Originally published in Engineering and Technology Magazine
China is pursuing an ambitious programme of hydroelectric expansion, with a series of ‘mega-dams’ on the way even bigger than the controversial Three Gorges project. But are there signs that this kind of hydropower might actually make climate change even worse?
“When I visited China’s Three Gorges Dam in 2009, I certainly felt respect for an impressive feat of human ingenuity,” says Peter Bosshard, interim executive director of environmental campaign group International Rivers.“At the same time, it was depressing to see how the dam had reduced the once mighty Yangtze River to a stagnant garbage pond. Near the dam site, the debris is so dense that you can practically walk across it.”
It’s a startling description of the Three Gorges dam, but for Bosshard it’s just the tip of the iceberg. “This is only the most visible symbol of a project that has upended the complex ecosystem and fragile geology of the whole Yangtze Valley,” he says. For 30 years, International Rivers has been campaigning to protect waterways and their ecologies and communities from the building of new dams, but China’s enthusiasm for large-scale hydroelectric projects has taken their concerns to a new level.
Apart from displacing more than one million people and reportedly being responsible for the extinction of the Baiji dolphin, the Three Gorges has become the focus of a complicated scientific controversy that presents new challenges for policy makers, researchers, and engineers alike: that when it comes to reducing greenhouse gas emissions, hydroelectric dams may not be as green as they first seem.
Meet the gigatonne
The first hydroelectric dams were built in the late 19th century, and were considered a clean and efficient form of energy production nearly a century before climate change became a global concern. Leading the push for hydroelectric power were Canada and the United States, with the US home to more than 2,400 dams powering hydroelectric plants across 34 states. They range from small dams providing power to local communities to vast, iconic structures like the Hoover Dam. Only in the latter half of the 20th century were these dam-building efforts eclipsed by China and Brazil, both in number and scale.
Last year, Microsoft founder and mega philanthropist Bill Gates posted a stunning infographic on his blog that gave a sense of scale to China’s gargantuan industrial growth: in just three years – between 2011 and 2013 – the country used 6.6 gigatonnes of concrete. That was more than the US had used during the entire 20th century, a ‘mere’ 4.5 gigatonnes.
The Three Gorges Dam consumed just a tiny percentage of that total – about 65 million tonnes – but that was still nearly 10 times as much as the construction of the Hoover Dam, together with 460,000 tonnes of steel, enough to build 63 Eiffel Towers.
In 2006, China overtook the US for the first time as the world’s largest producer of greenhouse gases, and while coal still remains the main source for fulfilling skyrocketing energy demands, there’s a very serious and tangible political desire to shift the balance. As such, China is investing hard in hydroelectric, and the Three Gorges seems to be just a stepping stone.
In a bid to reduce its carbon footprint, China aims to generate 120GW of renewable energy by 2020, mostly from new hydropower plants on the Salween, Upper Mekong, the Yarlung Tsangpo, as well as more development further up the Yangtze.
It’s the equivalent of building one new Three Gorges Dam every year for the next five years. According to Bosshard, it’s more hydropower than any other country has built in its entire history.
The Three Gorges Dam consists of almost as much controversy as concrete. The region has been famed for its biodiversity, with 600 known species of plant, more than half of them endangered. The Yangtze River is not just home to 300 different species of fish, but is also a vital element in an ecosystem that supports hundreds more species along its banks.
Then there is the problem of water pollution. The Yangtze’s fate has been closely linked to China’s industrial revolution, both as a water source for manufacturing and a dumping ground for waste. Now that the river is even less able to disperse pollutants quickly and effectively, scientists say the quality of the water upstream is degrading rapidly. Reports claim over 1,600 abandoned factories were flooded as part of creating the dam’s vast reservoir, emptying unknown amounts of toxins into the river.
The human and cultural cost of the construction has also been unprecedented. The reservoir submerged 13 cities, 140 towns and over 1,200 villages. More than 1.3 million people were displaced. Over 1,300 archaeological sites, monuments, and temples were lost to the rising waters.
Yet, for Beijing at least, all this seems a small price to pay for the benefits the Three Gorges Dam is expected to deliver. With its 32 state-of-the-art turbines, the dam is the world’s largest power producing facility, capable of generating 22,500MW of electricity – more than 28 times the output of Japan’s Kashiwazaki-Kariwa nuclear power plant, the largest of its kind in the world. And the dam’s annual output is said to be the equivalent to burning 50 million tonnes of coal, which would release 100 million tonnes of carbon dioxide into the atmosphere.
