The rapid development and transition to sustainable and renewable energy have increased the demand for longer-duration and large-scale energy storage technologies. Among these technologies, compressed air energy storage (CAES) has drawn considerable attention as an ideal solution. Different CAES-based concepts have been proposed. Underwater compressed gas energy storage (UWCGES) is a flexible and scalable energy storage technology suitable for renewable energy firms in coastal, islands, and offshore regions. It has emerged as a vital facilitator of the renewable energy transition.
Although the UWCGES has been extensively studied, only a few studies have focused on the crucial issues of liquid accumulation that often occurs in underwater gas transmission pipelines. The liquid accumulation is attributed to the condensation and precipitation of water vapor after reaching the pressure dew point due to the decrease in temperature and the increase in pressure with the increase in the water depth. This results in the formation of slug flow, which affects the operation and performance of UWCGES systems. To effectively control the liquid accumulation movement and enhance the gas transmission efficiency, it is important to analyze the slug formation and correctly predict the critical slugging velocity.
For hilly-terrain pipelines, the interfacial instability is a possible cause of the slugs associated with the movement of the liquid accumulation under zero net liquid flow (ZNLF). However, the liquid accumulation movement in hilly-terrain pipes under ZNLF is poorly understood. This has been attributed to two main reasons. First is the lack of a highly adaptable and reliable slug velocity model for accurate analysis of the slug flow formation mechanism. Second is the rare or possible inaccurate classification of the causes of liquid slugs.
Herein, Ph.D. candidate Chengyu Liang, Professor Wei Xiong, and Associate Professor Zhiwen Wang from Dalian Maritime University in collaboration with Professor Rupp Carriveau and Professor David Ting from the University of Windsor conducted a thorough experimental and modeling investigation of the liquid accumulation flow process, slug formation and critical slugging behavior in a hilly-terrain pipeline under ZNLF. In their approach, the liquid slug formation process was divided into three stages based on the amount of liquid accumulation, pipe inclination, gas volume, and the horizontal length of the liquid film. Additionally, the absorption, merging , and outflow of the liquid slug in the pipeline were analyzed and its formation mechanism was divided into three categories. The work is currently published in the journal, Journal of Energy Storage.
The research team showed that the movement of liquid accumulation was affected by changes in gas velocity. Increased inclination angle and liquid accumulation volume were more likely to produce slug flow. Theidealized liquid slug unit could be divided into four regions bubble, liquid slug tail, liquid slug body, and head of liquid slug , leading to continuous absorption, merging, and outflow of the liquid slugs. At small gas velocities, liquid slugs were formed by convective extrusion and wave growth, and they consisted of five processes: slugging, backflow, liquid slug leaving, squeezing, and liquid bridge formation. At high gas velocity, the liquid slug was mainly formed by wave merging and was associated with the generation of small waves at the interface of the liquid and gas.
Furthermore, a theoretical model for the critical slugging velocity was established and the relationships between the gas velocity, liquid film thickness, and maximum growth factor were detailed. As a result, the critical slug velocity corresponding to the maximum growth factor was determined by analyzing the Kelvin-Helmholtz instabilities. The proposed model not only predicted the critical slug velocity effectively but also improved the liquid movement control in the pipeline. In addition, a reduction in energy loss during the transmission of the gas and an improvement in energy storage efficiency was reported.
In summary, the collaborative research team reported the liquid flow characteristics in a hilly-terrain pipeline under ZNLF. The predictions were consistent with the experimental data with a maximum error below 10%. In a joint statement to Advances in Engineering, the authors said their study would contribute to developing advanced high-performance UWCGES to facilitate the transition to renewable and sustainable energy.
Liang, C., Xiong, W., Carriveau, R., Ting, D., & Wang, Z. (2022). Experimental and modeling investigation of critical slugging behavior in marine compressed gas energy storage systems. Journal of Energy Storage, 49, 104038.