Due to rapid technology cost declines and significant potential value of energy storage, we could see hundreds of gigawatts of storage on the future grid. The Storage Futures Study (SFS) is designed to explore the potential role and impact of energy storage in the evolving electricity sector of the United States, specifically how energy storage technology advancement could impact the deployment of utility-scale and distributed storage, and the implications for future power system infrastructure investment and operations. This report—the sixth in the series— assesses the hourly operations of high storage power systems in the U.S., with storage capacities ranging from 213 GW to 932 GW.

The assessment builds upon a previously published report in the Storage Futures Study in which NREL added new capabilities to its publicly available Regional Energy Deployment System (ReEDS) model to build least-cost scenarios for a range of cost and performance assumptions for energy storage (A. W. Frazier et al. 2021). Scenarios showed the potential for U.S. storage capacity to exceed 125 gigawatts (GW) by the end of 2050, even in the most conservative estimates—a more than a fivefold increase over current U.S. storage capacity (A. W. Frazier et al. 2021).

This analysis returns to the ReEDS high storage scenarios with detailed production cost modeling to observe the hourly, daily, and annual operations and associated value of storage.

Overall, we find that the high storage (and often high variable generation) power system scenarios envisioned in ReEDS successfully operate with no unserved energy and low reserve violations,1 showing no concerns about hourly load balancing through the end of 2050. The successful hourly load balancing indicates the various improvements to ReEDS in previous work are effective in envisioning these future scenarios.

On a daily basis, we find storage operations are heavily aligned with the availability of solar photovoltaics (PV), which has a predictable daily on and off cycle that aligns well with the need for storage to charge and discharge. Wind, on the other hand, has a less apparent daily cycle and often experiences long periods of overgeneration stretching for many hours or days, which is much longer than the duration of storage we explore here2. Although storage can play a key role in utilizing energy from both PV and wind, the synergies with PV are more consistent. On an annual basis, storage effectively provides time-shifting and peak-load reduction services in all configurations and grid mixes. Although storage has a low annual capacity factor, which is inherently limited by its need to charge, it has a very high utilization (in many cases over 75%) during the top 10 net load hours across scenarios and years—when the system needs capacity and energy the most—indicating a strong contribution to the system’s resource adequacy.

Lastly, we also find that storage increases the efficiency of many types of power system assets.

For instance, we find that in these future grid scenarios, storage reduces total electricity system carbon dioxide emissions by utilizing overgeneration from zero-marginal emissions sources like wind and solar to displace generation from the coal and natural gas fleet. In addition, storage can prevent start-ups of those generators and thus reduce emissions of criteria pollutants, which can disproportionally impact those in poor health with low income, particularly those living near thermal power plants. Storage also impacts the operation of the transmission grid. We find that storage increases utilization of some transmission lines (quantified by the amount of observed congestion) while reducing the congestion observed on other lines. Exactly how storage impacts nearby transmission by either increasing or decreasing usage depends on the local conditions, but we find that more often than not, storage encourages higher utilization of transmission assets.

These findings indicate that further analysis should consider the unique interaction of storage and transmission when both deploying and operating the assets together.

Collectively, the results of this and previous Storage Futures Study analysis show the growing opportunity for diurnal storage (that is, storage with up to 12 hours of duration) to play an important role in future power systems. This analysis shows how greater deployment of diurnal storage can increase efficiency of operations by reducing overgeneration, decreasing generator starts and emissions, and increasing utilization of the transmission system. Furthermore, storage plays an important role in providing capacity during the top net load hours. Future work could examine the role of longer-duration storage resources, especially under highly decarbonized grid conditions, such as those approaching 100% clean energy.