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A new optimization model for pumped storage hydropower can help grid
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Sites can be fully closed-loop, or they can use existing reservoirs along river systems. Supply curves are available for 8-, 10, and 12-hour storage durations, dam heights of 40–100 meters, head heights of 200–750 meters, and a maximum conveyance length between upper and lower reservoir of 12 times the head height (leading to a maximum horizontal distance between reservoirs of 8,250 meters for a 750-meter head height system). The dataset includes sites that use existing reservoirs to connect new off-river reservoirs with upper reservoirs and sites that repurpose open-pit mines for use as PSH reservoirs.
View the interactive tool
Resource potential is often assessed in terms of geographic (or resource), technical, and economic potential—each of which represents a succession of additional complexity and input assumptions that leverage similar data and a common analysis flow.
The closed-loop portion of the PSH resource assessment uses high-resolution digital elevation models (30-meter resolution) to identify potential upper and lower reservoirs within the technology parameters specified by the NREL adaptation of the ANU model. Design specifications include minimum head height of 200 meters and a maximum head height of 750-meters, dam heights of 40, 60, 80, and 100 meters, and a maximum conveyance length between upper and lower reservoir of 12 times the head height. This yields a large set of potential reservoirs with many overlaps.
Once the reservoirs are identified, technical potential criteria are applied to refine the development areas. The criteria eliminate any reservoirs that intersect existing water bodies and waterways; glaciers and ice-covered areas, protected federal lands; urban areas; critical habitat areas; or reservoirs within 1,000 feet of a wetland. Optional criteria can eliminate reservoirs intersecting roads or farmland or allow reservoirs intersecting ephemeral streams.
The HydroLAKES dataset of reservoir locations and characteristics is used to find potential sites that pair new off-river reservoirs with existing reservoirs that could be along river systems—to incorporate existing reservoirs into the full set of potential PSH reservoirs.
To find potential sites that use open-pit mines as reservoirs, NREL researchers search for pit features using 10-meter-resolution digital elevation models, removing any pits smaller than 10,000 square meters in area and shallower than 1 meter in depth. Pits are then identified by searching within 1 kilometer of mine locations in the U.S. Geological Survey mine symbol dataset and visually inspecting the results to remove errors.
Ultimately, this exercise results in a spatially resolved characterization of the technical potential quantity, quality, and cost of PSH resources, which can be sorted to represent a "supply curve" for a specific scenario. The figure below plots the supply curve of closed-loop PSH capital cost in dollars per kilowatt versus cumulative generating capacity in gigawatts for the contiguous United States for 8-, 10-, and 12-hour storage durations and the default assumptions for where to prohibit or exclude closed-loop PSH construction. This supply curve includes both closed-loop sites and sites that use existing reservoirs. Resource and cost data binned by cost ranges are also included in the NREL Annual Technology Baseline beginning in the 2022 data year.
The open-pit mine PSH site assessment, where pits are paired with potential off-river reservoirs, finds 15 candidate locations, as shown on the map below. These are mostly scattered throughout the western United States, with one site in Massachusetts. Many of these sites include active mining operations, so PSH development would compete with existing site use.
*Sites utilizing open pit mines as reservoirs have a minimum head of 100 meters.
**The reservoir volume similarity criteria is not enforced when pairing existing reservoirs with potential off-river reservoirs.
Learn more about NREL''s renewable energy supply curves or check out this webinar demonstrating the resource assessment and supply curves tool.
A Component-Level Bottom-Up Cost Model for Pumped Storage Hydropower, NREL Technical Report (2024)
Closed-Loop Pumped Storage Hydropower Resource Assessment for the United States, NREL Technical Report (2022)
Pumped storage hydropower (PSH) is an established technology that can provide grid-scale energy storage and support an electrical grid powered in part by variable renewable energy sources such as wind and solar. Despite recent interest in PSH, questions remain regarding the overall sustainability of PSH projects, and information about the life cycle of greenhouse gas (GHG) emissions associated with PSH technologies has been limited—until now.
