Academic Reading — Test 168

Reading passage

Among the many proposals for generating electricity without burning fossil fuels, the tidal lagoon stands out as one of the most predictable. Unlike wind or sunlight, which fluctuate with the weather, the rise and fall of the sea is governed by the gravitational pull of the moon and, to a lesser degree, the sun. Because these movements can be calculated decades in advance, engineers know with great confidence when a lagoon will produce power and roughly how much. This dependability is a considerable advantage for operators who must match electricity supply to demand throughout the day. A tidal lagoon is, in essence, an artificial pool created by building a curved sea wall, or breakwater, that encloses a stretch of shallow water along a coast or within an estuary. The wall is usually constructed from rock, sand and concrete, and may stretch for several kilometres. Crucially, the barrier is not solid all the way along: set into it are a number of large openings fitted with gates and turbines. When the gates are closed, the lagoon is sealed off from the open sea, allowing a difference in water level to build up between the two sides. It is this difference in height, known as the hydraulic head, that the scheme exploits to drive its machinery. The operating cycle follows the natural rhythm of the tides. As the tide outside begins to rise, the lagoon's gates are held shut, so the water within remains low while the sea climbs higher. Once a sufficient head has developed, the gates open and water rushes through the submerged turbines into the lagoon, spinning their blades and generating electricity until the levels inside and outside have equalised. The reverse occurs as the tide retreats: the gates are closed once more to trap the now-higher water inside the lagoon, and when the outside sea has dropped far enough, the water is released back out through the turbines, producing a second burst of power. A lagoon that generates on both the incoming and the outgoing tide in this way is described as operating in a two-way, or bidirectional, mode, and it can deliver electricity four times within a single day. The turbines themselves are central to the design. They are typically of the bulb type, in which the generator sits inside a sealed, bulb-shaped casing positioned directly within the flow of water, much like the machines used in run-of-river hydroelectric plants. Because the head produced by a lagoon is modest, often only a few metres, the turbines must be large and capable of working efficiently at low pressures and high volumes. Many modern designs are reversible, meaning the same unit can capture energy whether the water is flowing into the lagoon or out of it, which simplifies construction and reduces the number of separate components required. Choosing a suitable location is far from straightforward. The most valuable sites are those with a large tidal range, that is, a great vertical distance between high and low water, since the amount of energy available rises sharply as the range increases. Estuaries with broad mudflats and a funnelling shape tend to amplify the tide and are therefore attractive. Yet these same places are frequently rich in wildlife, supporting wading birds, fish and other organisms that depend on the regular flooding and draining of the mud. Conservationists have warned that enclosing such an area could disturb feeding grounds, alter the movement of sediment and trap migrating fish. Engineers respond by incorporating features such as fish passes and by timing operations to limit harm, but the ecological questions remain genuinely contested. The economic case is similarly complex. A lagoon requires an enormous initial outlay, chiefly because the sea wall is so massive and expensive to build. Once finished, however, the structure should last for well over a century, far longer than most power stations, and its running costs are comparatively low because the fuel, the tide, is free. Supporters argue that when the cost is spread across such a long lifetime, the electricity becomes reasonably priced and entirely free of carbon emissions during operation. Critics counter that the upfront sum is so large that public money or guaranteed prices would be needed to attract investors, and that other renewable technologies may now deliver power more cheaply. For the present, then, tidal lagoons remain a promising but largely unproven idea, admired for their reliability yet held back by their formidable cost.