Rust is Key to Hydrogen Storage

­The largest hold-up to renewable adoption is its intermittent nature. If the wind drops, or blows too strongly, then wind turbines are useless. The same goes for solar panels, which can only generate power during daylight hours, and lose functionality on cloudy days. We require power 24/7, so there is a need to fill in the gaps. Batteries have been the favourite choice for this role, but getting cost-effective batteries that have high enough capacity, and can hold it over a period of time has proven difficult. Another method of storing energy is by using excess renewable power to manufacture hydrogen by splitting water molecules. That hydrogen could then be stored and used when needed.  

Hydrogen can fill multiple roles in the sustainable economy. It can be burned for fuel with no environmentally damaging emissions, it can be used in fuel cells to top up vehicle batteries, or it can be used directly in combustion engines. There are still a couple of drawbacks with the use of green hydrogen. The first is raising the efficiency of the conversion of water to hydrogen, where researchers around the world are tackling the problem. The second is storage. It is very difficult to build a leak-proof tank that will hold the gas for long periods of time. Hydrogen is flammable, volatile and can make materials exposed to it brittle. Current methods of storing hydrogen rely on pressurized containers that require cooling and large amounts of energy. Extensive safety measures are also needed.     

Researchers at ETH Zurich are the latest to tackle the problem of long-term hydrogen storage. The team rejuvenated a well understood technology known since the 19th century in a novel way that it hopes will prove safer and cheaper than existing solutions. The steam-iron process feeds hydrogen into a stainless steel reactor filled with natural iron ore at 400oC. The hydrogen then extracts the oxygen from the iron ore, resulting in iron and water. The energy in the hydrogen can be stored as iron and water for long periods with very few losses. When the energy is required, hot steam can be fed into the reactor to turn the iron and water back into iron oxide and hydrogen, which can then be extracted. The steam is generated using waste heat from the discharging reaction.

The reactor doesn’t need any special safety measures. It has stainless steel walls that are only 6mm thick and the reaction occurs at normal pressure. The researchers demonstrated the technical feasibility of the storage technology at a pilot plant on the Hönggerberg campus. It uses three stainless steel reactors with a capacity of 1.4 cubic metres, each containing 2–3 tonnes of untreated iron ore.

The technique seems to work well enough, as you’d expect from one that has been understood for so long, however, the largest problem to be overcome is the wastage. Around 60% of the potential energy is lost in production and conversion, but the process is in its infancy, and that waste figure should fall in time. 

PSD