Last summer there was a revolving buzz when GKN Powder Metallurgy announced the plans to develop an innovative hydrogen storage system for residential homes using solid state metal hydride. Since then we have developed the demonstrator system, where our engineers overcame several technical challenges to adapt sizes and capacity of the required modules.
We started our research project with the objective to provide an integrated zero emission heat and energy storage system for an off-grid alpine chalet. Our engineers aimed for a system capacity of above 130 kW, equivalent to providing a normal four-person household for about 12 to 14 days with electrical power and no recharging. Taking advantage of clever heat management, the GKN Powder Metallurgy team additionally wants to re-use the process temperature for heating.
Intensive scientific and engineering ground work took place because our project is 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, our engineers together with our partners must master several challenges.
The storage challenge
Bringing our 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 our process, the storage tanks are loaded with hydrogen gas at pressure levels below 40 bars. The pelletized 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.
GKN Powder Metallurgy's storage module consists of eight separately controlled storage tanks and can store 133 kWh electric power.
The electrolyzer challenge
Another important aspect was the development of an electrolyzer unit suitable in size for a residential application to deliver high quality hydrogen gas. Electrolyzers 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 electrolyzer 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. Working together with our electrolyzer partner iGas, a complex hydrogen purification system was developed to achieve a gas quality of 99.999%.
The efficiency challenge
Looking at the entire system, producing and storing the gas is only half of the story. It requires a third key module 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 our 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.
GKN Powder Metallurgy is on the way to utilizing 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 utilize natural non-carbon energy sources like wind, water and sun.
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