Series of articles concerning hydrogen: Electrolyzer operating modes

In the course of the last few years, an immense dynamic can be felt in the field of hydrogen. At the same time, great progress has been made in research and development at many points along the hydrogen value chain and important findings have been made. In the first series of articles on hydrogen, the basics of the gas were explained along its value chain from production to transport and storage to application.

In this second series, individual focus topics are dealt with in detail and the current state of knowledge is summarized. This fourth article lists different operating modes for electrolyzers.

When hydrogen is discussed as an energy carrier of the future, it is usually green hydrogen produced from renewable electricity by means of electrolysis that is referred to. Electrolyzers use electricity to split water into its components – oxygen and hydrogen. If the electricity comes from renewable sources, hydrogen production is also largely CO2-neutral.

While in the simplest case, hydrogen production through electrolysis takes place continuously at constant nominal power, a number of alternative operating models are also possible. These are enabled by flexible part-load operation of the electrolyzer: In part-load operation, the electrolyzer produces less hydrogen but also consumes less electricity. This allows for a potential reduction in the cost of producing hydrogen by shifting electricity purchases to the hours with the lowest electricity costs or by tapping additional revenue streams, such as providing system services.

The electrolyzer can be operated not just according to one, but several of these optimization goals. Accordingly, a variety of combined modes of operation are conceivable. Such more complex operating modes are of interest to electrolyzer operators primarily because hydrogen production by electrolysis on its own is economical only in a few cases. Levelized Cost of Hydrogen (LCOH) is often used for comparison, which is typically much higher for renewable hydrogen than for fossil production from methane.

When operating an electrolyzer to produce green hydrogen, technical and regulatory constraints must be considered in addition to economic optimization goals. These are first described below. Then, an overview of five operating modes is given, which serve as cornerstones for the design of electrolyzer load profiles, and can also be combined in real operation.

Technical operating conditions

In the flexible use of electrolyzers, two technical capabilities of electrolyzers are in the foreground: the ability to change between operating states (load gradient) and to dynamically operate at partial load [5].

An electrolyzer is in one of three operating states: ON, STANDBY, or OFF. Regular operation takes place in the ON state. For this purpose, the electrolyzer is warmed up to operating temperature. During longer breaks in operation or during maintenance procedures, the electrolyzer is shut down. For this purpose, the product gas tanks are emptied and the temperature of the electrolyzer adapts to the environment. Thus, to resume operation from the OFF state, a longer heat-up phase is necessary. Depending on the size and operating temperature of the electrolyzer, this can take between 5 minutes (compact PEMEL at 50 °C) and 14 hours (high-temperature SOEL at 800 °C) [1, 4] and lead to a reduced lifetime of the electrolyzer due to thermal stress [10].

To allow short breaks in production, the electrolyzer can also be brought to STANDBY mode. Here, no hydrogen is produced, but temperature and pressure are still maintained [4]. Startup from STANDBY mode, for both PEM and modern AEL, typically occurs within a few seconds [1, 11]

Electrochemical effects cause degradation of electrodes or electrolytes even in STANDBY mode [3, 14, 18], which is why manufacturers often specify a maximum number of ON-STANDBY cycles with guaranteed efficiency and lifetime. This can be specified as a daily limit (about five cycles per day [6]) or over the lifetime (maximum 5000 cycles [18]).

Electrolyzers can also be operated at partial load. The range of output in which this is possible is called the flexibility range. Its limits are set by operational safety and plant technology and can range from 5% to 100% of nominal capacity [4, 12]. Within the flexibility range, efficiency remains nearly constant [11], and power changes are typically possible within seconds [5, 8, 13], again regardless of the type of electrolyzer.

Green electricity procurement

In order for the hydrogen produced to be considered renewable, the electricity used must also meet certain requirements. To this end, there are a number of legal stipulations and definitions that are relevant for different applications. Of particular relevance is the definition of renewable fuels of non-biological origin under the RED II at EU level, which – originally intended for the transport sector – is likely to be extended to other sectors according to the current status (February 2023) of the EU legislative process. Here, hydrogen may be designated as green if the electricity used is either supplied from a renewable energy plant (RE plant) via a direct line to the electrolyzer, or if a power purchase agreement (PPA) has been concluded with a green electricity producer and some additional conditions (including simultaneity and additionality) are met. When grid electricity (or a mix of grid and green electricity) is used, hydrogen is considered green to a proportion of the share of RE in the electricity mix (European Commission 10.02.2023).

Although the regulatory framework under consideration here is still under development and other definitions may be necessary depending on the application, the trends are clear. For example, operating modes based on direct deliveries of wind or solar power or PPAs will have a high potential for green hydrogen production, while the purchase of grid electricity will only result in partial green hydrogen production.

Continuous Operation

Here, the electrolyzer is operated continuously at nominal power. Either pure grid electricity can be used or a renewable power source is combined with grid electricity. This mode of operation makes no special demands on the technical properties of the electrolyzer. The predictable, constant hydrogen production makes this operating mode attractive for an application in industry. However, hydrogen produced in this way is significantly more expensive than conventionally produced hydrogen [9, 15].

Electricity-cost optimized operation

Offering control reserve

With flexible partial load operation, electrolyzers can contribute to a stable electricity supply. In particular, offering frequency containment reserve or automatic frequency restoration reserve (FCR, aFRR) is a suitable option. For this purpose, the electrolyzer is operated at a fixed set point by default, and when the grid frequency deviates from the base frequency (FCR) or the aFRR case is called up by the grid operator (aFRR), its power is ramped up or down within 30s or 5 min for a limited period in a previously determined range. Both symmetrical and positive and negative asymmetrical control reserve are possible, whereby the set point must be adjusted accordingly. The compensation is paid by the grid operator and consists of a premium for the mere provision of the control power and, in the case of aFRR, an additional labor price for the work actually performed.

This additional source of income reduces the dependency on electricity and hydrogen prices. In grid areas with highly fluctuating electricity production, this form of electrolysis operation can already compete with fossil production [9, 16, 20].

Operation following renewable energy production

In this operating mode, the electrolyzer power profile follows that of a (often directly connected) renewable energy (RE) installation. The relative dimensioning of electrolyzer and RE installation can be optimized based on the operating specifications of the RE installation and demands on hydrogen supply. This way, green electricity production is optimally used for hydrogen production. A grid connection may not be necessary or is only used to feed in surplus electricity. At current capital costs and revenues due to hydrogen sales, this operating mode does not yet form a basis for a solid business case.

A problem for operators of RE installations is that in times of high green electricity production, electricity prices tend to be low, or that RE installations are not allowed to feed in electricity due to redispatch measures. Here, an electrolyzer can come in helpful: While generated power is primarily fed into the grid, at low electricity prices or high production, a part of the renewable power can be diverted to the electrolyzer.

In regions with highly fluctuating electricity prices and high curtailment rates, this operating mode can be an economically feasible basis for a business model [16, 17]. In this case, however, electricity sales represent the main revenue stream.

Realistic electrolyzer operation as a combination of operating modes

To generate profit with the operation of an electrolyzer, it is beneficial to consider as many different income streams and operating modes as possible. A promising scenario, for example, is the combined use of electricity from a neighboring renewable source and the electricity grid while simultaneously offering operating reserve. The continuous operation at nominal power, on the other hand, is likely to stay an exception – research projects expect a yearly utilization of 50 – 75%.

Hydrogen production from electrolysis is a growing business opportunity. In this dynamical environment, new knowledge and expertise will be developed over time. The FfE will continue to monitor developments and undertake research on relevant topics.

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