Research

With interest in hydrogen globally rising, safe, environmentally friendly and economically competitive storage of hydrogen becomes more important. One such storage method is hydrogen in liquefied form (LH₂) at -253^oC. Its inherent safety, scalability and zero emissions at point-of-use make it especially attractive for large-scale and long-term storage and transport. A major challenge for LH₂ is boil-off. This phenomenon occurs as the inevitable heat ingress from ambient environment into a LH₂ tank causes the liquefied hydrogen to evaporate.

Although boil-off can be minimized by employing advanced insulation materials and schemes, cost considerations prevent BOR minimization to the point where boil-off would be negligible. With state-of-the-art insulated tanks, the daily boil-off rate (BOR) is 0.5-1.0% of the tank’s volumetric content for medium-sized applications (20-100 m²). This corresponds with a heat leak (and therefore a required cooling power) on the order of ~100 W at 20 K. Given these high losses and the high cost of liquefaction (~30% of hydrogen energy content), there is a strong motivation for developing and implementing active thermal transfer technology to achieve a state of zero boil-off (ZBO).

Apart from being efficient and cost-effective, the advantage of such ZBO cooling systems compared to other boil-off management solutions is that it provides the ZBO LH₂ tank as a flexible, stand-alone unit, applicable across different storage and transport scenarios. Alternatively, compression (and consequent storage) of boil-off gas or the usage of boil-off gas in a fuel cell requires scenario-dependent system integration.

Courtesy of: Radebaugh, Ray. "Review of refrigeration methods." Handbook of Superconductivity. CRC Press, 2022. 501-518. 

An efficient and cost-effective ZBO system for LH₂ storage has several requirements that relates to the cooler itself and the architecture to bring this cooling power to the application (the LH₂ tank). Typically, there are two candidate cooling cycles for cooling powers on the order of ~10² W at 20 K. For higher power liquefier systems (10³-10⁴ W) typically the Turbo Reverse-Brayton (TRB) cycle is employed. Scaling this technology down, however, to accommodate the envisioned ZBO LH₂ systems, causes the turbines to be less efficient and capital cost to relatively high. On the other hand, commercially available GM cryocoolers are currently limited to 50-100 W of cooling power and second law efficiencies of 9-11%. These are sufficient for the first demonstrator ZBO system. Going to higher cooling powers of several hundreds of watts necessitates rethinking the choice of cooling cycle, as scaling up GM cryocoolers requires prohibitively expensive regenerators. Now, ZBO LH₂ systems of these capacities have been developed for space applications, yet these solutions are technologically complex and expensive and require full-on integration with the LH₂ tank. For emerging LH₂ technologies in industrial applications efficient, compact, serviceable, modular, scalable and economically competitive ZBO systems are required. It is therefore that the Applied Thermal Sciences (ATS) research group aims to fill this gap by developing an innovative ZBO system as a sub task within the HyTROS project, a consortium initiated by GroenvermogenNL.

In aiding this research, the ATS research group focuses on developing a supporting structure:

• Development of LH₂ testing facilities to support the development and testing of a hydrogen boil-off recondenser.
• Expansion of network by sharing & collaboration with other research and industry partners.
• Development of techno-socio-economic frame of implementation for the researched LH₂ technologies.

Harro Beens

For the last one hundred years, humanity has soared through the skies. First, this was done using piston engines. While these were quite inefficient, they got the job done. Not long afterwards, turbojet and turbofan engines reshaped air travel, and with every iteration, aircraft efficiency improved significantly while reducing carbon emissions. Unfortunately, kerosene-based aircraft have the downside that they will never reach carbon neutrality. Now it is time again to reimagine the aerospace sector through the introduction of hydrogen as a fuel to move toward carbon-neutral flight.

A downside is that the volumetric energy density of gaseous hydrogen (GH2) (5 MJ/L at 700 bara) is quite low compared to kerosene (34 MJ/L at 5 bara). One solution is to use liquid hydrogen (LH2) (8.5 MJ/L at 5 bara). Using LH2 allows fuel tank sizes to be reduced significantly, but it also introduces a new challenge: the fuel infrastructure must be substantially redesigned for LH2 operations.

