Hydrogen as an energy source

Reading time – approx. 6 minutes – The second article in our three-part series discusses hydrogen as an energy source. It deals with the possibilities of using hydrogen and the components required for this.

Hydrogen, the most common chemical element in our universe, is very well suited as a clean and efficient source of energy. Unlike in a heat engine, chemical energy is converted into mechanical energy through combustion. Electrical energy is generated from this using a generator. Hydrogen can release electrical energy directly through “cold combustion” in a fuel cell. This elimination of intermediate steps in the conversion process is characterized by a significantly higher efficiency of the overall system.

The principle of “cold combustion” has been known since 1838, when Christian Friedrich Schönbein developed the galvanic gas battery, as the fuel cell was called at the time. However, the invention of the dynamo machine, the electric generator, by Werner von Siemens prevented its breakthrough at the time. The complex functioning of the fuel cell also played a part. New technological achievements in terms of catalyst material and manufacturing expertise, combined with the goal of decarbonizing the energy sector, are bringing the fuel cell back into focus today.

Electrical energy from hydrogen

As described in the article “Hydrogen as an energy storage medium”, hydrogen offers fundamental advantages for storing energy. To make the energy stored in hydrogen accessible to the end user again, it needs to be converted in a fuel cell.

This makes it clear that the fuel cell is not an energy storage device, but merely an energy converter. Similar to a battery, the fuel cell consists of two electrodes coated with a catalyst. These are separated from each other by an ion conductor, the so-called electrolyte. The electrical energy is supplied by a chemical reaction of a fuel, in this case molecular hydrogen H2, with an oxidizing agent. Usually, molecular oxygen O2 must be continuously fed in via the electrodes. Due to the standard electrochemical potentials of involved substances, a fuel cell achieves a maximum theoretical voltage of 1.23VDC. To use the electrical energy economically on a larger scale, several fuel cells are usually connected in series. These form so-called stacks that can supply a wide range of outputs and voltages.

DC/DC converters for controlling fuel cells

The current hype surrounding green hydrogen is also reflected in research activities surrounding fuel cells. While the focus used to be on material-specific problems like finding suitable membranes or catalysts, system behavior and optimal operation now play a key role.

The window in which a fuel cell works most efficiently is extremely small. Therefore, numerous system parameters must be considered. These include gas quantities, gas compositions, cooling temperatures, and load behavior.

As a rule, loads are not supplied directly from fuel cell stacks but are fed from intermediate batteries. In addition to simpler control behavior, this offers the advantage of smaller fuel cell systems. Short-term overload situations are buffered by batteries. A DCDC converter transforms the output voltage of the fuel cell to the required battery charging voltage.

DCDC converters play a decisive role in maintaining the optimum operating point of the fuel cell system. Instead of continuously adapting control variables to the current load, controllable DCDC converters can adapt the load. This can be adjusted to current operating conditions of the fuel cell. Additional external sense lines allow the DCDC converter to record and react to relevant operating parameters, such as temperature or moisture content. The output voltage is adjusted as required. The DCDC converter can be controlled using a higher-level control unit via a communication interface.

A fan often provides oxygen, cools, and removes product water from the cells. Its control influences the stack’s operating temperature and cell moisture content. Efficiency drops if the cell is too dry due to poor conductivity. On the other hand, a too humid cell favors condensation, hindering gaseous reactant transport. Such a fan with associated control electronics can also be supplied via a second DC/DC converter output, in addition to the power path.

Applications for hydrogen as an energy source

Hydrogen is suitable for central, large-scale power plants for supplying end consumers or stabilizing networks. Hydrogen is ideal for smaller, decentralized applications, like mobility or home heating. Fuel cell heating offers the advantage of achieving self-sufficiency year-round, depending on the system. Surplus photovoltaic electricity is used to produce hydrogen using an electrolyzer in summer months. This hydrogen supplies electrical energy in winter when there is little photovoltaic power. Additionally, waste heat from the fuel cell can be used to heat buildings.

Conclusion

Thanks to new technologies and a political and social rethink regarding sustainability, hydrogen offers excellent opportunities. These are evident in a wide range of applications. The advance of “cold combustion” will be unstoppable in the coming years.