While 28 tonnes appears to be a lot of material for generating 1.2MW of energy, it compares favourably when you look at wind turbines. How often does this need maintenance though?
> How often does this need maintenance though? Moving parts in salt water? Frequently. The positive thing about this design is that maintenance is potentially much easier than on an seafloor mounted turbine: if the tether is long enough, you can just make the craft surface next to a maintenance vessel with a small crane. No divers necessary, and no giant crane platforms like for offshore wind turbines, either.
This is critical to the economic viability of the project.