Hydropower is by far the largest source of renewable electricity globally and is projected to grow further in the coming decades. However, some researchers are concerned it could have a larger climate impact than is often acknowledged.
Studies show some hydropower plants can have emissions comparable to fossil fuel plants, adding to a long list of other problems associated with the technology, from impacts on river flow and ecosystems, to the displacement of local people. Many countries and institutions promote the expansion of hydro as a low-carbon way of meeting the world’s growing electricity demand, but can it shed its image as the dirty man of renewables?
The third renewable pillar
Water has been used as a source of energy for thousands of years, and hydro was one of the earliest forms of mechanical energy used in factories.
“Hydro has been the renewable energy technology from the purely mechanical days of grain grinding and textile mills of the mid and late-1800s,” says Robi Robichaud, a renewables expert at the World Resources Institute, a non-profit organisation based in the US. “Even as we [began] to generate electricity, hydropower was a leader in the 1890s [to] 1940s where it was available. It competed primarily against coal.”
Hydropower remains by far the largest source of renewable electricity, supplying around 16% of global power in 2019 – roughly three times the generation of wind power and six times that of solar. (It is worth noting that many environmental groups do not consider large hydropower a truly “renewable” resource due to its negative ecological impacts.)
Electricity production from hydro has increased by around two-thirds since 2000, and is expected to remain the largest source of renewable power for decades. Around 1,000 dams are under construction, largely in Asia, and the International Energy Agency (IEA) expects hydropower generation to rise another 50% by 2040. However, the recent rapid increase in wind and solar power means hydro’s share of the renewable electricity mix will drop below 50% for the first time by 2024, says the IEA.
In short, while solar and wind are the renewable superstars of the 21st century due to their rapid cost reduction and growth, hydro remains by far the largest overall source of renewable power, and will for some time. The IEA expects it will still account for 16% of the world’s electricity generation in 2025.
“They are still installing new [hydro], but they’re not going gangbusters,” says Robichaud, underlining that the best places for hydro are already taken.
China leads the world in hydro generation, followed by Canada, Brazil, the US, Russia, India and Norway which are all large producers. Hydropower is different in many ways from solar and wind, not least in its geographical distribution. The majority of dams are also not used for hydropower, but for storing water or as flood protection.
This storage ability is a key difference between hydro, and wind and solar.
“Hydropower is like a big battery, you can store power in the form of water,” says Mark Mulligan, a hydro expert from Kings College London. “And then you can release the water when you want and generate your electricity on demand.”
Advocates of hydro say this storage ability makes it the ideal partner to more variable wind and solar power: water can be pumped up a dam into the reservoir when excess electricity is available and released when needed. Such pumped storage hydropower provides 94% of the world’s battery storage, even if this proportion is steadily shrinking with the rising capacity of lithium ion batteries.
There is significant interest in whether large hydropower plants could play a smaller role in global energy production. One study found that replacing all hydropower generation in the US with solar PV would use just 13% of the space of existing reservoirs.
The cost of hydropower overall remains slightly below wind and solar, shows the latest report on renewable power generation costs from the International Renewable Energy Agency (IRENA). The report found that the global weighted-average cost of newly commissioned hydropower projects in 2019 sat at $0.047 per kilowatt-hour (kWh), compared with $0.053/kWh for onshore wind, $0.115/kWh for offshore wind and $.068/kWh for solar PV.
However, while wind and solar costs are coming down year by year, globally, hydropower costs went up by 27% between 2000 and 2019 due to rising installed costs, especially in Asia. The IRENA report said this was likely due to projects being in more challenging sites, noting that nine-tenths of hydro capacity commissioned in 2019 was cheaper than the cheapest new fossil fuel-fired cost option.
Hydro’s climate problem
The International Hydropower Association claims global emissions from fossil fuels and industry would be around 10% higher if electricity from hydro was provided by coal. However, a number of emissions studies have raised questions over hydro’s status as a clean power source.
Hydropower emissions can come from forest clearance to make way for dams, the sourcing and transport of materials, and construction. Methane produced from inundated vegetation fermenting in the reservoir can be another major source, especially in the shorter term. The climate impact from these causes tends to be highest in the first decade of a dam’s operation.
In some cases, methane and other sources of biogenic carbon from hydropower plants can be comparable with emissions from coal plants, concludes a 2016 paper led by Laura Scherer, now an assistant professor at the University of Leiden in the Netherlands. The study of around 1,500 hydropower dams concluded that the overall carbon footprint of hydropower was on average far higher than previously thought.
However, it also found there was huge variability between plants: the highest-emitting ones tended to be in tropical areas with a high density of vegetation (and carbon), Scherer says, while taller dams with a small surface area of reservoir in temperate areas were lower emitting. Emissions from run of the river hydropower, which avoids reservoirs altogether, were the lowest, she added.
