As hydrogen (H₂) scales up to meet decarbonization targets, one critical infrastructure question arises: How ready is the supporting machinery for deployment in large-scale liquefaction plants? Understanding where a technology stands in terms of its maturity can be a complex matter, especially in emerging markets such as H₂. Thankfully, industry does not have to speculate. Technology readiness levels (TRLs) offer a structured way to evaluate how close a given technology is to full-scale deployment.

This article explores the importance of scaling liquefaction, outlines the role of turbomachinery and applies the TRL framework to assess machinery readiness for large-scale H₂ liquefaction.

Why liquefy H₂? Liquid H₂ is 800 times denser than in its gaseous form at atmospheric pressure. This increase in density enables significantly more H₂ to be transported in a single trip, making liquefaction a more effective option for large-scale or long-distance distribution. While compression is suitable for short distances and small volumes, liquid H₂ becomes more practical as delivery volumes grow. A single liquid H₂ tanker can carry as much H₂ as eight standard gaseous H₂ tube trailers, and it often requires less infrastructure at the refueling station, improving overall distribution efficiency.

H₂ liquefaction requires one or more refrigeration cycles to cool H₂ gas to below –253°C. Original equipment manufacturers (OEMs) such as Atlas Copco Gas and Process provide the critical machinery to enable liquefaction, including refrigeration compressors and cryogenic turboexpanders. H₂ liquefaction is energy-intensive, especially at small scales; therefore, a variety of process configurations have been created to improve the performance of large-scale liquefiers, including the use of different refrigerants and multiple refrigeration cycles.

The main objective of optimizing a liquefier design is to improve two key performance indicators:

1. Specific liquefaction cost (SLC) which includes capital, operation and maintenance costs
2. Specific energy consumption (SEC), measured in kWh/kg LH₂).

Larger-scale liquefiers can significantly improve both metrics, as described below.

              Lower cost (SLC).  One of the key benefits of scale is an improved cost basis, which is largely due to the turbomachinery’s ability to scale with process flow without requiring additional units. Turbomachinery size increases with the volumetric flow of the process and is often quantified by a manufacturer’s frame size (or wheel diameter). For example, scaling a turboexpander from 30 tpd to 60 tpd does not double the machine cost; instead, it results in only a marginal cost increase, primarily due to the need for slightly more raw material in the larger frame size.

              Higher efficiency (SEC). The second key benefit of scale is improved performance. Larger-scale liquefaction enables more liquid H₂ per kilowatt-hour, resulting in a significant reduction in the SEC of the liquefier. As the turbomachinery scales to higher flows, the secondary losses—like tip clearances and seal gas—have a proportionally lower impact on performance, enabling higher overall efficiency.

In small-scale liquefiers, speed is often the limiting factor for turbomachinery. As flow increases with larger-scale liquefiers, the machines can run at lower speeds and avoid mechanical speed limitations. Running at optimal speeds allows the turbomachinery to achieve peak aerodynamic efficiency. Increased machine size also enables advanced bearing configurations, such as expander-generator or expander-compressor setups, which enable power recovery and further reduce energy consumption. However, these performance gains can only be realized if liquefier capacities scale fast enough to meet growing H₂ demand.

The importance of scale. As H₂ infrastructure expands to support a growing market, every part of the value chain must scale, including liquefiers. Scaling liquefaction capacity is essential to enable efficient H₂ distribution in support of growing mobility applications. While production capacity has advanced significantly, H₂ liquefaction trains have plateaued around 30 tpd. This represents meaningful progress over earlier 5-tpd to 10-tpd systems, but it remains insufficient to meet the needs of a high-demand future.

The gap becomes clear when considering that a single 30-tpd liquefier can supply enough H₂ for only about 300 heavy-duty trucks per day (assuming 100 kg of H₂ per truck). According to the International Energy Agency’s Energy Technology Perspectives 2023 report, approximately 400,000 heavy-duty fuel-cell electric vehicle (FCEV) trucks are expected to be deployed globally by 2030, most of which will rely on liquid H₂ distribution. The challenge of scaling up to meet projected truck demand is illustrated in FIG. 1. Clearly, liquefier capacities must scale rapidly to 60 tpd, then 100 tpd, 200 tpd and beyond. As liquefier sizes grow, the machinery enabling this transformation must also scale. To determine whether turbomachinery can keep pace with this growth, we turn to a framework originally developed for space systems: TRLs.

 

FIG. 1. Fueling capacity of a 30-tpd H₂ liquefier (approximately 300 trucks/d) compared to projected global demand for heavy-duty FCEV trucks by 2030 (~400,000 trucks). At current capacities, and assuming trucks refuel every other day, meeting this demand would require more than 650 liquefiers.

