I'm assuming it's a cylinder 500 m tall, lifted by 500 m so it almost clears the hole (assuming basalt at 2.9 g/cm^3, that's only 1.6 TWh, but close enough.) Watching a 1 km-wide 150-story structure rise from a hole and then disappear would certainly be a majestic sight. However, a few issues:
- The Mir mine, 4th deepest open-pit mine in the world, has roughly these dimensions (1,200 m wide x 525 m deep) [1]. Of course, its approximately a cone, so its total volume would be about half the proposed cylinder. The mine took 40 years to excavate, albiet in very harsh conditions.
- Simply removing the rock from the hole, with 100% efficiency, would expend 1 TWh of energy (average lift 1/2 of total height). If diesel powered construction equipment is used, theoretical maximum efficiency of just the engines is 50%, realistic is more like 20%. I'd be surprised if you could get a total efficiency of over 1 or 2%.
- Of course, you'd also spend energy moving the overburden away from the hole. If the angle of repose was 1-in-5, then the pile would be roughly 150 m (45 stories) at the tallest point, and form a circle 4 km wide centered around the hole.
- You could be clever, and just dig out the circumference and bottom of the cylinder. At the bottom every square meter would have 500 meters of rock sitting on top of it. That's 1.5 million kg, or 14,000 kPa (2,000 PSI). That doesn't sound... impossible... but it would require a dense forest of supports. It would be the world's most expensive room-and-pillar mine [2], by several orders of magnitude. Integrity of the rock would be a problem, too. A fault wouldn't just risk a tunnel wall blow-out or cave in, it could litteraly drop a 500 meter mountain on your head.
- Speaking of pressure... how do you perfectly maintain the integrity of a 3 km circumference piston ring? The water is going to really, really want to slip past the rock piston. Anywhere it does it will have very serious erosion. How do you fix a problem? Picture one of those submarine movies with water spraying everywhere, but unable to shut it off.
- Speaking of pressure, again... how do you maintain the integrity of the rock? You'd have to girdle it in a 3 km x 500 m tall wall.
Good clarification. Perhaps it would be better then to build these at smaller scales (as they suggest on their website) ;)
In the video this example is chosen to compare it with the area requirements of conventional pumped hydro as well as to show what magnitude of energy storage would be necessary to compensate for fluctuating solar/wind production.
- The Mir mine, 4th deepest open-pit mine in the world, has roughly these dimensions (1,200 m wide x 525 m deep) [1]. Of course, its approximately a cone, so its total volume would be about half the proposed cylinder. The mine took 40 years to excavate, albiet in very harsh conditions.
- Simply removing the rock from the hole, with 100% efficiency, would expend 1 TWh of energy (average lift 1/2 of total height). If diesel powered construction equipment is used, theoretical maximum efficiency of just the engines is 50%, realistic is more like 20%. I'd be surprised if you could get a total efficiency of over 1 or 2%.
- Of course, you'd also spend energy moving the overburden away from the hole. If the angle of repose was 1-in-5, then the pile would be roughly 150 m (45 stories) at the tallest point, and form a circle 4 km wide centered around the hole.
- You could be clever, and just dig out the circumference and bottom of the cylinder. At the bottom every square meter would have 500 meters of rock sitting on top of it. That's 1.5 million kg, or 14,000 kPa (2,000 PSI). That doesn't sound... impossible... but it would require a dense forest of supports. It would be the world's most expensive room-and-pillar mine [2], by several orders of magnitude. Integrity of the rock would be a problem, too. A fault wouldn't just risk a tunnel wall blow-out or cave in, it could litteraly drop a 500 meter mountain on your head.
- Speaking of pressure... how do you perfectly maintain the integrity of a 3 km circumference piston ring? The water is going to really, really want to slip past the rock piston. Anywhere it does it will have very serious erosion. How do you fix a problem? Picture one of those submarine movies with water spraying everywhere, but unable to shut it off.
- Speaking of pressure, again... how do you maintain the integrity of the rock? You'd have to girdle it in a 3 km x 500 m tall wall.
- You still need a very large water reservoir.
[1] https://en.wikipedia.org/wiki/Mir_mine
[2] https://en.wikipedia.org/wiki/Room_and_pillar_mining