Squeezing the ecological balloon
Things, however, may not be that simple. Since the early 2000s there have been growing concerns in the scientific community that cast doubt on the green credentials of hydroelectric dams. They worry about the ‘reservoir emissions problem’, which could have potentially devastating consequences for climate change. Most hydroelectric power comes from driving turbines with the controlled flow of dammed water, in effect creating a store of potential energy above the dam through the building of a large reservoir. These reservoirs are made by flooding a large area of land around the upstream river, and more often than not this land was either heavily forested or previously used for agriculture.
There are two problems with that; an important mechanism for absorbing CO2 is lost, and the plants and vegetation killed by the flooding decay and their stored carbon is converted into methane and even more CO2 – both greenhouse gases. Methane is much worse than CO2 when it comes to trapping heat. “In extreme cases, these emissions are much higher than the emissions from a fossil fuel plant with the same generating capacity,” says Bosshard. “The Balbina Dam in the Brazilian Amazon, for example, emits 12 times as many greenhouse gases as a coal-fired power plant of the same size.”
Not all researchers share Bosshard’s concerns. “On the whole, I think reservoir emissions from hydropower dams are minuscule compared to fossil fuel generation and its significance is exaggerated in many people’s minds,” says Bradford Sherman, an ecologist at the University of Canberra in Australia and dam engineering expert, whose interest in reservoir emissions dates back 15 years. “Reservoir GHG (greenhouse gas) emissions are highly site-specific and it is hugely misleading to extrapolate from one reservoir to the next.”
Sherman visited the Three Gorges in 2012, and is more upbeat about what he saw. “I was somewhat surprised and reassured as I learned about the steps to minimise the harm to the environment and water quality in the reservoir,” he says. “We sampled only a very small part of the reservoir but we tried to find areas that would be high emitters. We spent a lot of time surveying the tributaries that we could access, using hydro-acoustic methods to detect the presence of bubble plumes, and were very surprised by how few bubble-emitting sites were observed.
He says he spotted only “a single bubble event in many dozens of measurements” and argues that this was much less than he had observed in Australian water reservoirs. “If I had to guess, I would classify Three Gorges as a relatively low-emitting reservoir because of its depth and the steep topography, but more measurements are required along the length of the reservoir, especially upstream, to make an accurate estimate.”
Methane for electricity
What Sherman highlights when he mentions depth and topography is perhaps the most pressing issue when it comes to constructing new dams – the placement and preparation of reservoirs before construction even begins. Avoiding flooding heavily forested or agricultural areas will obviously limit the amount of vegetation at risk.
Amy Townsend-Small, a researcher at the University of Cincinnati, Ohio, recommends especially avoiding farmland. “Our work indicates that building reservoirs in agricultural areas may lead to increased emissions of methane from the reservoir surface,” she explains, because the excess of nutrients and nitrates in the soil – introduced artificially during agriculture – may compound the issue.
She also says that designing dams to draw water from nearer the surface – and avoiding water near the decomposing plant matter at the bottom of the reservoir – may help. “There is some indication that methane emissions may be reduced by using surface water, not bottom water, to supplement streamflow below the dam, but this needs to be investigated further,” she says.
Sherman adds that we now know a lot more about the processes that drive GHG emissions from reservoirs: “I would expect that this understanding is incorporated into the assessment of potential hydropower dams, so that only the more suitable sites from a GHG emissions perspective are considered.”
However, what if we could turn this problem into an opportunity – and capture the methane produced by a reservoir to generate electricity? It’s certainly a possibility, according to a team of researchers from Brazil, led by Fernando Ramos. They suggest trapping methane emitted by the reservoir using a system of catamaran style barges, which would separate the methane from the water and compress it for easy storage in tanks. From there it could be transported to methane-burning power stations to produce electricity. Although methane is one of the most dangerous greenhouse gases, when burned it supplies surprisingly clean energy, releasing 29 per cent less carbon than oil and 43 per cent less than coal. So far, the work of Ramos’ team has yet to leave the lab. However, Townsend-Small is dubious whether it will work: “The methane emissions are diffuse and spread out over time, and the reservoirs have large surface areas. It wouldn’t be cost effective to trap the methane and purify it for commercial use.”
The reservoir emissions issue, though, isn’t going to go away. “Hydropower remains the cleanest, in terms of GHG emissions, of the very large scale centralised electricity generating technologies as far as I’m aware,” says Sherman. “It’s no contest when compared with thermal energy sources as a general rule.”
But International Rivers’ Bosshard remains sceptical. “Mega-projects such as the Three Gorges demonstrate that the benefits of large dams are almost always over-estimated, while the costs and impacts are under-estimated… We can’t sacrifice the arteries of the planet to save her lungs, particularly when better solutions such as wind and solar are now widely available.”