One of the primary challenges is that hydrogen must be maintained at very low temperatures, near 20 K. At these temperatures, nearly all compounds except helium and hydrogen exist in a solid state. This means much of the infrastructure used in kerosene fuel systems cannot be reused, as it relies on lubricants, gaskets, or other components that deteriorate at such low temperatures. Another challenge is managing heat inleak in hydrogen fuel systems, as this causes evaporation of the liquid. While turbines or fuel cells require LH2 to be converted into GH2, this phase change can be undesirable in certain locations, such as fuel tanks or pumps. Nevertheless, much of the infrastructure including fuel tanks, piping, pumps, compressors, and sensors must be designed to withstand these conditions.

As LH2 moves from a tank toward the engine through piping in an aircraft, any heat inleak or pressure drop can cause the subcooled or saturated liquid to transition to a multiphase state. In other words, some of the LH2 evaporates into GH2, resulting in what is called multiphase flow.

Little is known about the fluidic and thermal behavior of multiphase hydrogen flowing through tubing, which is essential for designing safe and efficient hydrogen-powered aircraft. In addition, reliable fueling systems require mass flow meters to measure the amount of fuel being transferred from the tank to the engines at any given time. Unfortunately, the nature of two-phase flow makes traditional flow measurement methods such as Coriolis meters, anemometers, or turbine meters ineffective.

The COOL Pipe project focuses on closing existing knowledge gaps through the following objectives:

·         Study the thermal and fluid dynamics of multiphase hydrogen within fuel systems.

·         Develop predictive models for multiphase flow in fuel piping systems.

·         Design mass and void fraction sensors for use in multiphase hydrogen.

The consortium brings together complementary expertise. The University of Twente contributes extensive experience in cryogenics, while TU Delft provides expertise in aerospace engineering.

On the infrastructure side, Rotterdam The Hague Innovation Airport (RTHIA) provides a hydrogen testing environment, and the Dutch metrology institute (VSL) supports sensor calibration efforts.

Industry partners including Airbus, TNO, and Collins Aerospace contribute practical knowledge and real-world insights.

Wouter Eppink

https://hollandhightech.nl/nieuws-agenda/nieuws/mijlpaal-in-cryogene-koeling

Het team van Prof. Vanapalli van de Universiteit Twente en de engineers van MEC Stirling Technology uit Doetinchem hebben eind 2024 een belangrijke mijlpaal bereikt op het gebied van Cryogene Koeling.

De midsize cryogene koeler waar Prof. Vanapalli en de mensen van MEC Stirling Technology gezamenlijk sinds eind 2023 aan werken, bereikte bij 80 Kelvin een stabiele koelcapaciteit van 45 watt bij een kleine 10% of Carnot. Dit laatste is een verdubbeling van de efficiency van de nu beschikbare GM/Pulse Tube koeling systemen.

Succesvol
De Cryogene koeler van MEC is gebaseerd op de vrije zuiger Stirling motor van MEC. MEC  was de eerste onderneming in de wereld die succesvol vrije zuiger Stirling technologie wist te industrialiseren. Meer dan 20.000 stuks van de vrije zuiger Stirling motoren zijn de afgelopen 15 jaar geproduceerd. Deze hebben niet alleen hun weg gevonden naar on- en offgrid toepassingen voor industrieel en particulier gebruik, wereldwijd hebben ook vele researchorganisaties voor hun onderzoek en projecten een beroep gedaan op de technologie van MEC.

Commercialisatie
Op deze manier en langs deze weg is een aantal jaren geleden het contact tussen MEC en de Universiteit van Twente tot stand gekomen. Een contact dat nu geresulteerd heeft in een concreet, onderscheidend product. In de loop van het komend jaar gaat de commercialisatie starten en zullen eerste medische en industriële applicaties op basis van de MEC-koeler gedemonstreerd kunnen worden. Een groot voordeel is natuurlijk dat MEC zijn cryogene koeler vrijwel op dezelfde manier kan bouwen als de MEC Stirling Engine. Ook een groot deel van de reliability data van de MEC  Engine is ook in koeling mode relevant.

