Review of Technologies to Achieve Sustainable (Green) Aviation

Among all major modes of transportation, people travel by airplanes and automobiles continues to experience the fastest growth. As shown in Figure 1 [1], the travel as measured by Passenger Kilometers (PKM) is forecasted to more than double from the current 2010 level of ~ 40 trillion PKM to approximately 103 trillion PKM by 2050. Among these two modes of transportation, air travel is experiencing the faster growth. The number of Passenger – Kilometers Travelled (PKT)/ capita by various modes of transportation in different countries is shown in Figures 2(a) 2(d) [1]. Figures 2(a) and 2(c) also show that the use of personal vehicles compared to public transport (in PKT) is highest in U.S. followed by the wealthier nations. Furthermore, as the per capita income of a nation increases, the travel demand will increase (Figure 3) [1] resulting in greater demand for personal vehicles as well as for air transportation as shown in Figure 1. These projections are based on 3% growth in world Gross Domestic Product (GDP), 5.2% growth in passenger traffic and 6.2% increase in cargo movement. Only major policy changes and intervention by governments through development of infrastructure for public transportation is likely to slow down these trends shown in Figure 1. Most of the energy for transportation is currently provided by the fossil fuels (primarily petroleum). Figure 4 shows the oil consumption for transportation in U.S. and its forecast for the future [2]. Figure 5 shows the relative percentage of fuel consumption by various categories of vehicles in U.S [2]. The consequence of burning fossil fuels is well established in their long term impact on climate and global warming due to Greenhouse Gas (GHG) emissions, primary being the CO2 and NOx. Table I gives the current level of CO2 emissions worldwide by ground and air transportation [3] and Figure 6 shows the forecast for the future if the current Business as Usual (BAU) scenario continues [3]. The reduction in GHG emissions due to the burning of fossil fuels is the major goal of “Green Transportation.” The “Sustainability” goal is to explore both the technological solutions to increase the efficiency of transportation as well as the alternative carbon neutral fuels (e.g. biofuels among others).