Effects of piston speed, compression ratio and cylinder geometry on system performance of a liquid piston

Abstract

Energy storage systems are being more important to compensate irregularities of renewable energy sources and yields more profitable to invest. Compressed air energy storage systems provide sufficient of system usability, and large scale plants are found around the world. The compression process is the most critical part of these systems and different designs must be developed to improve efficiency such as liquid piston. In this study, a liquid piston is analyzed with CFD tools to look into the effect of piston speed, compression ratio, and cylinder geometry on compression efficiency and required work. It is found that, increasing piston speeds do not affect the piston work but efficiency decreases. Piston work remains constant at higher than 0.05 m/s piston speeds but the efficiency decreases from 90.9 % to 74.6 %. Using variable piston speeds has not a significant improvement on the system performance. It is seen that, the effect of compression ratio is increasing with high piston speeds. The required power, when the compression ratio is 80, is 2.39 times greater than the power when the compression ratio is 5 at 0.01 m/s piston speed and 2.87 times greater at 0.15 m/s. Cylinder geometry is also very important because, efficiency, power and work alter by L/D, D, and cylinder volume, respectively.

Flow and heat transfer characteristics of air compression in a liquid piston for compressed air energy storage

“…Similar tendency was also reported by Coetzee et al [27] by applying the moving boundary of a solid piston in a 2D axisymmetric CFD model. Mutlu & Kiliç [28] showed that the compression efficiency decreased with the increasing piston speed, especially at high compression ratio (CR) and high L/D ratio of the LP column.…”

Review on Liquid Piston technology for compressed air energy storage

“…Since a constant liquid volume flow rate is often assumed by the liquid pump, the liquid velocity as well as the Re L are considered thereby as constant during the compression process. On the contrary, the gas flow is much more complex than a simple pipe flow because the gas does not have the same velocity as water all over the piston column, as reported by Mutlu & Kiliç [52] and Schober et al [45]. It has been observed by Neu et al [53,27] that during compression many re-circulation zones could appear next to the walls (upper, bottom and lateral) of the piston column.…”

“…Mutlu & Kiliç [52] have numerically examined the effect of piston speed, the CR and the cylinder geometry on η c and required power of a LP. It was found that the η c decreased from 87.93% to 68.89% when CR increased from 5 to 80.…”

“…Their study recommended to use long cylinders with small diameters to achieve a higher η c , e.g., up to 91.42% with the proper length/diameter ratio L/D [52]. For low pressure compression, an important flow rate (velocity) is usually imposed to compensate the power level and the system performance.…”

Simple model of liquid piston compressor

“…Traditional Liquid Piston Compressor can be parametrised in the context of geometry and operations. Effect of the compression speeds and compression ratios was the topic of the study performed by Motlu et.al [5]. It was also found out that efficiency of such device rise when length to bore diameter ratio is increased [6].…”