Foreword: Navigating the Future of Aviation with Hydrogen-Powered Sustainability:

Sustainability is at the forefront of technological advancements, particularly in aviation. As we grapple with the need to control CO2 emissions from transportation, a key focus emerges on the propulsion systems of the next generation of commercial aircraft.

We are living in a time when the concept of sustainability must be taken into account in all aspects of technology, and especially in aviation.

More and more people are becoming aware that the CO2 emissions produced by means of transport must be controlled. Even considering that the emissions per person per kilometers of an airplane are lower than those of an internal combustion car, it is also true that it is precisely on long journeys (which is where we use airplanes) that we concentrate most of the emissions of air transport users. If we compare the number of kilometers travelled by car with those travelled by plane, the latter is probably higher.

The aim of this article is to address a few key aspects related to propulsion in the context of advanced mobility systems, such as hydrogen-powered air vehicles. It explores some crucial issues such as hydrogen fuel cell technologies, hybrid hydrogen fuel cell/battery systems, energy density, hydrogen, and electricity storage, as well as the economic and technical challenges involved in the implementation of these technologies.

These aspects can help us understand how propulsion systems work and how they can be optimized for greater efficiency, performance, and sustainability in advanced mobility.  

Propulsion System Factors: Navigating Hydrogen-Powered Sustainability in Aviation:

In this sense, to understand the new generation of commercial aircraft, we need to consider as a key aspect, the type of engine and propulsion that are going to be used.

If we think about the new engine of the next generation of aircraft, we must consider sustainability as a fundamental aspect, considering sustainability in the sense of not contributing to the increase of greenhouse gases.

Here we have three ways of approaching the problem:

1.Synthetic fuels. There are projects to extract CO2 from the atmosphere, using clean energies, to manufacture synthetic fuels, thus achieving a net “0” impact on the atmosphere (we emit what we have previously absorbed).

2.100% Electric. If we go to a system of this type, the main line we have to consider two significant aspects that will condition the operating parameters: Photovoltaic generation and energy storage.

3.Generation:
The low energy density that we would achieve with photovoltaic energy limits the power of the aircraft in relation to the usable surface area to capture the energy. As we can see in the projects that have been carried out to date, this configuration barely provides us with the energy necessary to fly a very light aircraft (glider type), although with an enormous autonomy.

4.Storage:

Although batteries are evolving unstoppably, they are still far from achieving a level of energy density similar to existing fuels. In other words, each kg of battery has less energy than one kg of fuel. This is fundamental in an aircraft, as weight is one of the elements that conditions the entire flight envelope. It must be taken into account that when the battery is consumed, the weight does not decrease, which does occur after the fuel is consumed.

Hybrid-Hydrogen Battery:

This configuration allows us to solve part of the limitations of the 100% electric system, as both the performance and energy density of the system could be comparable to what we currently use today.

For the purposes of this article, we will focus on the characteristics, potential and challenges of this last configuration.

Principle of Hydrogen Fuel Cell Engines:

For years, hydrogen fuel cell engines have been considered a key technology in the search for more sustainable and cleaner mobility solutions, especially in electric vehicles, including AAMs. Let us give a basic description of their working principle:

• Hydrogen Generation: The process starts with the generation of hydrogen (H₂). Hydrogen can be obtained from various sources, such as electrolysis of water using renewable electricity or natural gas reforming with carbon capture.

• Fuel Cell: The fuel cell is the heart of the propulsion system. In this stage, hydrogen is combined with oxygen (from the air) in the presence of a catalyst in an electrochemical reaction. This produces electricity, water and heat as by-products.

• Electricity Generation: The electricity generated by the reaction in the fuel cell is used to power the batteries which in turn power the electric motors that drive the vehicle. This electricity is the primary energy source that powers the operation of a hydrogen fuel cell electric vehicle.

• Water vapor emission: The reaction between hydrogen and oxygen in the fuel cell produces water (H₂O) as a by-product. This means that the only exhaust product of a hydrogen fuel cell engine is water vapor, making it a pollutant-free propulsion technology.

• Advantages of Hydrogen Fuel Cell Engines

Let’s try to summarize the most positive aspects of this type of solution:

• Zero Emissions: The main advantage of hydrogen fuel cell engines is their zero emission of pollutants and greenhouse gases at the point of use. The only by-product is water vapor.
• Longer range: Compared to conventional batteries, hydrogen fuel cell systems offer a higher energy density, which allows them to store more energy and provide longer vehicle range, a crucial aspect for AAM.
• Fast Refueling: Hydrogen refueling can be faster than battery refueling, which is beneficial for the efficient operation of AAM vehicles, where downtime is sought to be reduced.
• Versatility of Use: Hydrogen fuel cell engines are suitable for a variety of applications, from light-duty vehicles to freight and AAM vehicles, making them versatile in the mobility field.

