Hydrogen fuel cells don't technically store energy; they convert the chemical energy of hydrogen into electricity. The question of "how do hydrogen fuel cells store energy?" is really about how is the hydrogen fuel itself stored, which is then used by the fuel cell. The hydrogen can be stored in several forms, as detailed below.
Hydrogen Storage Methods
The key challenge with hydrogen fuel cells is the storage of hydrogen. Due to its low density at ambient conditions, hydrogen needs to be compressed or converted into a different form to store a useful amount of energy. According to reference [43], hydrogen can be stored in the following ways:
- Compressed Gas: Hydrogen is compressed to high pressures (e.g., 700 bar) and stored in high-strength tanks.
- Liquefied Gas: Hydrogen is cooled to extremely low temperatures (-253°C) to become a liquid, which is then stored in insulated tanks.
- Metal Hydrides: Hydrogen reacts with certain metals to form metal hydrides, which are solid materials that can store hydrogen. The hydrogen can be released by heating the metal hydride.
- Carbon Nanostructures: Hydrogen can be adsorbed onto carbon nanostructures like nanotubes and fullerenes.
The choice of storage technology depends on technical, economic, and environmental performance characteristics [44].
Comparison of Storage Methods
The following table provides a summary of the different hydrogen storage methods:
| Storage Method | Description | Pros | Cons |
|---|---|---|---|
| Compressed Gas | Hydrogen gas compressed to high pressure. | Relatively mature technology. | High pressure requires strong, heavy tanks; lower energy density compared to liquids. |
| Liquefied Gas | Hydrogen cooled to cryogenic temperatures to become a liquid. | Higher energy density than compressed gas. | Requires significant energy for cooling; boil-off losses. |
| Metal Hydrides | Hydrogen chemically bonded to a metal. | Safer than compressed or liquefied hydrogen; higher volumetric energy density. | Heavy; relatively slow kinetics for hydrogen absorption and desorption; expensive materials. |
| Carbon Nanostructures | Hydrogen adsorbed onto carbon nanostructures. | Potentially high surface area for hydrogen adsorption; lightweight. | Still under development; hydrogen storage capacity may be limited; expensive materials. |
Conclusion
Hydrogen fuel cells themselves don't store energy. The energy is stored in the form of hydrogen fuel, which can be stored as a compressed gas, a liquid, chemically bonded to metal hydrides, or adsorbed onto carbon nanostructures. The most suitable method depends on the specific application and desired performance characteristics.
