The present computational research focuses on fluid flow analysis and heat transfer enhancement in support of the design of a hydraulic Compressed Air Energy Storage (CAES) system. A CAES system compresses air to high pressure during high power generation periods, stores the compressed air, and expands it to generate power during high power demand periods. The main benefit of using CAES is that it overcomes the mismatch between power generation and power demand. An innovative liquid piston method is used in the present research, where liquid (water) is pumped into the lower section of a compression chamber, and the gas (air) is compressed by the rising liquid-gas interface. Important to the efficient operation of CAES is reducing the temperature rise during compression. The work input process for compressing the air requires two main steps. In the first step, compression work compresses the air to a high pressure. The air temperature rises during compression, and this leads to a second step, where the compressed air cools. In order to maintain the work potential, the pressure of the compressed air is maintained during cooling, volume decreases and cooling work is done. As a result, higher temperature rise during compression requires greater amount of total work input for a given amount of air and a given pressure ratio. For similar reasons, it is also desirable to reduce the temperature drop during expansion of compressed air. The use of a liquid piston offers opportunities to insert heat exchanger matrices into the compression chamber to improve heat transfer. Computational Fluid Dynamics (CFD) and design analyses on the heat exchangers are done in the present research. Two main types of heat exchangers, a commercially available, open-cell metal foam and an in-house designed interrupted plate matrix, are investigated, mainly through computational methods, and with experimental validations. CFD modeling of the liquid piston chamber inserted with exchanger matrices requires closure models that characterize the heat transfer and flow resistance characteristics of the heat exchanger elements. For the metal foam matrix, characterization is done by measuring the flow pressure drop and comparing existing heat transfer models to a liquid piston experiment. For the interrupted plate matrix, large numbers of CFD simulations on the unit cells of the exchanger and experiments are applied for developing correlations for these terms. Based on the unit cell simulations, models for three-dimensionally anisotropic heat transfer behavior of porous media have been developed. Using these models, 3-D global-scale CFD simulations of the liquid piston chamber have been done. As the liquid chamber represents an application of two-phase flow through porous media, the simulation combines a VOF (Volume of Fluid) method and a two-energy-equation modeling method for porous media. The choice of the interrupted plate matrix offers flexibility to vary the shape (e.g. plate height, thickness and separation distance) based on an optimum design to further improve CAES efficiency. Design sensitivity analyses typically require large numbers of computational runs and would demand extraordinary computational resources if combined with 2-D or 3-D CFD simulations. Developed in the present research is a simplified, one-dimensional (1-D) code that is much less computationally expensive, and preserves the main physics of the two-phase flow in porous media. The 1-D code is used for the design analysis of the heat exchanger shape distribution along the axial direction of the chamber. CFD Simulations have been done to also compare a no-insert chamber to chambers with exchanger inserts, for both compression and expansion processes. The expansion process allows the expanding compressed gas to push the liquid out of the chamber to generate power. It is shown that in both the compression and expansion processes, using a heat exchanger matrix in the liquid piston chamber can significantly reduce the rise or drop in gas temperature, thus reducing the losses. Using the CFD modeling tools, a design exploration is also done to investigate the effect of changing the profile of the chamber's cross sectional radius along the axial direction to design a gourd-like shaped chamber to agitate flow and enhance heat transfer.
University of Minnesota Ph.D. dissertation. June 2015. Major: Mechanical Engineering. Advisor: Terrence Simon. 1 computer file (PDF); xviii, 263 pages.
CFD Simulations and Thermal Design for Application to Compressed Air Energy Storage.
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