Tech Insights

Channeling Hydropower: How Waterways Can Support the Grid

April 12, 2024 by Shannon Cuthrell

From repurposing waterways to advancing projects with higher-performing technologies, the U.S. can invest in hydropower for reliable electricity and storage capacity. 

Thousands of non-powered dams and 3.5 million miles of sea-connected rivers and canals in the U.S. can be tapped to meet the growing demand for firm, flexible power and storage supply amid the transition to renewable energy. A National Renewable Energy Laboratory (NREL) report highlights opportunities to invest in retrofitting existing dams or adding new capacity featuring non-conventional designs. 

Non-powered dams, which account for 97% of all U.S. dams, could be transformed to generate electricity for medium-sized projects with at least 2 GW of capacity. These facilities usually provide flood control, water supply, irrigation, and other functions. 

 

The 61-year-old Cowans Ford Hydroelectric Station completed a multi-year round of upgrades in 2020. It’s North Carolina’s largest conventional hydropower plant, producing over 320 MW.

The 61-year-old Cowans Ford Hydroelectric Station completed a multi-year round of upgrades in 2020. It’s North Carolina’s largest conventional hydropower plant, producing over 320 MW. Image used courtesy of Duke Energy

 

Besides dams, existing waterways like canals and rivers can also be used for in-stream hydropower systems, which harness the kinetic energy of flowing water. No dam or head infrastructure is needed to deploy in-stream turbines. 

Hydropower is a proven technology. The earliest plants in the U.S. were built in the 1880s. The resource accounts for 96% of America’s utility-scale energy storage capacity, around 6% of all electricity generation, and 26% of the renewable energy mix. 

NREL notes hydropower is especially suited for small- and medium-sized projects producing under 30 MW. Many utilities today seek reliable technologies to offset volatility from variable renewable energy resources like solar and wind, which accounted for 14% of utility-scale generation in 2023. Hydropower resources can be tapped for much-needed long-duration energy storage. 

 

Utility-scale power and energy storage capacity by technology. Image used courtesy of the DOE (Page 17, Figure 1)

 

Hydropower in the U.S.

The U.S. has over 2,200 conventional hydropower plants with 80.58 GW of generating capacity. Washington, California, and Oregon are the top states in installed hydropower capacity. 

Standard hydroelectric plants store river water in a reservoir. When released, the water flows through a turbine and activates a generator. Another design is diversion (or run-of-river) plants, which utilize the natural decline of river elevation. 

While hydropower is generally reliable, it isn’t immune to periodic outages. Failures in turbine/rotor or generator components like stator windings, bushings, and terminals (usually in old units outliving their design life) accounted for more than two-thirds of lost generation due to forced outages between 2013 and 2021. Other top reasons include lack of water and failures in main transformers. 

 

Hydropower development pipeline.

Hydropower development pipeline. Image used courtesy of Oak Ridge National Laboratory

 

Still, hydropower remains a highly durable, low-cost form of clean energy, contributing to its steady relevance in the electric power sector. More than 1 GW of medium-sized projects are in the U.S. development pipeline today, including retrofits for non-powered dams (with 63% of the total or 670 MW), capacity additions (243 MW), closed- and open-loop pumped-storage (91 MW), stream-reach projects (46 MW), and generation in conduits (5 MW). NREL estimates that the required investment to complete this pipeline would range from $3.16 billion to $9.5 billion. 

 

Pumped-Storage Hydropower

Pumped storage hydropower (PSH) plants provide critical storage functions aiding the transition to renewable energy. They work like battery energy storage systems, which are increasingly popular for balancing supply and demand when renewables are unavailable. Similarly, PSH stores renewable energy for use in peak demand. Plants typically have two reservoirs at different elevations. Water moves down to discharge, passes through a turbine and is recharged by pumping back into the upper reservoirs. 

PSH lends substantial storage capacity, with most facilities topping 30 MWh. While battery installations have grown in recent years, PSH remains dominant, with 70% of utility-scale storage capacity and 96% of all storage. More than 40 PSH plants totaling 22 GW of generation capacity and 554 GWh of energy storage are operating today. The total has grown over the last 10 years due to upgrades at existing facilities. 

 

Pumped-storage hydropower animation.

Pumped-storage hydropower animation. Image used courtesy of DOE 

 

PSH offers more extended energy storage than batteries. The median storage duration among the 445 utility-scale batteries in the U.S. is just two hours, while PSH plants cover eight to 12 hours. PSH systems pump water from the lower reservoir into the upper one, then release the water during high demand back into the lower reservoir to spin the turbine. 

Closed-loop PSH systems represent most (80%) projects in today’s PSH development pipeline. Once completed, they’ll be the first such facilities in the U.S., as all existing PSH facilities are open-loop configurations continuously connected to naturally flowing water sources. California, Virginia, and South Carolina lead the nation in PSH capacity. 

Since closed-loop systems are not connected to natural bodies of water, they offer more flexibility in selecting locations and a lighter environmental impact than open-loop facilities. NREL estimates that 3.5 TW of closed-loop potential capacity exists across nearly 15,000 sites when assuming a 10-hour storage duration. 

 

Emerging Hydro Tech

Several emerging technologies can supercharge America’s hydropower potential, according to NREL. 

For example, small modular conduit hydropower systems can be applied to larger systems if needed in the future. NREL’s report cites technology from Georgia-based Emrgy, which offers modular turbines at 5 to 25 kW for existing canals. The company’s twin vertical axis turbines have demonstrated up to 70% water-to-wire efficiency, and the system doesn’t require a gravity drop and head pressure as in conventional hydropower. It optimizes the kinetic energy of the volume of water flow. 

 

Emrgy's hydrokinetic turbines

Emrgy's hydrokinetic turbines. Image used courtesy of Emrgy

 

Other innovations cited by NREL offer compelling benefits. Reversible pump-turbine flow inverters can divert the water flow to enter and exit a well, thus allowing installation at existing infrastructure. Open-pit mines can also be redeveloped to reuse the upper and lower reservoirs for projects. 

Geomechanical PSH systems pump water into underground rocks, and the formations act like a pressure spring to pass water through a turbine and generate electricity. A few companies in the U.S. are working to advance this technology. For example, Texas-based Quidnet Energy offers a modular solution grouping multiple units into larger configurations for 10-hour long-duration energy storage. 

 

Video used courtesy of Quidnet Energy

 

NREL’s report also highlighted hybrid plant configurations, which combine hydropower with other renewable and non-renewable resources. They typically have two or more generators paired at one interconnection point. As of 2021, there were nearly a dozen biomass-plus-hydro hybrids (mostly at paper mills), 26 fossil fuel-plus-hydro plants, and one with a nuclear-plus-hydro configuration. 

Hydropower-plus-battery storage is gaining interest because integrating batteries into a hydro plant with little to no water usage (like a small river facility) allows owners to provide peaking power, frequency regulation, or black start ancillary services. This helps with variability in net load from the increasing penetration of solar, wind, and distributed energy resources in the power grid. Also, hybrids with batteries can extend the life of hydropower units up to 5%, avoiding maintenance costs.