The mechanism is apparently simple: when electricity is cheap but not being used (for example, on a windy night, when the power generated by wind turbines supplies a network with little demand), the water is pumped from the lower reservoir to the higher one. It is left up there until there is a peak in demand, when it is allowed to fall to produce a large amount of electricity. The Tajo de la Encantada hydroelectric plant, owned by Endesa, is located in El Chorro, very close to the Caminito del Rey, and is one of Spain's most important power stations of its type. It has just undergone a major technical upgrade to ensure that it will continue to function for at least another three decades, and it is one of Malaga province's principal power suppliers. The total output is 400 megawatts (400,000 kilowatts) and, given that the average household energy consumption is 3,300 watts, some 120,000 homes can be directly powered by the plant while it's producing electricity. That is approximately the entire population of the city of Malaga. Andrés Muñoz and Antonio Velasco, Endesa engineers in charge of the management of this and other local facilities, took SUR into the heart of this eco-friendly power factory.
Tajo de la Encantada was built in 1973. It is a pure pumped storage plant, operating a closed cycle, thanks to the reversible machinery.
“The turbines pump the water from the lower reservoir to a higher dam at the top of the mountain, and when the conditions of the power grid and the market required it, it is allowed to fall through a pressure pipe, onto the turbines and generate electrical power,” they explain. The agua isn't lost or used, it just accumulates in the reservoir below. To power the turbines, three cubic hectometres of water need to fall.
The water is pumped back up to the top reservoir when there a surplus electricity production locally.
“Wind turbines can generate electricity during the night, when there's wind, but there's little consumption,” explain the experts.
However, when the electricity network is experiencing a spike in demand, and prices are more attractive, electricity can be produced “with added value”.
The engineers clarify that it was designed from the start to be an emergency power supply. In difficult situations, such as a voltage dip or the loss of other power sources such as thermal and nuclear energy, this power plant can maintain the high voltage of the power network.
Initially the plant produces 13.8 kilovolts (13,800 volts) and, through the use of transformers this is raised to 220,000 volts. Then, the Tajo de la Encantada substation, part of Spain's national electricity network, transforms this power again up to 400,000 volts, ready to be introduced into the network.
In Andalucía, Endesa has two plants of this type, this one and another in Seville (Guillena). In the Pyrenees there are several more. In total, there are 14 hydroelectric plants across Spain.
A key issue to keep in mind, according to the experts, is that the plant does not function continuously; it generates electricity on demand.
“It can supply electricity for eight hours, but after that the water from the higher reservoir has to be replaced.”
The plant currently has the capacity to keep the network supplied during peak demand times, which arecurrently between 1pm and 5pm andfrom 7pm to 10pm, although it does vary according to the season.
It doesn't function on a regular, daily basis, therefore but works as needed, depending on the market and the chance to make the most of other power sources, especially wind power. “There are often weeks where there is so much wind that it isn't used.” Similarly, when there is no surplus power, the water cannot be pumped up to the top ready to be dropped again.
A super generator
“The pumping stations were designed in the 1970s as an emergency measure, to prevent power outages. Now, the market has changed, but its primary function is still to maintain the stability of the electricity network.”
One of its principal functions, therefore, is to back up thermal and nuclear power stations. If there were a major blackout, the power generated could help to restart the thermal power plants in Los Barrios (Cádiz) and Carboneras (Almería), which would need 1,500 megawatts of power. “It's like a huge backup generator for the big thermal and nuclear plants, which can't start up by themselves.” According to the engineers, this kind of infrastructure shows the difference between developed and developing countries, because developed countries have a strategic plan. This allows for, for example, the construction of high speed railways, airports and industry, “because a secure network exists”.
One of the most striking sights for visitors to El Chorro is the tower on top of the mountain, next to the higher reservoir. It is a surge shaft with a diameter of eight metres and a height of 50 metres, with a further 50 underground, which connects directly to the pipes that draw the water up. Each one of the four turbines uses between 25 and 27 cubic metres, or more than 25,000 litres, per second. This produces a lot of energy, especially if the gates have to be closed, such as in the case of a shortcircuit or other issue in the network.
“When you turn a tap off very quickly, the pipe can shake or even burst,” they explain by way of an example. This high pressure has to be reduced, and that is the purpose of the surge shaft. Emergency stops are very exceptional, however.
The pipe also has special characteristics: it starts with a diameter of 5.4 metres, and as it approaches the turbines it gets thinner, in order to increase the pressure, until it divides into two and then into four, one for each turbine. It is 1,500 metres long, with an underground and overground section, and climbs 380 metres in total.
Finally, the visit ends at the power station itself, which is a sort of submarine, fully submerged in the lower reservoir. It is the length of a 15-storey building, around 50 metres. Inside there are the four turbines, each one 120,000 horsepower. At this scale, everything is worked out with complex calculations to be able to produce such great quantities of power.
The power station is integrated into a larger system of ecologically friendly ways of generating electricity. The dam also serves as a mixer for the waters from the Guadalhorce and Guadalteba reservoirs. The water is salinised and has to be mixed before it can be sent to the El Atabal treatment plant.