What technologies do airplanes use
How does the energy transition succeed in aviation? - Ideas for the electric aircraft propulsion systems of tomorrow
It is no secret that air traffic contributes to the environmental pollution caused by greenhouse gases. However, there are already measures and developments that reduce the fuel consumption of aircraft and thus less pollute the environment. The switch from fossil kerosene to climate-friendly energy sources is therefore also one of the core topics of aeronautical research at the German Aerospace Center (DLR).
New generations of aircraft can already fall back on optimizations of gas turbine technology and improved aerodynamics or on new types of lightweight construction materials and structures. Modern aircraft are already more environmentally friendly and efficient than they were a few years ago. Nevertheless, the scientists are constantly faced with new challenges - also in the area of environmentally friendly drive solutions.
Emission-free drive alternatives and their impact on the climate
There are several ways to use climate-neutral energy storage technologies in aircraft with internal combustion engines. One of them is synthetic kerosene. During the manufacturing process, synthetic gas consisting of carbon monoxide (CO) and hydrogen (H2) is used to produce fuels. This alternative fuel is already being added to conventional kerosene as a so-called drop-in fuel and is already being used in very small quantities. Another possibility is offered by gas turbines powered by hydrogen.
When evaluating the environmental impact of air traffic, however, it should be borne in mind that in addition to CO2 there are other climate-damaging emissions. For the efficient operation of gas turbines, combustion at high temperatures and under pressure is necessary. Nitrogen oxides (NOx), which also represent a burden on the climate. When using hydrogen, CO2-Emissions completely eliminated and also by-products such as sulfur oxides (SOx) or nitrogen oxides (NOx) can largely be avoided. However, water vapor, as a by-product of kerosene combustion and the main emission of hydrogen combustion, continues to influence the effects on the climate, among other things with the formation of contrails and contrail cirrus, which will have to be examined in more detail in the future.
So it becomes clear: Carbon dioxide-neutral technologies in combustion-based concepts emit fewer pollutants, but still have an impact on the climate. These influences need to be better understood and assessed in more detail. Electric drives are currently the only known alternative that works completely without emissions on the aircraft, because they do not require any combustion for their operation.
Are you paving the way for emission-free aviation? And what are the options here?
The electrification of aviation
A climate-neutral energy source for electric motors could be batteries charged with renewable energy. Another possibility is hydrogen fuel cells, which do not produce nitrogen oxides and whose water vapor is not as harmful as long as the aircraft is flying at a sufficiently low altitude. For a CO2In addition, it only makes sense to use hydrogen that has been generated from renewable sources.
The advantages are obvious: battery-electric drives are the only ones that can get aircraft into the air completely without emissions. They have sufficient power density to be used as the primary power source. In addition, increasing altitudes hardly cause any loss of performance, which means that they can reach fast speeds at very high altitudes.
But of course battery-powered aircraft also have their disadvantages: Compared to fuels, they have a very low energy and power density. This results in a relatively short range. According to the current technological maturity, batteries are around 60 times heavier than chemical energy sources such as kerosene. State-of-the-art batteries enable appropriately designed aircraft to have a maximum range of around 200 kilometers in the short term. However, technical advances promise higher performance in the medium term, allowing ranges of more than 500 kilometers. However, according to the current state of research, intercontinental flights that take place exclusively with battery drives will not be possible in the long term either.
Aircraft with battery-electric propulsion are predestined for travel within metropolitan areas or as feeder aircraft to larger airports. In addition, they are suitable for vertical take-offs at high airports and are therefore of interest for the operation of air taxis.
Aircraft with battery hybrid electric propulsion
However, it is also possible to combine different types of drive with one another and thus benefit from the respective advantages. If, for example, the battery-operated system is supplemented with an internal combustion engine, the low energy density of batteries can be compensated for by hybrid-electric architectures of various designs. The problem of the limited range of battery-powered aircraft can therefore be solved with the help of a combustion-based range extender - for example a gas turbine.
