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
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.
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.