Researchers from three national laboratories run by the Energy Department shows that smaller scale “run-of-river” hydropower can provide just as much baseload stability as reservoir-based hydropower plants while being responsive to real-time grid and market changes.

Idaho National Laboratory is leading a research effort known as "Integrated" to evaluate the ability of run-of-river hydropower to provide grid balancing through integration with an energy storage system.

A 1 MW run-of-river power generation site. River water backs up and flows over the weir. A portion of the flow is diverted to the blue penstock for use in a downstream turbine hall. The diverted water then flows back into the river. Source: BC HydroHydropower plants produced roughly 7% of the electricity in the United States in 2017 and about 40% of the nation’s renewable energy. Roughly two-thirds of those 303,000 gigawatts came from dams with reservoirs behind them. The rest came from run-of-river (ROR) projects, where running water in a river or canal spins a turbine.

ROR hydropower generates power intermittently, based on how much water is flowing on any given day. A project funded by the U.S. Department of Energy’s Water Power Technologies Office is exploring new ways to use energy storage devices such as batteries, flywheels and supercapacitors to add storage to ROR hydropower plants. Integrated storage could enhance ROR power by making it possible to ramp power up or down on demand.

Researchers said that the growing presence of wind and solar on the grid means energy generation can change with little warning. Increased contributions from these variable power sources have intensified the need for plants that can be ramped up and down quickly to balance supply with demand. That is one reason for the recent growth of natural gas-fired plants.

The National Hydropower Association said that hydroelectric generating resources can offer a range of services to support the grid. These include:

• Load-following and flexibility reserve
• Energy imbalance service
• Spinning reserve
• Supplemental (non-spinning) reserve
• Reactive power and voltage support
• Black start (restoration) service.

Flexibility

On a macro scale, Idaho National Laboratory researchers said that dams like Hoover, Bonneville and Grand Coulee provide dispatchable energy. When power is needed, more water can be released by plant operators. But ROR hydropower combined with integrated energy storage technologies may be as flexble as natural gas when it comes to load following, researchers said. By using storage technology, they see possibilities to create energy reservoirs from ROR hydropower.

In ROR systems, running water is diverted from a flowing river and guided down a channel, or penstock, which leads to a generating house. The force of the moving water spins a turbine and drives a generator. The water flows back into the main river further downstream.

How they work

Unlike large hydropower dams, ROR systems do not dam the river to create a reservoir. Instead, most use a small dam, or weir, to ensure enough water enters the penstock. They also may have a small reservoir called pondage to store small amounts of water for same-day use.

As a result, if or when the river’s water levels are depleted because of drought or water extraction, the "fuel" for the ROR system is reduced or becomes entirely unavailable. This means the capacity factor of ROR projects varies between 40% and 80%. (Large hydroelectric dams with reservoirs have capacity factors closer to 85% to 90%.) The lack of a reservoir also means that ROR plants are typically feasible only on rivers with large year-round flow-rates.

Typical ROR system design that incorporates battery storage, flywheels and a supercapacitor. Source: INLThe DOE laboratory researchers said that storage options include batteries, which may provide longer duration energy than other options. Flywheels involve heavy shaft-mounted rotating discs that speed up when electrical energy is applied to them. When energy is needed, a flywheel is slowed and the kinetic energy is converted back to electrical energy, so it can be transmitted to demand centers.

Like flywheels, supercapacitors provide a charge in response to short-term power gaps that may last from a few seconds to a few minutes. They can be recharged quickly, so pairing supercapacitors or flywheels with batteries can reduce battery stress, resulting in longer service life.

Next steps

Phase I of the Integrated project confirmed the idea that ROR hydropower combined with energy storage systems can be as responsive as reservoir-based hydropower. One test case demonstrated the potential for the technology to increase ROR hydropower revenue by 12% to 16% due to the additional services that the ROR hydropower plants could provide to the grid.

For example, ROR hydropower owner/operators could sell electricity to the grid at the most opportune times and avoid having to drop their price when demand is low. Known as load-following or peak-shaving, this option makes it possible for a plant to keep electricity on hand until it’s most needed.

ROR hydropower plants with integrated storage systems could also be compensated for helping improve grid stability. A service known as frequency regulation can help prevent brownouts when a transformer faults or people get home from work and turn on their air conditioning. Storage systems at ROR plants would be able to rapidly discharge electricity to respond to supply or demand fluctuations.

Phase II will include two field demonstrations of the technology. The first field demonstration will be a black start test to demonstrate that ROR hydropower plants with energy storage can restore electric power without assistance from the transmission system. Lessons from the field demonstrations, with Idaho Falls Power and another utility, will be used to refine the technology and ultimately prove its reliability under real-world conditions.

The black start test will involve Idaho Falls Power, a city-owned utility with four ROR hydropower plants on the Snake River. Black start capability is essential for small hydro to be able to operate a microgrid to power critical loads in the event of an extended large area outage. The test will investigate how much energy storage is needed to provide adequate frequency and voltage stabilization for a system during a black start.

Idaho Falls Power, a collaborator on the project, has four ROR power plants on the Snake River.

Along with INL, which is leading the project, Argonne National Laboratory and the National Renewable Energy Laboratory are taking part in the Integrated work.