We use the approximation in eqn. 12 to estimate that, for an adsorbed storage system to give a lower optimal tankage fraction than the bulk storage system, the saturation loading of xenon in the adsorbent must exceed 94 mol Xe/kg adsorbent. Based on the relationship between M and ρ ads in Fig. , such an adsorbent must not be dense. Lighter adsorbents (lower ρ ads ) also tend to give adsorbed Xe storage systems with lower optimal tankage fractions; see Fig. .