However, it is also important to mention that hydrogen fuel cell engines face challenges, such as sustainable hydrogen production and distribution, charging infrastructure and the cost of the technology. Despite these challenges, hydrogen fuel cell technology remains a promising area for moving towards cleaner and more sustainable mobility, especially in scenarios such as AAM.

Components of a Hydrogen Fuel Cell:

The key element of the system is the Hydrogen Fuel Cell, also known as a fuel cell. It is an electrochemical device that converts the chemical energy of hydrogen and oxygen into electricity and water, generating electrical energy without polluting emissions. Let’s see how it works:

• Anode: This is the part where the oxidation of hydrogen occurs. Hydrogen (H₂) enters the anode, and in the presence of a catalyst, it decomposes into hydrogen ions (protons) and electrons.

• Cathode: This is the part where the reduction of oxygen occurs. Oxygen (O₂) from the air enters the cathode, and electrons from the anode, through an external circuit, combine with the oxygen ions and protons to form water molecules (H₂O).

• Electrolyte: An ion-conducting material present between the anode and cathode. The electrolyte allows hydrogen ions (protons) to pass from the anode to the cathode, while blocking the passage of electrons, forcing them to flow through an external circuit, generating electricity.

• Catalyst: Used to speed up chemical reactions at the anode and cathode without being consumed in the process. Platinum is a catalyst commonly used in hydrogen fuel cells.

• Operation of the Hydrogen Fuel Cell:

The Hydrogen Fuel Cell: Empowering Sustainability in Aviation Propulsion:

It is based on an electrochemical (exothermic) reaction between hydrogen and oxygen to generate electricity and water as the only by-products. The sequence of the process is as follows:

• Hydrogen (H₂) is supplied to the anode of the hydrogen cell.

• At the anode, the hydrogen is decomposed into protons (hydrogen ions) and electrons through an ionization reaction: H₂→2H⁺+2e^– 

• Protons flow through the electrolyte to the cathode, while electrons flow through an external circuit, generating electricity that can be used to power an electric motor.

• At the cathode, the protons and electrons combine with oxygen (from the air) and generate water as a reaction product: O₂+4H⁺+4e^–2H₂O

• The electrical energy generated in the external circuit can be used to power the vehicle or any connected electrical device.

• Apart from the heat generated, the only by-product of this reaction is water, which is released as steam.

Perhaps we should focus a little on step 2, the splitting of the H2 atom into a pair of protons. The ionization of hydrogen at the anode requires a certain amount of energy to separate the electrons from the hydrogen atoms. This required energy is called “activation energy” and is provided by the heat supply or by the catalyst present at the anode.

In a fuel cell, the catalyst helps to reduce the activation energy needed for the oxidation reaction to occur at a reasonable rate. In the case of hydrogen fuel cells, a catalyst such as platinum is used to facilitate the separation of electrons and protons from hydrogen at the anode. This allows the ionization reaction to occur at a sufficiently fast rate to make the process practical.

Sustainability Unveiled: The Eco-Friendly Alchemy of Hydrogen Fuel Cells

Although the ionization reaction at the anode is not exothermic (as is the case at the cathode) the energy input required to initiate the reaction is recovered in the form of electrical energy generated in the external circuit as the electrons flow from the anode to the cathode.

It is important to note that, in a hydrogen fuel cell, the overall process is exothermic in net terms, as the generation of electrical energy at the cathode exceeds the energy required to separate the electrons at the anode. Overall, the electrochemical process in a hydrogen fuel cell is a balanced interaction between several reactions that work together to generate electricity efficiently and sustainably.

Sustainability Unveiled: Sustainable Efficiency in Hydrogen Fuel Cells:

The energy efficiency of a hydrogen fuel cell can vary depending on several factors, such as the specific design of the cell, the operating temperature, the quality of the materials and the cell management system. Generally speaking, however, hydrogen fuel cells are known to have a very high efficiency in converting the chemical energy of hydrogen into electricity.

In a typical hydrogen fuel cell, the energy yield can be in the range of 40% to 60%. This means that 40-60% of the energy contained in the hydrogen is converted into usable electricity, while the rest is lost as heat and other secondary processes. The proportion of heat loss can vary, but it is estimated that, in a well-designed hydrogen fuel cell, these heat losses can be in the range of 30% to 40%.

It is important to note that these values are approximate and are continuously varying as specific technology, design efficiency and operating conditions develop. However, despite thermal losses, hydrogen fuel cells are still very efficient compared to internal combustion engines burning fossil fuels, which can have much lower efficiencies.

In addition, one of the key advantages of hydrogen fuel cells is that the thermal energy generated can be recovered and reused. In applications such as hydrogen electric cars, this thermal energy can be used for cabin heating or other purposes, further improving the overall efficiency of the system.

It is important to consider that hydrogen fuel cell technology is still under research and development. So it is possible that future improvements in efficiency and further reductions in losses will be achieved.

(To be continued)