Reading http://physics.ucsd.edu/do-the-math/2011/09/got-storage-how-hard-can-it-be/ I was inspired to think about gravitational potential energy storage, which at 3 m of head is only about 29 J / kg, about 500× worse than lead-acid batteries’ energy density.
But what if you had a personal water tower? You could totally make it
30 m tall. Better still, if you live in a very dry area, you could
drill 100 meters down and put a water turbine down the pipe.
100meters * gravity is 981 J/kg.
He suggests that maybe for a house you want to have like 30 or 100 kWh of storage capacity to ride out cloudy days when your solar panels don’t produce energy. 50 kWh is 180 MJ, so at 100 meters of head, you need 180 tonnes of water. That’s a spherical tank some 7 meters across.
This is far from an impossible construction project, but it’s considerably harder than building the rest of the house.
If you have 180 tonnes of clean, potable water stored, that could in itself be a useful form of preparedness; water and oxygen are the only life-essential resources that it’s not usually considered practical to store a year’s supply of.
Alternatively, you could drill a 1km-deep well and lower a weight into it on a rope, perhaps slowly enough that the water resistance wouldn’t cause too much inefficiency. If you have a 200mm-diameter borehole and a 150mm-diameter weight that is 500m long, it has a volume of 8.8m³; if it’s mostly made of quartz, as inexpensive things of that size probably would, it will have a specific gravity of about 2.6, or 1.6 if we subtract buoyancy. So it will weigh about 14 tonnes, net of buoyancy, and thus have an energy capacity of almost 70 MJ (19 kWh).
You might think that a rope capable of lifting 14 tonnes safely would be too large, but Dyneema can handle that load at about a 6mm diameter (although at retail that rope might cost you a couple thousand bucks). A 20mm-diameter rope would be a more than adequate safety factor; it will occupy 160 liters of space above ground when lifted all the way.