The combination of electric drives with alternative fuels enables emission-free battery-electric operation at the airport, for example, while at least CO during the cruise2-neutral would be possible. In the future, environmentally neutral aircraft that can fly the range of an Airbus A320 would be feasible with such a combination.
Another possibility is to use batteries only to support the primary drive. The operation of the aircraft in the inefficient operating states, such as taxi or descent, can be supported with the highly efficient electric drive. In addition, such batteries can also support flight states with high performance requirements, such as take-off or go-around. The aim of this is to relieve the design constraints of the primary drive, which can increase the overall efficiency of the drive.
Aircraft with fuel cell hybrid electric propulsion
Although there is still a great need for research, significant progress has been made in recent years with regard to the power-to-weight ratio and service life of fuel cells, so that they are also of interest for aviation. There are basically different types of fuel cells, such as the solid oxide fuel cell (SOFC) or the proton exchange membrane fuel cell (PEM). Usually hydrogen is used as a fuel. From today's perspective, the fuel cell in connection with sustainably produced hydrogen has the long-term potential to provide sufficient power and range for commercial aviation. This would make emission-free air traffic possible. However, fuel cell development is not yet so advanced that it can easily be used in aviation. It is in the research and development stage.
Due to its degree of maturity, the high power-to-weight ratio and short response time, the PEM is the only technology that can currently be used in aviation. Experience over the last 20 years and investments in the automotive sector ensure that durability, performance and integration have been greatly improved.
Among other things, there are major challenges in energy storage and, in particular, in cooling. The fuel cell converts about half of the energy from the hydrogen consumed into usable power. The remaining 50 percent generate waste heat that has to be cooled to temperatures below 100 degrees Celsius. For example, a nine-seat feeder aircraft with a PEM fuel cell needs one megawatt of power. About one megawatt of heat is then released into the atmosphere. One megawatt corresponds to the energy of around 1000 hotplates that have to be placed somewhere in the aircraft.
This example makes it clear how necessary an effective cooling system is. It must be able to dissipate very large amounts of the heat produced by fuel cells into the outside air. This in turn leads to considerable additional mass and significant air resistance, which in turn affects the optimal design speed of the aircraft. On the one hand, a slower aircraft requires less power and thus less cooling, but on the other hand this lower speed limits the practical range of such aircraft.
Challenges of technology change
The question quickly arises why drives with batteries or even fuel cells are already possible in other transport sectors. New electric drive technologies are constantly being introduced here in order to minimize the impact on the environment. Why can't that also work in aviation?
A technological change in aviation is a lengthy and cost-intensive process. One of the greatest challenges is the approval of new technologies. The approval process for every new aircraft model, including its subsystems and subcomponents, is extremely time-consuming and very costly, as it naturally has to meet the highest safety standards. In addition, no approval requirements for electric drives have been defined so far. In addition, the technological maturity of alternative drive solutions for aviation is still very low. After all, airplanes require far more power than vehicles, but there are still no megawatt-class electric motors for aviation applications, for example. In addition, new propulsion solutions have effects on the entire aircraft that have yet to be investigated. Aviation research is currently working with industry on medium-sized to large electrically powered demonstration models that provide a certain basis for solving these problems. When introducing these new technologies, one must also consider the current air transport infrastructure, which is designed for fueling with kerosene. This applies in particular to the use of hydrogen as a fuel, but also to charging or replacing batteries while the aircraft is on the ground. New infrastructures at airports are of course very expensive.
Due to the investment required in research and development, licensing and infrastructure, and the associated risks, the transition to environmentally neutral aviation is a slow and costly process that may only be possible with constant political support.
