Performance and Health Estimation for Battery Energy Storage Systems Under Frequency Regulation Applications

Abstract

Increased penetration of renewables into the power grid can produce challenges for grid operators such as integrating non-dispatchable distributed generation, optimizing grid operations and controls, and providing support during outages. Energy Storage Systems are an important alternative for supporting electric grid operation and stability. Given their declining costs and cycling characteristics, Lithium-ion (Li-ion) batteries have become attractive for Battery Energy Storage System (BESS) applications.

A review of the state of the art shows that battery degradation is dependent on many different factors related to the application in which the battery is operating. The majority of research in this space focuses on batteries for electric (EV) and hybrid electric (HEV) vehicles. Comparatively little analysis has been performed on modeling the performance and assessing the degradation of Li-ion batteries in BESS or grid-tied applications. Thus, to enhance the performance, reliability, and sustainability of batteries in BESS applications, better models to study the battery’s degradation under different applications are required.

This research analyzes the performance and provides an initial estimation of the health of Li-ion batteries in frequency regulation applications. Frequency regulation applications appear to produce a great strain on batteries as they are constantly cycled as a fast-ramping resource to regulate the frequency in the grid. Currently, there is no accepted standard or profile for testing batteries in frequency regulation applications. Thus, the process to create a battery cycling profile for Frequency Regulation Applications is presented. First, frequency regulation battery data was downloaded from PJM to analyze its main characteristics and extract a viable profile for the battery to be cycled. 

Once the battery profile was established, a 25.6V, 185Ah Li-ion battery from EnerSys was cycled at 25^◦C and 40^◦C for a duration of 1500 and 4271 partial cycles. The aim was to analyze and compare the performance at 25^◦C and 40^◦C, and potentially identify the trends that show battery aging and capacity fade. To that end, different methods including capacity tests, throughput, Coulombic efficiency, and equivalent full-cycle analysis were performed. Electrochemical Impedance Spectroscopy (EIS) tests were done every 100 or 200 cycles to analyze the health and aging of the battery. An equivalent circuit model (ECM) was also developed from EIS test results. One of the objectives was to study how the model parameters changed with the number of cycles and start to identify the main degradation mechanisms associated with the battery operation under frequency regulation.