Background
Since powered flight was first achieved airplanes have needed one thing to be able to fly, fuel. The early piston engine aircraft used very heavily refined, high octane fuels to produce power. When jet engines were first considered for commercial aviation many people believed the idea to be economically unfeasible because they assumed that jet fuel would need to be more refined than the fuel used in piston engines. What they did not realize was that jet fuel was no more than kerosene. Kerosene is a very basic, inexpensive, and easily produced fuel and because it burns with a lot of energy, made an idea fuel for a jet engine. In aviation this kerosene based fuel is called Jet A. While Jet A is a good fuel because of how energetically it burns, it is still a fossil fuel and like all fossil fuels produces pollution, mainly CO2, NOx, SOx, and unburnt hydrocarbons (e.g. smoke). All of these are harmful to the environment both locally and globally. There needs to be a fuel source that has both the energy potential of kerosene and is a cost effective solution or else it will be harder to convince manufactures and operators alike to use it.
Synthesized Fuels
One group of fuels that is being looked into is synthesized fuels which include biodiesel, syngas, and hydrotreated oils. The advantage in using a synthesized fuel is that it is known as a “drop-in” fuel meaning it would avoid a costly and time consuming redesign of aircraft engines. That would make it easier to deploy worldwide since it could be put to use in existing aircraft.
Biodiesel, also known as fatty acid ester or FAE, produced from the transesterification of bio-oils into bio diesels using either methyl (FAME) or ethyl (FAEE) (Blakely, et. al, 2011). Another fuel type is called syngas which is a mixture of hydrogen and carbon monoxide which is derived from coal, gas, or biomass. It is created using the Fischer-Tropsch process which creates chains of carbon that form chains of alkanes, this then becomes synthetic crude which can be refined into synthetic kerosene (Blakely, et. al, 2011). Fuels created from hydrotreated oils are another option for an aviation fuel source. Hydrotreated oils are created from the hydroprocessing of vegetable oils which removes undesirable materials such as oxygen, sulfur, and any metals in the oil and breaks it down in to smaller carbon chain lengths which is necessary for a fuel (Blakely, et. al, 2011). There are many benefits to using synthetic fuels in aviation. For one, they can use current engine technology, their chemical compositions can be precisely controlled, and they do offer reductions in emissions, for example Syngas offers up to a 20% reduction in carbon emissions while only loosing 5% in specific energy output (Boretti, Dorrington, 2013). While these fuels do offer a reduction in emissions, they are not emission free. For that we need to look at a simple element, which also happens to be the most abundant in the universe.
Using Hydrogen
The use of hydrogen in aviation is not a new idea; German Zeppelins which were large ridged airships used hydrogen as a means to stay airborne. The most famous Zeppelin is the Hindenburg which flew transatlantic flights in the mid-1930s. The Hindenburg was destroyed by a fire when landing in New Jersey. That accident is the reason why hydrogen is not used as a ballast gas in airships anymore. But now there is another potential use for hydrogen in aviation and it is not for ballast, but instead as a fuel source. There are many advantages to using hydrogen in aircraft. It is lighter than kerosene, far more abundant, renewable, and would produce zero emissions. It would also enable aircraft to have smaller wings which would reduce takeoff weight by 30% and operating costs by 3% because the wing size would not be restricted to storing fuel (Verstraete, 2006). However there are complications to using hydrogen as a fuel. For one the production of hydrogen would need a lot of electricity not only for the electrolysis of water but to also run the plant that would be producing it. It is estimated at with a plant running at 80% efficiency would require 105 megawatts of electricity and 28 cubic meters per hour of desalted water to create 50,000 kilograms of hydrogen per day, and to liquefy all of that hydrogen it would take an additional 25,555 kilowatts of electricity (Khandelwal, et. al., 2013). Another issue would be that the most feasible way to store hydrogen is as a liquid which at atmospheric pressure means temperatures as low as 20.4 kelvin (or about -252.75°C) (Khandelwal, et. al., 2013). Another issue is that hydrogen atoms are much smaller than kerosene molecules, which means that traditional storage methods will not work as the hydrogen will find a way to escape. This means that new types of fuel tanks as well as new types of engines would need to be developed. That would mean that these hydrogen powered aircraft would be more complicated as well as more expensive to build and maintain. These caveats would mean that it could be decades before hydrogen powered airplanes take to the skies. While both engines and fuels are good places to turn to in order to improve the environmental impact of aviation. One of the more interesting ways that this could be done is with an aircraft’s structure and design.
