Pumped Storage

Pumped Storage Hydropower is America’s Energy Storage

Everything old is new again. Hydropower is making its comeback, and not just as a generation source. Water can act as a battery, too. It’s called pumped storage and it’s the largest and oldest form of energy storage in the country, and it’s the most efficient form of large-scale energy storage.

Hydropower was America’s first renewable power source. It is often mistakenly considered a tapped resource, but according to the U.S. Department of Energy’s 2016 Hydropower Vision report, hydropower’s capacity can sustainably add 50 new gigawatts by 2050 — 36 GW of which is pumped storage.

Pumped storage today makes up 97 percent of utility-scale energy storage in the United States at 42 sites with a total of 23 GW of capacity.

Pumped Storage Explained

Pumped storage facilities are built to push water from a lower reservoir uphill to an elevated reservoir during times of surplus electricity. In pumping mode, electric energy is converted to potential energy and stored in the form of water at an upper elevation, which is why it is sometimes called a “water battery”. Pumping the water uphill for temporary storage “recharges the battery”.

From there, gravity takes care of the rest .During periods of high electricity demand, the stored water is released back through the turbines and converted back to electricity in generating mode like a conventional hydropower facility.

Illustration source: U.S. Department of Energy Hydropower Vision Report

Pumped Storage Enables More Wind and Solar

Pumped storage hydropower enables greater integration of other renewables (wind/solar) into the grid by utilizing excess generation, and being ready to produce power during low wind and solar generation periods. It also has the ability to quickly ramp electricity generation up in response to periods of peak demand.

As the capacity of available firming resources continue to be stretched to their limit to support the growth of
variable renewable energy resources, the U.S. electric industry is moving more toward the deployment of emission-free energy storage resources. Pumped storage provides predictable, consistent generation.

Pumped storage facilities are like sponges; they can suck up this excess energy from the grid and store it for later.

Pumped Storage and the Grid

Other renewables, like wind and solar generation, while less variable with adequate geographic diversity, can present new
challenges for the U.S. grid. The power output in variable generation resources can fluctuate widely as weather patterns change and, while the changing weather patterns may be well understood, the magnitude of renewable energy generation ramps (in particular, when not in correlation with changing load) can be challenging to grid operators when renewable energy resources are a large component of their generation portfolio. This variable output can lead to frequency and voltage fluctuations, which adversely affect grid stability.

Pumped storage, however, meets increased transmission system demands for reliability and system reserves. It shifts, stores, and reuses energy generated until there is the corresponding demand for system reserves and variable energy integration.

This shifting, when performed at a grid-scale, can also avoid transmission congestion periods (i.e., absorb or consume surplus generation to levels consistent with transmission transfer capability), to help more efficiently manage the electric grid (e.g., quick access to significant and sustained energy ramping), and to avoid potential interruptions to energy supply (e.g., supply operating reserves, spinning inertia, etc.). Advanced adjustable speed technology also allows pumped storage to provide an even greater range of fast ramping, both up and down, and frequency regulation services in both the generation and pumping modes.