Thermal Mass

Effectiveness of Materials for Seasonal Heat Storage

Water is the most robust, bulk heat storage medium, 5x the specific heat capacity of rock - and 2.5 the heat capacity by volume since rock is more dense than water (rocks sink).

A practical storage medium could thus be rock/water mix, where the water has good heat exchange qualities. This is for seasonal storage.

Since water-tight vessels at high temperature are not low-cost - an easier route to seasonal heat storage may be an insulated volume of higher temperature rock. For example, if we increase the temperature of the rock to 200C or so - then we have achieved a low cost way to store heat in materials using the same volume of material as water. This route would require that we insulate the heat-containing vessel, so there is additional cost involved with insulation.

Here is a constant interplay of excess energy being available in the summer - compared to winter - though the winter insolation only works in case of having a larger power generation system such as more PV.

Notes

• 830J/kgC specific heat of sand.
• 830kJ/tonC. Assume a 140 to 90F drop, or 30C.
• 25,000kJ/ton30C
• 25MJ/ton of storage for normal conditions.
• Sand weighs 1.3 ton/cubic yard
• 32 MJ/yard
• Wood is 16MJ/kg
• Equivalent heat of 2 kg of wood
• 3 cubic yards are 6 kg of wood.
• Conductivity of sand - .25W/MK.
• 4.186J/gC° specific heat of water.