As taught in our NAFTC premier course “Clean Air & Energy Independence,” there are three methods of transporting hydrogen and storing it onboard the vehicle. Three methods under development today include:
- Gaseous high pressure storage, with which the fuel is simply stored under high pressure in a storage cylinder at ambient temperature
- Liquefied storage, in which the hydrogen is liquefied to cryogenic temperatures (-411’F) at which point the hydrogen becomes a liquid
- Hydride storage. With this method, the hydrogen gas bonds with special metal alloys to form a metal hydride. The stored hydrogen can then be withdrawn from the metal hydride at a controlled rate.
Today hydrogen can be transported and stored by compressing to high pressure. NAFTC Photos
Hydrogen also can be transported by chilling until the gas is liquefied. Both compressing and chilling have their own limitations and safety concerns.
All three of these methods have seen tremendous research and development work over the past few years. However it has appeared that of the three, either high pressure storage or liquefied storage would win out over metal hydride storage methods.
This may change with the recent announcement by Energy Conversion Devices, Inc. (trade name: Ovonics) of the development of a highly efficient metal hydride storage device which can store up to three times as much hydrogen in the same volume as can be stored using high pressure methods.
The Ovonic metal hydride system uses a specially developed alloy powder which is put into a conventional composite storage tank where it will absorb and release hydrogen on demand. The tank is fitted with an internal closed loop heat exchanger which is used to control the transfer of heat either out of or into the metal alloy. Photo courtesy of Ovonics
As can be seen above, the metal hydride storage tank is about the same size as a vehicular gaseous storage tank and much less complex than a liquefied storage vessel. Photo courtesy of Ovonics
Ovonics engineers claim they have developed a proprietary metal alloy which is far more efficient at bonding with hydrogen than previous substances. This method may emerge as the leading technology for hydrogen transport and storage!
How it works:
Hydrogen is pumped into a cylinder filled with small particles of metal alloy. The alloy absorbs the hydrogen by bonding to it and becoming a metal hydride. Heat is released during this process and must be removed from the cylinder for the process to continue. During charging excess hydrogen pressure must be maintained to facilitate the formation of the metal hydride. The maximum pressure reached is approx. 1700 PSI. To obtain pure hydrogen from the tank, heat is added and the pressure in the tank is allowed to drop. This causes the hydrogen to separate from the hydride molecule and flow out of the tank. During this operating mode, tank pressure falls to less than 500 PSI. The molecule is thus returned to its original condition as an alloy and the process can then be repeated. When all the hydrogen has been removed from the tank the pressure will fall to 0 PSIG.
Graphic courtesy of Ovonics
- Gaseous high pressure storage is the simplest method, but inefficient and potentially dangerous. A trunk sized tank may contain only the equivalent hydrogen energy that is provided by 4 gallons of gasoline and in order to do so must be stored at a pressure of 10,000 PSI!
- Liquefied storage is more efficient as hydrogen in liquid form is far more dense than as a gas, but liquefaction carries its own set of problems which revolve around the cryogenic (extremely cold) temperatures needed to maintain hydrogen as a liquid.
- Metal hydride requires neither high pressures nor low temperatures. The operating pressure of the Ovonics system during its discharge phase is between 200 to 500 PSI (About the same as your backyard gas grille). Metal hydride systems in the past required high temperatures as well. The Ovonics system uses a closed loop liquid heat transfer system that works within the temperature range of engine coolant (Approx. 200’F).
A typical in ground liquefied hydrogen storage facility is complex, expensive and will lose at least ½% of its hydrogen per day as the gas warms in the tank. NAFTC Photo
Until today, however, no one has been able to find an alloy that was efficient enough to justify metal hydride as a viable storage system.
A comparison of storage density illustrates the superior efficiency of the Ovonics metal hydride.
Graphics courtesy of Ovonics
This new system from Ovonics may solve one of greatest engineering problems that have stood between us and the hydrogen economy- An efficient, simple and safe method of hydrogen transport & storage.
Development of a safe, simple, low cost and efficient hydrogen storage system will hasten the day when every corner service station may look like this! NAFTC Photo