I have another idea that fits this topic, alluded to by Jo. Why not use the ground itself as the storage medium, rather than water tanks? Ground can be heated, or cooled to provide a highly efficient storage medium. Energy losses from a monolithic block of earth heated or frozen will be very low, and the larger the area used as storage the more efficient it will be.
In the case of Manitoba, our climate is extreme, deadly cold in winter, and often uncomfortably hot in summer. Often I have wished for a whisp of snow in July, or some summer heat on a day like today. Storing the cold and heat for later use would address our needs.
I was meaning to get a ground loop set up this fall for an offgrid refrigerator, but haven't set it up yet. Our ground is cold enough at depth to serve for refrigeration, about 42F, or 5.5C. The idea is to bore 10 foot holes, and set 2 or 3 loops of 1/2" copper pipe into holes, then run glycol through insulated hoses to a steel / aluminum block inside a mini fridge, circulated by a 12v pump powered by a designated solar panel. During the winter when the cabin is at ambient temperature, the ground loop will eventually be frozen to perhaps -20C, then providing excellent refrigeration the following year. The only question will be how much ground loop for effective refrigeration.
A larger system could use a central air heat exchanger, or perhaps something larger, to exchange heat from a large mass of earth, producing a large block of permafrost.
The same concept will apply to heating. If coolant is used, (or perhaps steam), a mass of ground could be raised to very high temperatures for later use, if using an adequate heat source, say a charcoal kiln, solar flat panels or a solar concentrator array: http://www.builditsolar.com/Projects/Concentrating/concentrating.htm.