Hydrogen as an energy storage medium

Reading time – approx. 7 minutes – The first article in a three-part series on the topic of hydrogen as an energy carrier deals with energy storage in hydrogen and the components required for this.

While a large proportion of the hydrogen produced today is a so-called “by-product” of chemical processes in industry, the basic principle of electrolysis of water is probably familiar to many from simple oxyhydrogen experiments in school. Electrolysis is a chemical process in which an electric current forces a redox reaction. When water is electrolyzed, it is split into hydrogen and oxygen at the electrodes. The increasing proportion of decentralized and regeneratively generated energy in conjunction with major technological advances in electrolysers is generating ever greater interest in hydrogen as an energy storage medium.

Electrical energy for hydrogen

Whether generated from nuclear, fossil or renewable sources, a fundamental problem in the provision of electrical energy is to ensure a balance between supply and demand. As there is generally neither a constant supply nor a constant demand for electrical energy, in the case of energy generation, possibilities must be created to store it efficiently and sensibly. The amount of energy generated from renewable sources in our latitudes is usually subject to strong fluctuations. In the event of an oversupply of energy, hydrogen is a suitable storage medium.

While there is a reduction in efficiency compared to storage in batteries, hydrogen as an energy storage medium offers an energy density that is over 200 times higher by weight and around 5 times higher by volume (at 700 bar) than conventional lithium-polymer batteries. In terms of costs, energy storage in hydrogen can also be scaled up significantly better than in batteries due to the relatively simple storage in tanks.

Another advantage is that the transportation of hydrogen using the existing road and rail infrastructure is just as possible as distribution via the natural gas network. Excess hydrogen from industrial processes has already been added to natural gas as a small percentage for several years.

System components of hydrogen production

Various components are required for hydrogen production using electrolysers, which can be designed differently depending on the output, the type of electrolyser and the desired degree of purity of the hydrogen.

In addition to a suitable power supply using ACDC or DCDC converters, water treatment is generally required when using feed water. In addition to purification, bases or acids are added to the water, depending on the type of electrolyser. A gas separator is located downstream of the electrolysis, which removes the product gases. Immediately after discharge from the cells, there are still impurities in the hydrogen, which can be removed in gas aftercoolers and condensation separators. Depending on the required purity, water contained in the hydrogen in cleaning and drying systems can condense and thus achieve a purity of around 99.9% by volume. For use in vehicles, the ISO 14687-2 standard only permits a residual water content of 5ppm. The purity can be increased to approx. 99.999% by volume by means of pressure swing adsorption. The hydrogen is usually stored in pressurized storage tanks in which the hydrogen is compressed to around 700 bar by a gas compressor.

DC voltage for operating the electrolyser

In simple terms, an electrolyser, or electrolysis cell, is the counterpart to a galvanic cell, e.g. a battery. While batteries supply DC voltage, the electrolyser must be supplied with DC voltage.

The minimum voltage across the electrodes required for electrolysis to take place, which is determined based on the reaction enthalpy, is 1.482V for the electrolysis of water under standard conditions and decreases accordingly at higher water temperatures. However, higher voltages are quite common in practice due to electron passage inhibition during electrochemical reactions and the ohmic resistance of the electrolysis cell. As a rule, so-called stacks are used, i.e. several interconnected electrolysis cells, analogous to the use of battery cells, so that the voltage required to operate the electrolyzer increases depending on the number of cells.

The DCDC converter during electrolysis

 Since the electrical supply of an electrolyser is of central importance for its service life and for the efficiency of the entire process, the requirements for a DCDC converter will be explicitly addressed here.

The supply from renewable sources or battery storage is subject to certain voltage fluctuations, which must be covered by the input voltage range of the DC/DC converter. For optimum operation of the electrolyser, both the voltage and the current must be adapted to operating conditions such as temperature, operating fluid pressure and quality as well as the desired load case. This requires a largely controllable output of the DC-DC converter.

In addition to the actual supply to the electrolysis cells, the electrolyser also requires additional energy for integrated electronics, for example, to operate an operating fluid pump or the gas separator. Ideally, this energy is also provided by the DCDC converter via an additional output.

In order to increase the efficiency of the application, the waste heat generated by the process and the components involved should be used to preheat the equipment or in downstream applications. This also requires flexible thermal management of the DC/DC converter in order to be able to react to the respective requirements of the application. As a rule, heat is dissipated via contact or liquid cooling.

This allows the electrolyser to be operated at the optimum operating point with maximum efficiency of the overall system. In the next part of our hydrogen series, we will deal with the topic of “DC voltage from hydrogen”.