Martin Hannan for The National: HOUSEHOLDERS on the Shetland Isles were not aware of it, but when they plugged in their kettles recently, they were sharing in a bit of history. For the Shetland Tidal Array at Bluemull Sound, installed by Nova Innovation of Edinburgh, has become the world’s first tidal power array to be connected to a grid and deliver power on a commercial basis – to dozens of homes on the islands. The achievement has been hailed by environmentalists and the renewable industry as a turning point in the development of marine power. Nova had shown its technology could work with a single turbine which generated electricity in March. But the installation of second turbine that is also working to the grid proves that large tidal power arrays can and do work. Commercially viable tidal power is seen as something of a Holy Grail by the industry, since it is one of the few renewable energy sources that is entirely predictable – as one industry source on