Principle Of A Hybrid Compressed Air And Supercapacitors Energy Storage System With Maximum Efficiency Point Tracking

This paper presents a hybrid energy storage system based on Compressed Air Energy Storage (CAES), where the extraction of energy is done within maximum efficiency conditions. As the maximum efficiency conditions impose the level of converted power, an intermittent time-modulated operation mode is applied to the thermodynamic converter to obtain a variable converted power. A smoothly variable output power is achieved with the help of a supercapacitive auxiliary storage device. The paper describes the concept of the system, the power-electronic interface circuits and especially the Maximum Efficiency Tracking algorithm and the strategy used to vary the output power. Practical results are also presented, recorded from a low-power pneumatic motor coupled to a small DC generator. Introduction World wide, considerations have been made on energy resources, about traditional generation and distribution systems as well as renewable means. Traditionally, energy storage has been of high interest, as in the case of not flexible nuclear or thermal power versus daily cycles [1], [2]. In the context of energy storage in relation with power generation, the issues of fast and strong load variations have also been studied [3], [4]. For the future concepts of energy production and distribution, like decentralized generation, balancing of production and consumption is one of the main concerns. Because of the loss of the effect of a "statistical mean value" of loads, that normally smoothes their profile, the principle of decentralized generation could present some risks both for generators and loads, which will reinforce the need of storage facilities. Concerning the integration of renewable means, like photovoltaic and wind power, that are intermittent sources, additional storage means are also required. Recently several new storage technologies have appeared, as alternative solutions to classical batteries, and are integrated in real networks [5], [6]. CAES technologies belong to these innovative developments, with their high potential of producing less problematic waste materials. In fact, they have been proposed from longer date, in the middle and high power range, as well as in transportation and generation of electrical power. Ultra-large size facilities have been proposed recently based on dedicated turbo machinery [7]. Several small size accumulators have also been proposed using intermediary mechanical/hydraulic conversion with the so-called "liquid piston” principle. [13] In the present study, a low cost vane type pneumatic motor has served as example of a thermodynamic expansion converter, with the goal of developing operation and control strategies in order to avoid the use of dissipating valves between the air tank and the converter and optimize the energy conversion. A supercapacitive auxiliary storage device is used to smooth the intermittent resulting power. The concept of the hybrid CASCES (Compressed Air and Supercapacitors Energy Storage) system is described first, then the Maximum Efficiency Point Tracking algorithm is presented and finally, the strategy for varying the electric output power is proposed. 1Principle of hybrid CASCES System The idea of associating a high capacity energy storage medium (compressed air), and a high power density storage device (supercapacitors), is implemented in the context of an off-line UPS system whose principle is presented in figure 1. All the devices of the system are reversible. During the storage process, the electric machine works as a motor and drives the pneumatic machine, which operates as a compressor to fill the vessel with high-pressure air. During the generation process, the compressed air is directly expanded in the pneumatic machine, which operates as motor to power the DC generator. The following technical solutions have been implemented to improve the reliability and efficiency of the system: There is no pressure regulation device between the air tank and the pneumatic machine in order to avoid the important losses in such a device. The motor inlet pressure therefore varies continuously. 1 [email protected]