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How green is Hydroelectricity?

Posted by Alex Barrett on 5 May 2017 at 4:24 pm

Large hydropower schemes account for the majority of the world’s renewable energy supply. Unlike wind and solar, hydroelectricity can provide a constant stream of renewable power, regardless of the weather or the time of year. Hydroelectric dams can be used to store power, and can quickly be brought online to meet peaks in electricity demand. All of these factors make hydroelectricity a great option to replace fossil fuels, and one that is being adopted all over the world. Unfortunately this system is not without its faults.

Hydroelectric power plants don’t use fossil fuels, so one would expect their greenhouse gas emissions to be low. Unfortunately hydroelectricity this is not the case, at least for industrial scale hydroelectric facilities. In order to guarantee a steady flow of water a dam is constructed across a river valley, and the area behind flooded. This creates a large reservoir of water which can be used to run the generators, and which is constantly replenished by rainfall and runoff. Unsurprisingly this sort of landscape modification effects the local ecosystem, but it also results in the release of large volumes of greenhouse gasses.

Firstly building a dam is a very energy intensive process. It requires vast volumes of steel concrete, both of which have a very high embodied energy. The cost to make them is high, so it will take a long time for the dam to pay back the energy involved in its construction. The production of concrete actually emits greenhouse gases, meaning that the dam will have a large carbon footprint before it is even completed.

When the valley is flooded it is impossible to entirely clear it of biomass, and in most cases no attempt to do so is made. Most hydropower installations leave all of the plants and foliage in place. Once underwater this biomass quickly decays and produces methane. This is especially problematic in the tropics, where many of these installations have been built. Here the warm conditions and extensive biomass make methane emissions pronounced.

Methane is a very effective greenhouse gas, it is 25 time stronger than carbon dioxide, although it doesn’t have quite as long a residence time in the atmosphere. Methane is produced when vegetation decays in natural lakes and wetlands too. However in these systems it is usually oxidised as it slowly seeps from the lowest depths of the lake to the surface. This converts it into carbon dioxide which is then released to the air. Unfortunately this does not occur in hydroelectric reservoirs. Instead water is drawn through turbines in order to generate electricity, releasing the methane in the process.

The greenhouse gas emissions are highest at the start of a reservoirs life, when lots of biomass is breaking down. However new biomass is constantly being introduced to the reservoir as a result of its rising and falling surface, so while emissions drop off they never stop completely.

The Brazilian National Institute for Space Research estimates that the world’s largest hydroelectric stations release as much as 104 million tonnes of Methane per year, accounting for 23% of all methane released by human activity [1]. The greenhouse gas emissions from hydropower do not compare favourably to those of fossil fuel burning power stations.

The good news is that these concerns are mainly related to large scale industrial hydropower stations. Smaller “run of river” systems, which do not require a reservoir have much lower greenhouse gas emissions. Most domestic hydropower falls into this category, so if you have a stream that is suitable for micro hydro then you can install it without fear.

So should we be building new hydroelectric stations? This is a complicated issue. They may have comparable greenhouse gas emissions to fossil fuels, but they are still renewable. The water cycle is not disrupted by generating hydroelectricity, whereas burning fossil fuels depletes the world’s supply.

Hydropower may contribute to greenhouse gas emissions, but reservoirs don’t release many of the other harmful pollutants that have to be controlled from the burning of fossil fuels. The disruption to the environment from building a reservoir is no worse than the destruction caused by coal mining.

Reservoirs have a variety of other benefits, such as providing a reliable water supply and preventing dangerous floods downstream. Hydropower also has the ability to “ramp” power production very quickly. This is important in meeting sudden peaks in electricity demand. It is also more reliable at producing electricity around the clock, as it is not dependant on having strong winds or bright sunlight in the way that other renewable energy sources are.

These advantages mean that hydropower remains a good option for reducing our fossil fuel use. Hydropower is likely here to stay, even if its green credentials aren’t as good as they may at first appear.

References

Emissions from Hydroelectric Reservoirs

  1. International Rivers
  2. New Scientist
  3. McGill Reporter
  4. Global Post

Advantages of Hydroelectricity

  1. US Government, Department of Energy Efficiency and Renewable Energy
  2. United States Geological Survey

Image credit: The Three Gorges Dam, by Felix's Endless Journey via flickr

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