How power storage affects the return on energy investment ratios of wind and solar resources

By Charles J. Barnhart, Michael Dale, Adam R. Brandt, and Sally M. Bensonab

The authors present a theoretical framework to calculate how storage affects the energy return on energy investment (EROI) ratios of wind and solar resources. Our methods identify conditions under which it is more energetically favorable to store energy than it is to simply curtail electricity production. Electrochemically based storage technologies result in much smaller EROI ratios than large-scale geologically based storage technologies like compressed air energy storage (CAES) and pumped hydroelectric storage (PHS). All storage technologies paired with solar photovoltaic (PV) generation yield EROI ratios that are greater than curtailment. Due to their low energy stored on electrical energy invested (ESOIe) ratios, conventional battery technologies reduce the EROI ratios of wind generation below curtailment EROI ratios. To yield a greater net energy return than curtailment, battery storage technologies paired with wind generation need an ESOIe > 80.

5 EROIgrid values as a function of storage or curtailment fraction, ø, and EES technology paired with solar PV (top panel) and wind (bottom). Note x-axis is shared, but y-axis scale for wind is 10 greater than the y-axis for PV.

5 EROIgrid values as a function of storage or curtailment fraction, ø, and EES technology paired with solar PV (top panel) and wind (bottom). Note x-axis is shared, but y-axis scale for wind is 10 greater than the y-axis for PV.

We identify improvements in cycle life as the most feasible way to increase battery ESOIe. Depending upon the battery’s embodied energy requirement, an increase of cycle life to 10 000 to 18 000 (2 to 20 times present values) is required for pairing with wind (assuming liberal round-trip efficiency [90%] and liberal depth-of-discharge [80%] values). Reducing embodied energy costs, increasing efficiency and increasing depth of discharge will also further improve the energetic performance of batteries. While this paper focuses on only one benefit of energy storage, the value of not curtailing electricity generation during periods of excess production, similar analyses could be used to draw conclusions about other benefits as well.

Broader context

Rapid deployment of power generation technologies harnessing wind and solar resources continues to reduce the carbon intensity of the power grid. But as these technologies comprise a larger fraction of power supply, their variable nature poses challenges to power grid operation. Today, during times of power oversupply or unfavorable market conditions, power grid operators curtail these resources. Rates of curtailment are expected to increase with increased renewable electricity production. That is unless technologies are implemented that can provide grid flexibility to balance power supply with power demand.

Curtailment is an obvious forfeiture of energy and it increases the lifetime cost of electricity from curtailed generators. What are less obvious are the energetic costs for technologies that provide grid flexibility. In this study we employ net energy analysis to compare the energetic cost of wind and solar generation curtailed at various rates to the energetic cost of those generators paired with storage. We find that energetic cost depends on the generation technology, the storage technology, and the rate of curtailment. In some cases it is energetically favorable to store excess electricity. In other cases, it is favorable to curtail these resources. Our goal is to stimulate the identification of new and optimum uses for excess renewable energy and research and development directions for technologies providing grid flexibility.  To read the full paper, click here:



The Project is proposed as a pumped storage hydroelectric electric generating  facility. The Project will involve construction of new water storage, water  conveyance and generation facilities at off-channel locations where no such facilities  exist at this time. The Project location is in a portion of Township 21 North, Ranges 4 and 5 West, Salt River Meridian of Yavapai County, Arizona, approximately 5 miles southeast of the unincorporated area known as Seligman, Arizona, and in the vicinity of what is known locally as the CF Ranch.

The Project will use off-peak energy to pump water from a single lower  reservoir to one or two upper reservoirs during periods of low electrical demand. The  Project will provide an economical supply of peaking capacity, as well as load  following, system regulation through spinning reserve and immediately available  standby generating capacity, among other ancillary services. The Project will  develop, conserve and utilize in the public interest the water resources of the region.

Renewable energy development in the southwestern United States will  continue to grow during the next decade. Significant public policy goals have been  established in California and Arizona, as well as surrounding states, to reduce  greenhouse gas emissions and fuel cost uncertainty associated with thermal  generation. Thousands of megawatts of renewable generation capacity are planned to be added to the grid by 2020. Large-scale energy storage is essential for successful  integration of variable energy resources while maintaining reliable grid operations.

