One would not agree less with Leonardo Da Vinci when he said, “Once you have tasted flight, you will forever walk the earth with your eyes turned skyward, for there you have been, and there you will always long to return”. There is a never-ending search for a means to make the aviation business better and safer, because man has become lovestruck to the sky as Da Vinci earlier suggested.
From 1808, when Sir George Cayley used gunpowder as fuel in a caloric engine, to when James Watt built heavy underpowered steam engines, and then from 1885 till date that gasoline has been in use as aircraft fuel, man is still in search for a source of motive power that would be economical and clean. Can hydrogen be that alternative fuel to run an airplane?
Yes, an airplane can run on hydrogen. Hydrogen fuel offers cleaner emission in running an airplane but there are doubts as regards its practicality on a commercial scale.
Hydrogen fuel is a zero-emission, eco-friendly fuel that when burned with oxygen produces no CO2, but only water and heat. Hydrogen can blend with the current aviation fuels, and is easily transported and stored. For now, hydrogen is considered a future source of energy.
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The Use of Hydrogen in Airplane Engine Design
Recently, the use of hydrogen as an alternative fuel has been emphasized for three reasons: the need to save fossil resources, the deceleration of the impact connected with the eventual depletion of the fossil resources, and the obligation to reduce environmentally harmful emissions. Hydrogen can either be used as an alternative fuel in aircraft engines or it can be used in a fuel cell to generate electricity to power a propeller.
In an internal combustion engine, hydrogen offers about 25% thermodynamic efficiency, meaning that most of it only turns into heat and doesn’t help to propel the aircraft.
However, in an electric aircraft, a fuel cell that converts hydrogen and oxygen into electricity for an electric motor operates at a minimum of 60% efficiency level.
Furthermore, liquid hydrogen can be used as a heat sink in airplane engines aiding the efficient cooling of hot sections of the engine.
The development of a hydrogen-fueled engine for an aircraft takes advantage of hydrogen’s unique characteristics of high flame speed, its ability to ignite similar mixtures at low comparable ratios, and an effective octane rating to attain high thermal efficiency while fulfilling low emissions conditions. The exhaust from hydrogen turbines is basically water and heat which makes the use of less-expensive materials possible for air preheaters fed by the non-aggressive exhaust gases.
Current Limitations of Hydrogen
There are critical limitations facing the use of hydrogen in an airplane engine. This has cast a lot of criticism on its use by aviation stakeholders. Though, further studies are ongoing to see how hydrogen can be harnessed efficiently to serve as a future source of energy for aircraft engines.
Hydrogen cannot be stored on the wings of the aircraft but rather in the fuselage because of its volatility, warranting a larger-sized fuselage, which in turn increases the overall size of the aircraft. This causes more skin friction drag and wave drag which inhibits the thrust of the aircraft.
The loss of hydrogen due to boil-off during the daytime, coupled with the bulkiness of aircraft powered by hydrogen fuel makes it difficult for use in military combat aircraft as they require very high maneuverability and supersonic speed.
Hydrogen is also less feasible in large aircraft because it is stored at very low temperatures, high pressure and requires a complex refueling process.
Development of New Types of Airplane Engine
It is claimed that the commercial aviation industry discharges an average of 700 million metric tons of carbon dioxide annually. Unfortunately, there is proof that these harmful carbon emissions have a huge effect on the atmosphere because they are released at altitudes close to the ozone layer.
Many in the aviation industry believe the pathway to obtaining cleaner jets is through further development in engine technology rather than cleaner fuels, especially because the alternative use of biofuels has so far failed to meet the target.
This reality has led to the constant advancement of the current hydrocarbon fuel combustion jet engines to meet the 2050 CO2 emissions reduction goal.
Asides reduced CO2 emissions, fuel economy, enhanced performance, noiselessness, and high power density are the objectives of aircraft engine development.
Right now, the leading innovations in aircraft engine technology are open-rotor design, boundary layer ingestion, and electric aircraft.
Open-Rotor Engine
The open rotor is also known as propfan because it is intended to possess the fuel economy of a turboprop together with the efficiency and speed of a turbofan.
The open rotor consists of non-enclosed, dual counter-rotating propellers driven by a gas turbine core. It is also defined as an Ultra High Bypass (UHB) because of its superlative bypass ratio of 15:1 or even more.
Propfans, though quite similar to the turboprops, differ by the shape and number of blades, tip speed, Mach number, bypass ratio, and cruise altitude. The open-rotor engine offers 28% aerodynamic efficiency, 35% fuel savings, and 30% CO2 emissions reduction over a similar turboprop.
The fuel efficiency of propfans comes with a noise penalty especially in the cabin which is higher than the equivalent turbofan engines. This noise could be minimized by reducing the blade tip speeds, decreasing the blade loading, and adjusting the length of the blades of the front and back propellers in such a way that the efficiency at high flight speed can be retained. The open-rotor engine may become available by 2030 as predicted by manufacturers.
Boundary-Layer Ingestion
In order to reduce drag and thereby save fuel, the engines are located near the rear of the aircraft.
So how does this save fuel?
Well, when the airplane flies, the air layer closest to the airplane frame, which is called the “boundary-layer” is slowed down. As it then comes out at the back, it will cause drag, which slows the plane down.
