Fuel cell vehicles are entering the automotive market with significant potential benefits to reduce harmful greenhouse emissions, facilitate energy security, and increase vehicle efficiency while providing customer expected driving range and fill times when compared to conventional vehicles. One of the challenges for successful commercialization of fuel cell vehicles is transitioning the on-board fuel system from liquid gasoline to compressed hydrogen gas. Storing high pressurized hydrogen requires a specialized structural pressure vessel, significantly different in function, size, and construction from a gasoline container. In comparison to a gasoline tank at near ambient pressures, OEMs have aligned to a nominal working pressure of 700 bar for hydrogen tanks in order to achieve the customer expected driving range of 300 miles. Beyond the need to contain pressure, the hydrogen tanks also differ from gasoline fuel tanks because of the additional vehicle space needed due to the lower hydrogen energy volumetric density even with the highly efficient fuel cell (four times the external volume of a gasoline tank including the fuel cell efficiency benefit). The main difference and challenge of hydrogen tanks is the construction and design that depends on a high utilization of carbon fiber in order to reduce the weight of the pressure vessel although substantially increasing the cost. In 2012, the U.S. Department of Energy (DOE), Office of Fuel Cell Technologies recognized these challenges and initiated a project to research enhance materials and design parameters to reduce the cost of hydrogen storage tanks. The project was led by Pacific Northwest National Lab (PNNL) and involved several other organizations in the value chain of hydrogen tank development: AOC, Ford Motor Company, Hexagon Lincoln, and Toray CFA. The project took a holistic approach to improving performance by investigating: (1) composite matrix resin alternatives including adding nano-reinforcing particles and fiber-matrix sizing for improved adhesion, (2) carbon fiber alternatives, (3) tank design alternatives using hybrid fiber layups, and (4) opportunities with cold gas operating conditions to maintain the hydrogen density while reducing the tank composite utilization. In each of these areas, the project successfully identified the potential benefits: (1) demonstrated new resin with 50% cost reduction at equivalent or better performance than traditional epoxy, (2) identified 4% to 12% improvement with fiber alternatives, (3) developed validated tank design models with improved failure prediction capability, and (4) confirmed value and system level viability of cold gas storage with a combined 22% cost reduction opportunity. This paper examines these modifications and considers the outlook for on-board 700 bar compressed hydrogen tank systems to achieve the commercialization goals for fuel cell vehicles.
