Proto Systems is in the process of developing a range of solid state metal hydride hydrogen storage tanks.
Introduction to metal hydrides –
Proto Systems solid state hydrogen storage devices are filled with metal hydride powders (typically made from rare earth metals). The metal hydrides can absorb or release hydrogen depending upon the gas pressure which is exerted above them. The metal hydrides materials are contained within aluminium or stainless steel vessels.
Typical metal hydride containers will supply a few atmospheres of hydrogen at room temperature. The metal hydride will release hydrogen if the pressure in the container falls below what is known as the” plateau pressure” and absorb hydrogen if the pressure rises above the plateau. The plateau pressure can be altered by either changing the material specification (alloy composition) or by heating/cooling the store.
Once connected to your device (eg- a hydrogen fuel cell), gas will flow from the vessel as your application consumes it. As gas flows from the storage vessel the pressure inside will fall below the plateau pressure of the metal hydride material which will result in release of hydrogen. The material will release hydrogen until equilibrium is reached once again (ie- the pressure above the material reaches its plateau). Therefore when there is no flow of gas from the vessel the pressure above the material will be at its plateau pressure (typically a few atmospheres). Likewise, if the material is not fully absorbed with hydrogen and the pressure is raised above its plateau then the material will absorb hydrogen until it is saturated. This is how the stores are re-filled. In order to do this a hydrogen source and regulator will be required to deliver the gas at around 10 bar pressure.
When hydrogen is absorbed or released from the material a thermodynamic effect occurs. On absorption the store will become warm to the touch and on release of hydrogen the store will become cold to the touch.
The metal hydride tanks come equipped with a relief valve (to avoid facture off the tank if mis-used or heated) and a valve to enable opening/closing of the vessel once connected to your application. A pressure regulator is then required to maintain the downstream pressure at your device (eg- a hydrogen fuel cell) and to maintain a constant pressure when re-filling. For re-filling of the stores a dry hydrogen source is required with a purity >99.9%. Proto Systems Ltd is planning on offering a re-filling service in the near future which will negate the need for a hydrogen source by the customer. The stores have a female Swagelok ¼” quick fit connector as standard which seals on disconnection to avoid air exposure of the materials. We will alter the design of the store to meet a customers connection requirements.
Proto Systems Ltd will make bespoke solid state stores to meet a customers design requirements from 50 litres H2 to 50,000+ litres H2.
Photograph of prototype store containing 400litres of hydrogen
Advantages of metal hydride hydrogen storage
Hydride stores will produce a very high purity gas supply as impurities in the feed gas will be absorbed onto the surface of the material (this will degrade the material over time if the gas supply is very impure,99.9% purity will be sufficient for long term durability).
By storing hydrogen in a metal hydride the amount of gas per unit volume can be increased compared to compressed gas tanks. For example one of our stores has an internal volume of 1.5 litres but can store over 400 litres of gas.
Storing hydrogen in a metal hydride is a lot safer than storing it as a gas or liquid. Compressed gas cylinders will store hydrogen at high pressure (typically 200 bar+), whilst liquid hydrogen will “boil off” over time and require venting to avoid significant pressure build up. Metal hydride tanks will store gas at low pressure (typically around 2-10 bar gas pressure).
If you have have a hydrogen storage material which requires measurement and characterisation please see equipment section of the website.