Flow control for Hydrogen storage

Hydrogen-fuelled trucks, buses or cars are very much related to the common battery-powered ‘electrical’ cars that we see more and more every day. Hydrogen-fuelled vehicles are electrical vehicles as well, but the way of powering is somewhat different: hydrogen and oxygen react in a fuel cell to generate electricity that powers an electric motor. While battery-powered vehicles get their energy from pre-charged lithium ion batteries, the hydrogen for hydrogen-fuelled vehicles is nowadays generally stored in on-board pressurised tanks.

For a maximum energy density, the stored hydrogen needs to be compressed to pressures as high as 700 bar to be able to fit in the limited tank volume for an adequate mileage. These tanks need to be strong enough to withstand the high pressure and should also be imperviable to hydrogen to prevent the gas from leaking. However, to avoid safety issues related to the extreme pressure and to avoid wasting energy when compressing the hydrogen to that pressure, alternatives for these tanks are looked for.

DLR, the German Aerospace Center in Stuttgart, investigates alternative ways to store hydrogen for use in fuel cells or vehicles. For the storage of hydrogen in metal hydride containers, they requested Bronkhorst’s distributor, Wagner Mess- und Regeltechnik, to provide a solution for the controlled supply of hydrogen gas into the container, and to measure the released hydrogen gas out of the container.

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Application requirements

In metal hydride containers, hydrogen is stored via reversible chemical reactions between a metal alloy and gaseous hydrogen. The solid metal hydride acts like a sponge that absorbs and releases the hydrogen. To investigate under which process conditions the loading/unloading of hydrogen works best, hydrogen flows and the process pressure need to be measured and controlled accurately. Furthermore, as we are dealing with an R&D environment, the setpoints and measurement values need to be recorded adequately for analysis purposes.

Important topics

Flow-pressure control
Reproducibility
Secure method to store hydrogen
Application at relative low pressure compared to traditional storage

Process solution

The Bronkhorst solution consists of a set of flow instruments at the inlet and the outlet side of the metal hydride container. For the introduction of hydrogen to the metal hydride, instruments are used from the IN-FLOW flow meter series in combination with Vary-P valves. The pressure in the metal hydride container is controlled with a certain pressure, to investigate the storage reaction.

For these purposes, at the inlet and outlet side of the metal hydride container, pressure controllers of the IN-PRESS series are present, connected to Vary-P valves. The parallel valve at the outlet side is a ball valve, which is used to enable the pressure to be reduced to atmospheric pressure.

The PROFIBUS-DP protocol is used for communication between the Bronkhorst devices and the control part of the setup, to set the setpoints and to read out the measured parameters for analysis at a later stage.
The entire setup is also available for a ATEX Zone 2 hazardous area.

Flow scheme

The focus of the investigation is in reducing the pressure and thus making hydrogen handling much safer. In this research environment pressures up to 100 bar are used, but 30 bar is a typical operational pressure for the metal hydride container to be operated. The storage of hydrogen is an exothermal process in which the heat generated must be dissipated. On the other hand, the release reaction is endothermal, which means that hydrogen is only released when enough heat is supplied. This leads to an inherently safe inclusion of the hydrogen gas in the metal hydride compound.

The reference variable for the investigation is usually the pressure. At the inlet side of the metal hydride container, the pressure controller and the mass flow controller work together as a flow-pressure controller. When introducing the hydrogen, the valves at the outlet side are closed and the hydrogen storage is initiated. When releasing the hydrogen, the inlet side is closed and the valves at the outlet side are opened. A complete experiment is a sequential process: first the hydrogen is introduced, and then it is checked how much can be loaded under certain conditions, and what is the stability of the introduced hydrogen into the metal hydride, and how reproducible this process can be conducted. Upon releasing the hydrogen, it is investigated how much hydrogen can be removed under certain conditions.

Again, stability and reproducibility are key in the release process.

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Hydrogen storage in metal hydride

Application requirements

Important topics

Process solution

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