We've all heard of pumped water to store energy, and your pump seems a feasible two-way pump/generator idea. Not outlandish to my eyes.
You won't have a pump-curve for this old pump on fire sale, so you won't know ahead of time if it can even take 30 L/min or be run backwards for that matter.
Bit of a risk (unless your moonshine isn't that good and you're happy to part with it anyway).
Figuring the motor/pump combination is probably only 50% efficient, then running at 100 Watts will do 50 Watts of work pumping the water. Then when the water comes back down it will be a 50Watt supply being converted back into electricity by a 50% efficient generator, getting you 25 Watts of electricity. I'm sure you can make the system more efficient than that, but on day 1 when it's first put together just to see that it does work, that's what I'd expect.
The head at your tank isn't likely to be 28 meters, so for a more modest 10 meter high header tank, you will need to flow 30 liters per minute to get your 50 Watts back. You may need a bigger pipe for the trip down than the trip up. Or just really oversize one pipe for both up and down. There will be a volume trapped in the pipe but maybe that won't matter. It all comes down in the end.
