| Hydrogen
is not congested. When combined, these regulatory restrictions are likely to make high-utilisation green hydrogen schemes uneconomic in most European countries. However, there are two exceptions to these regulations that soften their impact dramatically. First, electricity will be regarded as renewable and additional if it is produced from renewable power plants (of any age), drawn during an imbalance period and avoids downward redispatch of the renewable power. Furthermore, if the electricity for the electrolysis is purchased at a low price (<€20/MWh) the temporal correlation requirement is assumed to be met. The basis for the above exceptions is that this electricity may otherwise have been curtailed or is being generated at a period of very low demand from other users. The implications for intentionally intermittent operation are positive.
Balancing electrolyser capex and efficiency When an electrolyser is to be operated continuously at full load, efficiency is of primary importance in technology selection. A small improvement in the amount of hydrogen produced per MWh of electricity consumed will comfortably pay for the additional capital cost of a more efficient electrolyser. In these high utilisation green hydrogen schemes, solid oxide electrolysers or advanced alkaline electrolysers may be favoured.
On the other hand, intentionally intermittent operation of an electrolyser drives down utilisation because it is idle for a portion of the time. Low utilisation puts pressure on minimising capital. Also, since the electrolyser is not operating for so many hours, its efficiency plays a smaller role in the economic viability of such a scheme. With intermittent operation, low electrolyser capex becomes more important than high efficiency when considering technology selection. For many green hydrogen schemes, pressurised alkaline electrolysers are selected due to their availability, technical maturity and low capital cost. A limitation of these electrolysers is that hydrogen begins to cross over the membrane to the oxygen side at low loads. This generally limits their safe operating range to 30%-100% of nominal power consumption. However, at 30% power consumption, their hydrogen production efficiency can be cut in half. In the ideal case, pressurised alkaline electrolysers like to be turned on, ramped up and left alone.
Best fit technology
The key attribute of an electrolyser to exploit the concept of intentionally intermittent operation is hyper-flexibility. Hyper-flexibility means that an electrolyser can turn on and off rapidly, and can turn down to consume almost zero power. Proton exchange membrane (PEM) electrolysers (Figure 2) are the best-fit for bankable projects exploiting intentionally intermittent operation today. PEM electrolysers can be more tolerant of rapid ramping and daily idle periods associated with intentionally intermittent operation.
Figure 2. Pressurised PEM electrolysis process. Source: sbh4 consulting
The capital cost of a PEM electrolyser stack can be up to twice that of a pressurised alkaline electrolyser stack sourced from China. However, this cost differential is progressively being eroded as PEM technologies mature and manufacturing scales up. Furthermore, when considering the balance of stack and total project costs, the difference in cost between PEM and pressurised alkaline stacks becomes less significant.
Complementary infrastructure Balancing intermittent hydrogen production with the demand profile of end users requires hydrogen storage. The build out of hydrogen
pipelines with integrated storage will serve the dual purpose of connecting suppliers with offtakers and be a hydrogen storage buffer. The European Hydrogen Backbone plans to integrate pipelines in addition to high capacity underground storage (see Figure 3). In northern Germany and the Netherlands, salt cavern storage is proposed. Where the geological conditions make this possible, this technology offers the lowest levelised cost of storage (LCoS). In Southern Europe and the Nordics the option to use salt cavern storage does not exist. Here, lined rock caverns are likely to be the technology that offers the lowest LCoS at scale.
Figure 3. European Hydrogen Backbone: the vision. Source: European Hydrogen Backbone
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