As I understand tidal turbines currently are expensive to maintain and keep operating. There are not that many houses on Orkney, so producing 7% of their power is not much. The problem is scaling these prototype systems up for large scale use. It will also be necessary to have large exclusion zones for ships and other watercraft around them, which may cause problems.
Now, the Welsh had plans for huge tidal lagoons along the their southern coast where the tidal rise and fall is large. These could have generated huge quantities of electricity for hundreds of years, unfortunately it required some government money and they decided against it.
The technology for these tidal systems, using the rise and fall of the tide passing through static turbines has a long history and proven technology. The tidal barrage system across the Rance estuary in France was opened in 1964 by General de Gaulle. It is still in operation over 50 years later and likely to continue operating for the foreseeable future
I have visited it and it is most impressive.
Saving running away money - 'leaving fund'
What loutish behaviour - Boris manspreading
I'm uglier than yesterday - I'm aghast