GKN Powder Metallurgy announced plans to develop an innovative hydrogen storage system for residential homes using solid state metal hydride in August 2017. Since then, engineers at the company have developed the demonstrator system, overcoming several technical challenges to adapt sizes and capacity of the required modules.
The objective of GKN Powder Metallurgy’s research project is to provide an integrated zero emission heat and energy storage system for an off-grid alpine chalet. Engineers are reported to have aimed for a system capacity of over 130 kW, equivalent to providing a normal four-person household for around twelve to fourteen days with electrical power and no recharging.
The project is said to be among the first in the world to use metal hydride pallets for hydrogen storage in a residential application. Working on several key modules of the system, engineers together with project partners encountered several challenges.
The storage challenge
Bringing the concept from a lab-based model to a full-scale system brought the challenge of scaling the system concept and developing new designs for the hydrogen storage tanks.
Typically, hydrogen is stored as gas and requires very large storage tanks operated at high pressure up to 300 bars. Using metal powder as a medium to store hydrogen has some obvious benefits: the same amount of hydrogen gas can be stored in a tank not even half the size compared to gas. Additionally, the metal powder based process works at a lower pressure and is easier to control in terms of temperature levels.
In GKN’s process, the storage tanks are loaded with hydrogen gas at pressure levels below 40 bars. The pelletised metal alloy inside the tank reacts with hydrogen and builds metal hydrides. Loading the tank with hydrogen is an exothermic process, meaning the absorption of hydrogen into the metal framework of the tank needs to be cooled and maintained at 20°C to keep the loading process stable and efficient.
For the unloading or desorption, the tank needs to be heated up to 60°C as the chemical reaction to remove hydrogen out of the metal lattice is endotherm. The higher the flow of hydrogen into or out of the tank, the more intensive is the chemical reaction. To increase the kinetic capacity for quick loading and unloading and for safety reasons, thermal management is a key aspect of the metal hydride-tank-system.
“Our engineers have worked on double tube tanks to achieve an optimal heat transfer between the active material and the cooling/heating media. The new tank design speeds up processes and reduces energy losses for heating and cooling, and has also improved and shortened the process to activate the metal pellets to enable the metal hydride process” stated GKN Marketing Manager Ralf Radschun.
The electrolyser challenge
Another important aspect was said to be the development of an electrolyser unit, suitable in size for a residential application, to deliver high quality hydrogen gas. Electrolysers available on the market are for applications with high electrical power (usually from 50 kilo watts to several mega watts). “A huge number of modifications were necessary to downsize the electrolyser for our requirements and electrical capacity range of 5 kW. Additionally, much effort was spent to achieve the high gas quality that is required to enable hydrogen storage in metal hydrides,” added Radschun. “Working together with our electrolyser partner iGas, a complex hydrogen purification system was developed to achieve a gas quality of 99.999%.”
The efficiency challenge
A third requirement is to gain back electrical power from the stored hydrogen and to use the heat resulting from the transformation process. Managing and using the resulting process heat is the important second element that ensures the high round-trip energy efficiency of GKN Powder Metallurgy’s concept.
“The unloading and transformation of hydrogen gas back to electrical power works through a proton exchange membrane fuel cell which is the third key module of our system. Developing a clever and reliable control scheme creates efficient heat, power and hydrogen management,” added Radschun.
“GKN Powder Metallurgy is on the way to utilising a well-known chemical process and material capability of metal hydride and bringing it to a new level for a real-life residential application. This will help reduce CO2 levels and bring the opportunity to better utilise natural non-carbon energy sources like wind, water and sun,” Radschun concluded.