Hydrogen & Alternative Sources of Energy First-hand engineering expertise

In general, an electrolyser is a device for separating, breaking down and transforming a material or molecule (redox reaction) with the help of electrical energy. In a water electrolyser, water molecules (H₂O) are converted into hydrogen molecules (H₂) and oxygen molecules (O₂).

The actual reaction takes place in electrochemical cells at voltages of approx. 1.4 V. For practicality reasons, a number of these cells (electrical series connection) are piled up into stacks. All the peripheral equipment around the electromechanical cells comes under the term "Balance of Plant".

Balance of Plant (BOP) is a term that is generally used in connection with energy technology. It refers to all supporting components and auxiliary systems required for the conversion of energy – with the exception of the generation unit or transformation unit itself.

For electrolysers, this includes energy management (transformers, inverters, power controllers etc.), fluid and gas management (water conditioning, separation of fluid and gas phases, gas drying, gas compression) and thermal management (cooling systems for power electronics, stack and condensation drying).

A fuel cell consists of two electrodes – the anode (hydrogen side) and cathode (air side). Both electrodes are separated by an electrolyte. In the PEM fuel cell, this is a semi-permeable membrane that is only permeable to protons.

The hydrogen is fed to the anode. It is then split into protons and electrons with the help of a catalyst (usually platinum). The protons then migrate through the membrane to the cathode. The electrons flow to the cathode via an electrical consumer and electrical energy is supplied. At the cathode, protons and electrodes combine with oxygen from the surrounding air to form water.

The term "electrochemical cell" is an umbrella term for different types of cells such as electrolysis cells, accumulator cells, battery cells or galvanic cells. These types of cells can sometimes be reversible, such as accumulator cells. These can be charged and discharged – this means that they can convert electrical energy into chemical energy and release it again as electrical energy. In addition, some types of electrolysis cells can be operated as fuel cells. This means that the conversion of hydrogen and oxygen into water releases electrical energy and heat.

Electrolysis cells and fuel cells consist of bipolar plates, electrodes and, depending on the technology, gas diffusion layers (GDL) and membranes. When "proton / anion exchange membranes" (PEM / AEM) are used, these are often connected directly to the electrodes and referred to as a "membrane exchange assembly" (MEA).

The membrane electrode assembly (MEA) can be interpreted in more than one way. In some cases, this is understood to only mean the membrane with the catalyst layers coated on it (on one side for the cathode reaction; on the other side for the anode reaction). Often, however, the gas diffusion layer(s) is/are included, as this/these must also be electrically conductive.

Depending on the technology, the membrane consists of different polymers or ceramics, each which can selectively transport protons, anions (e.g. hydroxide anions = OH) or oxygen. The gas diffusion layers serve to transport the gases produced (electrolysis) and in particular the gases used (fuel cells) as homogeneously as possible away from or towards the reaction sites (catalyst layers). These gases are led out of or into the electrochemical cells via channels in the bipolar plates.

Bipolar plates which have been installed in a multiple-cell or stack configuration are above all responsible for physically and electrically connecting the anode from one cell with the cathode of the neighbouring cell. Bipolar plates in fuel cells are also responsible for leading the reaction gases to the reaction zone. For this purpose, flow profiles (flow fields) are milled or pressed into the plates on both sides, through which hydrogen flows on one side and air is supplied on the other.

A bipolar plate consists of the two poles of a single fuel cell: the hydrogen-carrying anode plate (the negative (-) pole) and the cathode plate (the positive (+) pole) for feeding the reaction air. The plates also regulate the removal of water vapour and the output of thermal and electrical energy. In electrolysis cells, they are mainly used to cool the electrolyser, supply reaction gases to the anode side and remove the hydrogen and gases produced in the reaction.

In electrolysis and fuel cell technology, a stack is a stack of electrochemical cells connected in series, including the housing/frame/clamping elements. The series connection makes it possible to increase the supply voltage and reduce the current with the same power consumption according to P=U*I. Independently of this, the series connection in a stack also simplifies the overall design of the system.

