@article{patil_acharya_ro_2020, title={Experimental investigation of water spray injection in liquid piston for near-isothermal compression}, volume={259}, ISSN={["1872-9118"]}, DOI={10.1016/j.apenergy.2019.114182}, abstractNote={Near-isothermal compression is desired to achieve high efficiency in many compressor applications. Low heat transfer characteristic of conventional compressors is a major bottleneck in attaining a near-isothermal compression. A high heat transfer rate is possible with an injection of a large number of water droplets using a spray nozzle inside the compression chamber. In this paper, the effectiveness of spray injection to achieve near-isothermal compression is investigated experimentally in a liquid piston compressor for a compression ratio of about 2.5. Parametric investigations are performed by varying injection pressures of spray from 10 psi (69 kPa) to 70 psi (483 kPa), using different spray nozzle angles (60°, 90°, and 120°), and by changing the stroke time of compression. It is observed that water spray injection is highly effective in abating the air temperature rise during the compression process. The pressure-volume plots indicate a significant reduction in the compression work, and they approach near-isothermal compression with spray at higher injection pressures. The isothermal efficiency of compression consistently increases with an increased injection pressure of spray and reaches up to 95% at the highest injection pressure studied (70 psi). Furthermore, the spray nozzle angle marginally affected the isothermal efficiency with a 1–4% improvement with the use of a 60° nozzle angle over a 120° spray angle at all injection pressures. Also, comparable isothermal efficiencies are observed for compression with different stroke times between 3 and 5 s especially at higher injection pressures which highlight the efficacy of spray injection in attaining a high power-density along with high efficiency. Overall, with an optimized spray design, water spray injection can achieve a highly efficient near-isothermal compression in liquid piston.}, journal={APPLIED ENERGY}, author={Patil, Vikram C. and Acharya, Pinaki and Ro, Paul I.}, year={2020}, month={Feb} }
 @article{patil_ro_2020, title={Experimental study of heat transfer enhancement in liquid piston compressor using aqueous foam}, volume={164}, ISSN={["1359-4311"]}, DOI={10.1016/j.applthermaleng.2019.114441}, abstractNote={Efficiency of gas compression can be significantly improved by achieving isothermal compression. A high heat transfer rate in the compression chamber is desired to achieve the isothermal compression process. A large surface area and a high heat transfer coefficient of aqueous foam can be used to achieve a significantly high heat transfer rate in the compression chamber. In this study, a novel heat transfer enhancement technique using aqueous foam is investigated in a compressor for achieving near-isothermal compression. Experiments are performed with the use of aqueous foam generated inside a liquid piston compressor. The volume of aqueous foam in the compression chamber, the air flow rate for foam generation, and various foam generator designs are considered in this parametric investigation. It is observed that the use of aqueous foam in the compression chamber is highly effective in reducing air temperature during the compression process. A higher volume of aqueous foam in the compression chamber leads to a significant increment in isothermal efficiency, however, with higher variability. The higher variability in efficiency is due to the higher cyclic variation of the temperature profiles during compression. A compression chamber completely filled with aqueous foam shows a 4–8% improvement in the efficiency for a compression ratio of 2.5. Moreover, several foam generator designs were tested to identify if there is any dependency of cyclic variability on foam generator design parameters. The results show some promise on optimizing the design to reduce the variability. Overall, the heat transfer enhancement using aqueous foam is effective in achieving an isothermal efficiency up to 92% compared to 86% for the no-foam case in a liquid piston compressor.}, journal={APPLIED THERMAL ENGINEERING}, author={Patil, Vikram C. and Ro, Paul I.}, year={2020}, month={Jan} }
 @article{patil_acharya_ro_2019, title={Experimental investigation of heat transfer in liquid piston compressor}, volume={146}, DOI={10.1016/j.applthermaleng.2018.09.121}, abstractNote={The use of liquid pistons is a promising approach for attaining efficient near-isothermal compression. One of the key factors affecting the efficiency of a liquid piston compressor is heat transfer. Understanding the heat transfer mechanism during compression is crucial for the design and development of an efficient liquid piston compressor. In this paper, heat transfer in the liquid piston compressor is studied experimentally for air compression. An analytical model is presented based on a thermal resistance circuit. Experiments are performed using compression chambers of different materials for a compression ratio of 2.05–2.35 with various stroke times of compression. It is observed that the rate of heat transfer increases with faster stroke time of compression. However, a faster compression process requires a higher compression work and results in a higher air temperature. The convective heat transfer coefficient of air decreases rapidly as compression proceeds and approaches a steady value towards the end of compression. Thermal resistance analysis for compression with different chamber materials indicates that convective thermal resistance of air has a significant contribution in the total thermal resistance. During the initial phase of compression, the high conductivity of the chamber material helps improve the overall heat transfer coefficient; however, it has a marginal effect during the later phase of compression. An isothermal compression efficiency of 84–86% is observed with the liquid piston.}, journal={APPLIED THERMAL ENGINEERING}, author={Patil, Vikram C. and Acharya, Pinaki and Ro, Paul I.}, year={2019}, pages={169–179} }
