U.S. Hydropower Capacity Trends: What’s Behind the Decline?
27-08-2024 | By Liam Critchley
Hydropower plants in the U.S. provide clean and renewable energy that is flexible for meeting the adapting grid environments in the U.S. Achieving a decarbonised electricity supply in the U.S. is going to hinge on hydroelectric power stations upholding historical levels of energy generation while simultaneously integrating many more distributed energy resources (DERs)—such as solar panels and wind turbines—into the grid. The combination of all these renewable energy sources will not only help to decarbonise the grid, but it will also provide flexible load balancing characteristics to support the grid during peak loads environments and power outages.
However, this scenario could well be challenging in the years to come. Hydropower plants depend on the flow of rivers within allocated sites, and there are many regulations that govern the storage of water behind a dam, as well as the pattern of water release through penstocks and spill gates. There are many of these constraints surrounding the flow of water and maintaining certain levels—to promote fish passage, downstream amenities, safety, and water—which can limit the flexibility of hydropower at certain times.
Another key aspect of hydropower plants in the modern-day world is that the market rewards hydropower producers if they operate at below capacity and only increase electricity production when needed. There are initiatives for flexible hydropower operations that improve hydropower profitability—especially when combined with other DER integration—but many of these incentives tend to reduce the annual electricity generation capabilities of hydropower plants.
As it stands, there are a number of aspects getting in the way that are reducing the levels of hydropower in the U.S., and if things don’t change, then these factors will also have a hand in the continued decline of hydropower within the U.S. Some of the main factors that are affecting U.S. hydropower include:
- Geophysical Changes – Climate and hydrological change
- Sociopolitical trends – flood control and environmental preservation
- Changes in the electricity market
A recent study has looked at the different factors affecting the decline of hydropower in the U.S. and what the future might look like if it continues along the same path.
The Importance of Hydropower in the U.S.
The decline of hydropower in the U.S. is important, not only from a sustainability perspective but also from a power demand perspective in the face of growing energy demands. In the U.S. today, hydropower plants generate 6% of the total electricity and 32% of utility-scale renewable generation. However, this percentage is declining as wind and solar technologies continue to expand in many regions of the U.S.
While the integration of more DERs is important for increasing energy demand and adding more flexibility to the grid, hydropower is also a vital energy resource in many U.S. states. For example, in the Pacific Northwest (Washington-Oregon-Idaho tri-state area), 63% of the annual electricity generated comes from hydropower plants. While other states are not as high as this, hydropower still significantly contributes towards a good percentage of annual power generation in some states, including 21% in the Northeastern United States (New York, Vermont, Maine, and New Hampshire), 13% in California, and 9% in Tennessee and Alabama.
Hydroelectric power is such a vital resource in some states that drought-caused losses in hydropower generation can affect electricity supply and grid reliability. This often leads to higher power generation of fossil fuel plants that results in higher grid costs and greenhouse gas emissions. Given the importance of hydroelectric plants in some regions, if the energy generation decline continues, then these states are going to be more susceptible to power outages and more reliant on carbon-intensive energy generation mechanisms—which is the opposite of what a lot of places are striving for.
How Hydropower is Declining in the U.S.
The recent study showed that the annual capacity factor of hydropower plans has declined, with four fifths of power plants showing a decreasing trend in their power generations since the 1980’s. The capacity factor of hydropower plant is defined by the total energy generation divided by the maximum possible energy generation implied by the nameplate capacity of the plant within the calendar year.
This capacity decline has seen a net loss of 13% generation across all plants despite there being capacity upgrades at more than half of the plants. After accounting for the capacity upgrades and removing them to see the loss and to look at the loss based on the plants all being the same and in their original state, the cumulative energy generation loss was shown to be 23% since 1980. This capacity loss since the 1980s is the same as retiring a Hoover Dam every 2 to 3 years.
The study looked at the annual energy generated by 610 out of 808 US hydropower plants that have a capacity of at least 5 megawatts. While not all plants fit into this category, and it equates to about three-quarters of all plants in the U.S, these plants correspond to 87% of the total conventional hydropower capacity in the United States (and only minor production plants were left out based on these characteristics, meaning that their exclusion wouldn’t have a big skew on the results).
The study found that changes in water availability were the main driver towards capacity factor decline in U.S. power plants over the last 4 decades and accounted for 21% of the energy decline. This obviously doesn’t explain all the factors involved with the decline, but it shows that other non-climate-based factors—such as deterioration of equipment, infrastructural changes, and changes to dam operations to support non-power objectives—have played a key role in the decline of U.S. hydropower generation.
The Potential Future of U.S. Hydropower
Even though there is a growing demand for electricity in today’s modern and technologically driven society, the majority of U.S. hydropower plants generate significantly less electricity each year than they did in the 20th century—even though there are ongoing efforts to decarbonise the grid.
Hydropower plants rely on water, and while lack of water has contributed to lower energy generation from rising temperatures in the wake of global warming (and the increased droughts it has brought), there are more societal and human-based factors that are driving the loss of power at hydroelectric sites.
It’s thought that changes to dam operations are the biggest human-led factor in reducing the capacity of power plants, which have primarily focused on initiatives to reduce power. The study reflects that the U.S. energy industry has perhaps traded the total energy-generating performance of their hydropower plants to meet the ever-changing needs of downstream communities—including preserving the local ecological systems and stabilising local energy grids.
It’s expected that the current decline trends will continue due to the continued environmental goals that are being set by the U.S. government and global think tanks. These environmental goals cover both river systems and grid infrastructure, so leniency in these areas is unlikely to occur, so it’s expected that the current trend will continue down the same path. However, in spite of this, there are ways in which the U.S. Department of Energy can facilitate more power from the nation’s hydroelectric plants, including new installations and turbine upgrades. Even though there’s a drive to reduce the environmental impact of hydroelectric plants—which reduces their capacity—new infrastructure upgrades could help to mitigate and stabilise, or even reverse, the long-term trends that have been occurring over the last 4 decades.
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