If engines are mounted on the back, the efficiency of the aircraft is improved since the drag on the aircraft itself has been “ingested” by the engines, and won’t act as drag on the main airframe. This “Boundary-Layer Ingestion” (BLI) technology can reduce as much as 8.5% of the aircraft fuel burn by today’s jet engines.
So, in short, BLI is a drag and turbulence reducer thereby increasing thrust just by reducing drag. But there are two major challenges with the BLI technology.
First, airflow along the boundary layer affects the performance of the turbine fans. Hence, a stronger fan is required to withstand the continuous hammering caused by the distorted flow of air.
Second, dirt is sucked along with air over the wings’ surface, which closes up the multitude of holes drilled into the wings skin. This leads to the further blockage of miniature air tubes and bigger air pipes. An air blower is needed to solve this problem. The installation of air blowers increases the weight and power demand on the engine. With the weight penalty comes manufacturing and maintenance costs.
Electric Aircraft (Hybrid and Battery)
Electric motors generate zero carbon emissions during their operations, this makes them a vital technology feature in attaining the 2050 environmental goal.
The following are some of the categories from the different electric engine considered for future aircraft:
Hybrid-Electric Systems
Hybrid-electric systems depend on gas turbine engines for charging batteries and for generating propulsion power.
In 2017, an alliance was formed by Airbus, Rolls-Royce, and Siemens to build the E-fan X engine in three years. The E-fan X powers a 2-megawatt electric motor using a serial hybrid-electric technology.
The future goal is to build a commercial aircraft equipped with the E-Fan X technology that can take 50-100 passengers on board across short-range routes.
Turbo-Electric Systems
Turbo-electric systems do not depend on batteries as a power source during any flight phase, they rely solely on gas turbines. The demonstration aircraft, STARC-ABL, for example, is a single-aisle, large turboelectric aircraft that relies hugely on electric power for propulsion and also to operate electrical onboard systems.
To make the design feasible, the current electric-motor power density of 4-5 horsepower/pound (hp/lb) needs an additional 5-6hp/lb through cryogenic superconducting technology.
Battery-Powered Aircraft
Battery-powered aircraft offer the climax of CO2 emissions reduction, minimal noise, and overall eco-friendly benefits.
At the moment, plans are ongoing by Wright Electric to provide zero-emissions regional flights with battery-powered aircraft by 2035. The design by the company is based on shared propulsion with plenty of electric fans fused into the wings, and on batteries that can be easily replaced during airport turnaround.
The aircraft on completion is expected to accommodate 150 passengers and fly up to 290 nautical miles.
Electric Blended Wing Body
Electric blended wing body (BWB) offers about 70% fuel savings, less noise, easier boarding and disembarking, and shorter turnaround time. The strength of the electric BWB is that it would merge the benefits of electric propulsion and the blended wing body airframe design.
There are, however, a number of challenges facing the development of the electric BWBs such as the adaptation of ground service infrastructure for large BWBs, uncommon design processes, and high uncertainties in investments.
Benefits of the Electric Aircraft
- Energy efficient
- Low noise
- Electric propulsion offers no pollutant emissions
- Upgradability from already existing very small aircraft to larger electric aircraft.
Challenges of the Electric Aircraft
- Installation, commercial arrangements and other logistics involved in electricity supply at airports
- Facilities for battery recharging and storage
- New certification processes
- Battery safety
- An efficient cooling system that can dissipate 50-800 kilowatts of heat during flight
- Availability of superconductive and supercooled electronics to lower the electrical resistance of the aircraft.
Battery Vs Hydrogen-Powered Aircraft
Lithium-ion battery for aircraft engines
Recent innovations within both the battery-powered aircraft technology and hydrogen technology are moving the aviation industry closer to achieving decarbonized transportation. Though both technologies are quite similar in the results they produce, there are still some striking differences between them.
Hydrogen-powered fuel cells are not dependent on scarce earth metals needed to produce lithium-ion batteries. Hydrogen fuel cells also outperform batteries in cold environments.
In addition, a battery stops working when stored electrolytes are used up. On the other hand, a hydrogen fuel cell continues to work as long as its external fuel supply lasts. This makes the hydrogen fuel cell more reliable and more durable.
Most airliners agree that using fully electric aircraft powered by lithium-ion batteries, have limited flight time which makes it an economical choice for aerial transit over relatively short distances.
Conversely, infrastructure is a major constraint for the use of hydrogen. Hydrogen fuel though abundant in nature is not cheaply available for industrial use and would require on-site storage. Also, hydrogen fuel cells compared to the Lithium-ion batteries have a slightly lower power density, which means small payloads adaptable for light aircraft over a short distance.
However, as development continues, there may be farther-range applications of hydrogen fuel cells and battery-powered aircraft in passenger and cargo vertical lift in the near future.
Conclusion
Over the last decade, hydrogen has found proficient use in vehicular and spacecraft applications. However, despite the theoretical groundwork and tremendous research into the use of hydrogen, particularly for commercial aircraft, there is no feasible economic scenario in the next few years that makes its usage appealing or desirable for aircraft manufacturers.
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