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 available in almost unlimited quantities, but 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.
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 either transported by pipeline or tanker. The use of pipelines is especially suitable for supplying gaseous hydrogen to major customers 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 tests the everyday use of various technologies for making hydrogen available at filling stations. Depending on the type of vehicle, it is possible to refuel them with gaseous hydrogen at pressures of 350 bar and 700 bar, with car manufacturers and several gas and plant manufacturers agreeing on the SAE TIR J2601 global standard for refuelling cars.
In their work with the CEP, the industry partners are jointly defining technical standards. For instance, there is a worldwide standard for filling ports, as well as for the entire fuelling process.
In the case of high-speed 700 bar fuelling, the hydrogen must be cooled to a temperature of between -33° C and -40° C. If the hydrogen is stored in liquid form, an evaporator is used. For gaseous-form storage, a refrigerating machine is used.
REFUELLING & PAYMENT
Drivers of hydrogen-powered vehicles will hardly notice a change while refuelling, 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. When refuelling with compressed gaseous hydrogen at a pressure of 700 bar an infrared interface on filling port sends data about temperature, pressure and the amount of hydrogen from the vehicle to the service station and vice versa.
A consistent payment and card system has been set up at the CEP filling stations.
As hydrogen has such a low density, huge tanks would be needed to store relevant quantities under normal atmospheric pressure and ambient temperature. Therefore, hydrogen is compressed or liquefied prior to storing it. Changing it from an uncompressed gas to a liquid state reduces its volume by 99.9%. For hydrogen refuelling with gaseous storage there are three different pressure gas levels: low-pressure storage, medium-pressure storage and high-pressure storage.
The high, round gas tanks (‘cigars’) used for low-pressure storage tanks can accommodate up to 200 kg of gaseous hydrogen at 45 bar.
Medium-pressure storage is often in the form of groups of cylinders - standard bottles for pressures of between 200 and 500 bar.
High-pressure stores are custom-built for pressure levels up to 1,000 bar. Most of these pressure storage tanks consist of several layers: an inner shell made of metal or composite materials for maximum corrosion resistance (usually alloys made with aluminium, stainless steel or plastic); and an outer layer to ensure stability (durability under high pressure), which consists of multiple layers of fibreglass, cabon fibres or a combination of both, stuck together with resins. Some filling stations also use liquid hydrogen storage in liquid hydrogen tanks.
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.