PhD Thesis - Energy Systems Research Unit - University of Strathclyde

5.7 Electrolyser Model
An electrolyser uses excess electricity to create pure hydrogen from water,
which may then be used for transport, for the production of heat or electricity, or
stored for later use. It is, effectively, a fuel cell operating in reverse, and shows
the same increase in efficiency at lower running levels. Again, complex models
such as TRNSYS and SIMELINT do exist to predict the output of specific
electrolysers [25,26], but these require large amounts of data specific to each
electrolyser, and give more detail than is required here. Therefore, easily
available manufacturers’ data will again be used to provide a sufficient
approximation of the performance of a range of electrolysers.
The performance of an electrolyser is defined by its electricity consumption
(kWh/Nm 3 ), which varies at partial loads, and may be quoted at a range of
percentage loads, and by its maximum hydrogen production rate (Nm 3 /hour).
The unit Nm 3 represents one normal cubic metre of hydrogen (measured at 0°C
and 1 bar), which is equivalent to 3 kWh. The amount of water required is
around one litre/Nm 3 , and there is usually a minimum acceptable percentage
load of around 25%. If the hydrogen produced is to be compressed or liquefied
for storage, the energy required to do this must be taken into consideration, and
this is also given in kWh/Nm 3 . The performance of an electrolyser is not
significantly affected by ambient temperature or altitude, and as it operates at
ambient temperature, it does not require to run on standby. Little waste heat is
produced, and this is not enough to be of use for CHP [27,28]. An example of
the electrolyser definition window can be seen in Appendix 1, Figure A1.50.
5.7.1 Electrolyser Performance Algorithm
To determine the amount of hydrogen that may be produced, the amount of
electricity available for use over each timestep must be known. Therefore, the
rate at which electricity is available (kW) must be divided by the number of
timesteps per hour to give the total amount of electricity available for use
(kWh). The maximum amount of hydrogen that may be made over the timestep
(Nm 3 ) must also be determined by dividing the maximum production rate by the
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