Hydrogen is the lightest and most abundant element in the universe. It is a colourless, odourless and tasteless gas, and with the atomic weight of 1 is the first element on our periodic table. Hydrogen consists of one proton and one electron and naturally occurs as diatomic molecules (H2).
Hydrogen is found on Earth only as part of chemical compounds, e.g. as water (H2O), in a variety of hydrocarbons (oil, gas, coal, biomass, etc.), and in other organic compounds. However, it can be released using energy, thereby itself becoming an energy store – a source of energy.
Most of the hydrogen produced today occurs as a by-product in the chemicals industry, and is then consumed by other processes in the same industry, especially in petrochemicals. At present, the industrial-scale production of hydrogen mainly involves reforming natural gas. First, a synthesis gas (hydrogen, carbon monoxide, carbon dioxide, water vapour and residual hydrocarbons) is produced. Carbon monoxide can be broken down into hydrogen and carbon dioxide via a conversion reaction with water. Hydrogen is separated from the gas mixture by absorption, adsorption or by using membranes.
Water electrolysis makes zero-emissions production possible if the electricity needed for electrolysis is produced from renewable energy sources. In water electrolysis, water (H2O) is mixed with a liquid, which improves the ion transport. Electrical energy is used to split the water into its components, hydrogen (H2) and oxygen (O). The hydrogen migrates to the negatively charged pole, while the oxygen migrates to the positive pole. The electrical energy used is converted into chemical energy and stored in the hydrogen. The principle can be used conversely in a fuel cell – the energy stored in the hydrogen is converted back into zero-emissions electrical energy. The CEP has successfully tested on-site production of hydrogen by electrolysis since the launch of the project. Action!
HYDROGEN FROM BIOMASS
In combination with renewable primary energies, the gasification of biomass is also a good option. Biomass, broadly defined, includes not only residues from agriculture or forestry and organic waste from households, but also organic industrial waste. The CEP operates a pilot plant in Leuna (Saxony-Anhalt) where hydrogen is generated from crude glycerine. Glycerine is a by-product of biodiesel production from vegetable oils. The hydrogen is produced using a ‘pyroreforming’ process in which desalinated crude glycerol is broken up under high pressure and at temperatures of several hundred degrees Celsius. This produces hydrogen-rich gas, which is then purified and liquefied. This process already offers a potential 50 percent reduction in greenhouse gases compared to conventional hydrogen production from natural gas.
If hydrogen is not produced right where it is used, it needs to be transported to its destination. Depending on the existing infrastructure and the volume to be transported, hydrogen is transported either by pipeline or by tanker truck. The use of pipelines is especially suitable for supplying gaseous hydrogen to major consumers, for whom the economic cost of a pipeline is worthwhile. Smaller volumes of compressed gaseous hydrogen (200-300 bar) or liquid hydrogen (LH2) at -253° C are transported by tank truck. An LH2 trailer can carry about 3,500 kg of hydrogen. In the CEP, hydrogen is produced either directly at the filling station, or is delivered by tank truck as a compressed gas or as liquid hydrogen.
The CEP is testing the everyday use of various technologies for making hydrogen available at filling stations. Depending on the type of vehicle, it is possible to refuel with gaseous hydrogen at pressures of 350 bar and 700 bar. Car manufacturers and several gas and systems manufacturers have agreed on a global standard for refuelling passenger cars, known as SAE TIR J2601.
The industry partners jointly define technical standards in the CEP. For instance, there is a worldwide standard for the fuelling coupling, as well as for the entire fuelling process. Hydrogen has to be pre-cooled before refuelling. In the case of high-speed 700 bar fuelling, it has to be brought to a temperature of between -33° C and -40° C.
REFUELLING & PAYMENT
Drivers of hydrogen-powered vehicles will hardly have to make any changes to their refuelling habits, as the technology is very similar to that used in conventional refuelling. You connect the filling port to the filler neck in the usual place on the car and start the refuelling process by pressing a button. It takes about three to five minutes to fill up. During the refuelling process with 700 bar of compressed hydrogen, data on temperature and pressure can be transmitted from the vehicle to the filling station via an infrared interface in the fuelling coupling. This transmission of data ensures that the vehicle is filled safely and completely and can use its full range. To make it easier for customers to pay, the CEP has set up a uniform payment and card system.
LIQUID HYDROGEN TANKS
Liquid hydrogen tanks are available in sizes of 1 and 5 tons of hydrogen content. The containers are insulated. However, as warming cannot be avoided, approximately 0.5 percent of the cryogenic liquid hydrogen (-253°C) evaporates per day (boil-off loss). This hydrogen can also be used for refuelling or energy.
Vehicles with a fuel cell electric drive have chemical energy on board in the form of gaseous hydrogen in hydrogen tanks. This is converted into electrical energy in an electrochemical process and continuously transferred to the downstream electric motor. As the central energy converter, the fuel cell also takes on the function of the alternator and supplies electricity for all the electronics and other consumers in the vehicle.
The PEM fuel cell used in the vehicles consists of two electrodes, which are separated from each other by a proton-conducting membrane (polymer electrolyte membrane or proton exchange membrane), which is coated with a platinum catalyst on both sides. These layers form the electrodes of the fuel cell. To convert the hydrogen and ambient oxygen into water, the proton-conducting membrane must be moistened. During the process the anode must be continuously fed hydrogen, while the cathode constantly supplied with oxygen from the air supply. The reaction of oxygen and hydrogen to produce water occurs as two partial reactions: Anode: 2 H2 → 4 H+ + 4e- Cathode: O2 + 4e- + 4H+ → 2 H2O Overall reaction: 2 H2 + O2 → 2 H2O From single cells, cell stacks are created where the cells are placed one on top of the other as in a sandwich. The current is proportional to the electrode surface and can be regulated by increasing or reducing the electrode surface. The production of electrical energy in the fuel cell is completely emissions-free - only heat and steam are released, so a fuel cell vehicle is a zero-emission vehicle (ZEV).
FUEL CELL HYBRID BUSSES
Unlike diesel hybrid buses, hydrogen-powered fuel cell hybrid buses do not emit any harmful pollutants such as carbon dioxide (CO2). Only climate-neutral water vapour is emitted. Noise emissions are also greatly reduced. The latest generation of fuel cell hybrid buses has advanced fuel cell systems with significantly lower hydrogen consumption and longer useful lives. While driving, the bus emits no pollutants and is virtually silent. In particular, the energy recovered during braking (recuperation) significantly contributes to the vehicle’s economy.
VEHICLE FUEL TANKS
Modern vehicle fuel tanks for storing hydrogen gas are ‘composite material bottles’, which have a plastic core and with carbon fibre wound around it. They enable pressures of 700 bar. The tanks are typically designed with safety factors of about 2 relative to the operating pressure. In accidents involving hydrogen vehicles no damage to the tanks has been observed to date.
It is widely believed that hydrogen diffuses through materials and does not remain in the tank. Although hydrogen molecules are very small, hydrogen has been transported and stored in steel cylinders without problem at pressures of 200 bar and more. In metal containers, diffusion is not a problem in practice, as the process occurs much too slowly. In the vehicle tanks described above, the diffusion rate is generally higher, but also negligible in practice. Otherwise, these tank systems would not be allowed. So parking the vehicles in underground garages, tunnels or other enclosed spaces is not a problem.