UC Berkeley

Abstract

Design, Analysis, and Testing of Large Capacity Kinetic Energy Storage Systems

by

Siyuan Xin

Doctor of Philosophy in Engineering – Mechanical Engineering

University of California, Berkeley

Professor David Steigmann, Chair

Global warming, climate change and pollution caused by the traditional energy generation technologies have become some of the biggest threats in today’s world. And the need for more energy is ever increasing. Development of renewable energy, such as solar and wind, as well as distributed smart grids are needed to replace the old energy production methods. Governments have set goals of increasing the proportion of clean energy generation. One major limiting factor of the development of renewable energy is that these energy sources is not as reliable and as stable as the traditional power sources to meet the energy demand. This problem can only be effectively solved by the use of energy storage to store excess power produced by the renewable energy sources. Therefore, energy storage, especially large scale energy storage systems, will be the key factor for enabling renewable energy to become the primary sources of energy. Current energy storage technologies include batteries, flywheel, supercapacitors, pumped hydro, and so on. But due to the issues of capacity, efficiency, life cycles or costs of these current energy storage technologies, a large scale energy storage system that has low cost, long life cycles and high efficiency is yet to be developed to be integrated with renewable energy generation and smart grids. In this work, the design, analysis and tests of large scale flywheel energy storage systems are carried out. The flywheel energy storage system will feature a large steel rotor. Theoretical analysis of the stress distribution of the rotor is discussed to give a parametric view of the key elements in the design. Material testing such as tensile tests, fracture toughness tests, and fatigue crack propagation tests are conducted to characterize the behavior of the rotor material. Finite element method is used to analyze the stress-strain distribution of the rotor in order to optimize the shape of the rotor and determine the rotational speed. Also frequency analysis using finite element method gives the dynamic response of the rotor. Catastrophic failure of the rotor can cause big damages and need to be prevented from happening. And therefore, fracture mechanics is applied to analyze the safety margin and estimate the lifetime of the flywheel system. Post failure analysis is also discussed in case of rotor failure. Energy storage efficiency is a crucial factor in order for the flywheel energy storage system to be applicable. The rotor is kept in low vacuum pressure to minimize aerodynamic drag and improve energy storage efficiency. An analytical model of aerodynamic drag on the rotor is developed. Experimental measurements of drag power loss on a disc rotor inside a controlled vacuum chamber are compared to the predicted results of the analytical model. Design of sub-systems such as bearings and vacuum system are discussed at the end.