https://sunamp.com/en-gb/hot-water-solutions-thermino-range/
The phase change stuff has positives like taking up less physical space but it's also a much less mature tech than storing hot water.
In the UK there was a unfortunate trend of ripping out these energy storage devices and replacing hot water tanks with on demand electric hot water heating ( only heat the water you need ). And new builds often have no tanks ( as it saves space in the new tiny homes ).
Very short sighted in my view - a very simple way to store energy and everyone uses hot water directly.
Versus resistance, which is exactly as efficient at 0°C and 1000°C, and why those storage heaters used to make sense.
(And storage is directly proportional to temperature differential above interior ambient)
No one is storing 1000C water at home.
It is true that the temperature deltas affects efficiency. You can use the thermocline to draw from the cooler lower portion of the storage tank to push this further. Or less technically, just a bigger tank, though this has some tradeoffs.
In warmer countries they are set up differently can act as free air conditioning by extracting heat from indoor air at the same time as heating water.
Right, but UK has/had "storage heaters" which were bricks with nichrome wire. They would heat the bricks really hot during cheap electricity times, and use that heat the rest of the day.
EDIT: I misread "ripping out these energy storage devices" as https://en.wikipedia.org/wiki/Storage_heater
Of course heat pumps for DHW should all have a tank for smoothing demand across several hours.