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Versatile power plants

Water reuse in a hydroelectric system would increase the supply of electricity to São Paulo and the Baixada Santista metro area of São Paulo State

Henry Borden plant: water descends 720 meters to generate power in Cubatão

EMAEHenry Borden plant: water descends 720 meters to generate power in CubatãoEMAE

Located in the foothills of the Serra do Mar, in Cubatão, southeastern Brazil, the Henry Borden hydroelectric plant is an important source of electricity. Through a series of pipelines, the plant captures water from the Rio das Pedras basin, which is linked to the Billings Reservoir, in the São Paulo Metropolitan Region. From an altitude of 720 meters, the water moves the turbine assembly designed to generate up to 880 megawatts (MW) of power. This project, however, has only been semi-operational since the 1990s, because of a restriction on the uptake of water, such that the waters of the Tietê and Pinheiros Rivers are prohibited from being reversed for Billings. In the interest of better plant utilization, Sadalla Domingos, a professor of civil engineering with the Department of Mechanical Engineering at the Polytechnic School of the University of São Paulo (Poli-USP), proposes transforming the Henry Borden plant into a reversible hydroelectric one. This possibility has been discussed in Brazil—at the end of last year Eletronorte (Centrais Elétricas do Norte do Brasil), a subsidiary of Eletrobras, brought together 235 experts from Brazil, Portugal, France, Germany and Austria in the capital Brasilia, where they discussed regulatory and economic issues related to these projects.

Reversible hydroelectric power plants have, in addition to the main reservoir (upper), a second reservoir (lower), located beyond the powerhouse where electricity is generated. During the day when energy consumption is highest, the hydroelectric plant uses water from the upper reservoir to generate electricity, like any other plant, and stores it in the lower reservoir, a step not found in non-reversible plants. At night, when consumption drops, it pumps some of the water that passed through the turbines and returns it to the upper reservoir. A type of closed circuit with continuous recycling of water is thus established.

There are currently more than 127,000 MW of electricity generated in reversible plants worldwide—the potential equivalent of nine plants the size of Itaipu, which has a generating capacity of 14,000 MW. “The United States, Japan, China, Italy and France are the leading countries in this type of technology, which was created around 1890 in Europe,” says Carmo Gonçalves, the engineer managing Eletronorte’s technology and a specialist in hydroelectric power plants. Brazil has only two small hydroelectric plants of this kind, both in the Pinheiros River—the Traição Plant, with 22 MW, and the Pedreira Plant, with 108 MW.

The only comprehensive study on the potential of this energy source in Brazil was presented at the Technical Seminar on Reversible Hydroelectric Plants in the Brazilian Energy Sector, held in Brasilia in 2014. “CESP (Companhia Energética de São Paulo) did a preliminary inventory study in the early 1980s, which showed that the technical potential for this type of development in São Paulo State was approximately 200,000 MW,” says Paulo Sérgio Franco Barbosa, a professor of civil engineering at the School of Civil Engineering, Architecture and Urbanism of the University of Campinas (Unicamp). “While they have not carried out economic feasibility studies for implementation of reversible plants—which could reduce this estimate—the potential is great.”

070-073_Hidrelétrica_236Barbosa says that a key advantage of reversible systems is their contribution to Brazil’s electrical system, which operates in an interconnected way. “These plants could provide more power to the system at peak times, avoiding voltage and frequency fluctuations in the network, which often cause drops in supply,” says Barbosa. In addition, this type of hydroelectricity can support the expansion of intermittent renewable energy sources, compensating for periods of absence or reduced electricity generation due to lack of sunlight (on cloudy days and during the night) or wind in the production of solar and wind plants, whose use in Brazil is increasing. “For these reasons, reversible hydroelectric plants are advantageous, even if their energy balance is negative when consuming more power—to operate the pumping station—than generating,” says Barbosa. He says that the negative energy balance of reversible systems is between 15% and 25%. According to Barbosa, the major challenge for their inclusion in the National Interconnected System (SIN) is in establishing regulatory frameworks and their economic viability.

Feasibility and adjustments
“Transforming the Henry Borden plant into a reversible hydroelectric plant would enable the full use of its capacity and save up to 7,500 liters of water per second from Billings, thereby improving the supply to Greater São Paulo,” says Domingos, the author of the proposal.  “Such a solution is significant at this time of supply crisis, and would solve an old conflict in the system, between generating power and supplying water.” The proposal was submitted in June and August 2015 at Poli-USP and at the Institute for Technological Research (IPT) and accepted by Empresa Metropolitana de Águas e Energia (EMAE), the state government agency responsible for the operation and maintenance of the Henry Borden. “The project sounds interesting, but we need studies to prove its economic, technical, institutional and environmental feasibility,” says Fernando José Moliterno, a mechanical engineer and manager of EMAE’s Department of Planning and Engineering. Domingos believes the cost and benefits of making Henry Borden reversible will depend on a detailed environmental, economic and engineering plan.

Under Domingos’ proposal, it would take two major adjustments to turn the Henry Borden complex into a reversible hydroelectric plant, which consists of two plants, one external and one underground, carved into the rock. The first step would be to build the lower reservoir to store the water used to move the turbines—currently, the water is directed toward regional rivers. The other would be to install a pumping station and a pipeline to send the water back to the Billings system, high up into the mountain (see Infographic). “We already have the technology for this,” says Domingos, adding that the lower reservoir would be built between the existing drain channels now used to drain water from Billings.

Rio das Pedras at the top of the Serra do Mar overlooking the pipelines that carry water to Cubatão

EMAERio das Pedras at the top of the Serra do Mar overlooking the pipelines that carry water to CubatãoEMAE

The Henry Borden complex was installed in 1926 and in the years between 1930 and 1960 increased its capacity to 880 MW. To obtain a water flow compatible with its capacity, the engineer Asa White Billings, who designed the hydroelectric plant, devised the flow reversal system for the Pinheiros River, which rises in Billings, crosses the city of São Paulo and empties into the Tietê River. The reversal system allowed water to flow in the opposite direction: water from the Tietê can be diverted to the reservoir. This project, which carried more water to Billings, constructed in 1940, provided an important energy supply for Baixada Santista, boosting the growth of industries in Cubatão and the city of São Paulo.

In the 1990s, however, the increasing pollution of Greater São Paulo’s water sources led the state government to restrict the reversal system in order to prevent Billings from being polluted by water from the Tietê and Pinheiros Rivers. Originally designed to provide water for the operation of Henry Borden, Billings, over time, began also to be used for supplying water to the capital and surrounding cities. In 1992, a legal provision added to the São Paulo State constitution specified that pumping water from the Pinheiros to Billings could only be done under exceptional circumstances, such as to control flooding of the river. Thus, the flow of water intended for the Henry Borden complex was drastically reduced, impacting power generation. When the reversible system was in full operation, the plants produced around 470 MW but with the reduced flow, it generated only 108 MW. With the proposal to make the Henry Borden reversible, the captured water would be sent back—thus allowing an increase in the return flow and thereby increasing the power generation. “It seems like an interesting idea, with advantages both for industries and residents of Baixada Santista as well as the population of the São Paulo metropolitan region,” says Barbosa.