Due to customers‘ demand for greater safety, eco friendliness, performance and efficiency, manufacturers of mobile working machines, trucks and buses are increasingly interested in hydrogen as a fuel source. In hydrogen-powered vehicles, the gas in the H2 storage tank has to be regulated through a series of control valves before reaching the fuel cell. Pressure measuring instruments play an important role in this process. However, standard pressure sensors are not all suitable for hydrogen applications, due to the materials used. WIKA’s MH-3-HY is specifically designed for vehicles that use hydrogen fuel cells. Furthermore, it has EC79/2009 approval.
In principle, hydrogen-powered vehicles are much cleaner in operation than those with internal combustion engines, since only water vapour is produced as a by-product when hydrogen and oxygen are mixed. Zero emissions is a particular concern for vehicles used indoors – forklift trucks, scissor lifts, tow tractors, order pickers and transportation vehicles – as combustion engines release dangerous carbon monoxide, volatile organic compounds (VOCs) and other harmful byproducts into the air. Even if the vehicles are equipped with the latest emission control systems.
Battery-powered electric vehicles, on the other hand, are clean, but the main drawback is their limited range and, thus, the need for frequent recharging. Furthermore, each charging cycle takes several hours, which means that companies need to replace empty batteries e.g. of forklift trucks with charged spare batteries to keep their machines productive. This creates a high logistical cost, as a large number of such spare batteries are needed and their storage requires a huge amount of space. Hydrogen-powered vehicles, on the other hand, take only a few minutes to refuel, and are back into operation immediately. They therefore enable more efficient logistics with savings in storage capacity.
Countries in East Asia (especially Japan) and Western Europe are pioneers in hydrogen fuel-cell vehicles, with other countries also attaching increasing importance to the topic. Our on-demand webinar provides a deeper insight into this. Furthermore, this shows how WIKA can support you in overcoming the challenges posed by hydrogen applications. The focus here is on hydrogen mobility, including filling stations.
Hydrogen applications require special pressure sensors
The compressed gas in hydrogen fuel tanks is under high pressure: usually around 350 bar / 5,250 psi for commercial vehicles. Before the hydrogen reaches the fuel cell to be converted into energy, its stored pressure has to be reduced to the fuel cell working pressure. For greater control and more safety, this process usually takes two steps:
- A pressure reducing valve, located near the tank, lowers the pressure to below 20 bar / 300 psi before entering the system.
- A pressure regulator then further reduces the pressure to approximately 1 bar / 15 psi before entering the fuel cell injector.
In general, pressure sensors are fitted before and after the pressure reducing valve. These ensure that the system pressure stays within safe limits. If the system pressure is not regulated correctly, a pressure relief valve will vent the hydrogen to the atmosphere. If the system pressure becomes too low or the hydrogen tank is empty, the fuel cell won’t have enough pressure for operation and will eventually shut down the machine.
The pressure sensor must fulfil the requirements of the applications with respect to accuracy and robustness. In this way, H2 applications pose a particular challenge. Under certain conditions, hydrogen molecules dissociate on metallic surfaces. The hydrogen atoms can easily penetrate the crystal structure of many common metals, including stainless steel. The diffusion process accelerates at higher temperatures and pressures. This is called hydrogen permeation, and it can do two things in a pressure sensor:
- Signal drift: a change in the electrical resistance in its sensitive structures leads to a signal offset
- Hydrogen embrittlement: Metals become more susceptible to cracks and fractures, i.e. mechanical damage
To prevent hydrogen permeation, a pressure sensor can have a gold-plated flush diaphragm. However, this option is usually too expensive for most hydrogen fuel cell vehicles. A more cost-effective solution is to use a special alloy that is less susceptible to hydrogen permeation and immune against hydrogen embrittlement.
Robust pressure sensors for hydrogen-powered vehicles
The MH-3 from WIKA is already an OEM electronic pressure sensor that is suitable for use under extreme conditions. Based on this, we have developed a version specifically for hydrogen-powered vehicles. The MH-3-HY features the same accuracy, dependability and robust design of the MH-3, but with a welded metal thin-film sensor made of 2.4711 (Elgiloy®) and a cobalt-chromium-nickel-molybdenum alloy with high fatigue strength and excellent corrosion resistance. This pressure sensor has no polymer seals to the medium and no oil filling. This makes it ideal for hydrogen applications with a pressure range from 20 bar / 300 psi to 600 bar / 8,000 psi.
Due to its characteristics, the MH-3-HY has been given the EU approval EC79/2009 for hydrogen-powered vehicles. This makes it easier for OEMs to certify their entire hydrogen system. However, it should be noted that this approval will be revoked by the EU in July 2022. Existing EC79 certificates will remain valid, but no new certificates will then be issued on the basis of this directive. To stay up-to-date, you will find further information on this topic from July onwards in our blog.
Furthermore, the production of the MH-3-HY is carried out on a highly automated line, which enables us to supply our customers in large quantities with reliably high quality. In comparison, other manufactures use semi-automated or manual lines. We also 100 % check each sensor at the end of the line, and helium-test each for leaks.
Ultimately, more and more municipalities and companies are interested in reducing their carbon footprint without sacrificing performance and efficiency. One way to achieve this is through the use of hydrogen-powered buses and mobile working machines. WIKA’s broad portfolio of measurement products contributes to this move towards a cleaner future.
Alongside the MH-3-HY, we offer a wide selection of pressure transducers, temperature sensors, float switches and further accessories for hydrogen applications – from production, through distribution to the end use of hydrogen. Further information on the subject of hydrogen can be found on the WIKA website. If you have any questions, your contact will gladly help you.
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