Ahead the world, behind only steam
Most hydrogen cars used a fuel cell. We explain here how a fuel cell is built and exactly how it works.
Driving with fuel cells
Fuel cell systems are energy convertors. Unlike the combustion engine, which is based on thermodynamic principles, converting heat into kinetic energy or movement, fuel cells convert the chemical energy in hydrogen directly into electric energy which can be used to drive an electric motor.
All fuel cell vehicles are electric vehicles – and are highly efficient. Their energy loss is significantly lower than that of conventional engines and today they achieve an efficiency rate twice that of vehicles with combustion engines. A fuel cell vehicle that runs on hydrogen is also entirely emission free, because the only products of the controlled reaction between hydrogen and oxygen are electricity, waste heat and water.
The principle of the fuel cell
A fuel cell is built like a sandwich.
The external shell is made up of two metal bi-polar plates, which serve as the anode and the cathode. Fine channels run through them to distribute the gaseous oxygen and hydrogen equally in the cell.
Between them is a proton exchange membrane or PEM. It is impermeable for the oxygen, but permeable for the hydrogen protons. On each side of this membrane is a catalyst layer made of platinum and carbon. On top of this, again on both sides, comes a gas diffusion layer (GDL), which ensures the equal distribution of the hydrogen and oxygen on the surface of the cell. The fine water channels in the bi-polar plates are for cooling purposes. Hydrogen and the ambient air provide the cells with the ingredients for a continuous reaction which supplies the vehicle with energy. At the anode, hydrogen is introduced, while compressed air enters at the cathode, to supply oxygen. In the presence of the platinum and carbon catalyst layer, the hydrogen oxidises, producing hydrogen protons and electrons. The hydrogen protons travel through the membrane and react with the oxygen to produce water, while the negatively charged electrons move from the anode to the cathode, generating direct current. This reaction takes place at a temperature of about 80 °C. This is why we sometimes speak of a low-temperature fuel cell or “cold combustion”.
Components of the fuel cell powertrain
The fuel cell powertrain is a modular assembly. Various configuration concepts can thus be realised in the vehicle, with the spatial design of different vehicle models being taken into consideration. The fuel cell powertrain comprises the following key components:
- Fuel cell stack
- System module
- Hydrogen tanks
- Electric motor.
The fuel cell stack
The fuel cell stack is the heart of the powertrain. It is an extremely efficient convertor of energy, which generates the power needed to drive the electric motor. Since an individual fuel cell is only about two millimetres thick and generates less than one volt, several hundred cells of this sort are stacked. In every individual fuel cell hydrogen reacts with oxygen in the ambient air, generating electricity, which then drives the electric motor. The total power thus generated, of over 200 V, can power a vehicle.
The fuel cell system module
The components surrounding the fuel cell together make up the fuel cell system. It regulates the conditions in which the fuel cell operates. It ensures that enough hydrogen and oxygen are available for the pertinent driving conditions, and that these are supplied at the appropriate pressure.
With the help of the system, excess heat is also dissipated via the cooling system and water vapour is removed.
The hydrogen tanks
In the hydrogen tanks, gaseous hydrogen is stored at a pressure of up to 700 bar. Multi-layer tanks are generally used to store the compressed hydrogen. They are made of corrosion-resistant metal or composite inner layer and an external stabilising layer made of fibreglass, carbon fibres or a combination of the two. Several layers of the fibres are wound around the inner shell and secured with resins. The tank can be filled at a hydrogen pump – a process not significantly different from filling a conventional vehicle at a petrol pump.
Almost all fuel cell vehicles produced to date have a battery, which both supports the entire powertrain and improves the overall efficiency by recovering the energy lost during braking. The most modern vehicles use lithium-ion batteries, which are key to the consistent electrification of the car.
The lithium-ion battery is used to store the electrical energy from the fuel cell system and the recovered kinetic energy. It also helps the car accelerate. The lithium-ion battery offers the advantage of being compact as well as being significantly more efficient than conventional batteries. Charging time is lower and the batteries have a long service life as well as being accident-proof. They can be recycled and operate absolutely reliably irrespective of climatic conditions.
Series production that would bring down costs is a prerequisite for a wider use of these batteries in the automobile sector.
The electric motor
The fundamental properties of an electric motor – a high degree of efficiency and full torque at zero speed – have a positive impact on the vehicle dynamics and the overall degree of efficiency of a fuel cell vehicle, making traditional gears superfluous. The electric motor is powered by the stack of fuel cells and the battery.back