Harnessing the power of environmental flows: Sustaining river ecosystem integrity while increasing energy potential at hydropower dams

Hydropower comprises the largest portion of the world's renewable energy portfolio yet induces significant environmental effects in river and floodplain ecosystems. Many countries are seeking to increase hydropower capacity primarily through the construction of new dams; however, this overlooks the potential energy that can be gained from optimizing existing hydropower infrastructure. Environmental regulatory protocols globally, and especially in the US, require hydropower facilities to provide environmental flows to downstream riverine ecosystems. Most of these facilities, however, fail to capture additional hydropower energy from these releases, such as passing them through smaller turbines. Herein, we conducted a US-wide analysis of the potential additional hydropower capacity and generation that could be available if environmental flows were harnessed for energy. We used two approaches based on availability of information: one reliant on available stream flow data to identify specific optimal turbine sizes, and another using a design-envelope approach, to accommodate situations where flow data was unavailable. Using available historical flow data and plant characteristics from over 200 existing hydropower facilities, the addition of minimum flow turbines could provide up to 730 MW of capacity and generate nearly 1.8 TWh of energy annually. Across the remaining hydropower fleet, the envelope approach suggests there may be anywhere from 12 TWh to 66 TWh of additional energy, supplied from a capacity increase of 2.5 GW–13.5 GW annually. Ultimately, this suggests that uncaptured environmental flows provide anywhere from 4% to 18% potential increase in US hydropower capacity and 4%–24% increase in generation.