The Project’s location in north-central Arizona is well-situated to firm variable energy generation in Arizona and the southwestern region of the United States and to support the successful implementation of public policy goals.

Pumped storage hydroelectric generation is recognized as one of only two commercially feasible “bulk storage” technologies (Compressed Air Energy Storage being the other), and the only one to have been commercially proven on a large scale. Other emerging technologies (e.g. batteries and flywheels) are smaller in scale and have significant research and development timelines, but are expected to play a role in small scale applications and management of electricity distribution systems.

The Project site has the potential to develop up to 2,000 megawatts (MW) of generating capacity using the storage provided at two upper reservoirs (Upper Reservoir Site A: “UR-A” and Upper Reservoir Site B: “UR-B”) that will be interconnected to a lower reservoir by a system of tunnels and penstocks. A powerhouse at the lower reservoir will house reversible pump-turbine units. The Project reservoirs will be formed by excavation and compacted rock and/or earth fill impoundments. In addition, opportunities exist within the vicinity of the Project to integrate renewable technologies, especially solar generation.

The elevation difference between the two upper reservoirs and the lower reservoirs will provide an average net generating head ranging from 1,400 feet (URA) to 1,250 feet (UR-B). It is anticipated that the proposed energy storage volume will permit operation of the Project at full capacity for up to 10 hours each weekday, with 12 hours of pumping each weekday night and additional pumping during the weekend to fully recharge the upper reservoirs.

The amount of active storage in the upper reservoirs will be approximately 8,200 acre-feet at UR-A and approximately 9,200 acre-feet at UR-B, providing 10 hours of energy storage at the maximum generating discharge. Water stored in both upper reservoirs will provide about  20,000 megawatt-hours (MWh) of on-peak generation on a daily basis.

The locations and proposed general configurations of Project facilities are presented on a series of maps provided in Exhibit 3 to this application. The Applicant will be considering alternative project configurations between the upper reservoirs and the lower reservoir. One such alternative involves replacing the tunnel associated with UR-B with a shorter 25-ft diameter tunnel that would interconnect UR-B with UR-A. In this option, a nominal 140 MW pumping station would be required to lift water from UR-B to UR-A to support the full 10 hours of generation at peak load for the project. A second alternative configuration envisions a tunnel from UR-B to a flow control structure located on the tunnel from UR-A to the lower reservoir. This alternative would require up-sizing the tunnel from the flow control structure to the lower reservoir to 35 feet in diameter.

In addition, the Applicant will evaluate alternatives for optimal staging of project implementation. For example, the development between UR-A and the lower reservoir could be implemented first, with all of the lower reservoir capacity constructed initially or with a later raise of the lower reservoir dam to increase storage capacity. The timing for implementation of the major project features will be dependent on demands for the energy storage and ancillary services afforded by the Project.


  1. We can go beyond the theoretical. Gridflex has done numerous analyses of the combination of wind and PV with its candidate pumped storage projects across the western U.S. Using hourly data, we’ve determined that the gains in capacity value, time-of-day value, transmission load factor improvement alone make it more cost effective than the default alternative of building new gas turbines to complement the wind or PV. Savings on curtailment are the icing on the cake, as are additional revenues in the ancillary services market (where they exist). The greatest gains are achieved with significant storage time, which places pumped storage and traditional CAES in a huge lead over systems such as batteries that are not yet cost competitive beyond 2-4 hours. Then there is the lifetime advantage – 60, 75 years or more with pumped storage, vs. perhaps 10 or 15 years (yet to be confirmed) with batteries. And, contrary to popular belief, there are a large number of suitable sites with far lower footprint and impact than the projects of yesteryear. The newer technologies’ place is at the distribution level, or in regions where pumped storage options or underground CAES simply are not available. Fortunately, the market appears to be advancing fairly rapidly toward concrete, monetizable recognition of the full value of new pumped storage today.

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