Thursday, July 28, 2016

Advances in low pressure hydrogen storage

I just read an update on LinkedIn from Mark Cannon, CTO at Hydrogen in Motion (H2M). His company works on the development of a low pressure hydrogen storage system. The hydrogen is adsorbed to a material. This technology is interesting because hydrogen is hard to store. You need a large volume of hydrogen to get the energy you need. For this reason hydrogen is stored at extremely high pressure. Such high pressure storage systems are very expensive.

Mark wrote on LinkedIn: "A year ago we successfully synthesized a material which adsorbed 4.2 wt % hydrogen and desorbed 70% of it at 50 bar and ambient temperatures. For the past year we have been focusing on improving the synthesis of this material and understanding all the mechanisms at play. We are currently adsorbing 5 wt % and releasing 80% of the hydrogen, effectively a 4 wt % material. Gravimetrically comparable to 250 - 700 bar compressed tanks but with a volumetric capacity better than 40 g/l. Now we are increasing material production for demonstration tanks for our strategic partners"

Now 4 weight percent (wt %) doesn't sound like much, but you have to realize that hydrogen is the lightest existing material on the planet. A fuel cell vehicle needs only 5 kg to drive about 500 km. So 4 wt % means that a storage system for 5 kg would weigh about 125 kg, which does not add too much weight to a vehicle. A high pressure storage cilinder for 5 kg weighs about the same.

Now of course this kind of storage technology has downsides as well. The number of filling cycles is limited. When asked Marked answered that "Early tests show 100's of adsorption/desorption cycles. " If a vehicle can drive 500 km on a tank, that means 200 cycles gives the tank a lifetime of 100.000 km.

Filling times are also important. High pressure cilinders can be filled in only 5 minutes. Mark did not mention how long it took to fill, but similar materials require hours to completely adsorb the hydrogen.

The main advantage of these systems is a much lower price. According to Mark: "Goals is to be half the price of compressed tanks of equivalent performance."

If filling times are a problem then you may end up with a hybrid system. Say a 4 kg high pressure cilinder and a 1 kg low pressure tank. The 4 kg can be filled quickly and the final kg serves mainly as an emergency supply in case the main tank is empty. If used, the emergency supply can slowly refill itself from the main tank after this has been refueled. Use as emergency supply also means that the number of filling cycles will not be an issue.

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