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AEM electrolysers suffer from a short lifetime, while limited information about their long-term operation, reliability and robustness is available. The stability of the AEM polymer used to fabricate membrane and catalyst layers is well recognised as a major issue, especially when operated with KOH as the supporting electrolyte. The main degradation mechanism is hydroxide (OH-) attack on the polymer backbone, which leads to membrane collapse and catalyst layer dissolution within a few days. One solution is cross-linking chemical methods, but this comes at a cost of cell efficiency. Another approach is by operating the stack without a supporting electrolyte (i.e. using only pure water), which can lead to a durability beyond 5 000 hours, but this results in much lower efficiencies, or current densities. Efficiency of a hydrogen production facility The system efficiency of a green hydrogen production facility, measured in units of kilowatt hours consumed per kilograms of hydrogen produced (kWhAC /kgH2), is a result of the individual efficiencies of the cell, stack and balance of plant, as follows. Cell: The efficiency profile decreases linearly from lower to higher load levels, so the higher the current input, the lower the stack efficiency. Naturally, the higher the hours of operation, the lower will be the efficiency due to degradation, though the aforementioned dynamic remains. At the operational level, the cell voltage is the element actually measured to infer the system performance, in such a way that, the higher the cell voltage, the lower the stack efficiency. Alkaline and PEM operate at different power density ranges, which has an impact on each technology’s footprint: Alkaline: Typically operates in a range of 0.2-0.8 A/cm2, since the diaphragm and electrodes are not manufactured to operate at higher current densities. PEM: Operates at higher current densities compared to alkaline, of about 2.0-2.3 A/cm2, though more efficient at 1.6 A/cm2 (with 1 MW as reference). Balance of plant: A range of system elements such as cooling, purifiers, thermal management, water treatment and others, consume power in order to operate, which also needs to be considered in the facility’s overall efficiency. Efficiency losses can be minimised by: designing the electrolyser facility while taking a whole-of-system perspective; using commercially-available components rather than custom made ones; and maximising system efficiency including balance of plant, tailored for the specific application. Rectifiers are a key component of the balance of plant. Rectifiers generally have very low efficiency at lower loads, rapidly improving until 15%-20%, which then remains relatively high from this point on. For this reason, the balance of stack efficiency can be improved by using the same rectifier for multiple stacks. This approach would reduce the number of rectifiers needed (investment costs) while also maintaining operation at higher efficiency levels, suitable in particular for hydrogen facilities of 20MW or larger. As a drawback, the system becomes less flexible and, therefore, such a setting would be recommended for facilities operating at flat power input/hydrogen production levels. At very low loads (marginally above zero), when water starts circulating, the system efficiency is low because equipment is already in operation, but production has not started yet, or is very minimal. From this stage to roughly 30% load, efficiency progressively increases and peaks at this level. Beyond 30% load, the overall system efficiency starts to decrease towards the nominal rate value. This behaviour is different than in, for example, many thermal power plants, which exhibit reduced efficiency with lower than nominal loads. It also presents a new feature that changes the operational strategy of the electrolyser. This can be operated at a lower-than-design load, benefiting from a higher efficiency (i.e. lower electricity cost per unit of hydrogen), at the expense of lower hydrogen production (and lower total revenues). Therefore, there is a tradeoff between capital and operational cost that should be considered at the project level. As presented in Chapter 3, Section 3, compressors, SCALING UP ELECTROLYSERS TO MEET THE 1.5°C CLIMATE GOAL 45PDF Image | GREEN HYDROGEN SCALING UP ELECTROLYSERS
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