Page 20 - Introduction to Transfer Phenomena in PEM Fuel Cells

Introduction to Hydrogen Technology 9
1.1.2. Storage technologies
To store hydrogen, it is necessary to design tanks that are compact,
lightweight, safe and inexpensive. This is decisive since it is precisely this
storage option that makes hydrogen particularly attractive compared to
electricity [CEA 18].
1.1.2.1. Storage of liquids
Storing hydrogen in liquid form is an attractive solution, and also in this
form it is used in the space sector. However, hydrogen is, after helium, the
most difficult gas to liquefy. This solution leads to high-energy expenditure
and high costs, making it more difficult for the general public to use.

1.1.2.2. Storage of high-pressure gas

The storage of hydrogen in gaseous form is a promising option. However,
the constraints are numerous. Light and bulky, the hydrogen must be
compressed to the maximum to reduce the size of the tanks. Progress has
been made: from 200 bar (pressure of industrial cylinders), the pressure has
risen to 350 bar today, and the developments now concern tanks that can
withstand pressures of 700 bar. This compression comes at a cost; even
compressed at 700 bar, 4.6 liters of hydrogen are still needed to produce as
much energy as with 1 liter of gasoline. The risk of hydrogen leakage must
also be taken into consideration with regard to the flammable and explosive
nature of this gas under certain conditions. Due to its small molecule size,
hydrogen is able to pass through many materials, including some metals.
This weakens some, making them brittle. The study of high-pressure storage
therefore consists of essentially testing the resistance of materials to
hydrogen under pressure. These materials must be resistant but relatively
light (transportability requirements). The metal tanks currently used are still
expensive and heavy compared to the amount of gas they can carry.
Reservoirs, not metallic but made of polymeric materials, are being
developed to meet these constraints.

1.1.2.3. Low-pressure storage
An alternative to using gas pressure tanks would be to store hydrogen in
certain carbonaceous materials or metal alloys capable of absorbing
hydrogen and releasing it when necessary. This storage mode is currently the
subject of many studies.