27. April 2021
Even though they have been a topic of discussion since the 1970s, fuel cell vehicles still seem to be more an object of research than an everyday reality. The next stage in the development of the “on-board power generation” principle, as initiated by the internal-combustion engine, appears to be efficient and economical. But what exactly is behind it?
Technology of the future? Fuel cell-powered aircraft. Photo: ©iStock/audioundwerbung
Clean, quiet and not reliant on oil
The battery electric engine always takes the limelight in discussions on which alternative fuel source might replace the internal-combustion engine as the new sustainable standard for cars. In battery electric vehicles, the battery functions as a storage medium for the energy used for propulsion. The fuel cell, however, makes it possible to generate that energy on board the vehicle, which means the driver is no longer reliant on the volume of the battery to achieve long ranges. Battery and fuel cell vehicles have a number of things in common: they are clean, quiet and not reliant on oil. But while electric vehicles can take hours to charge when the battery is empty, fuel cells can be quickly refuelled at the pump the way we’re all familiar with – only using hydrogen rather than petrol.
How it works
“Cold combustion” is based on chemical reactions. Inside the cell, a continuous stream of fuel – hydrogen – reacts with the oxidising agent, oxygen. These are called polymer electrolyte fuel cells (PEFC). In this process, electrons are taken from the hydrogen on the anode, and the protons get to the oxygen chamber via a semipermeable exchange membrane. The electrons are redirected via the power circuit and flow to the cathode, where the oxygen absorbs them – producing heat, energy and water.
Because a single fuel cell only produces around 1 volt, a stack of hundreds of cells is needed to supply the required power. The energy generated is either converted immediately into movement or temporarily stored in a traction battery, giving fuel cell vehicles ranges of between 500 and 800 kilometres. Unlike in traditional electric cars, the range does not depend on the size of the battery, but on the volume of the tank and the corresponding number of cells.
Emissions savings subject to fluctuations
Because the process of hydrogen production is very energy intensive, the emissions savings potential is largely dependent on the method used in that process. Employed in around 90% of cases, the most common process by far is steam reforming, which uses fossil fuels. However, just as much CO2 is emitted in that process as when burning the fuel, so the emissions savings are practically zero.
The situation is quite different with water electrolysis processes such as in the above-mentioned PEFC – as long as the electricity used is from a renewable source. Until now, however, this kind of climate-friendly process has only ever been used when cheap energy is available – so the costs determine resource efficiency.
Moreover, a large part of the expended energy is lost at two points in the electrolysis process: only 50% of the power required for the electrolysis itself can be retrieved afterwards, and a further 25% disappears when the hydrogen is converted into electricity, leaving only a quarter of the energy invested. Even though researchers at the Massachusetts Institute of Technology (MIT) have developed a catalyst they claim increases the efficiency of electrolysis to almost 100%, the energy loss is, as of now, a relevant limiting factor.
Virtually pollution-free locally
When the vehicle is in operation, the fuel cell is virtually pollution-free and achieves an extremely high level of efficiency. It is also an advantage that – in addition to the electricity – thermal energy is also produced, which can, with the appropriate technology, be used. The engine is also hardwearing and low-maintenance.
However, the operation of fuel cells involves highly complex technical specifications, which is reflected in the cost and reduces the technology’s competitiveness. Hydrogen can only be competitive if it becomes cheaper to produce, transport and use than comparable fossil fuels. When it comes to the environmentally friendly electrolysis method, the cost difference compared to internal-combustion engines must be reduced, and that will only happen if the latter become more expensive. In addition, consumers must understand that a functioning hydrogen economy will lead to price rises across all forms of motorised transport. Ultimately, the question remains whether it wouldn’t make more sense to use the green energy ideally used in electrolysis to charge battery electric vehicles instead.
Pilot projects mainly in passenger transport
Although manufacturers such as Honda, Hyundai, Mercedes-Benz, Renault and Toyota are already producing hydrogen-powered vehicles, fuel cell technology has so far mainly been employed in passenger transport. Researchers at the German Aerospace Center (DLR) and the University of Ulm have developed a PEFC aircraft with a range of 2,000 kilometres and around 80 passenger seats. It should be ready for standard use in 2030.
Fuel cell technology is already being employed in public transport in individual cases. Between 2012 and 2020, around 150 hydrogen buses were put into service in Europe, 15 of those in Cologne and 10 in Wuppertal, where the electricity for the electrolysis is produced from the waste of the local population. Hydrogen-powered trains are already running in Lower Saxony and Bremen, and trials are scheduled in the Tübingen/Sigmaringen region from summer 2021. Deutsche Bahn is planning to test Siemens fuel cell trains in Baden-Württemberg from 2022 onwards.
In combination with environmentally friendly methods of hydrogen production, fuel cell technology can represent an alternative to traditional battery electricity vehicles – with certain caveats. To be competitive and sustainable in the private sector, the costs will have to be realistic. Experts agree that that will only happen if the prices of other forms of power are adjusted appropriately. To date, economic viability appears to be better in the passenger transportation sector. Only time will tell whether mobility by electrolysis can become established on our roads. But for now, it still seems that the future of mobility lies in the charging station and not in the fuel pump.