Technology: Hydrogen

Hydrogen production

Hydrogen is the most often occurring element in the universe. However, it is always bound in various organic compounds. Hydrogen is very energy-rich, even more than petrol, and is therefore interesting to be used as an energy source. [1]

Via electrolysis, electric, thermic, solar or chemical energy is used to separate chemical bonds such as H₂O or CH₄ in order to produce pure hydrogen. The spent energy is transformed and stored in the hydrogen as chemical energy. This process can be performed completely climate-neutral by using renewable energies. The produced hydrogen is so-called green hydrogen, as opposed to blue or grey hydrogen, which results from electrolysis producing carbon emissions as a side product. [1]

Another climate-neutral way to produce hydrogen is photocatalytic water splitting. Via light energy, water can be splitted with the help of a semiconductor, and H₂ forms. However, due to the very low efficiency of around 2%, this technology is not profitable and most likely won’t be for the next few years. If technological advancements can be achieved, increasing the efficiency to more than 10%, photocatalytic water splitting is considered to have an enormous potential for the world’s energy supply in the long run, since it does not require a preceding energy production. [2]

Hydrogen storage

Once hydrogen is produced, it needs to be stored until being used as an energy source. Due to its low density, storing hydrogen as gas takes a lot of space, even when using high pressure to compress it. Furthermore, hydrogen is highly flammable when getting in contact with oxygen and therefore comparably dangerous. [1]

Hydrogen can generally be stored as gas, liquid, or in chemical bonds. For storing hydrogen in its liquid form it must be as cold as -253°C, requiring an elaborate cooling chain and super insulated double-walled tanks in the vehicles. [3] However, even with those measures, a considerable amount of hydrogen evaporates. Hence, for passenger cars and commercial vehicles, gas storage is the most common option. Even though the energy density is lower this way, storing hydrogen under a pressure of 350 to 700 bar is the most cost-saving possibility for rather small amounts of hydrogen. [4]

Other options are the storage in metal hybrides or Liquid Organic Hydrogen Carriers, which is particularly valuable for large quantities of hydrogen. Such Liquid Organic Hydrogen Carriers is for example certain oil which the hydrogen binds to. The liquid is not easily flammable and can be easily transported over long distances. Before being used, the hydrogen must finally be isolated under the use of energy. [5]

Engine types

Hydrogen combustion engine

Hydrogen combustion engines are similar to petrol combustion engines in their functionality and structure. A fuel is burnt at high temperatures, transforming chemical energy  into thermal energy, which then converts into kinetic energy. Converting a petrol fueled engine into a hydrogen fueled engine is relatively simple, since around 80% of the components remain unchanged. [6] Instead of petrol, hydrogen is stored in the tank and from there injected into the engine. The tank therefore needs to be adjusted using tear-resistant materials which withstand the high pressure of gaseous hydrogen and are impermeable for hydrogen molecules at the same time. [7] The performance of hydrogen combustion engines is lower compared to petrol combustion engines because gaseous hydrogen has a relatively low energy density. [8] Interesting about hydrogen combustion engines is that its relative efficiency increases with increasing load and is furthermore a reliable and durable technology. It is therefore an efficient solution for heavy duty vehicles, while for passenger cars hydrogen fuel cells are the more efficient option. [9]

Hydrogen fuel cell

In order to generate electrical energy from the chemical energy of hydrogen in combination with oxygen, there is the hydrogen fuel cell. A fuel cell consists of two electrodes (anode and cathode), which are connected via an electric current, closing an external circuit. Between the electrodes, there is an electrolyte allowing electrons of fuel (H₂) and oxidant (O₂), which are injected,  to move freely between the two sides of the cell. Hydrogen is injected from the tanks, while oxygen is simply contained in the air and let into the fuel cell. At the anode, an oxidation reaction generates ions and electrons. While the ions move to the cathode via the electrolyte, electrons flow to the cathode through the electric current of the external circuit, resulting in electrical energy. The electrical energy is then directed to a small battery, which in turn powers the vehicle. Additionally, ions, electrons and oxygen react at the cathode, forming water, which leaves the fuel cell immediately. Apart from emerging water, hydrogen can be burned cleanly. [10]

