Abstract
Utility-scale battery storage systems typically consist of multiple smaller units contributing to the overall power dispatch of the system. Herein, the power distribution among these units is analyzed and optimized to operate the system with increased energy efficiency. To improve the real-life storage operation, a holistic system model for battery storage systems has been developed that enables a calculation of the energy efficiency. A utility-scale Second-Life battery storage system with a capacity of 3.3 MWh/3 MW is operated and evaluated in this work. The system is in operation for the provision of primary control reserve in combination with intraday trading for controlling the battery state of charge. The simulation model is parameterized with the system data. Results show that losses in power electronics dominate. An operational strategy improving the energy efficiency through an optimized power flow distribution within the storage system is developed. The power flow distribution strategy is based on the reduction of the power electronics losses at no-load/partial-load by minimizing their in-operation time. The simulation derived power flow distribution strategy is implemented in the real-life storage system. Field-test measurements and analysis prove the functionality of the power flow distribution strategy and reveal the reduction of the energy throughput of the units by 7%, as well as a significant reduction of energy losses in the units by 24%. The cost savings for electricity over the system’s lifetime are approximated to 4.4% of its investment cost.
Highlights
Energy storage systems are a promising option to provide flexibility and grid services in future electric grids [1,2]
We focus on systems consisting of multiple units, which each feature a battery and a dedicated power electronics (PE) and can be operated independently
The measured efficiency values here and in the following include the differences of stored energy through the changed battery SOC between the beginning and the end of the considered period
Summary
Energy storage systems are a promising option to provide flexibility and grid services in future electric grids [1,2]. Lee et al discussed and tested the provision of frequency frequency control through multiple energy storage systems based on hierarchical cluster structures, but with a focus on system availability and reliability and no consideration of energy efficiency [10]. Control through multiple energy storage systems based on hierarchical cluster structures, but with a focus on system availability and reliability and no consideration of energy efficiency [10]. In contrast to the reviewed studies, this work features an analysis of a utility-scale Second-Life battery system in detail for its energy efficiency and the development of an optimized PFDS and its implementation. A holistic system model for battery storage systems that enables a realistic calculation of the energy efficiency is parameterized with the system structure, the component data, and the application load profiles.
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