I was just thinking about water, which is such an excellent heat storage device. Heat it up, cool it down. BTU's in and out, almost no loss if things are well insulated. It's a very efficient way to store energy. If you could solar-heat up enough water during the summer, you could "coast" all winter. You can also capture waste engine heat if you're generating power. Then, when the water is too cool to heat the house, the system still works as a massive low temperature boiler, which can be wood fired etc.
Apparently the name for this is Seasonal Thermal Energy Storage (STES).
But I haven't seen anyone do the math. How much water do you need? How hot, how cold?
So, here's a few numbers, in Metric and SAE. Note, BTUs and kJs are roughly the same size, so I'll use the same number for both.
The safe maximum temperature is around 90 C (194 F). Similar to your hot water heater.
The minimum effective temperature is around 24 C (75 F), a little warmer than you keep the house.
So, your thermal range is 66 C (119 F).
Metric folks, 1 L of water can store 262 kJ (or BTUs)
- SAE folks, 1 gal of water can store 995 BTUs (or kJ)
Now, the million dollar question - how much energy do you need to store? This varies a LOT by geography and house construction. I'll base it on a round number, and you can adapt it to your own situation.
Let's imagine a big house that uses 100 million kJ (BTUs) for the heating season, 30,000 kWh, about 5 cords of wood.
- 100 million / 995 BTUs = 100,000 gallons (379,000 L) = 13,368 cu ft (378.5 cu m)
Now, to make this more realistic, I'm going to cut that by 2/3. Why? Because the system that created the heat will continue to run, and produce more heat that we can use. A solar water system sized to heat the house on a sunny day will still output heat on a cloudy cold day, perhaps only 1/3 as much. If half the days are sunny, then on average we're getting 2/3 of our heat direct from the solar. We then supply the difference from our storage system.
33 million / 995 BTUs = 33,000 gallons (125,000 L) = 4,411 cu ft (125 cu m)
This much water needs a cube shaped tank measuring 16' 5" each side (5 m)
Or a cylindrical tank 18' across, 17' 4" high. Weighing 138 tons.
EDIT: It looks like this might be a slight overestimation. I'm not sure exactly why, but real world tank sizes are even smaller. Perhaps they're better insulated homes.