Pumped Hydropower Energy Storage (PHES) helps electricity systems manage the growing share of renewable generation by storing excess energy for later use. The approach improves grid flexibility without consuming water in closed-loop configurations, thereby supporting more resilient and sustainable energy systems. Read how the San Vicente Energy Storage Facility evaluates a closed-loop pumped storage system that could store renewable electricity through reversible pump turbines and long-duration energy storage.
Energy Storage Fundamentals
PHES stores electricity by moving water from a lower reservoir to an upper reservoir when renewable generation exceeds demand. The stored water later flows back through turbines to generate electricity during periods of higher demand. This process shifts energy across time instead of creating new energy. The approach supports greater use of variable renewable resources while maintaining reliable electricity supply.
Grid Integration Mechanisms
Electricity systems require a continuous balance between generation and demand. Increasing shares of solar and wind generation can create surplus electricity during some hours and shortages during others. PHES provides long-duration storage that absorbs excess renewable electricity and returns it when needed. This capability reduces renewable energy curtailment, improves grid stability, and supports dependable operation during periods of changing demand.
Infrastructure and Operational Design
A typical PHES system consists of two reservoirs connected by tunnels or pipelines and a powerhouse containing reversible pump turbines. During charging, the pumps move water uphill using surplus electricity. During generation, gravity returns the water through the same machines to produce electricity. Closed-loop configurations recycle water between reservoirs, thereby limiting operational water consumption and enabling repeated storage cycles over many decades.
System Benefits and Long-Term Value
PHES offers large storage capacities and extended discharge durations that complement shorter duration battery systems. The technology can provide energy shifting, reserve capacity, voltage support, and operational flexibility for electricity networks. These functions help maximize renewable energy investments while reducing dependence on fossil fuel generation during peak demand periods. Long operating lives also allow infrastructure investments to deliver benefits across multiple decades.
Case Study: San Vicente Energy Storage Facility
The San Vicente Energy Storage Facility is a proposed pumped storage project being evaluated by the San Diego County Water Authority and the City of San Diego at San Vicente Reservoir near Lakeside. According to the project information, feasibility studies began in 2022, while the State of California invested $18 million in 2021 to support initial project design, environmental reviews, and the federal licensing process. The initiative remains under evaluation and requires state and federal regulatory permits and licenses before construction can proceed.
The proposed project would create an upper reservoir above San Vicente Reservoir, an underground tunnel system, and an underground powerhouse equipped with reversible pump turbines. These turbines would pump water uphill when renewable electricity is abundant and generate electricity when water returns downhill. The project is designed as a closed-loop system, so the exchange of water between the reservoirs would not consume water during operation. Planned performance is approximately 4,000 megawatt-hours of storage, equivalent to up to 500 megawatts for about eight hours, with project materials also referencing 8 to 12 hours of storage.
Institutionally, the City owns and operates San Vicente Dam and Reservoir, while the Water Authority owns approximately two-thirds of the reservoir’s storage capacity. Both agencies are jointly evaluating feasibility while maintaining the reservoir’s primary water supply function. Environmental reviews, geotechnical investigations, engineering design, surveying, and federal licensing form key implementation mechanisms. If approved, project revenues would help offset regional water service costs while supporting renewable energy integration, grid reliability, and California’s broader clean energy objectives.
Take-Out
Long-duration energy storage systems strengthen renewable electricity integration by shifting surplus energy across time, improving grid reliability, and supporting resilient infrastructure through proven, long-life technologies.