Energy storage has recently attracted significant interest as an enabling technology for integrating stochastic, variable renewable power into the electric grid. To meet the renewable portfolio standards targets imposed by 29 U.S. states and the District of Columbia, electricity production from wind technology has increased significantly. At the same time, wind turbines, like many renewables, produce power in a manner that is stochastic, variable, and non-dispatchable. These attributes introduce challenges to generation scheduling and the provision of ancillary services. To study the impacts of the stochastic variability of wind on regional grid operation and the role that energy storage could play to mitigate these impacts, Pacific Northwest National Laboratory (PNNL) has developed a series of linked, complex techno-economic-environmental models to address two key questions: A) What are the future expanded balancing requirements necessary to accommodate enhanced wind turbine capacity, so as to meet the renewable portfolio standards in 2020? Specific analyses are conducted for the four North American Electric Reliability Corporation (NERC) western subregions. B) What are the most cost-effective technological solutions for providing either fast ramping generation or energy storage to serve these balancing requirements?

PNNL applied a stochastic approach to assess the future, expanded balancing requirements for the four western subregions with high wind penetration in 2020. The estimated balancing requirements are quantified for four subregions: Arizona-New Mexico-Southern Nevada (AZ-NM-SNV), California-Mexico (CA-MX), Northwest Power Pool (NWPP), and Rocky Mountain Power Pool (RMPP). Model results indicate that the new balancing requirements will span a spectrum of frequencies, from minute-to-minute variability (intra-hour balancing) to those indicating cycles over several hours (inter-hour balancing). The sharp ramp rates in the intra-hour balancing are of significant concern to grid operators. Consequently, this study focuses on analyzing the intra-hour balancing needs.

A detailed, life-cycle cost (LCC) modeling effort was used to assess the cost competitiveness of different technologies to address the future intra-hour balancing requirements. Technological solutions considered include combustion turbines, sodium sulfur (NaS) batteries, lithium ion (Li-ion) batteries, pumped-hydro energy storage (PHES), compressed air energy storage (CAES), flywheels, redox flow batteries, and demand response (DR). Hybrid concepts were also evaluated. For each technology, distinct power and energy capacity requirements are estimated. LCC results for the sole application of intra-hour balancing indicate that the most cost competitive technologies include Na-S batteries, flywheels, and Li-ion assuming future cost reductions. Demand response using smart charging strategies was found to also be cost-competitive with natural gas combustion turbines. This finding is consistent among the four subregions and is generally applicable to other regions.

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