S. Khute, A. Kovacs

This project aims to develop a cryogenic energy converter based on the Stirling cycle that efficiently recovers cold energy from decentralised midsize and small-scale regasification systems.  For the Netherlands alone, this represents for liquefied Nitrogen 1.6 PJ of (input) energy which gets lost during expansion. If successful, it is expected at least 20-40 % hereof could be converted into renewable electricity. The initiative will focus on high-tech fundamental research and technology development related to oscillating flow, heat transfer, and electromagnetic behaviour under cryogenic conditions. By integrating academic and industrial expertise, the project seeks to produce significant advancements in cryogenic energy systems, contributing to sustainable energy technologies.

Adam Kovacs

Dynamic thermal and flow control at cryogenic temperatures is increasingly important for quantum systems, cryo-microscopy, aerospace instruments, and hydrogen architectures. ATS has developed a high-performance research line on piezo-actuated cryogenic heat switches and cryogenic valves, combining mechanical design, flexure guidance, and precise cryogenic measurement.

Piezoelectric-Actuated Mechanical Heat Switch (4–120 K)

A recent ATS design demonstrates:

• ON conductance: 8×10³ – 1×10⁴ W/m²·K
• OFF conductance: < 55 W/m²·K
• Switching ratio > 145
• Zero power consumption in ON state (disc-spring preload)
• No thermal drift, stable low-temperature behaviour
• Compact, low-vibration architecture

This outperforms traditional gas-gap and superconducting devices, making it suitable for space-limited and power-sensitive systems.

Cryogenic Valves (Piezo-Driven)

Using similar principles — preloaded mechanics and piezo flexure actuation — ATS is developing compact valves for:

• Liquid hydrogen and multiphase hydrogen flow
• Low-leakage operation at 4–80 K
• Rapid, precise flow control

Together, these technologies form a crucial building block for next-generation cryogenic architectures.

The dynamics of sublimating solid carbon dioxide

Dry ice is of fundamental importance in a wide variety of technical applications ranging from cleaning surfaces of very large telescopes, spray cooling, refrigeration, to biological processes like cryopreservation. The triple point temperature and pressure of dry ice are above normal ambient temperature and pressure, causing only the solid and gaseous phase to exist at atmospheric conditions, and the solid phase to continuously sublimate. Since dry ice is in permanently sublimating state at standard ambient condition, it exhibits interesting behaviour involving coupled heat, mass and momentum transport phenomena. The aim of this PhD project is to understand physical mechanisms of sublimation involving two phase flows. In particular, the problem is divided into two categories, namely, (i) dry ice sublimation in free ambient with varying conditions, and, (ii) dry ice sublimation when it is in contact with a substrate of varying thermal properties. A brief description of the research activities undertaken in the PhD project is summarised below.

Saturation temperature of solid carbon dioxide for varying boundary conditions

A common misconception about the basic understanding of dry ice temperature exists in industries as well as academia. The dry ice sublimation temperature of -78.5 °C / 194.65 K is widely reported in literature and used in industries. This value is only conditionally true, i.e. in an environment of saturated CO₂ vapor in equilibrium with dry ice at 1 bar pressure. But in real applications dry ice utilised in an unsaturated environment which results in dry ice temperature lower than -78.5 °C because of CO₂ diffusion in the unsaturated environment. In order to correct this widespread misconception, a well-controlled experimental setup was devised to accurately estimate the influence of different environments with varying pressure and CO₂ concentration on dry ice temperature.  Figure 1 shows a dry ice sphere suspended on a thermocouple in a controlled ambient. Based on the measurements, it is shown that dry ice temperature can drop below -98 °C / 174 K in normal atmosphere condition. This deviation of 20 K from the value normally reported in literature is of significance for modelling different thermal processes involving dry ice and to gain deeper insights into the sublimation process.

