Customer: Universal Hydrogen
Liquid hydrogen capsule built by NK Labs with Universal Hydrogen’s ATR 72 experimental aircraft in Toulouse (Image courtesy of Universal Hydrogen)
Universal Hydrogen is on a mission to make hydrogen-powered commercial flight a reality in the near future. They’re creating a technology that can push aviation towards an efficient, carbon-free, true zero-emissions future. They are accomplishing this through solving several key problems - the distribution of hydrogen in a usable form, and building a hydrogen powertrain that can be used to convert a conventional airframe. Their first product is a conversion kit for regional planes - the ATR 72 and De Havilland Canada Dash-8. The converted planes will fly commercial routes proving the efficient, practical use of hydrogen in the company’s modular capsules.
Universal Hydrogen hired NK Labs to build a trade study examining viable options for the modular hydrogen storage in aircraft. We set out to identify, explore, and select the key options and technologies to develop in order to best realize Universal Hydrogen’s vision.
Concept of modular hydrogen storage capsules for usage on a regional plane (Image courtesy of Universal Hydrogen)
In this trade study, NK Labs explored a number of areas that we felt were necessary to pick the best path forward. We identified and parameterized such areas as:
Existing hydrogen storage solutions - such as tanks used in industrial & scientific storage, hydrogen-powered automobiles, and spacecraft.
Phases of hydrogen - this includes compressed gas, cryogenic liquid, cryo-compressed fluid, and chemically stored hydrogen.
Container shapes - compressed gas and liquid solutions necessitate storage tanks with a certain strength, but these tanks also need to be lightweight, and efficiently packed - what are the most efficient shapes to consider?
Container materials - High pressure solutions require materials with extreme strength. Cryogenic solutions require extreme insulation. Hydrogen is both reactive and a risk for leakage. Hydrogen is even known to diffuse into certain metals over time and cause dangerous embrittlement.
Examples of shapes considered for hydrogen storage within various aircraft fuselage
Different technologies were then ranked according to competing evaluation criteria that the team had identified for the trade study. Each solution considered both the stored hydrogen in its appropriate container as well as the equipment necessary to safely carry and use the hydrogen as fuel onboard an aircraft. These attributes included:
Technology Readiness Level
Following the trade study, NK Labs performed studies to further explore technologies that would be used in an aircraft using a modular hydrogen fuel system. We built and studied a scaled-down cryogenic tank section to evaluate the practical use of high vacuum, multilayer insulation, and a prototype plumbing, instrumentation, and control system designed for the extraction of gas from a liquid cryogen capsule.
Test setup for high vacuum cryogenic insulation, under construction
Liquid Capsule Design
NK Labs was tasked with designing and building an ultra-lightweight, high-dormancy experimental liquid hydrogen capsule, following up on the results of our trade study and exploratory testing.
As part of the design process, a refined optimization study was performed, using Matlab, to drive the major design parameters of the storage tank. This study performed sweeps over parameters of size, shape, service pressure, tank and insulation thickness, in order to achieve the best possible mass fraction for ~50 kg of hydrogen that could be stored for over 90 hours without venting.
Thermal models to account for the pressure rise over time in a cryogenic tank were developed, and finite element analysis was performed as the tank began to take shape in mechanical design. NK Labs designed novel support structures and connection hardware to create a strong, lightweight, high insulated vessel. Electronic design included liquid level and pressure sensors, as well as a heater system that would be used to build pressure and produce sufficient fuel flow for the flight phase.
Early FEA study analyzing pressure on a liquid tank shell design
Liquid Capsule Build & Test
NK Labs was asked to build a prototype liquid hydrogen capsule that could demonstrate meeting performance targets required in an aviation environment. Our engineers also built a test plan to evaluate the capsules in terms of functionality, mass fraction, and insulation performance (measured in hold time). Drawings were drafted, parts ordered, and assembly plans built. With the help of a variety of specialized suppliers, NK Labs was able to build, test, and deliver two prototype capsules to be evaluated as a successful proof of concept for practical, lightweight hydrogen storage in an aviation environment. In testing, the capsules exceeded initial projections for insulation performance, and represented a strong step forward towards sustainability in aviation.
Prototype liquid hydrogen capsule undergoing cryogenic testing
Prototype Liquid Hydrogen Tank for Universal Hydrogen by NK Labs (Image courtesy of Universal Hydrogen)