In our previous blogs on hydrogen, we have mentioned the difference between gaseous and liquid hydrogen on several occasions. After being produced in gaseous form, hydrogen is often stored or transported in liquid form, after which it may or may not be re-gasified for use.
This blog focuses on the liquefier device that converts gaseous hydrogen into liquid hydrogen. We discuss what the liquefier is used for and look at three different techniques that allow hydrogen to be cooled to the point where it becomes liquid.
What is a liquefier for hydrogen?
A liquefier is a device that can be used to liquefy hydrogen gas. Hydrogen becomes liquid when it reaches the extremely low temperature of -252.9 °C at atmospheric pressure; the liquefier is a cooling system able to achieve this temperature under the right conditions.
Achieving cryogenic temperatures is not that easy. There is, however, active research into the best techniques for extreme cooling, and several well-performing liquefiers are available. Examples include the Pulse tube, the Stirling cryogenerator, the Joule- Thomson cooler, and the Gifford-McMahon (GM) cryocooler.
Highlighting three types of liquefiers
Three well-known liquefiers that are successfully used to liquefy hydrogen are the GM cryocooler, the Stirling cryogenerator, and the (Joule-Thomson) liquefier that liquefies hydrogen utilizing a medium that is even colder than the required -252.9 °C.
The GM cryocooler
The first liquefier to achieve extremely low temperatures using clever techniques is the Gifford-McMahon (GM) cryocooler. The GM cooler is an efficient cooling system that makes use of a so-called cooling cycle. By conducting hydrogen gas past the coldest part of the cryocooler, it becomes cold enough to reach the liquid form.
How does this work? The GM cryocooler uses a working medium, in this case, helium. The operation and cooling cycle of the system is explained through a schematic representation. The cycle includes four phases:
- During the first phase, the high-pressure side of the compressor is connected via a rotary valve to the “cold head,” which contains the displacer, the regenerator, and the heat exchangers. The cold heat exchanger is in contact with hydrogen, which condenses. The displacer moves to the far-left position, moving hot helium from the hot chamber through the regenerator to the cold chamber. The helium temporarily releases heat to the regenerator along the way and reaches the cold chamber with temperature Te.
- Then, during the second phase, the size of the cold chamber is maximized, and the cold head is connected to the low-pressure side of the compressor through the rotary valve. Some of the helium flows back to the hot side of the cold head. The helium expands isothermally, extracting heat from the hydrogen through the heat exchanger in the cold chamber.
- During the third phase, the displacer moves to the far-right position, and cold gas flows through the regenerator, where it reheats, into the warm chamber.
- Finally, during the fourth phase, the hot chamber is connected to the high-pressure side of the compressor through the rotary valve. The heat released from the isothermal compression is dissipated through the heat exchanger on the hot side of the cold head, after which the cycle repeats.
The GM cryocooler was developed as early as 1963 by Gifford-McMahon and has since been used in various configurations, mainly for small-scale systems such as MRI machines, cryopumps, and liquefaction of gases such as hydrogen.
A schematic representation of the operation of a GM cryocooler.
The Stirling cryogenerator
A second liquefier that is very suitable for liquefying hydrogen is the Stirling cryogenerator. This cooler, too, reaches sufficiently low temperatures that will turn hydrogen gas into its liquid form when passed through the cooler.
The Stirling cryogenerator is based on a very old method called the Stirling Cycle, which was developed as early as 1816 by Robert Stirling. Similar to the GM cryocooler, the cryogenerator can cool virtually any gas or liquid to extremely low temperatures (below -253 °C).
How this works. The cryogenerator is a closed system that uses helium gas to cool another medium or substance, such as hydrogen. Cooling is achieved by alternately compressing and expanding the helium gas. Compression occurs at room temperature to allow for heat dissipation, while expansion occurs at (and provides) the required low temperature.
The gas is passed through the system using a displacer, passing various heat exchangers and a regenerator. This smart system can efficiently reach extreme cold without emitting toxic gases, thus making this method very environmentally friendly.
Like the Gifford-McMahon (GM) cryocooler, the Stirling cryogenerator is primarily used for small-scale systems. For large-scale systems, most commonly, Joule-Thomson systems are used, as we describe in the next section of this blog.
A dated but yet fascinating video from Philips Cryogenics about the development and technology of the cryogenerator.
Do you want to know more about how the Stirling cryogenerator and the GM cryocooler work? This article, published in Cold Facts, details the similarities and differences between the two methods.
Liquefying using an even colder material
Finally, hydrogen can be liquefied by cooling it with an even colder material than liquid hydrogen: liquid helium. Liquid helium has a temperature of no less than -269 °C (at atmospheric pressure) and is therefore considerably colder than liquid hydrogen at -252.9 °C. In this type of system, helium is liquefied using the Joule-Thomson method.
An excellent example of a project where this method was applied is the European Spallation Source (ESS) study. A few years ago, the ESS researched the best design of a liquefier for hydrogen, using liquid helium as a coolant. The liquid hydrogen would eventually be used as a coolant during dispersion experiments.
How does this liquefier work? The design of the ESS consists of two separate parts, one for liquid helium and one for hydrogen. The helium cooler is linked with relatively long transfer lines to a turbine expansion system connected to the cold box for hydrogen. This is then attached to a moderator reflector plug, also with the necessary transfer lines.
Proposed flowchart for cryogenic hydrogen circulation and the helium cooler within the ESS project.
Source: Physics Procedia (2015)
Helium is passed through a circuit and adequately processed to reach the desired temperature to cool the hydrogen. The gas passes through various heat exchangers and is heated and cooled under the required pressure to finally achieve and maintain the required temperature.
Because of the radiation safety measures involved in this project, the locations and distances between the helium cooler and the turbine expansion system under the hydrogen cold box are quite long. The components of the system are connected by long transfer lines; this is why
Demaco’s expertise plays an important role here.
Demaco is an expert in designing and manufacturing transfer lines for cryogenic liquids and, therefore, proved the perfect partner to be involved in this project. In projects such as the ESS project, we brainstorm with our clients on complex issues, including those related to the liquefaction of the hydrogen. No challenge is too big for us, and we are proud of our expertise across various hydrogen projects.
Would you like to know more about the ESS project? In this article in Physics Procedia, Klaus and colleagues go into great detail about the advanced liquefier.
Would you like to know more?
Do you wish to learn more about our work with liquid hydrogen? Then take a look at this page or our recent blogs on boil-off gas and hydrogen pipelines. In these articles, you can read all about the characteristics of liquid hydrogen and Demaco’s involvement in developing the best cryogenic infrastructures.