Researchers aim to better quantify greenhouse gas emissions from hydropower reservoirs

As the United States moves toward more sustainable and renewable sources of energy, hydropower is expected to play a pivotal role in integrating more intermittent renewables like wind and solar to the electricity grid because hydropower can fill gaps in generation when these forms of energy are unavailable.

Before the hydropower sector can fully realize its benefits to the national power grid and the environment, more must be understood about the greenhouse gas, or GHG, emissions from reservoirs. Current reservoir emission estimates are highly variable, uncertain and incomplete. At the global scale, estimates can vary significantly and currently range from 0.14% to 6.6% of global GHG emissions.

Researchers at the U.S. Department of Energy’s (DOE) Oak Ridge National Laboratory (ORNL) are combining statistical modeling with advanced field methods, including drone technology, to more accurately assess GHG emissions from hydropower reservoirs.

Emissions are currently estimated by conducting field measurements, but with approximately 1,400 reservoirs that support hydropower generation across the nation, few have been sampled and even fewer, if any, are monitored regularly. This, combined with inconsistently applied methodologies and types of emission pathways studied, has resulted in spotty and uncertain results.

To make up for such a limited amount of field data, hydropower operators must rely on online modeling tools to broadly define the GHG footprints of their projects. However, emission figures are only as good as the underlying models, which themselves are developed based on field data.

Adding to the confusion is the fact that reservoirs often have multiple uses, including drinking water and flood control. So, attributing all reservoir GHG emissions to hydropower can be inaccurate.

“We’re committed to putting sound science behind the search for more accurate greenhouse gas emission estimates,” said Natalie Griffiths, research scientist in ORNL’s Environmental Sciences Division.

With funding from DOE’s Water Power Technologies Office, the ORNL project team will return to the sites of six hydropower reservoirs they sampled in the southeastern U.S. a decade ago.

Using more advanced measurement tools at each reservoir, the team will compare their findings to those from 10 years ago and to current model projections. The new assessment techniques are expected to provide greater resolution and explanation of variability, leading to more accurate models — all of which may shed light on potential mitigation efforts to further reduce emissions.

Understanding the pathways of GHG emissions

GHGs are emitted from all inland waters because of microbes in the sediment that naturally produce either carbon dioxide or methane. The ORNL research team will assess carbon dioxide emissions but will focus primarily on methane emissions, which have been understudied and pose a global warming potential 30 times higher than carbon dioxide.