Cost Optimization and Liquid-Air Interface Stability Analysis of a Liquid Piston Compressor/Expander
2020-05
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Cost Optimization and Liquid-Air Interface Stability Analysis of a Liquid Piston Compressor/Expander
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2020-05
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The present research focuses on the optimal cost driven design and liquid-air interfacial stability analysis of a Compressed Air Energy Storage (CAES) system. CAES technology can be paired with renewable energy harvesting devices in order to overcome the mismatch between power availability and power demanded by the electrical grid. The proposed novel approach to CAES utilizes a liquid piston to compress air during periods of excess power availability to be stored for later use. As liquid (water) is pumped into the compression chamber, the air is compressed as the liquid-air interface rises. Using liquid as the compression piston allows for heat transfer media to be distributed throughout the compression chamber to facilitate greater heat transfer. Heat transfer is fundamentally important to the compression/expansion efficiency of the CAES system. As the air is compressed, the internal energy of the air will rise resulting in a rise in temperature. If the air is stored in this state, the air will eventually cool to ambient temperatures and this energy will be lost. It is therefore paramount to minimize the rise in air temperature during compression. A near isothermal compression/expansion can be accomplished through high amounts of heat transfer during the process. Many increases in heat transfer capability can be realized through the use of a liquid piston as the means to compress/expand air due to the liquids ability to flow through a tortuous path of heat exchanger material. The high thermal efficiencies achievable by this CAES system design make commercialization viable if the overall system cost can be reduced. A relationship between system performance and cost has been developed with the goal to find the optimal system parameters that yield the minimal cost per power of the system. System parameters include compression trajectory, compression chamber shape, and heat transfer media distribution, which define system performance through a one-dimensional model of the air compression. These same parameters are also used to calculate the cost of the system which includes both the hydraulic pump cost (determined by compression trajectory) and compression/expansion chamber cost. Furthermore, a flow intensifier concept was introduced to further reduce hydraulic pump size/cost by amplifying the available flow. A comparison is then made between the previous power density optimized design and the cost optimized design presented in this research. The addition of the flow intensifier resulted in a cost reduction of 75% while increasing power density by 234% when comparing the cost optimal flow intensifier design to the previous power density optimized design. Using a liquid piston for air compression offers many benefits in terms of heat transfer, but raises concerns pertaining to the stability of the liquid-air interface. Interfacial instability results in undesirable mixing of the liquid and air, which results in wasted effort as the air is compressed, yet is unable to be ejected out of the chamber into the storage vessel. Due to the cross-plate heat exchanger design, the torturous path the liquid flows through creates many flow boundaries that can lead to instabilities in the interface. To determine how the cross-plate heat exchanger geometry disrupts the interface, a simplified 2-D CFD model of the liquid interface was developed utilizing a Marker and Cell (MAC) approach. The research presented shows a relationship between cross-plate separation distances, liquid piston operation frequencies, and the resulting maximum wave amplitude of the liquid-air interface. Experimentation was completed using highspeed footage and edge tracking software to experimentally validate the model.
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University of Minnesota M.S.M.E. thesis. May 2020. Major: Mechanical Engineering. Advisor: James Van de Ven. 1 computer file (PDF); xi, 121 pages.
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Gust, Aleksander. (2020). Cost Optimization and Liquid-Air Interface Stability Analysis of a Liquid Piston Compressor/Expander. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/223087.
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