|Pore structure of elastomeric polyurethane foam (27µm slice thickness)—one of the materials under investigation. Click to enlarge. Source: Ohio University MicroCT Facility.|
Researchers from Ohio University, Inergy and DaimlerChrysler are exploring the use of conformable microstructures of polymeric foams for on-board hydrogen storage.
The Hydrostatic Pressure Retainment Microstructure stores the gas in small bubbles of a foam matrix, thereby forming a series of small, spherical pressure vessels. The resulting stress in the material between the bubbles is in a hydrostatic state of tri-axial tension.
The HPR approach has three main advantages:
Safety. In the case of an accident, only the gas contained in the adjacent cells to the fracture location would dispel at once.
Weight Savings. In theory, because the matrix material is in a state of hydrostatic tension, the material is being utilized 100% in all 3 cartesian directions, thus requiring less material.
However, neither of the two polymeric foams studied met the 2007 FreedomCAR goal for volumetric density of 0.036 kg H2/liter. One foam (DF-630A) delivered 0.0104 kg/L, the other (H130) only 0.0056 kg/L.
Looked at another way, while the target is for 8.18 kg of hydrogen in a 60-liter tank, DF-630A can hold 2.5 kg in the same volume, and H130 can hold 1.2 kg of hydrogen in the same volume.
The work is still in a relatively early stage. The team has developed an efficient method for examining polymeric foam for HPR applications, and note that composite foams could be used, which could greatly increase tank performance.
Next steps would be pursuing a composite foam, expanding the work beyond static structural analysis to consider thermal loads, potential chemical reactions, permeability and sorption rates, tank refuelability and cost—and then moving on to experimental tests of an actual hydrogen-pressurized tank.