Academic Reading — Test 89

Reading passage

The Pentland Firth, a narrow channel of water separating the northern coast of mainland Scotland from the Orkney Islands, has long been feared by sailors for its violent and unpredictable seas. The same forces that once threatened wooden ships, however, are now being treated as a remarkable opportunity. Twice each day, the rising and falling tides of the Atlantic Ocean and the North Sea are squeezed through this constricted passage, producing some of the fastest marine currents anywhere in the British Isles. At their peak, these currents can flow at speeds approaching five metres per second. Engineers regard the firth as one of the most promising sites in the world for generating electricity from moving water, and several projects have been installed there over the past decade. The technology that makes this possible is the tidal-stream turbine. In principle, such a device resembles a wind turbine, but it is placed beneath the surface of the sea rather than on land. As the tidal current passes over the blades, it causes them to rotate, and this rotation drives a generator that converts the movement into electrical energy. Because seawater is roughly eight hundred times denser than air, a relatively slow-moving current can transfer a great deal of energy to the blades. A tidal turbine can therefore be considerably smaller than a wind turbine of equivalent output, although the marine environment in which it must operate is far more demanding. Most machines are anchored firmly to the seabed, either by heavy gravity bases that rest on the floor under their own weight or by piles driven into the rock below. One of the chief attractions of tidal energy is its predictability. Unlike the wind, which may rise or fall without warning, the tides are governed by the gravitational pull of the moon and the sun, and they follow a rhythm that can be calculated decades in advance. This means that operators of a tidal scheme know precisely when the current will be strongest and when generation will pause around the brief periods of slack water between tides. For an electricity grid that must constantly match supply to demand, this reliability is enormously valuable, and it distinguishes tidal power from most other renewable sources. The disadvantage is that the turbine cannot generate continuously, since the flow weakens to nothing four times a day as the tide turns. Building machines that can survive the Pentland Firth is no simple matter. The blades and bearings are exposed to enormous and constantly reversing forces, and salt water is highly corrosive to metal components. Maintenance is awkward and expensive, because divers or specialised vessels are required to reach equipment that lies many metres underwater. Some designs address this problem by allowing the entire turbine to be raised to the surface for inspection and repair, so that engineers do not have to work below the waves. Developers must also take care that their structures do not harm the marine ecosystem. Researchers have monitored the behaviour of seals, porpoises and diving birds around operating turbines, and the blades generally turn slowly enough that most creatures can avoid them, although the long-term effects on wildlife are still being studied. The Pentland Firth has become an important proving ground for the whole industry. The MeyGen project, situated in the Inner Sound near the island of Stroma, has connected a series of large turbines to the national grid and has demonstrated that arrays of such machines can operate together for years at a time. The lessons learned at this site, both about engineering and about the costs of installation, are being shared across the sector. Supporters argue that the seas around Scotland alone could one day supply a significant share of the country's electricity, while also creating skilled jobs in coastal communities that have few other industries. They point out that, unlike a coal or gas station, a tidal array produces no greenhouse gases once it has been built. Despite this promise, tidal-stream power remains expensive compared with established technologies such as onshore wind, and the number of turbines in the water worldwide is still small. The high cost reflects the difficulty of the engineering and the fact that the industry has not yet reached the scale at which prices typically fall. Many specialists nevertheless believe that the cost will decline steadily as more devices are manufactured and as designs are refined, much as happened with offshore wind in earlier decades. Whether tidal energy eventually becomes a mainstream source of power may depend less on the turbines themselves than on the willingness of governments to support the technology through its early and costly years.