As far as I’m aware, at least what has been explained to me, storage is a smaller problem. The bigger problem is how do you transfer that energy from where it is stored to where it is needed. Wind farms in the sea help nothing in Bayern.
Price falls below zero because there's a supply glut (and due to aggressive, excessive, or effective subsidies to accelerate our team transition to clean energy).
Is it the national or regional electricity prices that are falling below zero?
Is the price lower during certain hours of the day? If so, then energy storage could probably smooth that out.
The maximum power you can take from the grid depends on the max. power of your electrolysers. But you only want to run the electrolysers, when they are profitable, that is when the electricity prices are low enough due to oversupply. On the other hand the whole equipment is probably expensive to buy and keep running, so you want to run them as long and often as possible. Additionally you may use energy storage to prolong the profitable time windows of your electrolysers.
That said, it would be really great, if hydrogen and/or hydrocarbons would be synthesised at scale. There are many pilot projects and I hope one finds the right tech and trade offs to make this profitable.