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Hydrogen Storage in Metal Hydride

  • Hydrogen storage on metal hydride can make hydrogen fuel vehicles more practical.
  • Research is critical to make the technology safer and more practical for large-scale implementation.
  • Accurate and repeatable flow and pressure control is critical to investigate new solutions for hydrogen storage.

Hydrogen fuel trucks, buses, or cars are an alternative to the more common, battery power, electric cars that we see more and more every day. These hydrogen fuel vehicles are electric vehicles as well, but the way of powering is somewhat different. In hydrogen vehicles, hydrogen and oxygen react in a fuel cell to generate electricity that then powers an electric motor. While battery power vehicles get their energy from lithium ion batteries, the hydrogen for these vehicles is generally stored in on-board pressurized tanks. To store hydrogen in a tank for vehicles, storage in metal hydride solutions is being explored.

For a maximum energy density, the hydrogen needs compression to pressures as high as 700 bar to fit in the limited tank volume for adequate mileage. In addition to these tanks needing to be strong enough to withstand the high pressure, they should also be imperviable to hydrogen to prevent the gas from leaking. However, to avoid safety issues from the extreme pressure and to avoid wasting energy when compressing the hydrogen to that pressure, alternatives for these tanks are necessary.


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, a solution that’s able to accurately control supply of hydrogen gas into the container while also measuring the release of hydrogen gas out of the container is critical.

Hydrogen Storage in Metal Hydride

In metal hydride containers, hydrogen can store 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 the process conditions where the loading and unloading of hydrogen works best, hydrogen flows and process pressure need accurate measurement and control. Furthermore, in the R&D environment, the setpoints and measurement values need adequate recording for analysis purposes.

Flow Control in Hydrogen Storage

  • flow-pressure control
  • reproducibility / repeatability
  • secure method to store hydrogen
  • comparing application at relative low pressure to traditional storage methods 

Accurate Flow Control and Measurement 

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, the IN-FLOW Flow Meter Series, in combination with Vary-P valves, are ideal. Varying pressure setpoints control the pressure in the metal hydride container 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, in addition to Vary-P valves. The parallel valve at the outlet side is a ball valve, which enables pressure reduction to atmospheric pressure.

PROFIBUS-DP protocol communicates between the Bronkhorst devices and the control part of the setup. This allows users to adjust the setpoints and to read out measurement parameters for analysis at a later stage. The entire setup is also available for ATEX Zone 2 hazardous area locations.

Metal Hydride: A Safer Way to Store Hydrogen

The focus of the investigation is in reducing the pressure, thus making hydrogen handling much safer. In the research environment, pressures up to 100 bar are suitable. However, 30 bar is a typical operational pressure for the metal hydride container. The storage of hydrogen is an exothermic process in which the heat needs to dissipate. On the other hand, the release reaction is endothermic. So, the hydrogen only releases when enough heat is available. 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 hydrogen, the valves at the outlet side close and the hydrogen storage initiates. When releasing the hydrogen, the inlet side closes and the valves at the outlet side open.

A complete experiment is a sequential process. First, the hydrogen is enters the system. Operators then determine how much can load under the specific conditions. After, the stability of the hydrogen in the metal hydride and the reproducible of the process is evaluated. 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. Overall, this research is critical for developing this green-energy technology to be suitable for large-scale implementation.

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