I'm fascinated with seasonal heat storage using large underground water tanks. Apparently one drains the heat in winter, and dumps heat during the summer?
As @blahlabs notes in peer comment sand batteries / heating buried dirt works better than heating water for seasonal power storage and smoothing.
In simple terms there's better efficiency and ease of use from having a higher delta (temperature difference) and dirt / sand / salts heated to 600C are significantly hotter than water at 100C.
Once water turns to steam drama and expenses climb.
