kWh does not measure max power, it measures capacity. A kWh is a constant factor multiple of Joules. I’d think you could absolutely make the capacity larger (ie store more energy) by just making the tanks larger. The flow rate is the thing that seems to determine the power output.

Yes, but I wasn't talking about capacity. I was talking about max output power.

Increasing the size of the tanks to increase the energy storage capacity, without increasing output power would be... useless.

Extreme example: a huge expensive 100 MWh battery that you can only draw a max power of 100 W from = useless.