
Existing pumped storage hydropower (PSH) technology provides critical long-duration storage and supports decarbonization, but Argonne National Laboratory set out to baseline new technology types that could have potential with additional research and development.
A new report, A Review of Technology Innovations for Pumped Storage Hydropower, details these technologies.
Evolving technology
PSH has been around for many years, but the technology is still evolving. Many new PSH concepts and technologies are being proposed or researched, according to ANL. This study performed a landscape analysis to establish the state of PSH technology and identify promising new concepts and innovations.
The report said that most existing PSH plants use reversible pumps/turbines, which are typically Francis-type turbines designed for both generating and pumping. The Tennessee Valley Authority built the first reversible pump/turbine (59.5 MW, Hiwassee Unit 2) in North Carolina in 1956.
Before that, PSH plants used a pump and motor on one shaft, and a turbine and generator on another shaft not connected to the first. The report said that separate pumps and turbines are still used for some PSH configurations, such as in ternary, quaternary, and pump-back PSH plants that have a separate pumping station. A pump-back PSH plant can use natural inflows into the upper reservoir to produce electricity as a conventional hydropower plant, but can also pump the water back into the upper reservoir for additional storage as a PSH plant.
Most existing PSH plants use fixed-speed (or single-speed) technology. These make use of a synchronous machine as motor-generator, which operates in sync with the grid frequency. This is also the case with most other generating technologies, as they typically use synchronous machines to generate electricity.
However, while other technologies use synchronous machines only as generators, PSH plants use them as both motors and generators. That means that the synchronous machine is used as a motor when the PSH unit operates in the pumping mode, consuming the electricity from the grid to pump the water into the upper reservoir. The same synchronous machine is used as a generator when the water is released from the upper reservoir, reversing the direction of rotation, to generate electricity for the grid.
As part of its review, the ANL report looked at 12 innovative PSH technologies, using a set of predefined criteria. Because the innovative PSH technologies are at different technology readiness levels, the study did not attempt to rank or directly compare technologies to each other. Rather, the goal was to provide an independent review of various proposed PSH technologies and discuss their innovations to assess whether they have the potential to reduce the cost and time required for the construction of new PSH projects in the U.S.
Promising technologies
Based on the review, ANL identified what it said were several promising innovative PSH technologies: submersible pump-turbines and motor-generators, geomechanical PSH, open-pit mine PSH and hybrid PSH.
The report said that Obermeyer Hydro, Inc., is developing PSH technology that uses submersible pump-turbines and motor-generators in the United States. While conventional PSH plants typically use reversible pump-turbines that are submerged below water level and non-submerged motor-generators above them in the powerhouse, this technology proposes that both pump-turbine and motor-generator can be submerged in a vertical shaft (or “well”), thus avoiding the need for the construction of a powerhouse.
The report said that the geomechanical PSH technology is being developed by Quidnet Energy. The main idea is to pump water into the ground, between rock layers where the water would be kept under pressure. The natural elasticity of certain rock formations act like a spring and keep the water under pressure, until the valve is opened and the water is released through a hydroelectric turbine to generate electricity.
The open-pit mine concept would use the infrastructure of decommissioned open-pit mines to develop PSH projects. It said one PSH project of this type exists, the 1,728 MW Dinorwig PSH plant in the United Kingdom, which was commissioned in 1984. Dinorwig is a closed-loop PSH project that uses an abandoned slate quarry as lower reservoir. In the United States, meanwhile, the 1,300 MW Eagle Mountain25 PSH project is planned on the site of a decommissioned iron ore mine. Two open mine pits will serve as upper and lower reservoirs of the proposed closed- loop PSH project.
The report said that potential key benefits of using decommissioned open-pit mines include cost and time savings, because less civil works would be needed to construct the reservoirs.
Cost estimates
The study pegged current PSH project costs at $2,623/kW for a generic 100 MW PSH plant with 10 hours of storage, and $2,202/kW for a generic 1,000 MW PSH plant with 10 hours of storage. It said the larger PSH plant has lower specific cost per kilowatt due to economy of scale.
The study also discussed potential methods for adding PSH capabilities to certain types of existing hydropower plants and briefly describes several other innovative PSH technologies for which there was not sufficient information available to conduct detailed evaluation.
The study also presented innovative construction methods, including new excavation techniques and modular dam construction methods, that could potentially reduce the cost and time required for the construction of new PSH projects.
This work is part of the Hydropower and Water Innovation for a Resilient Energy System (HydroWIRES) Initiative, which the U.S. Department of Energy’s Water Power Technologies Office launched in April 2019 to understand, enable and improve hydropower and PSH’s contributions to reliability, resilience and integration in the rapidly evolving U.S. electricity system.