Adsorbed Xenon Propellant Storage: Are Nanoporous Materials Worth the Weight?

Xenon is used as a propellant for spacecraft. Conventionally, xenon is compressed to high pressures (75-300 bar) for bulk storage onboard the spacecraft. An adsorbed xenon storage system based on nanoporous materials (NPMs) could, potentially, (i) reduce the storage pressures, (ii) allow for thinner-walled and lighter pressure vessels, and (iii) if the NPM itself is sufficiently light, reduce the overall mass of the storage system and thus of the payload of the rocket launch.

To investigate, we develop a simple mathematical model of an adsorbed xenon storage system by coupling a mechanical model for the pressure vessel and a thermodynamic model for the density of xenon adsorbed in the NPM. From the model, we derive the optimal storage pressure, tailored to each NPM, with the objective of minimizing the mass of the storage materials (walls of the pressure vessel + NPM) required to store the xenon. The model enables us to: (i) rank NPMs for adsorbed xenon propellant storage, (ii) compare adsorbed storage to the baseline of bulk storage, and (iii) understand what properties of NPMs are desirable for adsorbed xenon propellant storage.

We use the model to evaluate several NPMs, mostly metal-organic frameworks (MOFs), for adsorbed xenon propellant storage at room temperature, using experimental xenon adsorption data as input. We find Ni-MOF-74 and MOF-505 outperform the traditional adsorbent, activated carbon. However, we find each optimized adsorbed xenon storage system is heavier than the optimized bulk storage system, owing dominantly to the large mass of the NPM itself. Our model suggests that, for a NPM to provide a lighter adsorbed xenon storage system compared to bulk storage, the saturation loading of xenon in the adsorbent must exceed ca. 94 mmol Xe/g adsorbent.