Hydrogen as an energy source

Reading time – approx. 6 minutes – The second article in our three-part series on the topic of hydrogen as an energy source deals with the possibilities of using hydrogen as an energy source 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, in which chemical energy is converted into mechanical energy through combustion and 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 and the complex functioning of the fuel cell prevented its breakthrough at the time. 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 when it comes to storing energy. In order 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 the structure of a battery, the fuel cell also consists of two electrodes coated with a catalyst, which 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, which must be continuously fed in via the electrodes. Due to the standard electrochemical potentials of the substances involved, a fuel cell achieves a maximum theoretical voltage of 1.23VDC. In order to be able to use the electrical energy economically on a larger scale, several fuel cells are usually connected in series to form so-called stacks, which 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 the research activities surrounding fuel cells. While the focus here used to be on material-specific problems such as the search for suitable membranes or catalysts, the system behavior and optimal operation of the fuel cell now play a key role.

The window in which a fuel cell works most efficiently is extremely small, so that a large number of different system parameters such as gas quantities, gas compositions, cooling temperatures and load behavior must be taken into account.

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

DCDC converters play a decisive role in maintaining the optimum operating point of the fuel cell system. Instead of continuously adapting the control variables of the fuel cell to the current load situation, suitable controllable DCDC converters can adapt the load to the current operating conditions of the fuel cell. With the help of additional external sense lines, the DCDC converter can record and react to relevant operating parameters of the fuel cell such as temperature, moisture content of the cell or the output voltage as required. It is also possible to control the DCDC converter using a higher-level control unit via a communication interface.

A fan is often integrated into the system to supply the stack with oxygen, to cool it and to remove product water from the cells. The control of the fan influences the operating temperature of the stack and the moisture content of the cell. If the cell is too dry, the efficiency drops due to poor conductivity. On the other hand, a cell that is too humid favors condensation, which hinders the transport of the gaseous reactants. Such a fan with associated control electronics can also be supplied via a second output of the DC/DC converter in addition to the power path.

Applications for hydrogen as an energy source

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

Conclusion

Thanks to new technologies and a political and social rethink with regard to sustainability, hydrogen as an energy storage system and energy source offers excellent opportunities in a wide range of applications. The triumphant advance of “cold combustion” will be unstoppable in the coming years.