title: Combined Kinetic and Electrochemical Energy Storage Systems Offering Balancing Services to Electrical Grids
Creators: name: Mouratidis, Panagiotis
Publisher: name: UNSPECIFIED
Year: 2023

description: Energy storage technologies have a wide range of applications. Besides consumer electronics and electric vehicles, stationary energy storages are used to improve the power quality of electrical grids by offering balancing services. The investigated energy storage system for the provision of grid balancing services combines kinetic and electrochemical energy storages. The considered kinetic storage comprises high-speed flywheel storages according to the parameters of the prototypes developed at the Technical University of Darmstadt. The considered battery storage is composed of lithium-ion cells with lithium nickel manganese cobalt oxides in the cathode and graphite in the anode. In order to estimate the operating cost of the combined energy storage system, the power losses of the lithium-ion cell, the flywheel storage and the corresponding power converters are modelled. The derived loss function of the flywheel storage predominantly depends on its speed and the current of its permanent magnet synchronous machine. Similarly, the derived loss function of the lithium-ion cell mainly depends on its state of charge and its current. To consider the effects of the lithium-ion cell degradation, an empirical degradation model is further developed and parametrized based on the manufacturer specification for the lithium-ion cells used. The frequency containment reserve constitutes the main application of the combined energy storage and therefore determines the load profile. The probability distribution of a sampled 24-hour profile of the grid frequency of continental Europe is used to collectively size the combined energy storage. The degradation of the lithium-ion cells is thereby considered, so that the battery fulfils the requirements of the application throughout the target service life. The energy management of the combined storage system involves not only the power split among the storage units, but also the control of the individual storages. Therefore, the stator current that minimizes the total losses of both the electric machine and the power converter of the flywheel storage is derived. To minimize the instantaneous energy conversion losses of the combined energy storage, the optimal power share of the storage technologies is analytically derived using simplified loss functions and evaluated through simulations. Subsequently, the energy management is implemented in a programmable controller and tested on a prototype combined energy storage system. Despite the high uncertainties involved in the experimental investigation, its results are in qualitative congruence with the corresponding simulations. The cost-efficiency of combined energy storage systems is compared with that of battery-only systems for the applications of frequency containment reserve, frequency containment reserve along with wayside energy recovery in railway networks and frequency containment reserve along with electric vehicle fast charging. To optimally size the combined energy storage systems, a cost-benefit analysis is conducted, in which the total cost of ownership serves as cost and a low degradation of the lithium-ion cells serves as benefit. Optimally sized combined energy storages result in a lower total cost of ownership than optimally sized battery-only storages, which is more pronounced in use cases that involve high and frequent alternating load.