* Corresponding author. Tel.: 612-625-5831 Email address: simon002@umn.edu 1 Copyright © 2012 by ASME Proceedings of the ASME 2012 International Mechanical Engineering Congress & Exposition IMECE2012 November 9-15, 2012, Houston, Texas, USA IMECE2012-88985 Storage Power and Efficiency Analysis Based on CFD for Air Compressors used for Compressed Air Energy Storage Chao Zhang Department of Mechanical Engineering University of Minnesota Minneapolis, MN, USA Terrence W. Simon* Department of Mechanical Engineering University of Minnesota Minneapolis, MN, USA Perry Y. Li Department of Mechanical Engineering University of Minnesota Minneapolis, MN, USA ABSTRACT The compression process in a piston cylinder device in a Compressed Air Energy Storage (CAES) system is studied computationally. Twelve different cases featuring four different compression space length-to-radius aspect ratios and three different Reynolds numbers are studied computationally using the commercial CFD code ANSYS FLUENT. The solutions show that for compression with a constant velocity, the compression can be approximated by a polytropic pressure vs. volume relation. The polytropic exponent, , characterizes the heat transfer and temperature rise of the air being compressed. For the cases computed, it varies from 1.124 to 1.305 and is found to be more affected by Reynolds number and less by the length-to-radius ratio. Since the efficiency and storage power of the compressor depend on pressure vs. volume trajectory during compression, they are written as functions of the pressure rise ratio and the polytropic exponent, . The efficiency is high at the beginning of the compression process, and decreases as the compression proceeds. The effect of temperature rise, or heat transfer, on efficiency and storage power is shown by comparing the efficiency and storage power vs. volume curves having different values of  values. Smaller temperature rise always results in higher efficiency but lower dimensionless storage power for the same compression pressure ratio. The storage power is used in this study to distinguish the compression process effect ( effect) and the compressor’s size effect on the storage power. The likelihood of flow transitioning into turbulent flow is discussed. A ε - k Reynolds Averaged Navier Stokes (RANS) turbulence model is used to calculate one of the larger Reynolds number cases. The calculated polytropic exponent was only 0.02 different from that of the laminar flow solution. The CFD results show also that during compression, complex vorticity patterns develop, which help mix the cold fluid near the wall with the hot fluid in the inner region, beneficial to achieving a higher efficiency. Keywords: Compressed air energy storage, compression, heat transfer, polytropic process, compression efficiency 1. INTRODUCTION 1.1. Motivation The current study analyzes the performance of compressors used for a Compressed Air Energy Storage (CAES) system. In the current analysis of the CAES system, air is compressed in a piston cylinder device. After compression, the highly pressured air has a high work potential and can be used for later work extraction by expanding it in a piston-cylinder region [1]. Although this approach is named “energy storage,” it is not the internal energy of air that is being raised and stored; it is the exergy of the air. Given this, we will use the conventional definition of “storage energy” following [2] and [3]. The storage energy is defined as the amount of work extraction from the compressed air as it would undergo an isothermal expansion process to the atmospheric pressure.      1    (1) Consequently, the storage power is defined as the storage energy divided by the time used to compress the air.                 (2) The total work input to the compressed air takes place in two phases. In the first phase, work is done to compress the air from atmospheric pressure to a high pressure. This compression work constitutes the major portion of the total work input and the phase to be analyzed in CFD in this study. During compression in the first phase, the temperature of the air rises. In the second phase, which is the post-compression storage period, the air cools. In order to maintain the work potential (storage energy) of the stored, compressed air as it cools, pressure is maintained, volume decreases and additional work is done on