Solid-State Compressor for Space Station Oxygen Recovery

John Finn

At present, the life support system on the International Space Station Alpha vents overboard the carbon dioxide produced by the crew members. Recovering the oxygen contained in the CO₂ has the potential to reduce resupply mass by 2000 pounds per year or more, a significant weight which might instead be used for experiment payloads and other valuable items. The technologies used to remove CO₂ from air and to recover O₂ from CO₂ are flight-ready. However, the interface between the devices is a problem for the Space Station system. NASA Ames has developed a new technology that solves the interface issue, possibly allowing for the first time closure of the oxygen loop in a spacecraft.

The relevant part of the air revitalization system is shown in Figure 1. CO₂ produced by the crew is removed in the Carbon Dioxide Removal Assembly, or CDRA. This device effectively produces a pure CO₂ stream, but at a very low pressure. Elsewhere the oxygen generation system, which makes O₂ by electrolyzing water, produces a hydrogen stream. In principle the CO₂ and H₂ can react to form methane (CH₄) and water (H₂O) over a suitable catalyst. Water produced in this methane-formation reactor can be returned to the water electrolyzer, where the O₂ can be returned to the cabin. However, the methane-formation reactor requires CO₂ at a much higher pressure than that produced by the CDRA. Furthermore, the CO₂ and H₂ are often not available at the same time, due to power management and scheduling on the space station. In order to get the CO₂ to the reactor at the right pressure and at the right time, a device or assembly that functions as a vacuum pump, compressor, and storage tank is required.

One obvious solution to this problem is to use a mechanical vacuum pump / compressor combined with a high-pressure buffer tank. This has implementation problems, however. The rapidly moving parts of a mechanical compressor wear out relatively quickly, requiring frequent maintenance or replacement. The mechanical compressor can add noise and vibration to the sensitive station environment, unless large amounts of insulating material is provided. There is so little space available for the buffer tank that the compression ratio would have to be quite high. Finally the power required to compress the CO₂ to high pressure is considered very high for the power-limited Space Station.

The solution being developed by NASA Ames engineers uses a technique they originally developed for compressing the very low pressure Mars atmosphere so that it could be used in an in situ propellant production plant. The compressor uses a temperature-swing adsorption cycle and has no rapidly moving parts. Low-pressure CO₂ from the CDRA is adsorbed in a cool cylinder containing a sorbent material that has a high capacity for CO₂. In this step, the device acts like a vacuum pump. Next, the cylinder stays in a standby mode until the CO₂ is required; i.e., the device acts like a storage tank. Finally, the cylinder is heated and the CO₂ is driven off the sorbent, producing CO₂ at a high pressure. The compressed CO₂ flows into the methane-formation reactor. Coolant from the Space Station’s thermal control system cools the cylinder back to its initial state, and the process is repeated.

Several such cylinders are combined in the device. They operate out of phase from each other, so that there is always a "vacuum pump" and a "compressor" available whenever they are needed by the processors on either side.

A single-bed prototype solid-state compressor was built at Ames and successfully tested with a high-fidelity CDRA at NASA Marshall Space Flight Center in 2000 (Figure 2). The temperature-swing adsorption compressor uses much less power than the mechanical compressor system and has far fewer parts. Its lifetime is estimated at ten years. It is free of vibration and noise, and is also smaller and lighter than its counterpart.

Figure 1. Carbon dioxide removal, carbon dioxide reduction, and oxygen generation planned for the International Space Station (methane-formation is not yet implemented). The compressor would be placed between the carbon dioxide removal and methane-formation reactor assemblies.

Figure 2. Single-bed prototype of a NASA Ames solid-state compressor.