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The article mentions that they heat the sand to 500°C, which is not possible with water (well, at least not without turning into steam along the way).

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To be pedantic, yes you can but you'd need to pressurize it to uuhh... According to this calculator [0], you can get water to 370 degrees C if the pressure is 207 atmospheres, which is about the pressure of the ocean two kilometers deep.

[0] https://www.engineeringtoolbox.com/water-vapor-saturation-pr...

How many kiloton of TNT equivalent?

Well, if you say the energy stored is the 100MWh from the headline figure, and say you can arrange release every joule of all at once by flashing high-pressure water to steam at 1 atm that's about 0.1kT.

So quite a bang - allegedly this is 200lb, so about the same: https://www.youtube.com/watch?v=ZDgvar7ON54

Interestingly that's also about the pressure of gas in scuba tanks. Can't imagine how much energy to pressurize water to that

Due to incompressibility of liquids, pressurizing a liquid is very cheap energy-wise - orders of magnitude cheaper than pressurizing a gas. The issue is that pressurized liquid also requires correspondingly strong and expensive vessels and pipes.

You don't need to pressurize it. You just put it in a tank and apply heat, the water evaporates and creates pressure. Like a pressure cooker.

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