Advantages and disadvantages

One advantage of hydrogen powered vehicles is the fueling process, which is comparable to those with combustion engines. Within 3 minutes, the tank is filled with hydrogen. Additionally, the reach of hydrogen powered vehicles is another advantage: With 500 to 800 km, it is higher than the reach of current electric vehicles. [11] Also, hydrogen exists in large quantities and - in the case of the fuel cell - can be used completely emission-free, if the electrolysis is conducted with renewable energies. Hydrogen combustion engines emit nitrogen oxide due to the high temperature at which hydrogen is burnt. [8] [12] In general, one must be careful in evaluating the environmental impact of hydrogen technologies, because it highly depends on the way hydrogen is produced. This ranges from a completely emission-free process (green hydrogen) to a highly environmentally harmful production of grey hydrogen. [13]

When looking at the efficiency of hydrogen as an energy provider, it is comparably inefficient. While electric vehicles have an efficiency of 64% [14], hydrogen fuel cells as well as hydrogen combustion engines can only translate 25-35% of the energy that is initially stored in the hydrogen compounds. [15] The process of isolating hydrogen from its natural compounds does not only involve an energy loss, but also doubles or even triples the price for hydrogen compared to natural gas. [13] On top of these costs, the acquisition cost of hydrogen-fueled cars is expensive nowadays. [11] Another disadvantage of hydrogen as fuel today is the infrastructure: there are only about 100 hydrogen fuelling stations in Germany today, although this number is steadily increasing. [11]

See Also

Sources

[1] Bdew. Flexible Herstellung: Wie wird Wasserstoff erzeugt? Retrieved: December 13, 2021 from https://www.bdew.de/energie/wasserstoff/flexible-herstellung-was-ist-wasserstoff-und-wie-wird-er-erzeugt/

[2] Dechema (2014). Herstellung von Wasserstoff - Photokatalyse. Retrieved from https://dechema.de/dechema_media/Downloads/Presse/Photokatalyse-called_by-dechema-original_page-125316-original_site-dechema_eV-view_image-1.pdf

[3] ADAC e.V. (2021). Wasserstoffautos: Technik, Modelle, Tests, Tankstellen. Retrieved from https://www.adac.de/verkehr/tanken-kraftstoff-antrieb/alternative-antriebe/wasserstoffauto-so-funktioniert-es/

[4] Emcel (2019). Welche Möglichkeiten der Wasserstoffspeicherung gibt es? Retrieved: December 13, 2021 from https://emcel.com/de/wasserstoffspeicherung/

[5] Süddeutsche (2019). So lässt sich Wasserstoff speichern. Retrieved: December 13, 2021 from https://www.sueddeutsche.de/bayern/brennstoffzelle-wasserstoff-speicherung-erneuerbare-energien-lohc-1.4562738

[6] Stahl, T. (2021). MAN baut Diesel auf Wasserstoff um: Ist das die Alternative zur Brennstoffzelle? Retrieved: December 13, 2021 from https://efahrer.chip.de/news/man-baut-diesel-auf-wasserstoff-um-ist-das-die-alternative-zur-brennstoffzelle_105197

[7] Energieagentur NRW. Brennstoffzelle & Wasserstoff. Retrieved: December 13, 2021 from https://www.energieagentur.nrw/brennstoffzelle/700_bar_wasserstofftanks_fuer_pkw

[8] Green-Motors. Wasserstoffantrieb. Retrieved: December 13, 2021 from https://www.green-motors.de/alternative-antriebe/wasserstoffantrieb

[9] Heid, B., Martens, C., & Orthofer, A. (2021). How hydrogen combustion engines can contribute to zero emissions. Retrieved from https://www.mckinsey.com/industries/automotive-and-assembly/our-insights/how-hydrogen-combustion-engines-can-contribute-to-zero-emissions

[10] Nice, K. & Strickland, J. How Fuel Cells Work. Retrieved: December 13,2021 from https://auto.howstuffworks.com/fuel-efficiency/alternative-fuels/fuel-cell.htm#pt2

[11] Lehne, S. (2019). Brennstoffzelle.

[12] Johnson, W. (2019). Here's why a hydrogen combustion engine will never be popular. Retrieved: December 13, 2021 from https://www.motorbiscuit.com/heres-why-a-hydrogen-combustion-engine-will-never-be-popular/

[13] Quaschning, V. (2021). Wasserstoff: Lösung oder Irrweg für die Klimakrise? Retrieved: December 13, 2021 from https://www.youtube.com/watch?v=icxlPGcTlX8

[14] Bundesministerium für Umwelt, Naturschutz und nukleare Sicherheit. (2021). Effizienz und Kosten: Lohnt sich Der Betrieb eines Elektroautos?. https://www.bmu.de/themen/luft-laerm-mobilitaet/verkehr/elektromobilitaet/effizienz-und-kosten

[15] Volkswagen AG (2019). Hydrogen or battery? A clear case, until further notice.