the same way that water towers work - when the pumps are running (power is on), you pump water into the tower; then, when the pumps fail, gravity continues to provide water pressure as water drains from the tower. The water tower effectively acts as a "battery" in the system, storing energy (and water) and then releasing it as needed
I didn't design the project, but I would imagine that moving water would be cooler than stagnant water. Other pre-industrial revolution pump designs like a hydram pump could keep cool water moving from the sea as well
