Land based Lifters have a height limitation that would not be an issue if the Lifter were to be placed offshore. A 300 foot Lifter could easily be expanded to a height of 1000 or 2000 feet. The limiting factor would be the support structure. The upper support would be an already proven technology. Barges and tankers are built to hold thousands of tons and are as much as 400+ feet long.
A quick calculation of a 400 foot tanker capable of holding 24000 tons of material placed over a sea bed anchoring site 2000 feet deep might be spec'd out as follows:
Block Size: 12' x 4' x 2.8' = 134 cu.ft.*
Weight of Block: 134 cu.ft. x 150 lbs per cu.ft. equal 10 Tons
Drop Time: 360 minutes (6 hours)
Potential Energy: 2.5 kW for a single block
Carousel Capacity: 2000 ft. / 2.8 ft. <= 714 blocks
Assume 571 blocks, 80% of 714, in the Carousel and a 180 minute evacuation time of all blocks. This would produce a maximum peak output of 2.9 megawatts. From this peak value it would start to decline. Adding additional blocks to the Carousel, as blocks are being removed from the bottom, would lengthen the time of peak output.
This is where the storage capacity of the surface ship and the number of blocks in the carousel are important.
Capacity of Carousel: 571 Blocks
Number of blocks stored on the barge: 1142 Block
(For better performance only half would be used for a given run.)
Possible full Carousel reloads: 1
Output time with one refill: 12 hours
Output with full Carousel: 1.5 megawatts per hour
See another drawing for an off shore design here:
Off Shore Lifter with Gravity Bank and Carousel
* Due to the buoyancy of a loaded Carriage, the actual size of the block will need to be some what larger in order to have the desired potential energy.
Output Time vs Reload Time is used as a measure of efficiency of any Gravity Storage system. Batteries can take hours or days to recharge and pumped hydro, with additional pumps, can reach or exceed 100%. Just as with hydro, a single pipe would not be expected to fill a large water reservoir. A working Lifter may be compared with the pumping and delivery relationship present in pumped hydro storage. A single Lifter reloading a Gravity Bank several hundred or thousand feet in height suffers the same efficiency problem. One would like to be able to replenish a Gravity Bank as quickly as possible. A single Lifter would need this long to deliver enough blocks to load a Carousel the size of the one described above:
Assume the following.
( Carriage speed up a Vertical Support of 1 foot per minute x 2000 feet) / 60 minutes = 33 hours
To reload the Gravity Bank with one Lifter would require:
33 hours x 571 Blocks = 18843 hours or 785 days
This is terrible and a real show stopper. So it becomes necessary to design Lifters with multiple Carriages and to use multiple Lifters.
A Block is coming off the Carousel every:
( 360 minutes run time / 571 blocks ) = .63 minutes
This means that a Carriage would need to deliver a Block every .63 minutes.
Let's say that each Block takes up 15 feet. ( 12 feet high + 3 feet spacing )
It would take this long to move a Block 15 feet.
15 feet / 1 foot per minute = 15 minutes
To get one Block delivered every .63 minutes would require:
15 minutes / .63 minutes = 24 Lifters
Each Lifter has room for:
2000 feet / 15 feet = 133 Carriages
This would mean that 24 Carriages would be delivered to the top of a Lifter every 15 minutes. ( 1 foot per minute )
Now, how would that look.
( 24 Lifters / 15 minutes) * 360 minutes = 576 (571 Blocks needed every 6 hours )
This is looking better. With this configuration we have an efficiency of:
( 6 hours of Carousel Output / 6 hours to reload ) x 100 = 100%
In the above example, 12 Lifters would be placed on each side of the barge. If a lower efficiency is allowable, then a smaller number of Lifters could be used, resulting in a longer reload time. Using motor powered tilting assemblies for each Carriage would be one way to handle delivering a tilting force to each Carriage. Other methods, using surface mechanical assemblies, might also be considered. Also, it is assumed that the Lifters are all primed, like a pump would be. The Carriages beneath the ones taken off the top of each Lifter would have to be moved up to fill the vacancy after each run. The Lifter (pump) is expected to stay primed as well as the Carousel. This may seem like a loss, since to keep the Lifters primed, you have to tilt up the remaining Carriages. Since each Carriage starts at a different height on a refill run, the total amount of energy put into the system remains the same as that removed.