Gaseous hydrogen can be stored in a tank after compression at high pressure. In transport, for example, a pressure level of 350 bar for commercial vehicles and 700 bar for cars has become established. At 700 bar the density is approx. 40 kg/m³ (24 kg/m³ at 350 bar). High-pressure accumulators offer a low-cost solution for small storage quantities and are therefore mainly used in mobile applications such as cars and commercial vehicles.
There are currently four different types of pressure vessels on the market:

• Type 1: Pressure vessel consists only of a metallic (usually steel) wall. Nominal pressures are in the range of 200 bar.
• Type 2: In addition to the metallic wall, pressure vessels have a jacket made of resin-impregnated glass or carbon fibre with a nominal pressure of up to 1000 bar.
• Type 3: Tanks have a liner made of metal (usually aluminium) and a jacket made of carbon fibre around the whole tank. Nominal pressures are typically 350 or 700 bar.
• Type 4: Accumulators have a liner made of plastic (typically polyamide or polyethylene) and the jacket is usually made of carbon fibre, as with the type 3 vessels. Nominal pressures are usually 350, 500 or 700 bar.

Compared to gaseous hydrogen storage, liquid hydrogen as a fuel offers advantages in terms of energy density (71 kg/m³). The pressure in the tank can also be kept low. This has a positive effect on the tank system in terms of the storage tank weight and space requirements, costs (especially for large storage volumes) and safety.

The production costs of cryogenic hydrogen (-253 °C) are not insignificant, however. The hydrogen also heats up if it is not constantly cooled. This leads to an increase in pressure within the tank. This can lead to "boil off" losses. In other words, the gaseous hydrogen is vented into the environment.

A hydrogen engine is a gas engine that runs on gaseous hydrogen instead of liquid fuel (such as diesel and petrol). There are pure hydrogen engines which are powered with pure hydrogen. There are also bi-fuel hydrogen engines which are powered by a fuel mixture of hydrogen and other gases (such as methane and natural gas).

A hydrogen engine is considered as an alternative to a fuel cell, as existing combustion engines can be converted with relatively little technical effort. Studies show, however, that the cost advantage will considerably decrease with the ramp up in fuel cells. In addition, hydrogen engines struggle with a lower efficiency, higher maintenance requirements and the label of not being 100% carbon neutral.

"Fuel cell electric vehicles“ (FCEV) are exclusively powered by an electric motor just as with "battery electric vehicles" (BEV).

Unlike BEV, the electrical energy required is not provided by a large drive battery (known as a traction battery). Instead, it is made available by converting chemical energy from the alternative energy source into electrical energy - made possible by the fuel cell.

At the moment, fuel cells are not yet designed for such rapid and long lasting load changes as combustion engines are. For this reason, a (small) drive battery is also installed which is fed when the load is low and supplies additional energy when the load is high. This allows the fuel cell to be operated at a relatively constant load when the FCEV is being driven.

Fuel cell power plants (FCPP), combined heat and power (CHP) and fuel cell combined heat and power plants (FC-CHPPP) impress with their high overall level of efficiency. Depending on the fuel cell technology used, the electrical efficiency is currently around 30-60 %. The overall efficiency can be over 95% as electricity and heat are generated directly from the electromechanical reaction without any further conversion steps.

Fuel cell power plants have so far been developed primarily in the power range of 10 kw to 3 MW. In recent years, however, the development in the lower power range has increasingly gone in the direction of micro and nano fuel cell combined heat and power plants with 0.3-1.5 kW electrical output and 0.6-2.0 kW thermal output for detached and semi-detached houses. In the upper power range, power plants with around 80 MW have already been achieved, and this is to be increased further in the coming years through modular designs.

Power-to-X (also called PtX or P2X) refers to the use of electricity surpluses from variable renewable energies to aid all kinds of technologies. For example, these surpluses can be stored directly in batteries (power-to-power), converted into heat (power-to-heat) or used to produce chemical energy sources (power-to-gas, power-to-liquid).

If the surplus electricity is used to produce chemical energy sources, further differentiation is often made (e.g. power-to-hydrogen, power-to-syngas, power-to-ammonia, power-to-fuel).

Ammonia (NH3) is a chemical compound of nitrogen and hydrogen, which is present in gaseous form under normal conditions. Since the development of nitrogen fertilisation by Justus Liebig (around 1840), it has been one of the most important basic chemicals. But it was not until the Haber-Bosch process was used on an industrial scale at BASF in Ludwigshafen around 1913 that a significant increase in annual production was possible. Today, ammonia is one of the most produced chemicals (146.5 million tonnes in 2021, 80 % of which for fertiliser) and the basis for the production of all other nitrogen compounds.

In view of the developing sustainable hydrogen economy, a further increase in annual ammonia production can be expected. The reason: it is better suited for transport and storage than pure hydrogen.

Due to the rather high boiling point of -33 °C, it is much easier and cheaper to liquefy ammonia compared to hydrogen (boiling point -252 °C). The higher volumetric energy value of ammonia compared to hydrogen (3.2 kWh/l compared to 2.8 kWh/l) is also highly relevant, especially for transport logistics.