MAE PhD Defense – Vikram C. Patil

Title: Efficiency Improvement Techniques in Liquid Piston Compressor for Ocean Compressed Air Energy Storage Application

Advisor: Dr. Paul Ro

Date and Time: Friday, March 15, 2019 at 10:00 AM

Location: EB3 – 3115

ABSTRACT: “Modern electricity infrastructure needs an efficient large-scale energy storage system to accommodate energy supply from intermittent renewable energy resources. Ocean compressed air energy storage system (OCAES) is a promising large-scale energy storage system. In OCAES, energy is stored in the form of compressed air under the Ocean. This research is aimed at achieving technological advancement of OCAES to attain an efficient large-scale energy storage system. Energy and exergy analysis of various OCAES configurations is performed first to identify OCAES configuration with high efficiency. Isothermal OCAES shows significantly higher efficiency over adiabatic and diabatic OCAES with potential to reach a 72% roundtrip efficiency of energy storage. However, attainment of such a high efficiency is conditional on achieving near-isothermal compression and expansion of air. Compression of air using liquid piston is researched to attain efficient near-isothermal compression. Understanding the heat transfer mechanisms during compression is crucial in the design and development of efficient liquid piston compressor. Therefore, a thorough investigation of heat transfer in liquid piston compressor is performed experimentally. It is observed that convective thermal resistance between air and chamber has significant contribution in total thermal resistance; therefore, heat transfer enhancement techniques to enhance the heat transfer coefficient between gas and chamber can lead to significant improvements in the isothermal efficiency of the liquid piston compressor.

Various heat transfer enhancement techniques are investigated in liquid piston compressor. First, the heat transfer enhancement using water spray injection is investigated in liquid piston compressor for efficiency improvement. Experiments are performed at various injection pressures of spray with different spray angles at different stroke times of compression. The higher spray injection pressure shows higher improvement in compression efficiency and an optimal spray angle which can create smaller droplets with minimum loss of droplets due to the impact on chamber wall can lead to a marginal improvement in efficiency. Water spray injection is a highly effective technique to achieve near-isothermal compression with an isothermal efficiency up to 95% at a high power density. Moreover, aqueous foam based heat transfer enhancement is investigated in liquid piston compressor. Experiments are performed with the use of aqueous foam generated under different foam generation conditions. The volume of aqueous foam in the chamber, the air flow rate of foam generation, and various foam generator designs are considered in this investigation. A higher volume of aqueous foam in the compression chamber leads to a further increment in isothermal efficiency, however, with higher cyclic variability. Also, experiments highlight the potential of reduction in cyclic variability through the foam generator design. Overall, the use of aqueous foam in the liquid-piston compressor is effective in achieving an isothermal efficiency up to 92%. Additionally, another heat transfer enhancement using metal wire mesh spirals of aluminum and copper materials are investigated in liquid piston compression. Metal wire meshes have a high thermal conductivity and a large heat surface area and therefore hypothesized to improve heat transfer from the gas to the liquid inside the liquid piston compressor. Both aluminum and copper meshes observed to improve isothermal efficiency of compression to 88-90% from the base efficiency of 82-84%.

Lastly, the end-to-end efficiency of liquid piston based OCAES with spray injection, aqueous foam, and metal wire mesh techniques is estimated. Heat transfer enhancement in liquid piston shows significant improvement in end-to-end efficiency of OCAES with spray injection as a highly effective efficiency improvement technique with potential to attain about 60% efficiency of OCAES from the base 45% efficiency.”

BIOGRAPHY: “Vikram Patil graduated with Master’s in Mechanical Engineering from Indian Institute of Science, Bangalore, India in June 2013 and with Bachelor of Technology in Mechanical Engineering from Walchand College of Engineering, Sangli, India in May 2011. He worked as an Assistant Manager in the Research and Development division of Bajaj Auto Limited in Pune, India from July 2013 to July 2015.
He joined North Carolina State University as a doctoral candidate in the department of Mechanical and Aerospace Engineering in Fall 2015. His research interests are in the field of energy analysis, heat transfer, and energy storage. He is a member of the American Society of Mechanical Engineering (ASME), American Society of Heating Refrigeration and Air-Conditioning Engineers (ASHRAE), and Institute of Electrical and Electronics Engineers (IEEE).”