Heat storage Granite rubble has a density of 165 pounds per cubic foot. It has a heat capacity of 0.21 BTU per pound. So 165 X 0.21 X 100 degree F gives heat capacity of 3465 BTU per cubic foot for 100 degree change between 500 degrees F and 600 degress F. One kWh is equal to 3412 BTU so 3465 divided by 3412 equals 1.02 kWh per cubic foot per 100 degrees change. If a city block surface area is 330 X 330 feet (about 100,000 sq feet) then a one block area one foot deep could hold 100,000 kWh of heat.
That would be 100 mWh . If the block area was 100 feet deep the capacity would be 10,000 mWh or 10 gWh (10 giga watt hours) per block one hundred feet deep. As I recall the electric utility of Alameda supplied between 300 to 400 gWh of electricity during one entire recent year (2007, I think.) It would require storage of 30 to 40 block areas of 100 feet in depth. Forty two blocks is six blocks by seven blocks. To allow for 20 percent efficiency of conversion of heat to electricity increase the required storage by five. It would require about 200 blocks surface area (about 14 blocks by 14 blocks.). That would be about 4200 feet on a side. A way to reduce the volume would be to increase the top temperature to 800 degrees F., for example. That would triple the heat capacity and also increase the efficiency of the conversion of heat to electricity. That alone could reduce the square are required from 40 blocks to about 10 blocks. To reduce the volume further, cast iron bricks can be substituted. That would require about one-third the volume to about 3 to 4 blocks. Although the volume seems large, one should realize that it covers the requirements for an entire city for an entire year. This storage doesn’t have to be in the City. It can be done in the country along with the collectors. (Solar thermal collectors can be used in cities, also. Smaller storage volumes can be used. Solar thermal can be used for heating as well as for electrical generation. Most of the fossil fuel consumption in commercial and residential use is to provide heat.) Storage could be below with the collectors on top.
If collectors on top were not desired, crops could be grown on top. They would have longer growing season and less risk of frost damage because of the warmth of the storage underneath.