Although hydropower offers significant potential for renewable electricity generation and storage, characterization of greenhouse gas (GHG) emissions from hydropower reservoirs is inconsistent and incomplete, leading to highly variable emission estimates that have ranged from 0.14% to 6.6% of global GHG emissions. This uncertainty can pose an obstacle to widespread adoption of these water power resources.
One way that reservoir emissions are currently estimated is by conducting field measurements, but few of the country’s 1,400 reservoirs that support hydropower generation have been sampled, and even fewer are routinely monitored. 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.
To more accurately quantify GHG emissions from hydropower reservoirs, researchers at the US Department of Energy’s Oak Ridge National Laboratory (ORNL) are using a combination of modeling and new field sampling measurements to quantify emissions from each of the most common emissions—diffusion, ebullition, and degassing.
- In diffusion, microbes in the sediment churn and release GHGs to the water surface. Warmer temperatures increase this rate of release.
- In ebullition, or bubbling, methane becomes trapped in bubbles, and if those bubbles rise to the water surface, the methane is emitted to the atmosphere. This bubbling pathway occurs primarily in shallow waters that do not have enough water pressure to keep the bubbles from traveling through the water column to the reservoir’s surface.
- Degassing occurs when reservoir water is passed through a pumphouse via hydropower turbines, or through flood control spillways. Pulling deep water, which often contains higher methane concentrations than shallower water, through turbines can release methane downstream.
The ORNL study marks one of the first times all three GHG emission sources are being measured simultaneously.
With support from the US Department of Energy’s Water Power Technologies Office, the ORNL project team is returning to six hydropower reservoirs in the southeastern United States that they sampled 10 years ago to collect new GHG measurements. Using more advanced measurement tools, including drone technology, the team is comparing its findings with those from 10 years ago and to current model projections.
In addition to collecting the new field measurements, ORNL researchers are analyzing the effects of local climate variability and dynamic reservoir operations using the International Hydropower Association’s G-res Model—a web-based tool to assess the carbon footprint of a reservoir. Results of these models will provide insight into environmental and other factors that influence GHG emissions and identify gaps in knowledge or data. The project team is also collaborating with the US Environmental Protection Agency’s national reservoir emission survey that is measuring GHG emissions at 108 reservoirs.
The new assessment techniques are expected to provide greater resolution and explanation of variability in GHG emission estimates, potentially leading to more accurate models. The work done in this study will shed light on potential mitigation efforts to further reduce emissions while enabling greater adoption of hydropower for a more resilient national power grid.