The Smithsonian blog has a post on a new study on the amount of tidal power available in Scotland's Pentland Firth - Is Scotland the “Saudi Arabia” of Tidal Power ?. A lot of the commentary elsewhere seemed to have a vast amount of political spin on it, claiming that earlier estimates of 10 - 20 GW were vastly overblown. if you actually look at the report (and particularly the image below) its fairly obvious that the new "1.9 GW" estimate is for an installation crossing the firth at one point - obviously you could build a number of installations along the firth (points A1 and A2 for example, or out to the Skerries) and get a rather larger number similar to earlier estimates, not that logic is a strong point for many critics of renewable energy.
The researchers looked at the construction of multiple rows of these sorts of turbines, placed in a variety of locations within the Firth. Their models took into account the depth of the water at each given location, observed tidal speeds and heights over the course of each month, and a number of other variables.
Ultimately, the team found that the maximum practical capacity of 1.9 gigawatts would be possible with three rows of turbines, built in the locations mapped below (B, C, and D on the map). Because each row slows down the movement of the tides that pass through it, building more then three would only marginally improve the power capacity, while increasing the overall cost of the project at a constant rate. (A, on the map, is a proposed alternate scheme that would produce a similar level of energy but at a higher cost.) ...
As of now, the biggest hurdle is price: without any carbon pollution regulation schemes in place, most renewable sources of energy, including tidal power, just aren’t as cheap as burning coal or other fossil fuels. But many energy companies have already recognized that, long-term, the cost of fossil fuel production will increase—both because of eventual regulations of the emissions of greenhouse gases and because of fossil fuels are becoming increasingly costly to extract—and harnessing the power of the tides could provide a reliable way to meet a portion of our energy demands.