[ efficiency ]
sub-systems and equipment that are treated as comprising the plant . The system boundaries are in turn influenced by the design of the project , the electrolyser technology , and the intended end use .
One of the largest sources of losses within the BoP is typically the power electronics , which consist of a transformer and a rectifier to convert incoming alternating current electricity into direct current electricity suitable for the stacks . Best-in-class commercially available rectifiers currently deliver conversion efficiencies of around 96 %.
Compression requirements are also an important variable . Electrolyser technologies that operate at pressures of around 30 bar – such as PEM and high-pressure alkaline – have minimal compression requirements for direct feed of hydrogen into most industrial processes , but still require compressors for on-site storage ,
tube trailer distribution , or grid injection . Conventional alkaline electrolysers that operate at atmospheric pressures have correspondingly higher compression requirements across the full range of end uses and distribution methods . Energy requirements for compression tend to increase at smaller system scales .
As a rule of thumb , BoP can be assumed to account for around 10 % of green hydrogen plant ’ s total electricity consumption , although in practice this will vary considerably between projects and technologies .
Stack efficiency improvements
Recent years have seen incremental increases in the stack efficiencies of the two dominant forms of commercial low-temperature electrolyser technology – alkaline and PEM . These increases have been achieved by targeting sources of losses within the stack , including through the use of more
100 % 90 % 80 % 70 % 60 % 50 % 40 % 30 % 20 % 10 %
100 % 90 % 80 % 70 % 60 % 50 % 40 % 30 % 20 % 10 %
0 %
0 % 100 MW Alkaline 100 MW PEM 100 MW Alkaline
100 MW PEM
BOP power consumption and losses Stack power consumption
Compression Gas purification Process cooling Circulation pump Losses from rectifier and transformer
Fig . 2 . Comparison of power consumption requirements at the system and BoP level for atmospheric alkaline and PEM electrolysers at BoL and rated load . Source : Adapted from Fraunhofer ISE
16 Hydrogen Tech World | Issue 16 | June 2024