 @article{patil_ro_2018, title={Energy and Exergy Analysis of Ocean Compressed Air Energy Storage Concepts}, volume={2018}, ISSN={["2314-4904"]}, DOI={10.1155/2018/5254102}, abstractNote={Optimal utilization of renewable energy resources needs energy storage capability in integration with the electric grid. Ocean compressed air energy storage (OCAES) can provide promising large-scale energy storage. In OCAES, energy is stored in the form of compressed air under the ocean. Underwater energy storage results in a constant-pressure storage system which has potential to show high efficiency compared to constant-volume energy storage. Various OCAES concepts, namely, diabatic, adiabatic, and isothermal OCAES, are possible based on the handling of heat in the system. These OCAES concepts are assessed using energy and exergy analysis in this paper. Roundtrip efficiency of liquid piston based OCAES is also investigated using an experimental liquid piston compressor. Further, the potential of improved efficiency of liquid piston based OCAES with use of various heat transfer enhancement techniques is investigated. Results show that adiabatic OCAES shows improved efficiency over diabatic OCAES by storing thermal exergy in thermal energy storage and isothermal OCAES shows significantly higher efficiency over adiabatic and diabatic OCAES. Liquid piston based OCAES is estimated to show roundtrip efficiency of about 45% and use of heat transfer enhancement in liquid piston has potential to improve roundtrip efficiency of liquid piston based OCAES up to 62%.}, journal={JOURNAL OF ENGINEERING}, author={Patil, Vikram C. and Ro, Paul I.}, year={2018} }
 @inproceedings{patil_ro_2017, title={Comparative assessment of different types of ocean compressed air energy storage systems based on exergy analysis}, DOI={10.1115/power-icope2017-3630}, abstractNote={Ocean compressed air energy storage (OCAES) is a promising large-scale energy storage concept. Different types of OCAES viz. - Diabatic, adiabatic and isothermal are possible based on the handling of heat in the system. In diabatic OCAES, compressed air is cooled in a cooler and heated using external heat source before transport to the expander. In Adiabatic OCAES, heat from the compressed air is stored in a thermal energy storage (TES) and reused to reheat compressed air before sending it to the expander. In Isothermal OCAES, air is compressed and expanded isothermally which results in the least compression work and highest expansion work. These OCAES configurations are assessed using exergy analysis in this paper. The exergy efficiency of individual components, exergy flow and overall efficiencies of diabatic, adiabatic and isothermal OCAES are presented. Results show that adiabatic OCAES shows improved efficiency over diabatic OCAES by storing thermal exergy of compressed air in TES and isothermal OCAES shows significantly higher efficiency over adiabatic and diabatic OCAES.}, booktitle={Proceedings of the ASME Power Conference Joint with ICOPE-17, 2017, vol 2}, author={Patil, V. C. and Ro, P. I.}, year={2017} }
 @article{patil_ro_ranganath_2016, title={End-To-End Efficiency of Liquid Piston based Ocean Compressed Air Energy Storage}, DOI={10.1109/oceans.2016.7761399}, abstractNote={Intermittent ocean energy resources need energy storage system for their optimal utilization. Ocean compressed air energy storage (OCAES) is a promising way for a utility scale energy storage. In this paper, a liquid piston based ocean compressed air energy storage is analyzed for end-to-end efficiency. An analytical model for end-to-end efficiency based on efficiencies of individual components in the liquid piston based OCAES is presented. Numerical analysis based on the analytical model and experimental data is done using Monte Carlo simulations. End-To-End efficiency of the liquid piston based OCAES is observed to be highly dependent on the polytropic index of compression and expansion. End-to-End efficiency increases from 24% to 72% with decrease in polytropic index from 1.4 to 1. Experimentally observed polytropic index shows 45% end-to-end efficiency. Comparison of results with end-to-end efficiency of existing compressed air energy storage systems indicates that the isothermal liquid piston based OCAES shows significantly higher end-to-end efficiency.}, journal={OCEANS 2016 MTS/IEEE MONTEREY}, author={Patil, Vikram C. and Ro, Paul I. and Ranganath, Kishore R.}, year={2016} }
 @article{ramakrishnan_ro_patil_2016, title={Temperature Abatement Using Hollow Spheres in Liquid Piston Compressor for Ocean Compressed Air Energy Storage System}, DOI={10.1109/oceans.2016.7761341}, abstractNote={This paper deals with a novel technique to curb the temperature raise during compression in a liquid piston compressor used in Ocean Compressed Air Energy Storage (OCAES) system. Hollow spheres made of various materials, viz. Silicon Carbide (SiC), High Density Polyethylene (HDPE), and Polypropylene (PP) were made to float on the top surface of the liquid column. It was observed that the temperature abatement in each of the three cases was very evident. The heat transfer does not depend on the material of the sphere, but the fact that there is a solid surface between water and air itself plays an important role along with the size of the sphere. The heat transfer per unit area from the simulation and the analytical model have been compared and the values are found to be very similar. Also, polytropic index of the compression process was evaluated in case without and with SiC spheres, and it was found to be closer to the isothermal index of 1 when the spheres are used.}, journal={OCEANS 2016 MTS/IEEE MONTEREY}, author={Ramakrishnan, Kishore Ranganath and Ro, Paul. I. and Patil, Vikram. C.}, year={2016} }