An experimental and numerical analysis to determine the viability of a liquid piston as an alternative compression technology for localised waste methane liquefaction

Methane is the largest non-CO2 greenhouse contributor, with a GWP of 86 over a 20-year period. Sources of waste methane are highly dispersed and include agriculture, waste processing and energy. Yet with densification methane can offer an energy dense alternative to fossil-based fuels for heavy duty transport. The challenge with densification is that current methods involve large scale liquefaction, which would incur significant cost and energy in transport of feedstocks. A small-scale liquefaction system is preferred but are inefficient due to limitations on compressor technology. To reduce the work of compression a gas should be compressed isothermally, requiring a large surface area to volume ratio for enhanced heat transfer. A liquid piston gas compressor has a liquid to gas interface during compression, allowing the use of cylinder inserts to increase surface area to volume ratio. This paper compares the performance of a modified Reverse Brayton cycle operating with a liquid piston with small diameter pipe inserts and single to three stage compression, using a hybrid experimental and numerical approach. The experimental analysis captured the temperature and pressure profiles of the gas when compressed using a liquid piston. The increased surface area to volume ratio from the pipe inserts resulted in a peak temperature rise of 7.5˚C for a 13.3 compression ratio, corresponding to a polytropic index of 1.01. The results indicated a near-isothermal compression of gas when using a liquid piston. Using the experimental data in the numerical simulation of the reverse Brayton cycle found that using a liquid piston reduced specific energy by an average of 27%
compared to conventional multistage compression. The specific energy showed that liquid piston compressors could be a promising alternative for small scale liquefaction compressor technology.