How Does Closed-Loop Pumped Hydro Storage Measure Up?

National Renewable Energy Laboratory (NREL) recently assessed the global warming potential of closed-loop pumped storage hydropower (PSH) systems compared to other grid-scale energy storage technologies. 

 

A rendering of the planned closed-loop Gordon Butte Pumped Storage Hydro Project in Montana. The site will have three turbine generators and 400 megawatts of installed capacity. Image used courtesy of Absaroka Energy

 

Running a life cycle analysis, NREL researchers found that PSH has the lowest greenhouse gas emissions among four other energy storage options: compressed-air energy storage (CAES), utility-scale lithium-ion batteries, vanadium redox flow batteries, and utility-scale lead-acid batteries. 

Grid-scale energy storage, a necessity for the U.S. energy transition, enables utilities to accommodate an increasing share of renewables like wind and solar, which accounted for 14% of America’s electricity generation in 2022, according to the U.S. Energy Information Administration (EIA). But, few studies compare the carbon impacts of different storage technologies. 

NREL noted that not all technologies offer the same services: PSH and CAES are best used in long-duration energy storage, but batteries supply short-term storage. While CAES provides grid-scale energy storage and other benefits like grid inertia and resilience, PSH produces about one-quarter of the emissions of compressed air. 

 

Closed-loop PSH’s global warming potential compared to alternative energy storage technologies. Image used courtesy of the authors (Creative Commons-BY license)

 

What Is Pumped Storage Hydropower?

To generate and store power, pumped storage hydropower facilities need water flowing between upper and lower reservoirs. This type of hydroelectric energy storage configuration has two reservoirs positioned at different elevations: water moves down to discharge, travels through a turbine, and is pumped back into the upper reservoir for the recharge process. The U.S. Department of Energy likens this concept to a giant battery, storing and releasing power on demand. 

 

The layout and functions of an open-loop vs. closed-loop pumped storage hydropower system. Image used courtesy of NREL

 

Unlike open-loop systems with a hydrologic connection to a natural body of water like a lake, closed-loop PSH is not connected to an outside body of water. 

According to the EIA, pumped storage hydroelectric systems in the U.S. had 22 gigawatts (GW) of utility-scale nameplate power capacity at the end of 2022—more than any other energy storage resource, including batteries and solar electric with thermal energy storage. There are 40 PSH plants and 152 generators scattered nationwide. 

A 2022 GIS-based analysis by the NREL found technical potential for 35 terawatt-hours (TWh) of energy storage at 14,846 sites across the U.S., Alaska, Hawaii, and Puerto Rico. Those sites could add 3.5 TW of capacity, assuming a 10-hour storage duration. 

 

More on the NREL Findings

The NREL study used public data comprising 39 designs of 35 proposed PSH facilities in the preliminary permitting phase in the U.S. The base scenario assumed renewables would cover the entire electricity mix. The researchers also assumed the closed-loop PSH facility would have 835 megawatts (MW) of installed storage capacity and deliver 2,060 gigawatt-hours (GWh) of stored energy annually. 

The lifecycle modeling boundary started with facility construction and ended with decommissioning. NREL wanted to gauge the global warming potential (GWP) of 1 kilowatt-hour (kWh) of stored electricity to a grid connection point at a nearby substation. 

The GWP of closed-loop PSH systems in the U.S. ranges from 58 to 530 grams (g) of CO2 per kWh, with the source of stored electricity having the largest impact, according to the researchers’ scenario analysis (base case GWP: 86 g CO2/kWh). Construction accounted for the second-largest source of emissions, with concrete and steel representing 4% of the GWP. Hydropower had the lowest GWP among competing energy storage methods, followed by lithium-ion batteries, lead-acid batteries, vanadium redox flow batteries, and CAES. 

PSH site characteristics can substantially impact this potential, as brownfield sites have 20% lower GWP than greenfield sites. Facility/project-level decisions also play a role, with larger facilities having a lower GWP than smaller ones. However, the choice of reservoir liner material and the expected facility lifetime had a marginal impact on the closed-loop PSH’s GWP. 

NREL’s study was published in Environmental Science and Technology.