Does Hydro Energy Emit Greenhouse Gases?

Hydroelectric power is a form of renewable energy that generates electricity by using the power of moving water. Hydroelectric power plants capture the kinetic energy of flowing water by directing it through turbines connected to generators (Britannica, 2024). The moving water spins the turbines, which then spin magnets inside the generators, producing electricity. This electricity is then fed into transmission lines and distributed (National Geographic, 2023). Hydropower plants are often built by damming a river and creating a reservoir, allowing control over the water flow. The water in the reservoir has potential energy, which is converted to kinetic energy as it falls through the dam turbines. Hydropower is considered a renewable energy source because it relies on the water cycle, in which water evaporates, forms clouds, precipitates, and flows back down to the ocean in an endless loop.

Greenhouse Gas Emissions from Hydroelectric Dams

While hydroelectric power does not directly emit greenhouse gases like fossil fuel-based power plants, reservoirs associated with hydroelectric dams can produce emissions of methane and carbon dioxide

Large reservoirs created by hydroelectric dams lead to the decomposition of biomass and carbon inundated by the dam. This decomposition causes the release of methane and CO2. Vegetation decays in an oxygen-deficient environment underneath the reservoir water, resulting in methane emissions. The emissions rates vary based on water depth, temperature, biomass availability and reservoir operation. Additionally, gases are released when water passes through the dam’s turbines and spillways.

According to the IPCC, hydropower reservoirs globally contribute approximately 1.3% of all man-made greenhouse gas emissions. The levels are modest compared to fossil fuel power plants.

Methane Emissions

Most methane emissions from hydropower facilities come from the breakdown of organic matter that gets flooded by the reservoir. As plants and other organisms decompose underwater, they release methane into the water. This methane eventually escapes into the atmosphere.

According to the EPA, methane makes up over 90% of greenhouse gas emissions from U.S. hydropower reservoirs (Research on Emissions from U.S. Reservoirs). Research shows that newly flooded reservoirs tend to emit more methane compared to older reservoirs. This is because new reservoirs have more decomposing vegetation and organic matter.

One study from Washington State University found that methane emissions from reservoirs are increasing over time as more dams are built globally (Methane emissions from reservoirs are increasing). Methane is more potent than CO2, so rising emissions are a major concern.

CO2 Emissions

Hydroelectric dams cause CO2 emissions primarily from the decomposition of flooded vegetation and deforestation. When an area is flooded to create a reservoir, vegetation that gets submerged decomposes in the absence of oxygen, resulting in emissions of CO2 and methane. According to the U.S. Department of Energy, the rate of decomposition depends on the climate, vegetation and soil types (Tracking the Carbon Footprint of Hydropower).

Additionally, the initial clearing of forests and vegetation to construct the dam and create the reservoir results in deforestation. This leads to a loss of CO2 absorption by plants and trees. Estimates by the International Hydropower Association show reservoir emissions from tropical regions are higher than boreal and temperate regions due to greater vegetation decomposition (Hydropower’s carbon footprint).

Emission Rates Vary

Greenhouse gas emission rates from hydroelectric dams can vary significantly depending on several factors. According to a 2021 study published in IOPScience (https://iopscience.iop.org/article/10.1088/2515-7620/acae24), reservoir GHG emissions are highly variable and depend on the climate, geology, and age of the dam.

Climate is a major factor determining GHG emissions. In warmer climates, reservoir decomposition and methane production tend to be higher. The geology of the flooded land, such as the amount of organic material and nutrients, also affects GHG production. Older reservoirs with more built up organic matter generally have higher emissions than newly flooded areas.

The study analyzed 28 reservoirs across North America and found over 300% variation in methane emissions and 30-40% variability in CO2 emissions. This highlights the challenges in precisely quantifying hydroelectric GHG emissions, as site-specific characteristics heavily influence the results.

Comparisons to Fossil Fuels

According to data from the International Hydropower Association, the carbon footprint of hydropower is significantly lower than fossil fuels like coal, oil, and natural gas across the entire lifecycle of energy production (Hydropower Association, 2022).

