Academic Reading — Test 28

Reading passage

Electricity is difficult to store in large quantities. Once it has been generated, it must usually be consumed almost immediately, because the batteries capable of holding industrial volumes of power remain expensive and limited in capacity. This creates a persistent problem for national grids, which must balance supply and demand from one moment to the next. Demand rarely stays constant: it rises sharply when millions of households switch on appliances at the same time, and it falls again overnight when most activity ceases. To smooth out these swings, engineers have long sought a means of saving surplus electricity when it is plentiful and releasing it when it is scarce. Pumped-storage hydroelectricity offers one of the few methods that can do this on a genuinely large scale. The principle behind the technology is straightforward, even if the engineering is not. A pumped-storage station relies on two reservoirs, one positioned high in the hills and the other in a valley below. When the grid produces more electricity than is needed, often during the night when demand is low and prices are cheap, the station uses that surplus to drive powerful pumps. These pumps push water from the lower reservoir up to the higher one, where it is held behind a dam. The water is not consumed; it is simply moved upwards, gaining what physicists call gravitational potential energy. In effect, cheap night-time electricity is converted into stored energy in the form of a great mass of water held aloft. When demand later climbs, the process is reversed. The water is allowed to rush back down through pipes, spinning turbines that turn generators and feed power back into the grid. The British station at Dinorwig, in the mountains of north Wales, is among the most famous examples of this approach. Completed in 1984 after a decade of construction, it was built largely inside a hollowed-out mountain so as to limit its visual impact on the surrounding national park. An enormous cavern, large enough to house a cathedral, was excavated to hold the turbine hall. The upper reservoir, Marchlyn Mawr, sits high above the lower lake of Llyn Peris, and the difference in height between the two bodies of water gives the falling water its considerable force. Dinorwig was designed not so much for the steady supply of electricity as for speed. Its great virtue is the remarkable rapidity with which it can respond to a sudden surge in national demand. That speed is the key to the station's economic value. Dinorwig can move from a state of near standstill to full output of roughly 1,800 megawatts in around sixteen seconds. Such a response would be impossible for a conventional coal or nuclear plant, whose boilers and reactors take many hours to bring up to power. The classic illustration of Dinorwig's usefulness is the television pickup, a surge that occurs when a hugely popular programme ends and millions of viewers simultaneously rise to switch on electric kettles. Within seconds the grid must find a vast amount of additional power, and a station like Dinorwig is one of the few facilities able to deliver it almost instantly. Without such rapid reserves, the sudden strain could cause the frequency of the grid to drop dangerously, risking widespread disruption. No method of storing energy is perfect, and pumped storage is no exception. Some energy is inevitably lost at every stage, through friction in the pipes, resistance in the machinery and the heat produced when water is pumped. A typical pumped-storage station returns only about three-quarters of the electricity that it originally consumes; the remainder is dissipated and cannot be recovered. Despite this loss, the system remains worthwhile, because it allows operators to buy electricity cheaply when demand is low and sell it at a premium when demand is high. The economic gain from this difference in price more than compensates for the energy that disappears along the way. In recent decades, the value of pumped storage has grown for a further reason. As countries rely increasingly on wind and solar power, they face a new difficulty, for these sources are intermittent and cannot be commanded to produce on demand. The wind may blow strongly at night, when little electricity is wanted, and fall calm during the busy evening hours. Pumped-storage stations provide a way to absorb the excess generated at awkward times and return it when it is genuinely needed. In this sense, a technology first conceived to cope with the rhythms of human habit may prove just as important in managing the unpredictable rhythms of the weather.