Looks to be a flow battery, so the power to energy ratio is tunable by changing the ratio of membrane surface area to storage tank volume. The chemistry is likely either sodium sulfur or lithium sulfur.

If the anolyte/catholyte can be made cheaply enough, it may actually be able to do seasonal storage economically. However as wind and solar get cheaper, I'm not sure we will need it. The cost of curtailment during peak season is going to fall just as quickly as the cost of panels falls, and that cost is ultimately what seasonal storage has to compete against.