Electrolysis

The Electrolysis Process

In the water electrolysis process the hydrogen is produced by electrochemically splitting water molecules (H2O) into their constituents hydrogen (H2) and oxygen (O2). The decomposition of water takes place in a socalled electrolysis cell and consists of two partial reactions that take place at two electrodes. The electrodes are placed in an ion-conducting electrolyte (usually an aqueous alkaline solution with 30 % potassium hydroxide KOH). Gaseous hydrogen is produced at the negative electrode (cathode) and oxygen at the positive electrode (anode). The necessary exchange of charge occurs through the flow of OH-ions in the electrolyte and current (electrons) in the electric circuit. In order to prevent a mixing of the product gases, the two reaction areas are separated by a gas-tight, ion-conducting diaphragm membrane. Energy for the water splitting is supplied in the form of electricity.

To achieve the desired production capacity, numerous cells are connected in series forming a module. Larger systems can be realised by adding up several modules. Two types of electrolysers are common, atmospheric and pressurised units. An advantage of the atmospheric electrolyser, working at ambient pressure, is its lower energy consumption but the required space for the unit is relatively high. Pressurised electrolysers deliver hydrogen up to 30 bar. This reduces energy demand for compression and may even make compressor stages redundant. Today, atmospheric electrolysers with capacities of up to 500 Nm3/h and pressurised units with a capacity range of 1 – 120 Nm3/h are standard products.

Electrolysis Units in the CUTE Project

A hydrogen demand below 100 Nm3/h and the aspects of reduced space demand and lower compression energy requirements led to the fact that all the sites in the CUTE project using electrolysers decided to install pressurised units. The two main process inputs are electricity and water. The electricity for the electrolysis is taken from the grid as AC voltage, stepped down by a transformer and converted to DC voltage by a rectifier. Energy demand is higher than for atmospheric electrolysis (4.8 ± 0.1 kWh/Nm3 compared to 4.1 ± 0.1 kWh/Nm3 H2). This equals an efficiency of ~65 % referring to the lower heating value of hydrogen (3 kWh/Nm3) for the pressurised electrolyser.

State-of-the-art electrolysers can be switched on and off in minutes. They are thus capable of using off-peak electricity with lower tariffs from the grid and even intermittent renewable energy sources such as wind or solar power. A hydrogen-powered vehicle will only contribute to CO2-emission reduction if clean sources for the energy supply are used. This is why the cities that employ an electrolyser for on-site hydrogen production base their energy supply partly or fully on renewable resources (see design values table for details). Water may be supplied from the tap.

The electrolyser needs pure water, and a feed water treatment system is installed. About 1 litre of water is required to produce 1 Nm3 or 0.09 kg hydrogen. The elevated pressure of 10 – 15 bar reduces the energy demand for compression, the size of the electrolyser, and the size and costs of the compressor. The electrolyser units include the main components: transformer, rectifier, water purifier, lye handling system (cooling and pump), dryer, deoxidiser, compressor and storage. As the buses require a gas quality better than 99.999 % purification is needed. The only impurities direct from the electrolyser are oxygen and water vapour. Vapour is removed by the dryer and oxygen by the deoxidiser. After purification the hydrogen is compressed and stored. The produced oxygen could also be dried and purified for use in other applications. At the CUTE sites, the oxygen is released into the air only.

Key Characteristics of Installed Electrolyser Technology

Technology Related

On site electrolysers are available as turn-key solutions. The fully integrated operating units are preassembled on skid-mounted frames allowing simple transport and installation. The modular design allows an adjustable capacity range. • The pressurised electrolysers feature compact space-saving design and automatic, unattended operation. • The units have a low maintenance and spare parts need since no or only few moving parts are used (depending on supplier). • The electrolysers can be operated in a production range of 25 – 100 % of the rated capacity and plant availability is projected to be 98 % or higher. • Energy consumption is 4.8 kWh/ Nm3 H2 ± 0.1 kWh (electrolyser and pumps) and 5.1 kWh/Nm3 ± 0.1 kWh (incl. transformer, rectifier and gas cleaning). These design values refer to operation at max. load and an output pressure of 10 – 15 bar.

Safety Related

The electrolyser plants are designed to fulfil the highest safety standards (EN regulations, labelling and EC directives). This includes e. g. a safe, controlled plant shut-down in case of any deviations from normal operation and the usage of leak-proof gas and lye flow ducts.