Maximum Power Point Tracking of a Small-Scale Compressed Air Energy Storage System

This paper is concerned with the maximum power tracking of a pneumatically driven electric generator in a standalone small-scale compressed air energy storage system. In this system, an air motor is used to drive a permanent-magnet direct-current generator, whose output is controlled by a buck converter supplying power to a resistive load. The output power of the converter is controlled such that the air motor operates at a speed corresponding to the maximum power. A maximum power point (MPP) tracking controller employs a hybrid perturb and observe method. The rate of change of the converter's output power with respect to its duty cycle and the change of the power and the duty cycle are used to correct the search direction under transient input power fluctuations. Small speed step changes are used in the vicinity of the MPP to improve the accuracy of the search algorithm. However, relatively coarse speed step changes are used when the operating point is far from the MPP to improve the dynamic response of the controller and to increase its speed of convergence. The analysis and design of the controller are based on a small injected-absorbed current signal model of the power converter. The controller is experimentally implemented using a real-time digital signal processor system. Test results are presented to validate the proposed design and to demonstrate its capabilities.