Hydrogen Storage
The hydrogen storage component of a hydrogen buffering system is required to store the hydrogen produced by the electrolyser. Hydrogen can be stored in one of three states:
• As a gas
• As a Liquid
Gaseous Storage
Two of the most promising methods of storing hydrogen in a gaseous state are:
1. Composite tanks
2. Glass Micro Spheres
1. Composite Tanks
The use of composite tanks to store hydrogen is the currently the most commercially used method and therefore the cheapest. It involves the use of a compressor to compress the hydrogen to 200-250 bar for stationary applications [1], such as a renewable energy buffering system, or 350-750 bar for transportation applications. The hydrogen is then stored in a tank made from lightweight, composite materials. The main issue faced when using compressed storage is the large tank size required due to the low volumetric capacity of compressed hydrogen (0.03 kg/L for 700bar). This results in a large expense in materials as well as a great degree of space required to accommodate the tanks.
2. Glass Micro Spheres
Another method of storing hydrogen as a gas is in glass microspheres. This is a premature technology that involves the heating and compression of hydrogen and a permeation process that allows it to be stored in small glass spheres. The advantage of this technology over compressed tank storage is the improved storage density of 5.4% wt and inherent safety. However, the 300°C temperature required to store the hydrogen is not achievable by PEM fuel cells therefore additional heating would be required for this method to work [1].
Liquid Storage
The most common way of storing hydrogen as a liquid is by cooling it to cryogenic temperatures (~20K) but this may also be achieved by storing it as a constituent in other liquids. This section of the website provides an overview of three of the most promising methods of liquid storage:
1. Cryogenic liquid hydrogen
2. NaBH4 solutions
3. Rechargeable organic liquids
1. Cryogenic Liquid Hydrogen
Hydrogen liquefaction increases the energy density of hydrogen by increasing its volumetric capacity to 0.07 kg/L. The liquefaction process requires a substantial amount of energy and results in a 30-40% loss in the hydrogen’s energy content. Other disadvantages of cryogenic storage are that in a dormant state liquid hydrogen suffers boil off losses and also requires a super insulated container. [1]
2. NaBH4 Solutions
An alternative method of storing hydrogen as a liquid medium to cryogenic storage is in a borohydride (NaBH4) solution. This is achieved through a catalytic hydrolysis reaction and produces hydrogen and a by-product, which must be converted back to NaBH4. This storage method has received some commercial attention but mainly in the transportation sector. [1]
3. Rechargeable Organic Liquids
This storage method is much alike the above storage in borohydride solution but yields a lower energy density. For this reason it has not been considered for our storage medium. [1]
Our Systems
For the purposes of the case_studies presented in the modelling and case studies section of this website, it is assumed that compressed gas storage is used. This is mainly due to the fact that compressed storage is the most developed of the storage technologies discussed and the high tank efficiencies involved. With recent developments in materials carbon fibre is now being used increasingly more for compressed storage in both transportation and stationary applications. With the combination of this and higher obtainable pressures through piston_compression compressed storage tanks take up less and less space.
References:
[1] http://www.che.iitm.ac.in/~arunkt/fuelcells/talkfiles/ssm_hydrogen.pdf
[2] http://www.ieahia.org/pdfs/HIA_Storage_G&P_Final_with_Rev.pdf
[3] http://www1.eere.energy.gov/hydrogenandfuelcells/storage/hydrogen_storage.html
