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GREEN HYDROGEN COST REDUCTION For PEM electrolysers, there are two modes of pressurized operation: balanced and differential. In the balanced mode, there is a similar pressure on the anode and cathode sides. This means the membrane, spacers and porous transport layer can all be thinner given the lower requirement for mechanical strength. This in turn translates into a higher efficiency, since there is less internal cell resistance. The downside of the pressurised mode is that the entire equipment needs to be designed for a higher pressure, resulting in a penalty cost and safety issues due to the pressurised oxygen. In the differential mode, only the anode is subject to the higher pressure, as it receives all the pressure from the cathode side, where the production of pressurised hydrogen occurs. Figure 12. 18% 16% 14% 12% 10% 8% 6% 4% 2% 0% Based on IRENA analysis based on BNEF, 2019. Energy losses for the multi-stage mechanical compression of hydrogen. 0 200 400 600 Pressure (bar) Max 800 1000 Min From an efficiency perspective, compressing to 30 bar in the electrolyser (or even 100 bar, depending on the specific membrane) has a relatively small penalty on efficiencies and additional costs for pressure vessels, while this region is the one with the steepest losses in mechanical compression10 (see Figure 12). Therefore, it makes sense to perform this step with the electrolyser to generate the largest savings for the system and compress to the final delivery pressure in a separate compressor. This means PEM, or pressurised alkaline, is preferred from this perspective. Otherwise, the additional energy penalty for compressing from 1 to 30 bar (typical operating pressures for the alkaline and PEM electrolysers respectively) is about 3.5%-4% equivalent of hydrogen lower heating value (this energy is used as electricity instead of directly from the hydrogen compressed). Yet, efficiency is only one dimension. From a cost perspective, for a 100-MW electrolyser, a compressor of about 4 MW would be needed to take the stream from a typical atmospheric alkaline electrolyser to the typical operating pressure of a pressurised electrolyser (30 bar). This size already benefits from economies of scale and only adds about 38 10 A critical parameter for compression is the pressure ratio. Going from 1 bar to 60 bar is a ratio of 60 and therefore, it requires more energy than going from 60 bar to 300 bar. Compression losses as a fraction of hydrogen LHV (%) Pipeline Underground storage Buses/ trucks Passenger vehicles TanksPDF Image | GREEN HYDROGEN SCALING UP ELECTROLYSERS
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