Tax coal, tax gas,

Agree. Taxing pollution is a simple way and perhaps politically most "stable."

..use the income to build grid interconnections

Why earmark taxes?

..and offer contracts for storage facilities.

Why should public build these?

- Again, without picking technologies.

Government has actually a good history in R&D. IMO favorable technologies should be supported with the help of public money in R&D, but not by micromanaging the market place.

Simply offer a flat payment per mwh storage capability at a given efficiency.

How do you calculate efficiency?