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At the bottom of the lake

Methane in hydroelectric power stations can generate further energy

A team of researchers from the National Institute of Space Research (Inpe), in São José dos Campos, in São Paulo State, have created a system for capturing the methane  accumulated in the reservoirs of hydroelectric power stations and using this to produce electricity. Estimates from the group indicate that all the reservoirs in the world together emit between 18 and 24 million tons of methane a year, which corresponds to between 5% to 7% of this gas liberated into the atmosphere by all human activities, such as, for example, growing rice, sanitary landfill sites, coal mining, oil drilling or livestock farming. Along with carbon dioxide (CO2), methane gas (CH4) is one of the main culprits of the greenhouse gas effect on the planet. The warming potential of methane is between 21 and 25 times greater than that of carbon dioxide. This is because, despite having a shorter life span than CO2, the methane molecule is more opaque to thermal radiation than that of carbon, causing more heat to be trapped on the earth’s surface.

The solution reached by the Brazilian researchers could raise  the production capacity of the hydroelectric power stations located in the Amazon River basin by 30%. The leaders in methane emission are those power stations built in humid forest areas, like the Brazilian stations Tucuruí, Balbina and Samuel, and Petit Saut, in French Guyana, all of them in the Amazon. Tucuruí alone, the biggest of them, liberates between 700,000 and 1.2 million tons of methane a year. “There’s evidence, albeit scientifically unproven, that there are great stocks of methane at the bottom of Itaipu. No one has yet been there to measure it, but the gas would originate from the waste thrown into the lake by the farms and animal breeding units that surround the reservoir”, says researcher, Fernando Manuel Ramos, from the Associated Computing and Applied Mathematics Laboratory (LAC) at Inpe, the project’s author along with three other colleagues from the institute, Luís Antônio Waack Bambace, Ivan Bergier Tavares de Lima and Reinaldo Roberto Rosa.

Methane from reservoirs is produced mainly by bacteria that form part of the underwater decomposition cycle of the carbon that exists in the organic material remaining from the time when the reservoir was formed or that has been carried there in the form of sediment by the rivers that run into it. The gas remains dissolved in the water, mainly in the deepest layers of the lake and escapes into the atmosphere when it goes through the turbines and the power stations’ spillways. “It’s like opening a bottle of fizzy drink. The water from the reservoirs is suddenly depressurized and liberates the gas dissolved in it, in this case methane”, says Ramos.

Physical barrier
The idea of the group from Inpe is to avoid the methane reaching the atmosphere by capturing it while it is still at the bottom of the lake. The first step is to put a physical barrier,  in the shape of a large membrane, to prevent the power station’s turbines, normally placed between 40 and 60 meters down, from sucking in the methane-rich water. This membrane would be made from an oiled tarpaulin, as used on trucks to protect loads, anchored by weights and piles to the bottom of the reservoir. “It would be located at a safe distance so that it’s not sucked into the turbines and it would be fixed to buoys on the surface that would control its position and height. With this barrier the water entering the turbines would come from the reservoir’s surface waters where methane concentrations are smaller”, explains Bambace.

A system of tubes connected to pumps similar to a swimming pool vacuum cleaner would collect the methane-rich water -imprisoned – at the bottom of the reservoir and carry it to barges on the surface where the gas would be extracted by a vaporization system within a closed environment. This equipment would reproduce the same process that occurs at the turbine outlet and on the power station spillways, when the gas is liberated by the formation of spray and the bubbling  water. The treated water containing residual methane would return to the bottom of the reservoir by another system of ducts and the extracted gas would be used for generating power. “The methane could be stored and transported to any thermoelectric power station, but the best thing would be for  power generation to occur in power stations alongside the hydroelectric power stations in order to take advantage of the same transmission lines”, says Ramos. So the collected methane will be burned in a gas turbine for generating electricity, in a similar way that kerosene is burned in an airplane’s turbine. The CO2 resulting from the burning of the CH4 goes into the atmosphere, but does not add to the greenhouse effect because it was captured beforehand from this very same atmosphere by vegetation and other living organisms from the reservoir and the surrounding areas.

The researchers from Inpe estimate that the construction of a system for capturing 1 million tons of methane a year would require investments in the order of US$ 100 million. This amount of gas would be sufficient to generate 1,780 megawatts (MW) of power, which is the equivalent of a medium-sized hydroelectric power station; for comparison purposes Tucuruí I has 3,960 MW of installed power capacity. “The installation of systems for taking advantage of methane in the Tucuruí, Balbina and Samuel power stations, the three largest in the Amazon, would increase their production capacity by 30%”, points out Ramos. “Besides generating clean energy and reducing a source of global warming this would also contribute to reducing pressure for the construction of new hydroelectric power stations in the region.”

Long road
The work of the group was published in the scientific journal, “Energy”, in June. At present the researchers are trying to obtain financing to construct the first prototype. They estimate that the experimental model would be ready in a year. The construction of a production system, such as for example, at the Balbina reservoir that has 250 MW of installed power capacity, would take between three and four years to become operational. The group foresees that the physical barrier installed at the bottom of the Balbina lake would be some 850 meters long and 25 meters high and would cost something around R$ 1 to 2 million. Ramos also believes that the depth of water between the top of the barrier and the surface of the lake of around 10 meters will be sufficient for fish and other aquatic species to move around. Specific studies in the reservoirs will be able to assess each detail of the environmental impact.

“No one is yet producing power on an industrial scale from the methane found in reservoirs, and there’s a long way to go before this idea matures”, says Ramos. An important step still to be done is mapping out the stocks of methane in the Brazilian reservoirs and in the world, which can be done by drilling or sampling, with analysis of the water taken from the lake bottom. For the system to be economically viable, explains Bambace, it is necessary for the reservoir to have concentrations in the range of 20 grams of methane per cubic meter of water, a level found 60 meters down in the lake at Tucuruí. “It’s possible also that there’s a lot of methane in São Paulo’s reservoirs, because of the pollution coming from the rivers. But only after mapping out stocks will we know the true potential of the solution we’re proposing.”