Storage and transfer of fluid commodities such as oxygen, hydrogen, natural gas, nitrogen, argon, etc. are absolute necessities in virtually every industry. These fluids are typically contained in one of two ways: as low-pressure cryogenic liquids, or as high-pressure compressed gases. Cryogenic liquids afford high energy and volume densities, but require complex storage systems to limit boil-off, need constant settling in zero-gravity environments, and are not well suited for overly dynamic situations where the tank orientation can change suddenly (in an airplane or car, for example). Conversely, high-pressure gas storage bottles are not affected by tank orientation, do not require settling, and can be kept at room temperature; they are considerably less complicated pieces of equipment. These vessels, however, are heavy due to the thick walls required to contain the high pressures, and storage densities — even at extreme pressures such as 10,000 psi — are dramatically lower.
These two options are traded depending on the system requirements, but few practical options exist that provide all the benefits while limiting the downfalls. Monolithic aerogels and other high-surface-area (>800 m2/g) materials — mostly powders — can achieve large storage densities via solid-state adsorption. Monoliths have been found not to work in practice due to catastrophic cracking during thermal cycling and/or gas charging/discharging, and powders are severely limited by means of containment.
The Cryo-Fluid Capacitor (CFC) approach uses a composite aerogel system to preserve integrity during all operational phases, and can store large quantities of fluid commodities at moderate pressures and low temperatures in a non-liquid state. Occupying the middle ground between the two extremes described above, the CFC concept presents a host of alternative and enabling applications, and provides the fluids designer with new and novel possibilities when engineering a storage/supply system.
Energy storage is not useful unless the energy can be practically obtained (“un-stored”) as needed. In the present case, the goal is to store as many fluid molecules as possible in the smallest, lightest-weight volume possible, and then supply (“un-store”) those molecules on demand as needed to the end-use application. The CFC concept addresses this dual storage/usage problem with a novel packaging approach. An integrated conductive membrane also acts as a large-area heat exchanger that easily distributes heat through the entire cylinder to discharge the unit quickly, and can be interfaced to a cooling source for charging; this feature also allows the cryogenic fluid to quickly and easily load the system. Another important note is that the unit can be charged with cryogenic liquid or from an ambient temperature gas supply, depending on the desired manner of refrigeration. Also, although the “standard” CFC is a cylindrical geometry, the salient and novel features of the technology are extensible to almost any shape, including conformal.
The CFC capitalizes on the energy storage capacity of liquefied gasses (i.e. cryogenic liquids) and relative simplicity of high-pressure gas bottles, while limiting the downfalls associated with both methods. By exploiting a unique attribute of nanoporous materials — aerogel, in this case — fluid commodities such as oxygen, hydrogen, methane, etc. can be stored in a molecular surface adsorbed state at densities on par with liquid, at low to moderate pressure, and then supplied as a gas, on-demand, to a point of interest.