On July 10, 2024
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Getting the most from Vermont’s hydro power

By Jonathan Dowds

Editor’s note: Dowds is the deputy director of Renewable Energy Vermont.

From 18th-century mills to current hydroelectric facilities, hydropower has been a valuable resource throughout Vermont’s history. Today, hydro is an important source of carbon-free electricity, accounting for nearly 50% of the power that we generate in our state. Hydropower has less variability in its power output than wind and solar, making it a good complement to these energy sources and an important part of the most cost-effective path to 100% renewable electricity: a combination of wind, solar, hydro, and energy storage. 

Most hydro facilities in Vermont are “run of river” meaning that they do not have large reservoirs and the flow of water entering and exiting the hydropower facility is essentially the same. Run-of-river hydroelectric projects are considered the most environmentally friendly form of hydropower. 

Even so, the number of hydro facilities operating in Vermont is unlikely to increase so optimizing the output of our existing fleet is critical. Fortunately, there are opportunities to manage some of these dams to produce more power from the existing infrastructure while continuing to protect our state’s water quality, mainly by being more creative and dynamic about how much water flows through the dam’s turbines and how much flows over the dam itself.

In general, water flowing to a hydroelectric facility follows two paths: a portion of the water is passed through turbines — generating electricity — and then discharged back to the river below the dam, and a portion of the water is allowed to flow directly over the dam. The more water that passes through turbines the more emission-free electricity is produced. The stretch of the river between the base of the dam and the location where the turbines discharge water back into the river is commonly called the “bypass reach.” The length of the bypass reach can vary among hydroelectric projects but is often on the order of 100-200 feet.

When an existing Vermont hydroelectric facility comes up for relicensing, the Vermont Agency of Natural Resources is charged with determining if the operation of the project meets state water quality standards. The agency evaluates the impact of the project on water quality (dissolved oxygen and temperature), aesthetics, and fisheries habitat — including but not limited to habitat in the bypass reach. Determining the volume of water that must flow over the dam to meet these standards is an essential part of this process. But this water volume is not static.

The magnitude of flow needed to maintain dissolved oxygen levels and aesthetic benefits changes seasonally. Colder water holds more oxygen than warmer water so the amount of water spilled over the dam can safely be decreased in the fall and winter allowing more water to flow through the turbines to produce clean renewable energy. As for the aesthetic benefits of spilling water over the dam, in an era where climate change is threatening some of Vermont’s most iconic plant species and habitats, it is fair to ask whether this should be a consideration at all. But even accepting that aesthetics should be a consideration, spilling water over the dam, year-round, provides little benefit overnight when visibility is minimal or in the winter when rivers are iced over. Maximizing flow through the turbines during these periods is another opportunity to increase power production.

The magnitude of flow in the bypass reach can also impact fish habitat but changing how we measure these habitat impacts may also open up opportunities for increased power generation without appreciably impacting fish populations. 

Currently, the Agency of Natural Resources sets thresholds for the amount of habitat that must be preserved in percentage terms rather than based on the absolute area impacted. If a bypass reach has relatively little good fish habitat at baseline — which is not uncommon given that it lies at the base of a waterfall and may be comprised of bedrock or ledge — then the loss of even a small area of potential habitat is large in percentage terms. Losing say 100 square feet of potential habitat when there are only 200 square feet to start with looks large in percentage terms (50%!) but is modest in terms of absolute habitat availability (and exponentially more modest in the context of the entire river). Getting back to absolute impact measurements would help us make more sensible trade-offs between two very important goals: habitat protection and clean power generation.

Vermont’s regulators at the Agency of Natural Resources play a crucial role in setting these water spill standards when dams are required to relicense their projects. In the interest of maximizing our valuable hydro resources, the agency would do well to get creative about maximizing the clean energy these facilities can produce.

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