Our yearly energy demand of the house is 3600kWh for space heating, and 4400kWh for DHW, total 8000kWh.
To store 8000kWh of energy as heat, here is one option:
- Not accounting for losses from time of heat input to time of heat usage,
- Store hot water in a Tank - say we can use water from 0 deg to 80deg C (delta T is 80K)
- Specific heat of water is 1.17Wh/kgK (=4.18kJ/kgK, as 1watt is just 1J/second).
- Volume of water (M) needed: M = E/(C*delta T) = 8,000,000Wh/1.17*80 = 85,470litres
- thats a lot of water - a tank about 3m x 9.5m x 3m tall
We can reduce the storage requirement further. We don't need the capacity to store the entire year's energy demand. Lets say we use the solar thermal system to it's max - then we just need to figure out what fraction of the energy needs are not met in winter, and store enough to cover that much. - surely much less than the 8000kWh. Later, we'll also have to deal with the storage losses and system inefficiencies - so the amount of storage needed will probably end up near the 8000kWh anyway...!