CO2 recycling and pressure measurement in space

In September 2018, a Japanese H-IIB rocket launched the HTV supply vessel containing the ACLS (Advanced Closed Loop System) module destined for the International Space Station. The ACLS is tasked with ‘recycling’ CO₂ from the air in the spacecraft and generating fresh oxygen for the crew by means of electrolysis. Swiss manufacturer KELLER has developed and manufactured highly reliable absolute and differential pressure transmitters to regulate these processes.

The International Space Station moves around the Earth at some 400 km above its surface. As barely any oxygen is present at this altitude, it must either be supplied from the ground or generated on board. Bringing oxygen to outer space is expensive, with transportation costs for 1 kg of payload coming in at around EUR 33,000. It is therefore a good idea to try to process the air exhaled by the astronauts in order to generate oxygen that can be inhaled again.

This is the purpose of the ACLS, which was transported to the American Destiny module (US laboratory) on 22 September 2018. Airbus developed the ACLS on behalf of the European Space Agency (ESA).

In the ACLS cycle, oxygen is generated by the carbon dioxide from the cabin air being turned into methane and water using hydrogen obtained from splitting water molecules and adding energy. The electrolysis process extracts breathable oxygen from the water.

According to Airbus, the system is designed for a crew of three astronauts and saves 450 kg of additional water load per year. At full performance, the ACLS extracts 3 kg of CO₂, supplies 2.5 kg of oxygen and produces 1.2 kg of water each day.

The ACLS requires extremely reliable components to ensure that these processes run safely, and KELLER won the contract to develop the pressure measurement technology. The project posed some extreme challenges as, at 400 km above the Earth’s surface, components cannot be replaced within a reasonable period of time if they fail. KELLER’s contribution to this mission comes in the format of absolute and differential pressure transmitters that work in the range of 50 mbar to 20 bar at 0–110°C.

“With its pressure transmitters that can undertake the most varied tasks in numerous types of aircraft and contribute to the security of all manner of systems thanks to their reliability, KELLER has proved that the demands imposed on sensor lifetimes (MTBF) in actual operations are many times greater than they need to be,” said Jürg Dobler, Managing Director of KELLER.

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