Submerged in about 40 meters (44 yards) of water off Scotland’s coast, a turbine has been spinning for more than six years to harness the power of ocean tides for electricity — a durability mark that demonstrates the technology’s commercial viability. Keeping a large, or grid-scale, turbine in place in the harsh sea environment that long is a record that helps pave the way for bigger tidal energy farms and makes it far more appealing to investors, according to the trade association Ocean Energy ...