Geo-Solar Seasonal Heat Storage

Solar Thermal is great - but if you size the system so there is no excess heat in summer, the available heat in winter is pretty low.  So the thing we all want to do is store excess heat from the summer to use in winter, and store the excess cold from winter to use in summer.

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 improve this dramatically if we utilize the phase change of water from solid to liquid, since the latent heat of melting of ice is 92.8Wh/kg  (=334kJ/kg).  If we utilize the water from say a couple of degrees below zero to about 80 degC, the volume of ice/water required reduces to about 1/2 of what was before (about 43,000litres, down from 85,500)  but there is then the technical problem of dealing with the expansion and contraction of the ice - which I don't know how to tackle as yet.  You can imagine that a container about 3m x 5m x 3m tall can certainly fit somewhere on our city lot.

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...!