Reed Doucette and Malcolm McCulloch at the University of Oxford (UK) have compared high-speed flywheels, batteries, and ultracapacitors on the bases of cost and fuel economy as the energy storage system in a fuel cell based hybrid electric vehicle. Their paper is in press in the Journal of Power Sources.
They built computer models to simulate the powertrain of a fuel cell based HEV where high-speed flywheels, batteries, and ultracapacitors of a range of sizes were used as the ESS (energy storage system). A simulated vehicle with a powertrain using each of these technologies was run over two different drive cycles in order to see how the different ESSs performed under different driving patterns.
The results showed that when cost and fuel economy were both considered, high-speed flywheels were competitive with batteries and ultracapacitors.
Reed T. Doucette and Malcolm D. McCulloch (2010) A comparison of high-speed flywheels; batteries; and ultracapacitors on the bases of cost and fuel economy as the energy storage system in a fuel cell based hybrid electric vehicle. Journal of Power Sources doi: 10.1016/j.jpowsour.2010.08.100