The defossilization of the economy to achieve climate neutrality is one of the biggest challenges mankind is facing in the next decades. This goal shall be achieved by transitioning to renewable energy generation and by coupling the sectors energy, industry, buildings, and mobility mostly via electricity. Due to the high volatility of renewable energy generation from wind or solar power, reliable and cost-effective storage and transport means for huge energy quantities are a vital prerequisite. Only when they are fulfilled can the goal of carbon neutrality can be achieved. Hydrogen offers exactly these required capabilities and can additionally also be used as a reaction partner in the process industry.
Compressors are the linking components in the H2 value chain, connecting the process steps from H2 generation to end-users. They play a central role for storing and transporting hydrogen. Even though the gravimetric energy density of H2 is excellent, its low density, the lowest of all gases, make compression, liquefaction, or binding it to other materials or elements necessary for sensible technical usage. For all these processes to increase the volumetric energy density of hydrogen, efficient compressors are required. To achieve a globally optimized solution for each case, an holistic integrated approach of adjusting and selecting the components along the value chain is paramount.
To successfully achieve this, it’s important to know the possibilities and requirements for the individual process steps in the value chain.
H2 Generation Methods
The still most widespread and cost-efficient method for H2 generation is the steam reforming of methane (SMR) from fossil natural gas. However, per ton of H2, around 10 ton of CO2, i.e., 300 g/kWh, are produced as well. By separating, storing, or utilizing (CCSU) the generated CO2, the carbon balance of this process can be significantly improved. H2 generated from fossil methane by SMR combined with CCSU is called Blue Hydrogen. Pyrolysis is another method for H2 generation. When fossil methane flows through a molten tin-filled bubble column reactor, Turquoise Hydrogen and carbon powder are formed.
Green hydrogen, which is the choice for the future, is mostly produced by using renewable electric power for water electrolysis. Currently, three methods have achieved market maturity, alkaline electrolysis (AEL), proton exchange membrane electrolysis (PEM), and high-temperature electrolysis (HTE) based on solid oxide electrolyzer cells (SO).
Storage of Hydrogen
Due to its low volumetric energy density, it does not make sense to store H2 under ambient conditions. In order to achieve an acceptable energy density, different basic methods can be used, including the binding to a carrier material like metal hydrid solid storage or organic carrier liquids (LOHC), the liquefaction (LH2) by cooling below the boiling point (-252°C) with a density of around 70 g/l, the pressurized gas storage (GCH), and the production of synthetic fuels by binding hydrogen to other atoms like carbon or nitrogen to store high energy quantities with high energy densities. All these methods have their individual characteristics and limits, but they have one thing in common: Hydrogen must be compressed by compressor plants to store it with a sufficient energy density.
Transport and Distribution
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 ton 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 150,000 cubic meters 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 use in mobility applications, for generating electricity and heat, or it can 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 is 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%.
Choosing the Best-Fitting Compression Technology
Even this short overview of the components of the H2 value chain shows that a detailed knowledge of its individual steps is necessary to come to optimized solutions. Additionally, it becomes obvious that compressors are of central importance, especially for storing and transporting H2. Therefore, choosing the best fitting compression technology for the individual use case, which is defined by the components left and right of the compressor in the value chain, is paramount. The low mol-weight of hydrogen makes compressors working by the displacement principle the solution of choice. They achieve isothermal efficiencies of more than 80%. When high-purity H2 without oil contamination is needed, water-injected screw compressors and especially non-lube piston or diaphragm compressors are the suitable answer. For these applications, water-injected screw compressors can go up to 15 bar, dry running crosshead-type piston compressors achieve more than 300 bar discharge pressure, and diaphragm and piston compressors with a hydraulic drive easily surpass 1,000 bar. For reasonable flow rates, the latter two depend on higher suction pressures. A diaphragm compressor can compress around 1,000 Nm3/h from 30-1,000 bar in three stages. A high-volume piston compressor on the other hand can compress more than 800,000 Nm3/h from 40-80 bar with a drive power of 22 MW while transporting 2.4 GW of power bound in the transported H2. When purity is not of central importance, oil-flooded screw compressors can be used up to 50 bar pressure and piston compressors with cylinder and packing lubrication can be used up to 1,000 bar.
Looking at the Individual demands of Use Cases
Already the rather simple case of an H2 trailer filling station shows how many different approaches and solutions exist. The task: In Germany, 300 tons of H2 shall be generated annually from renewable energy to supply a train and a bus refueling station. The station is less than 50 km away from the end-users. This offers several options.
The production of 300 tons of H2 via electrolysis requires around 16 GWh of electric energy. This would require either a 16-MW photovoltaic power plant or wind power of around 8 MW from typical onshore wind parks or 3 MW from a biogas fired combined heat and power plant (CHP). When relying solely on power from photovoltaics (PV), around 16 MW of electrolyzer capacity are needed. Due to the very volatile production pattern resulting, e.g., from cloud shadowing, PEM should be the technology of choice, as it offers the fastest load change capability. Additionally, a battery buffer might be helpful to flatten the load curve and to reduce the needed power. As PV produces electricity with direct current on which the electrolyzers are also running, the rectifier system, which is normally needed, is not necessary. With the rather constant power supply from a CHP unit, around 3 MW of cheaper alkaline electrolysis would be sufficient. The size of the electrolyzer determines the maximum hydrogen flow, which, in turn, determines the peak flow the compressor and the gas treatment unit must be able to handle.
Different Pressures cause different Systems
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 case study, 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 precompressor nullifies the investment cost advantage of a cheaper atmospheric electrolysis systems and add 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 1,000 kg and offers a significantly longer lifetime, but this comes with a higher investment price.
This little case study 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 more than eaten up 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.
Development Needs for Compressors and Outlook
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 type of 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 fuel 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 1,000 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 also allowing for condition based predictive maintenance concepts is another important development area. The one who masters the compression challenges and understands the process surroundings can provide significant added value to the clients.