An outlook: E-aircraft will be cheaper to operate and maintain
After initial investments in the development and implementation of the new technologies, the manufacture, maintenance and servicing of the electrical aircraft components will be cheaper than it is with current drive concepts. This is because the electrical components contain fewer moving parts than other types of drive. The lower complexity of the propulsion system in electric flying also has the potential to reduce maintenance costs. The entire value chain can benefit from the maintenance of the (hybrid) electrical concepts: from storage costs to the reduction of components and extended maintenance intervals - everything speaks in favor of the new form of drives. In order to estimate the maintenance costs, the scientists then have to develop current models that take into account the new types of aircraft system architectures and components.
How does the energy transition succeed in aviation?
For environmentally friendly aviation, future technologies such as sustainable fuels combined with new gas turbine concepts, hybrid-electric aircraft and hydrogen technologies must be promoted.
In the area of alternative fuels, the effect of drop-in fuels can be maximized through higher admixture rates. So-called aromatic-free near-drop-in fuels are ideal for minimizing climate impact. However, there is currently no approval for such fuels. Since the 50 percent admixture limit is a safety buffer, reliable methods of fuel evaluation and aircraft component design are required. In the interplay of alternative fuels and new gas turbine concepts with, for example, low-emission combustion chambers, the CO2-Emissions by up to 80 percent, soot and particulate emissions by up to 90 percent and NOx- Reduce emissions by almost 100 percent.
In addition to component development, there is a significant need for research on electric flying in the system understanding of the various hybrid-electric variants. In order to answer these questions, both numerical analyzes and systematic experimental, real flight tests with a suitable flight demonstrator are required. In addition, the power and energy density of all components in the system must be increased in order to increase the range of hybrid-electric aircraft. In the field of fuel cells, the scientists have to investigate and improve heat management. Questions about efficient hydrogen storage need to be answered. The experts must examine, research and further develop the performance management system and the question of what influence electrical drive systems and integrations have on the overall system should also be part of the research. Among other things, the area of distributed drives and boundary layer suction play a role here. In order to be able to make verifiable statements about the overall system, the researchers have to consider a close coupling of the engine and aircraft. For this it is necessary to consider the engine internal aerodynamics and aircraft external aerodynamics in parallel. The challenge lies in the acquisition and processing of the system status - starting with the control platforms through to the software embedding of the control algorithms. Power distribution and operational safety are the keys to the introduction of electric drive technology. Since there must be redundancies here, operational reliability can only be achieved with a modular power distribution.
And there are still many unanswered questions about hydrogen combustion, especially with regard to its impact on the climate. In order to find answers, real experiments with a flight demonstrator are necessary. By converting the engine of a regional aircraft for hydrogen combustion, flight tests could be carried out, which quantify the atmospheric influences and test the efficiency of the technology. The resulting water vapor emissions in the atmosphere could be examined in order to better understand their complex processes. First of all, the compatibility with gas turbines including new combustion chambers could be researched and at the same time the connection between flight control and the effect of water vapor emissions could be further developed in order to avoid the climate impact of this technology as far as possible.
The transformation of the aviation industry requires diverse investments in development, approval and infrastructure as well as the support of national and international political decisions. The successful introduction of key technologies for climate-friendly air traffic requires flight tests and thus a scheduled demonstrator program. Until then, current generations of aircraft must be equipped with retro-fit solutions in order to minimize their environmental impact.
How does a fuel cell work?
A fuel cell is an electrochemical cell that converts the chemical energy of hydrogen and oxygen directly into electricity. So far, the power-to-weight ratio of the fuel cell at around one kilowatt per kilogram has not been sufficient for aviation applications at the system level, as the electric motor and tank also contribute to the total weight. For comparison: A pure gas turbine system has a power density of around five to 15 kilowatts per kilogram. In contrast to the battery, the fuel cell is an energy converter in which the energy itself is stored in a tank that determines the range. Emissions such as carbon dioxide and soot, nitrogen oxide do not occur in fuel cells because the fuel cell is not based on a combustion process, but on a catalytic reaction. With the exception of water, fuel cells therefore cause no emissions and are also characterized by a high degree of efficiency of over 50 percent.
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