Figure 1: A dry ice sphere suspended on a thermocouple in a controlled environment (left); A backlight image of a dry ice sphere captured during an experiment (right)

Leidenfrost dynamics of sublimating solid carbon dioxide

When placed on a hot surface, dry ice that sublimates at atmospheric conditions, hovers on a cushion of its own vapor in the Leidenfrost state, see schematic in Figure 2 . Discovered in 1756, the Leidenfrost is mostly studied for liquids in contact with solid and liquid substrates. In this work, we investigated this phenomenon experimentally and theoretically for dry ice pellets on a hot sapphire substrate. While studying Leidenfrost effect, a key parameter for estimating the vapor pressure, heat transfer rate and the flow profile in the vapor layer is its thickness. A direct measurement of vapor layer thickness below dry ice pellet, and a systematic comparison with theoretical model is missing in the literature. For the first time, the Optical Coherence Tomography technique is adapted to measure vapor layer thickness and gain insights into the Leidenfrost phenomenon at a high temporal and spatial scale. Time evolution of vapor layer thickness, vapor layer profile and effects of key parameter like substrate temperature on the vapor layer thickness is measured and compared with the results of the phenological model based on lubrication theory.

Figure 2: Schematic considered for the formulation of theoretical model for Leidenfrost

Dry ice sublimation in an insulation box

Dry ice sublimation inside a polystyrene foam package is experimentally and numerically investigated considering macro scale conductive and radiative heat transfer processes. Because of the large latent heat of sublimation of dry ice (and small Stefan number), the front of dry ice inside the box moves very slowly. Based on this assumption, a three dimensional quasi-steady model is developed to predict variation of dry ice mass inside the insulation packages. The model uses an iterative approach in which the dry ice level is changed in every subsequent step until the dry ice interface reaches the bottom surface of the insulation package, see fig. The model is validated against mass and temperature measurements performed on two types of insulation packages made of different material and geometry. The model is able to fairly predict the reduction in dry ice mass over time and the end-of-sublimation time. Both experimental and numerical results show that the sublimation rate of dry ice can be reduced by covering the inner walls of insulation package with a reflective layer like aluminized mylar foil. The good agreement between model and experimental data of dry ice mass variation makes it possible for application engineers to use the modeling approach in estimating sublimation rates of dry ice inside a insulation package.

Figure 3: Front view cut section of the EPS box is shown at different instance of time. Q represent the heat transfer rate that goes into the EPS box. In the schematic, dry ice domain is represented by shaded area. The colors white and grey represent expanded polystyrene and carbon dioxide gas, respectively

Physics of absorption and evaporation of cryogens in a porous medium

The aim of this PhD project is to understand the absorption and evaporation of cryogenic liquids into porous materials. The absorption into porous materials is important for so-called Dry Shippers. A Dry Shipper is a container used to transport biomedical samples at a temperature below ‑150 °C by use of liquid nitrogen. The liquid nitrogen is absorbed into a porous lining inside the container to remove the hazard of spillage and comply with safety regulations. Although, the physics of absorption and evaporation of cryogens into these superheated porous materials and its channels is of great importance, until now this is not very well understood. Other application areas are heat pipe design, and propellant management systems for space applications.

The absorption of liquids into a porous material without evaporation is often explained using the Lucas-Washburn model, considering viscosity, surface tension, and gravity. In this model the porous medium is modeled as a bundle of capillary channels with governing  parameters the viscosity, density, capillary diameter, tortuosity, and wettability of the medium. In addition, for porous material at superheated temperatures compared to the saturation temperature of the liquid evaporation will significantly impact the imbibition process.

Physical scales

In this research the physics is studied on three scales; the macro-, meso-, and micro-scale. The macro-scale consists of the Dry Shipper autonomy. In the meso-scale we will study absorption and evaporation of liquids into the porous material. On a smaller scale the porous material consists of connected pores and channels, therefore as a model problem the imbibition of liquids into capillary channels will be investigated.

This work is performed in collaboration with Air Liquide Research.

Watch the video for a short introduction to this topic.

Tissue snap freezer for molecular medicine

Biopsies in daily clinical practice are often obtained with the aid of a biopsy (hollow) needle, core biopsies. After obtaining the biopsy, it should be preserved to keep the properties of the tissue as similar as possible to the in-patient situation. Ongoing biochemical reactions and decay are unwanted processes which will alter the biopsy properties. Most common fixation methods of tissue are: formalin (= formaldehyde solution), alcohols like methanol or ethanol and freezing.