Top Image Retrieved From http://upload.wikimedia.org/wikipedia/commons/a/a2/Aircraft_being_fueled_by_tanker.jpg
Since powered flight was first achieved airplanes have needed one thing to be able to fly, fuel. The early piston engine aircraft used very heavily refined, high octane fuels to produce power. When jet engines were first considered for commercial aviation many people believed the idea to be economically unfeasible because they assumed that jet fuel would need to be more refined than the fuel used in piston engines. What they did not realize was that jet fuel was no more than kerosene. Kerosene is a very basic, inexpensive, and easily produced fuel and because it burns with a lot of energy, made an idea fuel for a jet engine. In aviation this kerosene based fuel is called Jet A. While Jet A is a good fuel because of how energetically it burns, it is still a fossil fuel and like all fossil fuels produces pollution, mainly CO2, NOx, SOx, and unburnt hydrocarbons (e.g. smoke). All of these are harmful to the environment both locally and globally. There needs to be a fuel source that has both the energy potential of kerosene and is a cost effective solution or else it will be harder to convince manufactures and operators alike to use it.
Synthesized Fuels
One group of fuels that is being looked into is synthesized fuels which include biodiesel, syngas, and hydrotreated oils. The advantage in using a synthesized fuel is that it is known as a “drop-in” fuel meaning it would avoid a costly and time consuming redesign of aircraft engines. That would make it easier to deploy worldwide since it could be put to use in existing aircraft.
Biodiesel, also known as fatty acid ester or FAE, produced from the transesterification of bio-oils into bio diesels using either methyl (FAME) or ethyl (FAEE) (Blakely, et. al, 2011). Another fuel type is called syngas which is a mixture of hydrogen and carbon monoxide which is derived from coal, gas, or biomass. It is created using the Fischer-Tropsch process which creates chains of carbon that form chains of alkanes, this then becomes synthetic crude which can be refined into synthetic kerosene (Blakely, et. al, 2011). Fuels created from hydrotreated oils are another option for an aviation fuel source. Hydrotreated oils are created from the hydroprocessing of vegetable oils which removes undesirable materials such as oxygen, sulfur, and any metals in the oil and breaks it down in to smaller carbon chain lengths which is necessary for a fuel (Blakely, et. al, 2011). There are many benefits to using synthetic fuels in aviation. For one, they can use current engine technology, their chemical compositions can be precisely controlled, and they do offer reductions in emissions, for example Syngas offers up to a 20% reduction in carbon emissions while only loosing 5% in specific energy output (Boretti, Dorrington, 2013). While these fuels do offer a reduction in emissions, they are not emission free. For that we need to look at a simple element, which also happens to be the most abundant in the universe.
Using Hydrogen
The use of hydrogen in aviation is not a new idea; German Zeppelins which were large ridged airships used hydrogen as a means to stay airborne. The most famous Zeppelin is the Hindenburg which flew transatlantic flights in the mid-1930s. The Hindenburg was destroyed by a fire when landing in New Jersey. That accident is the reason why hydrogen is not used as a ballast gas in airships anymore. But now there is another potential use for hydrogen in aviation and it is not for ballast, but instead as a fuel source. There are many advantages to using hydrogen in aircraft. It is lighter than kerosene, far more abundant, renewable, and would produce zero emissions. It would also enable aircraft to have smaller wings which would reduce takeoff weight by 30% and operating costs by 3% because the wing size would not be restricted to storing fuel (Verstraete, 2006). However there are complications to using hydrogen as a fuel. For one the production of hydrogen would need a lot of electricity not only for the electrolysis of water but to also run the plant that would be producing it. It is estimated at with a plant running at 80% efficiency would require 105 megawatts of electricity and 28 cubic meters per hour of desalted water to create 50,000 kilograms of hydrogen per day, and to liquefy all of that hydrogen it would take an additional 25,555 kilowatts of electricity (Khandelwal, et. al., 2013). Another issue would be that the most feasible way to store hydrogen is as a liquid which at atmospheric pressure means temperatures as low as 20.4 kelvin (or about -252.75°C) (Khandelwal, et. al., 2013). Another issue is that hydrogen atoms are much smaller than kerosene molecules, which means that traditional storage methods will not work as the hydrogen will find a way to escape. This means that new types of fuel tanks as well as new types of engines would need to be developed. That would mean that these hydrogen powered aircraft would be more complicated as well as more expensive to build and maintain. These caveats would mean that it could be decades before hydrogen powered airplanes take to the skies. While both engines and fuels are good places to turn to in order to improve the environmental impact of aviation. One of the more interesting ways that this could be done is with an aircraft’s structure and design.
Top Image Retrieved From http://upload.wikimedia.org/wikipedia/commons/a/a2/Aircraft_being_fueled_by_tanker.jpg