The defossilization of the economy to achieve climate neutrality is one of the greatest challenges humanity faces. This goal is to be achieved by transitioning to renewable energy generation and by coupling the energy, industry, building and mobility sectors, especially via electricity. Due to the high volatility of renewable energy generation from wind or solar energy, reliable and cost-effective storage and transportation options for large amounts of energy are an essential prerequisite. Only if these are fulfilled the goal of carbon neutrality can be achieved. Hydrogen meets these requirements and can also be used as a reactant or reaction partner in the process industry.
Compressors connect the individual components and process steps of the H2 value chain from production to the end user. They play a central role in the storage and transportation of hydrogen. Although the gravimetric energy density of H2 is excellent, its low density, the lowest of all gases, makes compression, liquefaction or binding to other materials or elements necessary for useful technical utilization. Efficient compressors are required for all these processes to increase the volumetric energy density of hydrogen. To achieve a globally optimized solution for the respective case, a holistic, integrated approach to the coordination and selection of components across the value chain is of key importance.
In addition to our centuries of experience in the development, construction and maintenance of hydrogen compressors, our solution portfolio also includes hydrogen generation systems such as PEM electrolyzers and small-scale steam reformers.
Typical Applications for Our Compressors Are:
"Green hydrogen" can be produced from renewable energy sources such as solar, wind, hydro, biomass, geothermal or tidal power plants. If H2 is produced from nuclear energy, it is referred to as "red H2". Hydrogen can also be produced from fossil fuels such as natural gas. All these sources have individual characteristics in terms of volatility, investment costs and achievable full load hours per year.
The most cost-effective and therefore most widespread method of H2 production is the steam reforming of methane (SMR) from fossil natural gas. However, this produces around 10 tons of CO2 per ton of H2, i.e. 300 g/kWh. The carbon footprint of this process can be significantly improved by capturing, storing or using (CCSU) the CO2 produced. The hydrogen produced from fossil methane by SMR in combination with CCSU is referred to as "blue hydrogen". Pyrolysis is another method of H2 production. When fossil methane flows through a molten tin filled bubble column reactor, "turquoise hydrogen" and carbon powder are formed.
Green hydrogen is usually produced by using renewable electricity for water electrolysis. Currently, three processes have reached market maturity: alkaline electrolysis (AEL), proton exchange membrane electrolysis (PEM) and high-temperature electrolysis (HTE) based on solid oxide electrolyzer (SO) cells. A future option on the way to market maturity is anion exchange membrane technology (AEM), which is basically a mixture of AEL and PEM. Research is also being carried out in the field of biotechnology, e.g. photolysis or H2 based on algae.
Due to its low volumetric energy density, it is not useful to store H2 under environmental conditions. To achieve an acceptable energy density, the following basic methods can be used:
All these processes have their individual characteristics and limitations, but they have one thing in common: hydrogen must be compressed by compressor systems to store it with sufficient energy density.
To transport the H2 to the end user or to a storage site several methods can be used. Mobile pressure storage systems like trailers and containers can, depending on the pressure, store several kilograms to around 1.5 tons of H2. When using a freight train as a „rolling pipeline“, around 60 tons of H2, or speaking in energy terms, 2 GWh can be transported at a time. A truck trailer for LH2 contains 3-4 tons and a big LH2 tanker can transport 150.000m3 of LH2 which corresponds to 10.000 tons of hydrogen. Pipelines offer the possibility to transfer very high amounts of energy of up to 30 GW per pipeline and additionally also form a significant storage volume.
The energy stored in hydrogen can be reconverted for using it in mobility applications, for generating electricity and heat or it can also be used as a reaction partner for industrial processes. If fuel cells are used for the electrification around 50-60% of the lower heating value of H2 are converted into electric energy and the remaining energy is emitted as heat. Fuel cells require H2 of highest purities, this requirement can be neglected when using it as a combustible fuel for turbines or engines. The drawback of using it in this form is the lower efficiency of around 30-40%.
Even this short overview of the components of the H2 value chain shows that detailed knowledge of the individual steps is necessary to come to optimal solutions. Additionally, it becomes obvious that compressors are of central importance, especially for the storage and transportation of H2. Therefore, the selection of the best fitting compression technology for the particular application, which is defined by the components to the left and right of the compressor in the value chain, is paramount. Due to the low molecular weight of hydrogen, compressors that work according to the positive displacement principle are the right solution. Depending on the process configuration, they achieve isothermal efficiencies of 70 to more than 80%. If high-purity H2 without oil contamination is required, our non-lube piston or diaphragm compressors are suitable. For these applications, dry-running crosshead piston compressors achieve a discharge pressure of up to 500 bar and diaphragm and piston compressors with hydraulic drive achieve more than 1000 bar. The latter require higher suction pressures for reasonable delivery rates.
