[ efficiency ] electrodes , and heat dissipation . These losses increase over the lifetime of the stack due to the activity of various degradation mechanisms .
Power consumption values supplied by manufacturers often provide insight only into this first category . They are typically expressed as a kWh value of electricity input per unit of hydrogen produced , referring to the power consumption of the stack at the beginning of life ( BoL ) under optimal load conditions . To obtain the stack efficiency , this figure is often converted into a percentage value relative to the energy content of the hydrogen – either the lower heating value ( LHV ) or the higher heating value ( HHV ).
The second category covers inefficiencies caused by an electrolyser ’ s operational profile . The efficiency of the power-to-hydrogen process varies across an electrolyser ’ s operational load range , generally peaking at under half of the rated capacity .
In practice , electrolysers supplied by renewable energy sources will also undergo extended
periods of low electricity supply during which the system needs to be placed on standby . In standby mode , the system must continue consuming electricity to maintain internal temperatures and pressures , allowing for rapid start-up . Alternatively , an electrolyser can be shut down entirely , which increases both the time and the energy inputs required for start-up .
The actual efficiency of an electrolyser over its lifetime therefore depends on its typical load profile when operating , as well as the frequency and severity of standby and shutdown events . A more unpredictable operational profile will also accelerate the rate of cell degradation and consequently impact either the lifetime efficiency of the project or the expenditure required for stack replacements .
Into the third category can be placed inefficiencies from the surrounding balance of plant ( BoP ), such as power electronics , water purification systems , gas processing technologies , and compressors . The extent of these inefficiencies depends partly on the system boundaries – the collection of
$ 9 $ 8
- $ 1.3
$ 7 $ 6
- $ 1.0
LCOH ( USD / kg )
$ 5 $ 4 $ 3
- $ 0.7
$ 2 |
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$ 1 |
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$ 0 |
Upper bound onshore wind |
Average onshore wind |
Lower bound onshore wind |
System efficiency at BOL 60 %
System efficiency at BOL 70 %
Fig . 1 . Illustrative impact of a 10 % improvement in system efficiency on the LCOH of a PEM electrolyser project using upper , medium , and lower bound electricity costs for onshore wind . Source : Guidehouse Insights
Hydrogen Tech World | Issue 16 | June 2024 15