Understanding TRLs. TRLs, developed by the National Aeronautics and Space Administration (NASA) in the 1970s, provides a standardized framework for assessing the maturity of emerging technologies. The scale ranges from 1 to 9, with TRL 1 being the least mature and TRL 9 being the most mature. A simplified version of the TRL scale is shown in FIG. 2.

FIG. 2. Simplified definitions of TRLs.  

TRLs offer a common language for evaluating technology status and managing risk. In short, TRL 1–3 reflects early-stage research and development (R&D), TRL 4–6 focuses on validation and demonstration, and TRL 7–9 indicates systems proven and deployed in operational environments. While useful, the framework has limitations, as it may not fully align with the nuances of every product or system environment and often requires interpretation. Still, TRLs remain a valuable tool for assessing turbomachinery readiness for H₂ liquefaction.

Turbomachinery readiness for liquefaction. Cryogenic turbomachinery developed for liquified natural gas (LNG) provides a strong foundation for H₂ liquefaction. In both applications, turboexpanders operate at extremely low temperatures and are directly influenced by the boiling point of the feed gas: –162°C for natural gas and –253°C for H₂. While H₂ requires significantly colder conditions, established LNG cycle designs, like the nitrogen reverse Brayton and mixed-refrigerant cycles, can be directly applied to the pre-cooling stage of H₂ liquefaction. These cycles are already operating at TRL 9 in LNG and therefore bring a high level of maturity to H₂ liquefaction.

Relevant experience also comes from petrochemical processes where turboexpanders used in ethylene and propane dehydrogenation (PDH) operate in cryogenic conditions with H₂-rich process streams reaching up to 96% H₂ by mole-fraction. These high-capacity systems demonstrate that turbomachinery can reliably handle industrial-scale H₂ flows, directly supporting the technical requirements of large-scale H₂ liquefaction.

Multiple turboexpander configurations are used across these industries. In ethylene production, expander–compressors with magnetic bearings are commonly applied to recover energy and reduce downstream compressor load. In PDH plants, where no recompression is required, expander–generator systems with integrally geared designs are used, as shown in FIG. 3. These generator-loaded configurations incorporate dry gas seals and high-speed pinions. This provides experience with hydrogen-rich operation for multiple machinery configurations, further advancing the TRL for the corresponding subcomponents in cryogenic hydrogen service.

FIG. 3. Integrally geared expander-generator for H₂-rich PDH service for the petrochemical industry.

Direct H₂ liquefaction experience adds another layer of readiness. Turbomachinery has already been deployed in commercial H₂ liquefiers up to 30 tpd, where oil-free operation is essential to ensure a reliable and long-lasting plant. Active magnetic bearings (AMBs) are the preferred solution in these medium- and large-scale liquefiers, eliminating the risk of oil contamination of the brazed aluminum heat exchanger. Expanders with AMBs have been in H₂-rich petrochemical service for > 30 yrs, demonstrating TRL 9 performance under relevant conditions. AMBs offer high load capacity, allowing larger turboexpanders to handle increasing process flows with a minimal number of machines. Their hermetic design ensures reliable, oil-free operation in cryogenic H₂ environments. An example of a magnetic-bearing turboexpander used in a medium-scale liquefier is shown in FIG. 4.

FIG. 4. Magnetic-bearing turboexpander for H₂ liquefaction service in medium-scale liquefier

Together, this diverse experience forms a clear technology readiness pathway. Proven designs from LNG and petrochemical applications can be confidently adapted for H₂ liquefaction, accelerating progress from current levels to truly commercial scale.

Reaching TRL 9: The final step. Where does turbomachinery for large-scale H₂ liquefaction fall on the TRL scale today? Based on decades of relevant experience, it is already in the final stages of maturity at TRL 8 and ready for full deployment. What is needed now is not further technical development, but commercial-scale adoption. Reaching TRL 9 represents more than technical validation. It signals commercial readiness and gives project developers the confidence to invest at scale, knowing that performance, reliability and safety have been proven in relevant environments.

Fortunately, the risk is low. Proven turbomachinery from LNG and petrochemical applications have already demonstrated compatibility with cryogenic H₂, including in systems up to 30 tpd. Scaling this to 60 tpd and beyond is not a leap of faith, it is simply an engineering progression.

Takeaway. Turbomachinery for H₂ liquefaction is not an experimental concept; it’s a proven technology, shaped by decades of cryogenic service. The challenge now is not technical feasibility, but commercial deployment at scale. As large-scale H₂  production becomes central to the energy transition, mature and scalable liquefaction machinery will be a cornerstone of success. As H₂ infrastructure accelerates, now is the time to deploy proven turbomachinery that will make large-scale liquefaction a commercial reality.