Another study, published last year by the Environmental Defense Fund, a US NGO, similarly found a huge variability in emissions from different plants, with some on a par with fossil fuels. Overall, emissions were found to be better than fossil fuel plants, but worse than wind, solar and nuclear.
“If minimising climate impacts is not a priority in the design, construction and geographic placement of new hydropower facilities, we could end up generating electricity that yields more warming – especially in the near-term – than fossil fuels,” says Ilissa Ocko, climate scientist at EDF and lead author of the study.
Emissions from hydropower would on average be similar to emissions of fossil fuel carbon capture and sequestration (CCS) plants by 2050 under a ”climate protection scenario”, shows another study. Both hydro and CCS would average around 100g of CO2 equivalent per kilowatt-hour, it found, while nuclear, wind and solar power would emit just 3.5–12g.
Mulligan says the methane problem from hydro does not compare overall with equivalent emissions from coal, oil or gas. “But it is an indicator that there is no free card in electricity generation – everything has an impact,” he says.
The other hurdles
Climate impact is just one of many environmental concerns about hydropower. Dams require a lot of steel and cement rock, which needs to be mined and can cause local pollution during construction. Studies have also documented devastating impacts on freshwater river fish. Biodiversity hotspots, including the Amazon, Congo, Salween and Mekong watersheds – key sites among the recent trend of large hydropower dams being built in developing countries – have been especially hard hit.
Furthermore, activists are pushing for the hydropower industry to address its dubious human rights record. The industry has by far the largest number of human rights allegations in the renewable energy sector, says Jessie Cato from the Business and Human Rights Resource Centre, a UK-based non-profit.
These allegations cover everything from unsafe labour practices to abuse of land rights, but frequently they relate to free, prior and informed consent, says Cato, meaning the right of local and indigenous people to have a genuine say in whether and how projects go ahead. As well as the importance of considering impacts on local people, Cato argues that ensuring all renewables projects respect human rights is an imperative if the world is to achieve timely decarbonisation.
Many countries plan to use hydropower to meet their climate commitments under the Paris Agreement, but some will face hard choices. One study on Ecuador warned against an over-reliance on hydropower due to concerns that social-environmental constraints and climate impacts on hydropower, such as more frequent droughts, could limit its output. There is increasing interest from countries like Bhutan, Laos, Tasmania and Canada to export hydro energy to neighbours looking to cut their emissions.
It is easy to see why there continues to be hesitancy over hydropower, and there is an increasing push to ensure future development improves its social and environmental record. One 2018 review of hydropower advised moving away from big dams towards smaller in-stream turbines placed within a free-flowing river without using a reservoir, and partnering with other renewables like solar and wind.
The paper set out a framework of solutions for hydro. These included: environmental and social impact assessments carried out by companies serving citizens not dam builders; the right for locals to stop dams being built; sustainability evaluation measures that can stand public and independent scientific scrutiny; and the development of innovative technologies that do not require damming the river or resettling populations.
Another study warning about the high greenhouse gas emissions of many proposed Amazon dams that carbon emissions from the expansion could be lowered by placing dams in higher elevations in smaller streams. However, smaller dams are not automatically more environmentally friendly, says Mulligan. “They will spread the impacts far and wide,” he says, adding that environmental impact assessments rarely take into account other dams along a river. Smaller dams are expanding rapidly and can fall under the policy radar despite their potential cumulative effects.
There are many ways to reduce the impact of hydro but all will be “in incremental slivers”, says Robichaud. More sustainable technologies like pumped storage, run of the river and new stream-reach hydro can help he says. Another way is to add hydro-electricity generation to the thousands of dams built for other purposes such as water storage or flood control.
Steady progress is being made to try to clean up hydro’s less-than-shiny record when it comes to climate. A sustainability framework for dams developed by the International Hydropower Association was updated in 2018 to include best practice in climate change resilience and mitigation. Meanwhile, the Climate Bonds Initiative is developing a hydropower standard so investors can ensure hydropower projects are in line with the Paris Agreement. To be certified, projects must prove an emissions footprint of less than 100g of CO2 equivalent per kilowatt-hour, as well as strict standards for adaptation and resilience to climate change.
All the while, hydropower operators will be increasingly forced to deal with the effects of climate change, with rising temperatures melting glacial sources of water for hydropower in some parts of the world and drought drying up water basins in others.
As Mulligan points out, with many countries facing the dual challenge of cutting carbon and fulfilling energy demand, those with the potential to develop more hydro are likely to continue to exploit this resource.
Jocelyn Timperley is a freelance climate journalist based in Costa Rica.