Brief Literature Review

We were inspired to look into alternative energy solutions, and ultimately, liquid methane, by the increasingly pressing global issue of climate change.

As carbon dioxide levels reach their highest in 800,000 years (Duffy 2012), the effects of further greenhouse emissions are described throughout a variety of literature including UNESCO’s report on the impacts of climate change in China’s Yellow River Basin (2011). Concerning factors identified to be as a result of climate change by such studies include the decrease of river runoff due to glacier thawing, vegetation area change and economic effects to agricultural production. Resulting social concerns are strongly influencing the push to implement greenhouse reduction techniques. The use of a blend of biomethane and natural gas derived liquid methane would offer a reduction in the emission of greenhouse gases and tie in with such ventures as the European Pathway to Zero Waste and European Commissions ‘Flightpath 2050’. The production of biomethane from fresh waste products is a renewable and sustainable method of producing fuel that is chemically identical to fossil fuel sourced natural gas, in line with strategies proposed through Airbus’ ‘Smarter Skies’ initiative (Airbus 2012). A biofuel blend would assist to reduce and recycle energy and in turn further reduce the accumulative effects of climate change across the globe, ensuring the longevity of the aviation sector and our future career’s.

Initiatives such as the United Nations led Global Renewable Energy and Training program have inspired our passion to develop our proposal and lead the change for a cleaner and more efficient aviation sector. Another such program, UNESCO’s Engineering Initiative stresses the need to develop clean technologies and reduce industrial pollution, acting as a driving factor in the earths requirements for the engineers of tomorrow (UNESCO 2012).

As part of a continuing effort to reduce emissions and wastage, the possibility of onboard methane fuel cells, operating off gaseous boil-off methane will also be considered. Murray, Tsai and Barnett (1999) have shown comparable performance to hydrogen fuel cells can be achieved via ceria containing anodes in a solid oxide fuel cell. This would eliminate the need to carry two types of fuel, and produce electricity for auxiliary onboard systems in a cleaner, more efficient manner. Lin et al. (2005), describe achievable power densities of 0.52 W/cm² at 700°C and 1.27 W/cm² at 800°C using methane fuel cells, with improved efficiency at higher temperatures. The ability to integrate fuel cells operating at such high temperatures presents another challenge to be considered before Round 2, but could potentially be introduced near to the engines. It should be noted, detailed propulsive changes and requirements will remain outside of the scope of this investigation. The notion and innovation behind natural gas fuel cells can also be extended to the supporting infrastructure for the aviation industry. LNG tankers that experience boil-off may carry onboard fuel cells in order to produce electricity required in travel, as per Sattler (2005), again minimising wastage.

Interested in improving sustainability, the idea to modify existing aircraft to run on liquid methane arose from a propulsion class and the topic of the fuel shortage crisis. The recent introduction of the Carbon Tax in Australia and European Union’s Emissions Trading Scheme (ETS) both drew substantial attention and scrutiny, emphasizing the significance of alternative fuels to the prolonged sustainability of the aviation industry. The ETS in particular has received ongoing criticism from a range of world leaders, originally to target all airline operators with flights to, from and within the Union in an attempt to reduce greenhouse emissions to 97% of historic levels by 2013 (Leggett, Elias & Shedd 2012). A new, alternative fuel source must be relatively easy to introduce to existing aircraft, meet stricter worldwide greenhouse emission standards, available in large supply and be safe to handle. A biomethane and natural gas derived liquid methane blend was found to best satisfy such a criteria. Alternative plant derived biofuels had been considered; however, with their farming potentially competing with valuable food sources and a lack of suitable land, a liquid methane/biomethane blend was deemed to have a greater potential. Significant mining growth surrounding natural gas, including the recent $64 billion Gorgon Project in Australia, is set to further increase the supply of natural gas and could therefore support the introduction of this alternative fuel into the aviation industry.

Following on from this class, an essay had to be written as part of coursework on the topic of LNG use as a future aviation fuel. Approached by students from different viewpoints, one was comparing the chemical properties such as energy density and Thrust Specific Fuel Consumption of methane to those of kerosene. The amount of carbon dioxide produced and change in tank volume for a particular range could be found, allowing the viability of such an alternative fuel to be quantified. Secondly, LNG use as a future aviation fuel could be analysed from a financial feasibility viewpoint. For example, how much would fuel relative to energy output cost using prices for both kerosene and natural gas as given by Exxon Mobil? In both approaches, the use of LNG rendered positive results; reducing direct operating costs in the long run and overall greenhouse gas emission. The sustained high price of petroleum and higher cost of gas to liquid derived kerosene (Fischer Tropsch) reiterate the apparent need of an alternative aviation fuel, with natural gas a substantially cheaper and readily available solution. Gas to liquid (GTL) processes will be assumed to remain unviable due to current high production costs.

Historic literature such as the Lockheed/NASA funded project by Carson et al. (1980) suggested the optimal aircraft configuration to be two LNG tanks at either end of the fuselage. The minor shortcomings regarding safety presented through this report, along with the substantial lapse in time since being investigated has motivated us to find contrary and improved design solutions that satisfy all safety criteria. With technology having developed significantly since the time of this report; such as the use of Cryogel^TM, advanced composite materials, structural and computational techniques, the team believes that despite the necessary cryogenic systems, a means of storing the LCH₄ fuel within the wing is possible. As a result, minimal modifications to the remainder of the aircraft will be required.

Ongoing research and the recent success by Northrop Grumman and NASA into composite cryogenic hydrogen fuel tanks as part of their Next Generation Launch Technology program reiterates the potential for composite materials to be used under such circumstances. Testing has suggested that such tanks can offer between a 10 and 25 percent reduction in weight when compared to conventional tanks, providing for the greater payload required when using liquid methane fuel (Northrop Grumman 2004). Such outcomes have inspired the group to see the further potential associated with liquid methane propulsion and further refine our conceptual design.