One alternative to a freezing pool of water that I'm intrigued by is using thermochemical heat batteries to transport heat energy from summer to winter.
For example, evaporating a 40,000 L solution of calcium chloride brine can store 4.6 megawatt-hours(*) of thermal energy and release it in the winter by passing water vapour over the solution to dilute it. This can heat geothermal temperatures from 10°C to 27°C which can be used in in-floor heating.
But the big asterisk is that only 11% of that 4.6 megawatt-hours of that thermal energy being "stored" is held in the enthalpy of dilution, and 89% of that heat is coming from the enthalpy of vaporization, and acquiring water vapour in winter isn't so easy.
One would probably need a source of water and use geothermal heat to raise dry cold air temperature to the needed 10°C, while absorbing water vapour from a (relatively small) tank of water. The net result is more of a thermochemical heat pump rather than a thermochemical battery per se. But the point is that, other than some fans, this is not electrically powered.
Then there is the issue of storing such tanks safely underground, and more problems. Still, there might be some innovative solutions lying in this direction. For example, one can replace this open-loop system with a closed-loop vacuum system to boil water, but at the cost of far more complex and dangerous engineering.
For example, evaporating a 40,000 L solution of calcium chloride brine can store 4.6 megawatt-hours(*) of thermal energy and release it in the winter by passing water vapour over the solution to dilute it. This can heat geothermal temperatures from 10°C to 27°C which can be used in in-floor heating.
But the big asterisk is that only 11% of that 4.6 megawatt-hours of that thermal energy being "stored" is held in the enthalpy of dilution, and 89% of that heat is coming from the enthalpy of vaporization, and acquiring water vapour in winter isn't so easy.
One would probably need a source of water and use geothermal heat to raise dry cold air temperature to the needed 10°C, while absorbing water vapour from a (relatively small) tank of water. The net result is more of a thermochemical heat pump rather than a thermochemical battery per se. But the point is that, other than some fans, this is not electrically powered.
Then there is the issue of storing such tanks safely underground, and more problems. Still, there might be some innovative solutions lying in this direction. For example, one can replace this open-loop system with a closed-loop vacuum system to boil water, but at the cost of far more complex and dangerous engineering.