Our NEA|HOFER diaphragm compressor can compress more than 1000 Nm3/h from 30 to 1000 bar in three stages. A large-volume piston compressor, on the other hand, can compress more than 800,000 Nm3/h from 40 to 80 bar with a drive power of 22 MW and transport 2.4 GW of H2 bound in the transported H2. If purity is not of central importance, oil-flooded screw compressors can be used up to a pressure of 50 bar and piston compressors with cylinder and packing lubrication up to 1000 bar. All these compressor systems are available with our self-developed KO3 compressor system design software.
The case of an H2 trailer filling station shows how many different approaches and solutions there are. The challenge: In Germany, 300 tons of H2 are to be produced annually from renewable energy to supply a train and bus refuelling station. The station is less than 50 km away from the end users. This offers several options.
The production of 300 tons of H2 requires an installed electrolysis capacity of around 3 MW. In theory, it would be possible to supply the installed 3 MW electrolysis capacity with pure PV or wind energy. Due to the fluctuating energy production of these generation options, a combined energy supply from wind and PV is economically viable. For example, an installed wind power capacity of 13.5 MW in combination with an installed PV generation capacity of 3 MW leads to an economically viable plant utilization. On the other hand, an installed wind power capacity of 7.5 MW in combination with an installed PV generation capacity of 10.5 MW, for example, could also lead to a similarly useful plant utilization. The optimal system configuration is highly dependent on the location and can be determined individually, e.g. as part of our consulting services. PEM technology is best suited due to the highly volatile generation pattern caused by cloud shading, for example. It offers the fastest load change capability. In addition, a battery buffer could be helpful to flatten the load curve and reduce the required installed renewable energy generation. The size of the electrolyzer determines the maximum hydrogen flow rate, which in turn determines the peak flow rate that the compressor and gas treatment plant must be able to handle.
Typical trailers used to supply hydrogen refueling stations have maximum filling pressures of around 300 to 500 bar. Depending on the used electrolyzer the suction pressure for the compressors ranges from a few millibar to around 50 bar. In the given use case, a system with atmospheric discharge pressure from the electrolysis shall be compared to a system offering 30 bar on the hydrogen side, for the trailer 500 bar are selected as the filling pressure. At 30 bar suction pressure a diaphragm compressor can handle flow and compress to more than 500 bar in two stages. At atmospheric pressure the low stroke volume of a diaphragm compressor makes precompression a must. A four-stage piston compressor is needed to achieve 30 bar pressure. This pre-compressor nullifies the investment cost advantage of a cheaper atmospheric electrolysis system and adds more complexity through four more stages and having to mix two compression methods. The efficiency of the mechanical compression is very much in line with the efficiency of the electrochemical compression in the pressurized electrolyzer. Furthermore, the choice of the outlet pressure and the method of electrolysis has significant effects on the selection of the gas drying and deoxo units. Between the electrolyzer and the compressor a buffer vessel to decouple the systems should be used. This vessel must be bigger for atmospheric systems. This shows the significant impact the choice of power source and the outlet pressure of the electrolyzer have on the selection, design and complexity of the compressor and gas treatment systems.
For the transport of the hydrogen to the refueling stations the type of chosen vessel plays the determining role. Trailers with conventional steel tubes or bottles operating at 300 bar can transport around 500 kg of H2 and often are limited in the number of full load cycles. A 40-foot gas container (MEGC-type) with 380 bar pressure can transport a useful quantity of around 1000 kg and offers a significantly longer lifetime, but this must be paid with a higher investment price.
This use case emphasizes the importance of knowing the characteristics and the interdependence of the components along the H2 value chain. Cost advantages which seem to be achievable when using low pressure electrolysis are eliminated by the investment and operating costs of a then required much more complex compressor system. Here, providers of integrated solutions which also have a good eye on servicing the systems can generate a significantly higher customer benefit and thus achieve a competitive advantage.
The compressor technology for feeding hydrogen into pipelines and salt caverns is already available. The main challenge in these applications lies in the achievement of the required purities depending on the end user. Maintaining the highest purity continuously over the pipeline and cavern transport and storage systems currently seems to be hardly achievable.
With the ongoing conversion of mobility especially in heavy-duty applications like trucks, buses and trains the demand for high-purity H2 will significantly increase. This calls compressor systems for refueling stations and trailer filling being able to compress much higher flows to around 500 bar while maintaining the purity demands of fuel cells. The development push must focus on dry running piston compressors delivering more than 1000 kg/h to this high-pressure level. For providers of integrated solutions, the digital connection of the components for a better communication of the systems with each other and allowing for condition based predictive maintenance concepts is another important development area. Combining a strong compression technology with other elements from the H2 value chain while having an expert aftermarket organization with a good local footprint and a digital remote system monitoring concept is the key for an optimum end user experience. The one who masters the compression challenges and understands the process surroundings can provide significant added value to the clients.
