Hi,
My car weighs 2.4 Tonnes (without occupants), so would that give me more generating capability than you? Perhaps not so good when I need to drive it first thing in the morning. The only problem I can see.....
Theoretically Yes, but you would need to wait for the sun to shine for a few hours in the morning to wind the car back up before you could go to work (however that should be much less of a problem in Aus. than here in UK). And I can foresee a few
more problems : For example, if the pit is 2 x 5 = 10 square metres then you would need to dig out 10,000 cubic metres of "spoil", weighing up to 25,000 tonnes (typical rock density), to be lifted up by an average height of 500 metres (and dispose of it somehow). That would give a minimum "pay back" period of 5,000 cycles (of an ultimate 2.5 tonne weight) for you, or 12,500 for me. Not good, because the aim here is particularly for a system with a storage cycle of many days, weeks or ideally months
. Of course this particular technology is targeted at employing disused (coal) mineshafts, but google tells me that relatively few in UK have/had a diameter above 4 - 5 metres, or are as much as 1 km deep.
As for the "swimming bath"; a pair of water tanks 1.5 metres (5 feet) high above and below the garage could hold about 15 m
3 (15 tonnes) of water and lifting that though 3 metres (10 feet) could store up to 150 Watt.hours. That's equivalent to about 15 x Lithium 18650 cells or a handful of Laptop batteries.
Indeed, storing the energy as heat is an alternative approach and I calculate that the 15 tonnes of water could store our excess 1 MWh by heating it from say 40 degrees to 90 degrees Celsius (not too good for swimming though). But one problem is insulation; a typical hot water tank (100 litres, 25 gallons) with one inch (2.5 cms) of (the manufacturers') foam insulation loses about 1 - 2 kWh per day. That figure is actually "encrypted" on the European Energy Performance Label (of most HW cylinders) as a value such as "68 W". It's the number of Watts lost in one hour under "standard test conditions" (60 C inside and 20 C outside IIRC), i.e. 1.6 kWh/day. That number is surprisingly constant because larger tanks will generally have thicker insulation, so we could extend it to our 15 tonne tank which is about 5 times larger linearly (125 times by volume). But the surface area is increased by 25 times, so the insulation would need to be 25 times thicker, i.e. 2 feet or 62 cms. Even then, between summer and winter, around 30% of the (heat) energy would leak away.
However, 1 MWh is less than 10% of a typical UK home's winter heating requirement so we would need better storage than that tank. One possibility is to use sand which actually has a
lower thermal capacity than water (as do almost all materials) but can be heated (by internal air pipes) to many hundreds of degrees C (500 to 900 C has been mentioned). Insulation would be potentially more difficult but there aren't the problems of thermal convection currents in liquids such as water, so it might be possible to maintain a graduated temperature away from a very hot core.
We're never going to have >10 MWh of excess PV generation, so a "heat pump" seems more appropriate, but this needs primarily electricity not hot water. Using the pump in summer (aka an air conditioner) when the surplus PV electricity is available, is interesting because it could heat water (more efficiently) to a higher temperature in summer, perhaps to 60 degrees C. IF the house had a 2 metre high basement under its whole area, then filling this with water (i.e. 200 m
3)
might get close to a 10 MWh thermal store, but perhaps there's a better method?
Cheers, Alan.