Thermodynamic Analysis of a Liquid Piston for Small-Scale Liquefaction
: A Sustainable Approach to Anthropogenic Methane Upcycling

Student thesis: Doctoral Thesis


With the rise of global decarbonisation efforts, heavy-duty vehicle (HDV) emissions and anthropogenic methane have become significant barriers to achieving carbon neutrality. This thesis links the emitted methane and HDV energy supply through an original thought experiment called "sunlight to traction." The research demonstrates that anthropogenic methane could provide a less carbon-intensive fuel source for HDVs in the short to medium term, with total UK methane emissions providing 71% of the heavy-duty fleet's energy demand. In particular, the research focuses on two previously unaccounted-for methane emissions, grass cuttings and sewer gas, with total UK emissions at 480 ktonnes and 31 ktonnes, respectively. Furthermore, the research demonstrates that anaerobic digestion (AD) can increase methane yield from grass cuttings, with excess grass in the UK alone meeting 33% of the HDV energy demand.

In gaseous form, methane's energy density is 0.09% of diesel, necessitating densification or conversion. This study evaluates the following densification methods: methane to electricity, hydrogen, compressed and liquid methane and is based on three criteria: well-to-wheel efficiency, storage mass, and storage volume while considering the emitted carbon dioxide. Results show that all pathways analysed prevent more carbon dioxide emissions than are released (carbon negative). An overview energy efficiency analysis showed a difference of 5% between electricity, compressed, and liquid methane pathways, with hydrogen performing the worst. However, upon review of each pathway's storage mass and volume, it became clear that liquid methane offers a practical near-term solution with a storage volume of 999 L for the energy content of 750 L diesel, a commonly used HDV fuel size. Electric, hydrogen and compressed methane storage volumes were calculated to be 890 L, 2408L and 2030 L, respectively.

Based on the previously discussed storage analysis, liquefaction was selected as the process to focus on within this research. Due to the diversity and geographical dispersion of anthropogenic methane sources, this research proposes a small-scale liquefaction approach to increase the energy density of methane. A comparison of two fundamental liquefaction cycles (Linde-Hampson and Reverse Brayton) revealed that universally, both Reverse Brayton and Hamson-Linde cycles performed similarly regarding specific energy with only 0.1 kWh/kg between them. However, for this research, a modified Reverse Brayton cycle was selected due to the lower system pressure requirements, with the modification focusing on carbon dioxide removal and specific energy reductions. However, the proposed cycle was not competitive at a small scale and specific energy reductions were needed. A liquid piston was selected to replace a conventional compressor; a liquid piston is a technology that uses a liquid to compress gas directly. A core benefit of a liquid piston is the ability to increase efficiency by increasing the surface area to volume ratio, increasing the heat transfer and, consequently, the isothermal quality. For this study, multiple small-diameter pipe inserts were used. Using a liquid piston within a cryogenic cycle is the core novelty of this research.

A combined Modelling and experimental approach showed that the optimised Reverse Brayton cycle with an 80% adiabatic compressor had a specific energy of 2.49 kWh/kg, introducing a liquid piston decreased specific energy to 1.81 kWh/kg when using the liquid piston operating with no pipes as the compression technology. Adding the pipes decreased the specific energy to 1.51 kWh/kg, but the pipes could not be experimentally validated.

To further understand the characteristics of liquid pistons and to increase novel contributions, the specific liquid piston parameters such as polytropic index, P-V diagram, and stroke time analysis were assessed. A critical finding of this research was the impact fluid viscosity had on the liquid piston operation when using the pipe inserts. Water was the most researched fluid for liquid pistons, where viscosity issues were not reported. Nevertheless, this research used water-based hydraulic fluid (Fuchs 46M Red) to increase compatibility with off-the-shelf components and found that the diameter of pipes must be matched to the fluid used, as during experimental testing, the smaller pipe diameters were not operationally incompatible with the fluid, resulting in operational errors. A pressure drop analysis showed that the diameter of the small pipe inserts would need to be increased to 15 mm to omit the pressure drop.

Finally, accurate gas temperature measurements during compression are vital for isothermal quality quantification for a liquid piston. Nevertheless, previous research had alluded to the premise that thermocouples are too vulnerable to locational changes to measure gas temperature changes during compression accurately. This study examined the temperature differences measured in a T-type thermocouple in two locations. One thermocouple was placed within the cylinder (eventually submerged), whilst the other was located before the gas outlet to prevent submersion. Temperature measurements were taken before the thermocouple was submersed, and the results showed that the thermocouple placed within the cylinder, which would eventually be submerged, measured a temperature almost double that of the thermocouple placed in a non-submerged location. Therefore, this research shows that alternative methods for obtaining gas temperature during compression with a liquid piston are needed. The location of thermocouples significantly impacts temperature measurement accuracy in compression systems with liquid pistons, which could result in an overestimation of performance. This study is the first to address this issue, providing valuable insights into improving temperature measurements in such systems.
Date of AwardMar 2024
Original languageEnglish
Awarding Institution
  • University of Brighton
SupervisorPenelope Atkins (Supervisor), Robert Morgan (Supervisor) & Professor Andy Atkins (Supervisor)

Cite this