Formalin fixation is routinely used. The protein crosslinking properties are ideal for structure preservation, after imbedding in paraffin the biopsies can be cut in thin slices and the tissue can be studied. But the crosslinking of the proteins makes this method less well suited when one wants to study the loose proteins themselves. Also the quality of RNA and DNA is detrimental effected by this fixative.

Freezing the tissue: By freezing the tissue the proteins are preserved well, and also the RNA and DNA can be isolated in good quality. So this is the preferred method for e.g. mass spectrometry analysis  of proteins.

In collaboration with the group of Prof. Henk Verheul, VUmc we are working developing a tissue snap-freezer with the following requirements: Quickly freezes a sample, does not need dry ice or liquid nitrogen, can be used in a surgery or biopsy room, can be transported easily and is easy to use.

The developed snapfreezer. Details can be found here: Cooling of a vial in a snapfreezing device without using sacrificial cryogens’, by Michiel van Limbeek, Sahil Jagga, Harry Holland, Koen Ledeboer, Marcel ter Brake en Srinivas Vanapalli. Scientific Reports 9: 3510.

doi:10.1038/s41598-019-40115-6

Project Title: CryoOn- Cryogenics meets Oncology; A novel cryogenic device to snap-freeze and transport biopsies
Researchers: Steven van Lohuizen, Michiel van Limbeek and Sahil Jagga
Sponsor: NWO-TTW

Sample preparation for electron microscopy

As a partner in a research consortium, the group of Prof. Dr. Stefan Raunser, director at the Max Planck Institute (MPI) of Molecular Physiology in Dortmund, receives considerable BMBF funding. Together with the group of Srinivas Vanapalli from the  University of Twente, the Dutch companies Delmic, Demcon kryoz  and CryoVac from Troisdorf, the Dortmund scientists want to develop innovative methods and improved workflows for the investigation of biomedical specimens using cryo-electron tomography. The project is funded with 2 million euros by Eurostars, a joint funding programme of the European Research Initiative EUREKA and the European Commission.

Proteins regulate all important biological processes, they are responsible for the functioning of our metabolism and for our perception of the environment. The protein haemoglobin for example carries oxygen in the blood. Muscles consist of the proteins myosin and actin. Even the smallest change in the composition of these proteins can disturb their cellular function by changing their interactions or stopping them from being in the right place at the right time - often with serious disease related consequences.

Cryo-electron microscopy (Cryo-EM), awarded the Nobel Prize in 2017, makes it possible to unveil the three-dimensional structure of proteins in almost atomic resolution and thus to investigate disease-relevant structural changes. The MPI in Dortmund is one of the leading research institutions in this field. However, the Cryo-EM field seeks to use the great potential of cryo-EM also for the investigation of large biological preparations like whole cells and tissue samples. The challenge is to visualize proteins in their functional natural environment.

One possibility is offered by a variant of the method: In cryo-electron tomography, cells or tissues are frozen at high speed. As a result, the water contained in the cells solidifies into a vitreous state which prevents the formation of ice crystals that would destroy the cell structures. The frozen preparations are then transferred to the EM, where they are examined at approx. -190°C. This workflow has to be performed as contamination-free as possible and without interrupting the cold chain. Similar to computer tomography in medical diagnostics, two-dimensional images of the sample are taken by tilting it relative to the electron beam. The series of images is than converted into a three-dimensional image. 

However, due to the elaborate preparation and processing of the samples the method is associated with such a high technical effort that it is only accessible for a few laboratories in the world. The consortium, which has many years of experience in the fields of cryogenic technologies and vacuum systems, intends to use the funds to simplify the complex work processes involved in cryo tomography and thereby also to enable the examination of thicker samples like tissue sections. This includes the development of improved methods for the contamination-free transfer of samples into the microscope and the archiving of frozen specimens. With the introduction of the new workflow CETFlow (Cryogenic Electron Tomography WorkFlow), cryo-electron tomography is to be made accessible to a larger group of users, so that the technology can also be applied for specific medical questions.