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USD 0.07/kg H2. For higher pressures, the cost penalty would be the same for both technologies. This price differential translates to about USD 50/kW11,which represents the cost differential threshold for atmospheric alkaline stacks to be on a par with 30 bar (PEM) stacks. According to Saba (Saba et al., 2018), the cost increase for changing the design pressure from 1 bar to 15 bar is about USD 150/kW for an alkaline electrolyser. Another factor affecting cost is the type of design for the PEM electrolyser. Differential pressure mode is preferable, as it eliminates balance-of-plant high-pressure components on the oxygen side. Stack costs are at the same time increased, however, since stack components on the oxygen side need to sustain the set operating pressure. A tradeoff sits between 30 bar and 70 bar, depending on each business case and application. Compression also has a cost associated with the disposal of the compressor condensate, which can be as high as USD 1 000 per day for a 20 MW system. Compression costs are relatively small compared to overall production costs, even when considering operation at atmospheric pressure, with further benefits of a simpler design with cheaper materials for the lower pressures There are two other factors to consider. The first one is scale. Large compressors are more efficient than small ones and will result in a smaller cost addition to the hydrogen produced. Hence, as the scale increases, it tends to favour mechanical compression over electrochemical. The second factor is final delivery pressure. In case a pressure higher than the operating pressure of the electrolyser is needed, mechanical compression will be needed, in any case. In this instance, making the compressor larger and being able to achieve a cost reduction in the electrolyser, might be attractive. SCALING UP ELECTROLYSERS TO MEET THE 1.5°C CLIMATE GOAL Power supply system Power supply for electrolysers represents a significant cost component (20%-30% of the total), yet there is a high potential for cost reductions (see Chapter 2, Section 6). For small-scale electrolysis plants, power supply is often either part of the package sold by electrolyser manufacturers, or a custom design from engineering, procurement and construction (EPC) contractors for each individual facility. As the scale of the facility increases, standard utility scale power supply systems become available from leading manufacturers of electrical equipment. This can significantly reduce cost and increase the performance of the power supply for electrolysers. Further optimisation can be achieved by careful system integration of different components in the electrolyser facility, optimising the entire facility rather than individual components and leveraging efficiency gains in different parts of the balance of plant, including the power supply. The water electrolysis industry is benefiting significantly from the improvements made in the solar industry, and the power supply also has an important role to play in maximising the efficiency of the electrolysis facility. While the electrolyser stack has a linear efficiency decrease with the increase in output, due to the increase in voltage, rectifiers have very low efficiency at low load (Kim et al., 2013). Depending on the expected operating regime (e.g. fixed output, variable input driven directly by solar or wind), the sizing and design of the power supply can be optimised to maximise system efficiency, defined as the minimisation of efficiency losses from power input to hydrogen output at the required pressure. Optimised design affects not only the efficiency, but also the flexibility of the system, as discussed in Chapter 3, Section 5. Power supply system cost can decline through economies of scale, standardised designs and participation of specialised electrical equipment suppliers instead of electrolyser manufacturers 11 Assuming a 50% capacity factor for a hybrid PV-wind plant. 39PDF Image | GREEN HYDROGEN SCALING UP ELECTROLYSERS
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