The global shift to use of vehicles powered by fuels other than gasoline, like natural gas and hydrogen, has spurred substantial growth in the manufacture of pressure vessels. The low price of natural gas has motivated much work on compressed natural gas (CNG) tanks for vehicles, particularly in emerging economies. Eldib Engineering & Research Inc.’s (Berkeley Heights, N.J.) most recent market study on growth of demand of carbon fibers in reinforced vessels designed to store high-pressure hydrogen determined that carbon fibers are, and will remain, a key enabler of hydrogen fuel cell commercialization. Demand for fiber will rise quickly in this decade, reaching to about 7,000 metric tonnes (15.43 million lb) per year by 2020. The primary end-use segment will be automobiles (70 percent), followed by buses (18 percent) and in-transit/refueling storage (10 percent). Materials-handling vehicles will comprise about 2 percent of the demand in 2020.
Pressure vessels are used to store fuel in a car, truck or bus (15 to 150 liters in volume), akin to a gas tank for a combustion-type engine, or for bulk storage applications. Vessels are currently classified in four types:
Type I: All-metal construction, generally steel.
Type II: Mostly metal with some fiber overwrap in the hoop direction, mostly steel or aluminum with a glass fiber composite; the metal vessel and composite materials share about equal structural loading.
Type III: Metal liner with full composite overwrap, generally aluminum, with a carbon fiber composite; the composite materials carry the structural loads.
Type IV: An all-composite construction, polymer (typically high-density polyethylene or HDPE) liner with carbon fiber or hybrid carbon/glass fiber composite; the composite materials carry all the structural loads.
Each vessel type has its benefits and liabilities. Type I vessels are the least expensive, with estimated production costs of roughly $5 per liter of volume. The metalworking skills and equipment needed to produce them are widely available internationally. To their detriment, Type I vessels also are the heaviest, weighing approximately 3.0 lb/liter (1.4 kg/liter). By comparison, Type II vessels cost about 50 percent more to manufacture but can reduce the weight of the storage containers by 30 to 40 percent. Type III and Type IV vessels take the weight savings even further, weighing between 0.75 and 1.0 lb/liter (0.3 and 0.45 kg/liter). The cost of Type III and Type IV vessels, however, is roughly two times greater than Type II vessels and 3.5 times greater than the all-metal Type I tanks.
A fifth, all-composite, linerless Type V tank has been the pressure vessel industry’s holy grail for years. Recently, one company, Composites Technology Development Inc. (CTD, Lafayette, Colo.), successfully designed, tested and built such a tank for argon gas storage on the FASTRAC 1 (Formation Autonomy Spacecraft with Thrust, Relnav, Attitude and Crosslink) satellite, The 1.9L tank, at 6 inches (152 mm) in diameter, 7 to 8 inches (178 to 203 mm) in length, weighed only 0.44 lb (0.2 kg). It was filament-wound with T700 carbon fiber supplied by Toray Carbon Fibers America Inc. (Flower Mound, Texas) wet out with CTD’s proprietary KIBOKO toughened epoxy resin. It had an operational pressure of 200 psi, a proof pressure of 1,000 psi, and a burst pressure between 2,000 and 2,500 psi.
In 2013, The National Aeronautics and Space Admin. (NASA, Washington, D.C.) announced on July 2 that it recently completed a major space technology development milestone by successfully testing a large, pressurized cryogenic propellant tank made of composite materials. The almost 8-ft/2.4m diameter composite tank will hold cryogenic propellants (gasses chilled to subfreezing temperatures and condensed to form highly combustible liquids) critical to future long-term human exploration missions beyond low-Earth orbit. In the past, propellant tanks have been fabricated from metals.
Switching from metallic to composite construction holds the potential to dramatically increase the performance of future space systems through a dramatic reduction in weight. A potential initial target application for the composite technology is an upgrade to the upper stage of NASA’s Space Launch System (SLS) heavy-lift rocket. Built by The Boeing Co. (Chicago, Ill.) at its Tukwila, Wash., facility, with tooling developed by Janicki Industires (Sedro-Woolley, Wash., see photo at left), the tank arrived at NASA in late 2012. Engineers insulated and inspected the tank then put it through a series of pressurized tests to measure its ability to contain liquid hydrogen at extremely cold temperatures. The tank was cooled to -423°F/-253°C and underwent 20 pressure cycles as engineers increased the pressure to 135 psi.
A unique and specialized type of FRP pressure vessel also has become an essential component of desalination technology (see “The markets: Utility Infrastructure,” under “Editor’s Picks,” at top right).