In 2023, NREL researchers published a wide-ranging study that included a full life cycle assessment of new closed-loop PSH projects in development in the United States. The majority of GHG emissions from PSH are attributed to the grid mix of energy used to pump water from a facility''s lower reservoir to its upper one, as this mix is not usually made of 100% carbon-free energy sources. As such, GHG emission levels decrease in locations with a higher level of renewable energy sources in the grid mix. Additional emissions stem from a plant''s construction (e.g., from diesel-powered equipment, concrete, or steel) and ongoing plant operations.
In the study, researchers compared their results to published data on the GHG emissions of other energy storage technologies, including compressed air energy storage and different battery types. The results showed that GHG emissions associated with PSH were lowest among the group studied.
The success of the study inspired the creation of an interactive tool on OpenEI that uses the study data to enable developers to calculate the GHG emissions of potential PSH sites in the United States—with the goal of promoting PSH development with configurations and locations with the lowest global warming potential.
Users can input specifications for PSH facilities at varying levels of detail, such as reservoir volume, dam material and dimensions, number and capacity of turbines, and the length of the transmission line that connects the PSH system to the grid. They can then compare different PSH scenarios side by side and view the emissions by component, material, and life cycle phase.
To use the tool, users first select between a Basic and an Advanced scenario, in which they can specify a site configuration and explore GHG outcomes. Basic mode offers a smaller set of options for a simpler user experience, whereas Advanced mode allows the user to submit detailed specifications for PSH system components (e.g., number of reservoirs being built, dam material, and distance to grid connection).
Multiple scenarios with different inputs can then be viewed side by side and subsequently edited with different inputs to produce the desired outcome.
The tool was built using the data and methods from the 2023 study, where researchers conducted a life cycle assessment of closed-loop PSH under a variety of assumptions. This data includes all GHG emissions from facility construction, operation, and maintenance and exclude any emissions that might occur during decommissioning or any reservoir-based emissions. We do not consider nonpower uses of the PSH site, which in practice could bear some responsibility for life cycle GHG emissions.
The dataset was built using raw data from proposed PSH sites in the preliminary permitting phase with the Federal Energy Regulatory Commission. For sites with alternative plant design options, the additional configurations are also included in the overall dataset. Each configuration includes an expected annual amount of electricity delivered, which reflects how the plant is expected to operate.
Researchers used methods established in the literature to quantify the GHG emissions of materials and energy inputs in the closed-loop PSH system. A calculation was performed for every configuration (e.g., the GHG emissions per kilogram of concrete for a dam or per kilogram of steel for the powerhouse). Researchers then took a weighted average across the configurations to create a complete inventory of PSH GHG emissions for each material and energy input. View the tool''s supplemental information for further explanations of the calculations.
Guidelines from the Intergovernmental Panel on Climate Change were used in the study to calculate total GHG emissions (kilograms of carbon dioxide equivalent) from individual chemical emissions for each component, material, and life cycle phase as well as in total. Operational emissions associated with electricity used to pump water to the upper reservoir are based on the average emissions intensity of the contiguous U.S. electric sector as determined by NREL''s Regional Energy Deployment System grid planning model. In these calculations, all emissions are attributed to the power production function of the PSH plant and do not take into account any other uses of the site.
These methods allow the tool to estimate life cycle GHG emissions across a wide range of specifications for PSH site design and operation, such as alternative electricity mixes, plant lifetime, or plant size. GHG estimates can then be compared with those of other storage and grid technologies to help understand comparative technology tradeoffs.
Life Cycle Assessment of Closed-Loop Pumped Storage Hydropower in the United States, Environmental Science & Technology(2023)
Life Cycle Assessment for Closed-Loop Pumped Hydropower Energy Storage in the United States, Hydrovision International, NREL Presentation (2022)
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