A hydraulic accumulator, (similar to its namesake in electronics, now more commonly known as a capacitor) stores the energy. (This is done rather than just using the pumps directly as, say, you can use these weaker pumps over a long period of time to store a large amount of energy that the accumulator can release more quickly to do a job that requires more power.)
All that pumping next door has raised this accumulator—basically a big drum full of 80 tons of scrap metal—high off the ground where it normally sits these days. This is the stored energy that's sitting on top of the water in the hydraulic system, ready to be diverted to anything around the docks that needs power. Up until as recently as 2010, this was the power source that opened and closed the main Cumberland Basin lock gates. Without it no ship could have entered or left the harbour.
GPS Coordinates:
51.44672, -2.61831
Location: Cumberland Basin