They tested two approaches: "Open ventilation" (hose to the surface) and "Closed ventilation" (no air sources).

    "The most important finding was that an air-connection to the surface is not needed, reducing the technical effort significantly." [1]

    "Charging requires more time and energy with closed ventilation. In return the higher pressure difference results in a higher turbine power and energy during the discharging phase…ventilation is not beneficial in this regard." [2]

[1] https://en.wikipedia.org/wiki/Stored_Energy_at_Sea

[2] https://www.sciencedirect.com/science/article/am/pii/S235215... (PDF)