This invention relates generally to volatile liquid storage systems and, more particularly, to techniques for storing cryogens, both in space and in terrestrial environments. Liquid cryogens are not practically storable in space for extended periods using present technology. Without active cooling, liquid oxygen (LOX) and liquid hydrogen (LH2) boiloff rates have been estimated to be of the order of 0.5 to 4%/month for 1 AU orbits (orbits at one Astronomical Unit, which is the distance of the earth from the sun), and somewhat less for distances further from the sun. Storage system heat leaks and low vaporization enthalpies (heat of vaporization) for these cryogens, relative to fluids like water, principally determine the loss rate.
Prior to the present invention, efforts seeking to improve hydrogen storability have focused on increasing its volumetric and gravimetric storage efficiency through use of physical adsorption on materials with high specific surface areas, high pressure tanks, chemical absorption in the form of hydrides/compounds with higher hydrogen content, heat flux reduction via oriented cross-section and use of sunshields, and active cryocoolers. These conventional approaches necessarily add system weight, cost and complexity.
The availability of light weight, long term cryogen storage systems would enable the use of non-nuclear propulsion systems for civil space exploration, planetary ascent and descent propulsion capability and hydrogen generation and storage for planetary bases. There is a similar need for long term cryogen storage technology for defense space missions, to provide more efficient and cost effective satellite orbital transfer operations. The present invention satisfies these needs without significantly adding to the weight and complexity of a cryogen storage system.
The present invention resides in a long-term storage system that substantially reduces the vapor pressure of a stored cryogen and simultaneously provides lower heat leakage rates into the stored cryogen. Therefore, the stored cryogen experiences significantly lower boiloff rates than would be experienced by the same cryogen conventionally stored. Briefly, the storage system of the invention comprises a storage tank shell comprising a protective outer layer, an impermeable inner layer and at least one intermediate insulation layer between the outer and inner layers; and a nanoporous foam structure filling the storage tank within the inner layer of the storage tank shell. The nanoporous foam structure provides a storage medium for a cryogen, which exhibits a substantially reduced vapor pressure than when in its bulk state, resulting in a lower boiloff rate for a given rate of heat leakage through the storage tank shell. If the storage system is not actively cooled, it still greatly extends cryogen storage life. If active cryo-cooling is employed, the system reduces the cooling power required to maintain the stored cryogen at a desired temperature.
It will be appreciated from this brief summary that the present invention represents a significant advance in the field of cryogen storage. Specifically, the invention provides a storage system in which cryogen boiloff rates are dramatically reduced, extending storage life or reducing the cooling power needed to maintain a given temperature. Other aspects and advantages of the invention will become apparent from the following more detailed description, taken in conjunction with the accompanying drawings.
As shown in the drawings for purposes of illustration, the present invention is concerned with storage systems for cryogens. Stored cryogens are subject to boiloff over a period of time, depending on the environment in which the stored cryogens are located. The rate of boiloff is increased by the low vaporization enthalpies and high vapor pressures of typical cryogens and by heat leakage into the storage system.
In accordance with the present invention, emerging nanotechnology is applied to address and improve upon both of the above limiting factors. Materials may now be engineered with dimensional features and phonon scattering on a nanometer length scale, potentially enabling practical super-insulation materials with much lower thermal conductivities, possibly by as much as an order-of-magnitude or more. Similarly, ultra-light weight nanoporous foams, nanocapillary tubular and platelet shaped particles and aerogels may be used for storing cryogens. With a higher surface energy term due to extreme surface curvature, a “nano-containered” cryogen has an altered thermodynamic state from its bulk fluid state and has a substantially reduced vapor pressure or an effectively higher vaporization enthalpy (ΔHv), thus exhibiting a lower boiloff rate for a given heat leakage rate. The surface curvature effect on the vapor pressure of liquids in capillary pores is well understood and is described by the Kelvin equation1:
1D. J. Shaw, Introduction to Colloid and Surface Chemistry, 3rd edition, Butterworths Publishers Inc., Boston, 1980, p116.
RT ln(Pr/Po)=−(2γM/ρr)cos(θ)
where Pr/Po is the ratio of the capillary liquid vapor pressure to that of the bulk liquid vapor pressure; R and T are the universal gas constant and system temperature, respectively; γ, M and ρ are the liquid's surface tension, molecular weight and density, respectively, while r and θ are the pore radius and liquid contact angle with the pore wall.
As shown in
It will be appreciated from the foregoing that the present invention would permit the practical long term storage of cryogenic fluids for space exploration and science missions and would support the safe storage of hydrogen for future terrestrial transportation and energy systems. When coupled with a cryocooler, nanopore foam storage media would reduce the electrical cooling power required to maintain a desired temperature and to counter heat leaks. Furthermore, the need for extra or separate radiation shielding would be reduced or eliminated. The foam structure used in the storage system of the invention would effectively replace fluid containment screens often used in some storage tank vessels, and would facilitate fluid transfer in zero-gravity environments.
It will also be appreciated that, although a specific embodiment of the invention has been illustrated and described, various modifications may be made without departing from the spirit and scope of the invention. Therefore, the invention should not be limited except as by the accompanying claims.
This application claims, under 35 U.S.C. §119(e), the filing priority of Provisional Application No. 60/667,282, filed Mar. 30, 2005, and having the same title as the present invention.
Number | Date | Country | |
---|---|---|---|
60667282 | Mar 2005 | US |