/g°C, while estimates for sand run towards ~0.830 J/g°C. If we also assume the sand is 1.6x denser, then our below-boiling water still comes out ahead at ~3.15x the joules per volume.
There are hints [0] this system tops out around 600°C.
I think the original plan was to convert the heat back into electricity with a turbine. So the higher temperature of sand would greatly improve thermodynamic efficiency.
>I think the original plan was to convert the heat back into electricity with a turbine.
Is that just speculation or did you read it somewhere? IIRC the original motivation of PNE was a bunch of engineers at uni speculating on how to build the perfect building for engineers, and making it self-sufficient would require handling its own heating, which they originally thought would be best done with a big hot-water tank to store the heat. No turbine was suggested, IIRC.
In addition to what others have said, isn't one of the weird properties of water that it tends to take in energy easily, but not give it back so easily? I've never really understood how that works, but I think I've leqrned that at some stage. Hence why it's used in cooling so much. Somebody jump in and tell me how wrong I am, or if I'm on a track that doesn't lead completely nowhere.
Water vaporizes, and at that point blows up just about every container you can build around it. As will ice when it cools down. Sand is just sand, very little difference, very unreactive from way under freezing temps to about 1300 degrees.
And you might vent steam, but you should probably take into account that while water < 45 degrees or so is pretty innocent, steam will strip flesh from bone starting at 180 degrees or so, it won't "just" burn you.
That’s the other bizarre thing about water/ice: most things expand as you heat them, but very few things expand as you cool them. Water has maximum density at 4C, so even before you freeze it it’s already starting to expand as you cool it.
ah OK, I get your point :) It doesn't really matter though because water when liquid doesn't cause any problem when expanding or contracting, its level in a vessel will just slightly change. It's only the ice that can fuck up pipes etc
Note here is also that district heating uses water that is heated to 65-115C.
Which means that you have rather little of delta to work with. And at upper end it becomes somewhat risky to have large container of water that is beyond boiling in normal pressure...
With sand you can use very simple heat-exchangers. No need to use exotic heat pumps that require extra energy...