Research shows the greenhouse gas emissions from hydropower range from 2 to 48 g CO2/kWh over a project’s lifetime. Comparatively, life cycle emissions are 820 g CO2/kWh for gas-fired generation, 830 g CO2/kWh for oil-fired generation, and 860-1,250 g CO2/kWh for coal-fired electricity. Therefore, hydropower produces at least 50 to 200 times less greenhouse gases than fossil fuel-powered energy, including the methane emissions from reservoirs. As a result, hydropower is considered one of the lowest CO2 emitters of any energy source available today (Hydropower Association, 2022).

According to the U.S. Department of Energy, the average utility-scale hydropower facility produces over 30 times less greenhouse gas emissions than a conventional coal plant per unit of electricity generated over its lifetime (U.S. Department of Energy, 2022). Hydropower avoids 3.2 billion metric tons of carbon emissions compared to producing an equivalent amount of electricity from fossil fuels in the U.S. (U.S. Department of Energy, 2022).

Mitigation Strategies

There are ways to reduce the greenhouse gas emissions from hydroelectric dams. One strategy is to remove vegetation from the area that will be flooded before filling the reservoir. Studies have shown that clearing trees, shrubs, and other plants in advance can lower methane emissions from the reservoir by 25-85% (https://www.nature.com/articles/s41467-019-12179-5). Vegetation decaying underwater produces methane, so removing it ahead of time prevents some emissions.

Another mitigation technique is to aerate the water in the reservoir. Aerating the water introduces oxygen which helps decompose organic matter without producing as much methane. Strategically placed aerators in a reservoir can reduce methane emissions by over 50% based on preliminary studies (https://www.hydropower.org/factsheets/greenhouse-gas-emissions). More research is still needed, but reservoir aeration appears promising for reducing greenhouse gases from hydroelectric dams.

Hydro vs. Other Renewables

When comparing greenhouse gas emissions across renewable energy sources, hydropower generally has lower emissions rates than other renewables like solar, wind, and geothermal. According to the U.S. Department of Energy, the median life cycle emissions for hydropower are 24 g CO2-eq/kWh, compared to 46 g CO2-eq/kWh for solar PV, 12 g CO2-eq/kWh for wind, and 38 g CO2-eq/kWh for geothermal [1]. The emissions for hydropower mainly come from the reservoir itself, whereas the other renewables have emissions from manufacturing and materials. Hydropower’s emissions are generally lowest when the reservoir surface area is relatively small compared to the power capacity. Overall, hydropower remains one of the lowest carbon renewable energy sources available.

The Future of Hydro

Hydroelectric power has an important role to play in the transition to renewable energy. According to the International Energy Agency, global hydropower capacity is forecast to grow by 17% between 2021 and 2030, adding 230 GW of new capacity (IEA). The U.S. Department of Energy’s Hydropower Vision report predicts that by 2050, hydropower can grow capacity by up to 50% in the United States, adding up to 148 GW of new capacity and avoiding 5.6 gigatonnes of CO2 emissions (DOE).

Several factors will drive future hydro growth globally and in the U.S. Upgrading existing infrastructure can boost output at existing dams. New pumped storage can help integrate variable renewables like wind and solar. Developing countries still have major hydropower resources to tap. Climate change adaptation will also necessitate new reservoir and dam projects. Overall, hydropower is poised to remain the largest source of renewable electricity for decades while supporting grid stability and integration as more renewables come online.

Conclusion

Hydroelectric power has become an important renewable energy source, providing emissions-free electricity in many parts of the world. However, hydro dams are not completely free of greenhouse gas emissions. The reservoirs behind dams can produce substantial amounts of methane and carbon dioxide through the decomposition of organic matter. Methane has a global warming impact 25 times greater than CO2.

Emission rates vary widely based on the climate, geology and operations of each dam. Tropical regions tend to have higher emissions due to increased bacterial methane production. Overall, hydroelectricity produces far fewer emissions than fossil fuel power plants. But hydro does result in more emissions per kilowatt-hour than other renewables like wind and solar.

There are ways to mitigate greenhouse gas emissions from reservoirs, through management strategies like oxygenation systems. As we look to expand renewable energy, hydro will remain part of the solution. But other options like wind and solar may be preferable in some cases based on lower emissions. More research can help improve hydro technology and best practices to minimize its global warming impact.