the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 29 Oct 2024
Backup power supply for a hydrogen-producing offshore wind turbine – a technology comparison
Abstract. Hydrogen is an important energy carrier for the transition of the energy sector towards decarbonization. An attractive option for its sustainable production are independent offshore wind farms that include all systems for the hydrogen production directly on the wind turbine platform. However, these systems face a challenge in maintaining a constant electric power supply on the turbine platform during periods of calm winds as they are not connected to the onshore electrical grid. This study evaluates and compares different technologies for a backup power supply on the wind turbine platform. Due to the limited energy storage capacities of battery systems and thus, short energy bridging times, systems are investigated that make use of the platform-produced hydrogen to generate electricity and consequently provide long bridging times. Three different backup power supply systems are investigated: A fuel cell system combined with a battery storage system (H2-FC+BS) as well as a hydrogen internal combustion engine with and without a battery storage system (H2-ICE+BS and H2-ICE). These systems are examined in terms of efficiency (hydrogen consumption), lifetime, robustness, maintenance requirements, space consumption, and costs. The results suggest that the hybrid system of a hydrogen combustion engine with an accompanying battery storage unit provides an optimal solution, offering a balanced compromise between efficiency, robustness, and minimized maintenance demands.
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RC1: 'Comment on wes-2024-135', Anonymous Referee #1, 10 Dec 2024
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A nicely presented and well written paper. A few different options are compared, but the authors are reluctant to make a clear final decision. Hydrogen processing seems to be implemented at the individual turbine level, so it would be interesting to compare against a wind farm approach (offshore energy hub).
Why implement energy conversion and storage equipment at turbine level and not at farm level? The paper should acknowledge and evaluate the pros and cons of such an approach.
It seems to be assumed that a standard wind turbine design is being employed, but since the turbine is not grid connected, does this offer additional degrees of freedom, and cost savings?
Table 1 – include a bigger space between the 2021 and 2022 columns
Line 60 - words such as downtime, outage, interruptions, etc. imply that there is a problem with the wind turbine (which needs to be fixed), while actually it is either too windy or not windy enough which prevents the wind turbine from operating. Consequently, the terminology used here should be changed.
Is 2 years of data sufficient to make robust decisions?
Fig. 3 – all objects are labelled, but some labels are in German, and some are quite cryptic, so could more user-friendly labels be applied to all objects (in English)?
Fig. 4 – change angel to angle
Fig. 6 – a legend showing the significance of the different line colours would be helpful
Line 135 – maintenance time and access to offshore platform to replace filters?
Line 148 – refer to Table 3?
Table 3 – how is the specification determined?
Line 168 – some clarification on the nature of the Dymola simulations would be helpful – presumably the simulation was just determining the steady-state efficiency at different loading levels?
Line 183 – as above, the text is rather vague on the nature of the simulations performed
Line 245 – cost discussion is very short and very qualitative – can this be made longer and more quantitative in nature?
Clarify why waste heat for heating was deliberately left out
What is the final recommendation by the authors? A summary table comparing the different options would be helpful, perhaps with good, average, bad scores for the various criteria outlined in section 5
Citation: https://doi.org/10.5194/wes-2024-135-RC1 -
AC1: 'Reply on RC1', Linus Niklaus, 18 Dec 2024
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Thank you very much for your comment.
I can understand your comments very well, as similar questions arose during the project work. I will try to answer your questions as best I can.
Why implement energy conversion and storage equipment at turbine level and not at farm level? The paper should acknowledge and evaluate the pros and cons of such an approach.
The basic idea of producing the hydrogen at each individual wind turbine was dictated by the project framework. The underlying idea is to create a flexible and modular system based on container solutions that can be assembled on land and then only need to be connected at sea. At the same time, this results in a system that is independent of the overall size of the wind farm. Mass production of the turbines is expected to result in cost advantages. In some places, however, it would certainly be easier to realize one central system instead of many small ones.
It seems to be assumed that a standard wind turbine design is being employed, but since the turbine is not grid connected, does this offer additional degrees of freedom, and cost savings?
A “standard wind turbine” from our project partner is assumed. The reason for this decision is to concentrate on the development of the hydrogen production plant within the scope of the project and not to increase the complexity of the new development by redesigning the wind turbine. However, the ulterior motive is certainly to be able to use the same turbine for different applications (with and without hydrogen production) in order to reduce the costs for a single turbine by producing large quantities. By adapting the turbine, however, there is certainly great potential for optimization, especially with regard to the electrical connection of the electrolyser. This would have comparatively little impact on the backup power supply of the plant.
Is 2 years of data sufficient to make robust decisions?
The years 2021 and 2022 are two years with very different weather conditions in the German North Sea. 2022 was a very warm and relatively windy year. 2021, on the other hand, was much colder and less windy. These two years therefore represent the extremes for the system. It is to be expected that there will be further years in between. Despite the differences, there is no qualitative difference between the systems in the two years in the efficiency comparison (Figure 9).
Line 135 – maintenance time and access to offshore platform to replace filters?
The maximum maintenance interval for the air filters of the fuel cell is 1 year according to the manufacturer. However, more frequent replacement is recommended to ensure reliability. The aim is to create a system that can operate maintenance-free beyond the annual service interval of the wind turbine in order to avoid additional maintenance time on the wind turbine. However, replacing the filter itself does not require a great deal of time. The problem is the accessibility of the system.
Table 3 – how is the specification determined?
Systems available on the market were used to specify the components. These were selected so that they were suitable for this system and could fulfill the required boundary conditions.
Line 168 – some clarification on the nature of the Dymola simulations would be helpful – presumably the simulation was just determining the steady-state efficiency at different loading levels?
Line 183 – as above, the text is rather vague on the nature of the simulations performed
To determine the efficiencies (Figure 8), only the stationary end state of the respective systems was considered. For the consumption calculations (Figure 9), the four scenarios described in Table 2 were simulated dynamically (i.e. from the start of the wind failure and the discharge of the battery storage system, through the start and warm-up process of the energy converters, to the restart of the wind and the recharging of the battery storage system). Both the hydrogen consumed and any electrical energy drawn from the wind turbine were then evaluated.
Line 245 – cost discussion is very short and very qualitative – can this be made longer and more quantitative in nature?
Reliable, qualitative statements about the costs of such systems are actually very difficult to make. On the one hand, the price of such complex components is constantly changing as development progresses. On the other hand, the number of units has a considerable influence on the price. So it makes a difference whether you build 1 such system or produce it in series. We have therefore deliberately decided against quoting numerical values in order to avoid false statements.
Clarify why waste heat for heating was deliberately left out
The direct use of waste heat was dispensed with as part of the project, as this would have required additional lines and connections on the platform and on the components. The cost of a purely electrical connection, on the other hand, is significantly lower. This should therefore not be implemented, at least in the first generation of the platform.
What is the final recommendation by the authors? A summary table comparing the different options would be helpful, perhaps with good, average, bad scores for the various criteria outlined in section 5
Our final conclusion is that for the conditions here, a hybrid solution consisting of a hydrogen combustion engine and a battery storage system represents an optimal combination of efficiency and robustness.
I hope I have been able to answer your questions clearly. If you have any further questions, I will be happy to answer them. I will of course take your comments regarding layout, graphics and choice of words into account when revising the document.
Best regards
Linus Niklaus
Citation: https://doi.org/10.5194/wes-2024-135-AC1
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AC1: 'Reply on RC1', Linus Niklaus, 18 Dec 2024
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