Implications of Battery and Gas Storage for Germany’s National Energy Management with Increasing Volatile Energy Sources

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Interesting and highly realistic study on German energy transition. I can agree with authors regarding the reactivation of German nuclear power plants.

Still, the study seems to be rather superficial, without much calculations. It is impossible to launch the web tool stated in the paper and try to repeat some of the calculations. Data in the graphs are presented in daily resolution. Was this a time resolution in the calculations? Daily energy balance is just a part of the problem, but to really investigate the power system balancing phenomena one should use hourly or even 15- miuntes resolutions, to have a proper insight into the power system balancing capabilities. Please clarify on that.

It is hard to recognize a scientific novelty in the paper. Please clarify on that.

Author Response

Thank you for your constructive critical review.   I don’t understand, why the web tool did not work. In any case, it is now updated and documented as an Appendix of the paper. The Data have 15 minute resolution (95 data points per day), and all calculations have been done in this high resolution.  I took your advice serious, and added a dropdown to the tooll to select a specific month. Then the graphs are in a much higher optical resolution.  We added one such higher resolution graph into the paper

Due to your and other reviewers comments, we have added several new subsections to clarify the questions:

• In the introduction, we added a subsection “Modelling approaches for energy transition scenarios”, where we not only describe other related concepts, but also write the specific goal of our approach. One of the novelties of the paper is our aim not to prove that energy transition works, but to find the pain points where it might fail.
• We extended the subsection “Scope of this investigation” in the introduction where we point to the other novelty, the strict orientation on measured data rather than models.
• We have taken very serious your point that you are missing calculations. We have added a subsection “Outline of the simulation process”, where we describe the processing steps verbatim, followed by the extended subsection of the simulation details, where we describe the actual calculations in detail.
• As a consequence of these extensions, also the number of references has increased significantly. 

 

Reviewer 2 Report

Comments and Suggestions for Authors

This paper evaluates the existing energy policy for Germany's grid to meet the climate-neutral standard KND2045. It finds severe shortfalls in energy from the existing/planned wind, solar, and planned storage deployments on the electric grid. I find some overall deficiencies in the paper, which I have marked up attached and summarized below.

1. The paper spends relatively little time in the Introduction describing the context and motivation of the paper w.r.t. KND2045. At the same time, it also lacks a rigorous literature review of existing grid-wide models, including capacity-expansion models, which would address some of the shortcomings of the methods and conclusions in the paper.
2. Many key assertions in the paper are a) addressed far too quickly and with little to no quantitative description, and b) not supported or backed up by any sort of citation of existing literature.
3. The analysis of the paper is supported by a model that is a) poorly described in terms of its capabilities and optimization functions to determine the dispatch of energy resources on the grid, and b) is acknowledged by the authors themselves as overly simplistic and optimistic.
4. I find the quality of the figures quite poor, with small text, missing axis labels, poor cropping, and even labels that are not in English.
5. The results section could do more with the model and far strengthen the novelty of this paper, if it included capacity expansion optimization to determine what the least-cost deployment of energy resources could be, rather than simply running a model in open-loop with existing KDN2045 scenarios.
6. The paper concludes rather narrowly that only conventional power plants and nuclear power plants are the solution to the energy shortfall, rather than exploring other options, or even running a least-cost capacity expansion model, as suggested above.

Many other papers and studies are able to meet similar net-zero or carbon-neutral policy goals with low/zero-carbon fuel sources, via capacity expansion models that can optimize for least-cost solutions, rather than ad-hoc alterations of a static model.

I strongly recommend the authors to study more literature on capacity expansion models and their results, and to amend their model and methods to include such analysis. I suggest the authors to read reports from other existing capacity expansion studies, such as the Alternative Fuels Study produced by the Hawaii State Energy Office, which looks at a broad range of scenarios, technologies, and fuel sources, to meet a similar 2045 renewable portfolio standard. The California Energy Commission also has many studies relating to this as well.

Comments for author File: Comments.pdf

Comments on the Quality of English Language

Much of the language used in the paper is far too casual and unfit for academic publication. Exclamation points are rarely appropriate for papers.

Author Response

Your thorough and challenging review of the paper has impressed us. We have addressed each remark of yours. But we did not make changes everywhere. E.g. in the title - meanwhile the word "volatile" or "volatility" is also owned by the energy transition community.    It also may well be that the expressions "Dunkelflaute" (dark wind lull) and "Hellbrise" (the opposite, lots of sunshine and windy) may enter the English language (like "Kindergarten" or "Sitz im Leben") 

Our response to your points:

1. Following your suggestions the introduction has been rewritten, wie new subsections "Modelling approaches for energy transition scenarios", where the capacity-expansion models are discussed, and "Scope of this investigation"
2.  Several new subsection have been created to address areas like demand-side management, energy exchange within Europe, energy exchange beyond Europe, curtailing renewable energy production. Many new references have been added. 
3. The model description has been enhanced, by a new subsection "Outline of the simulation process", and also the "Simulation details" have been improved. 
   
   The term "overly simplistic and optimistic" has meanwhile been given its correct context in the paper: When we know that all assumptions are optimistic, then we are in the best possible position prove a possible failure of the concept. If the concept fails on a test with optimistic assumption, then it is proven to fail for less optimistic assumptions. As a matter of fact, the scientific approach is always to prove something wrong, to falsify.  Preliminary "truth" is when I fail to falsify. Your argument to optimize is only valid at the end of the paper, if you accept the paradigm shift that may prevent failure of the energy transition.
4. The figures have been remade, after change of the default settings of the graphics part of the Online-Tool from where the diagrams are taken. 
   The Figure with the German text has been removed and replaced with a table.
5. The goal of the paper has not been to optimize a model but to demonstrate the limits of volatile energy production in combination with storage. Therefore our focus are the KND2045 scenarios. We have accepted your argument, that an expansion of wind and solar be twice the volume of KND2045 is totally unrealistic and therefore dropped it.  
6. With the given parameters, after ruling out significant changes in renewable energy production, there are not many possibilities left. By definition we want to rule out demand reduction (demand-side management is discussed in the paper), which is a synonym for poverty, then there is only backup power left. If backup power is to be emission free, it has to be nuclear. 

We appreciate the suggestions for diving into the capacity-expansion models. That may well be a next logical step for a future paper. 

Reviewer 3 Report

Comments and Suggestions for Authors

This article critically analyzes Germany's energy transition plan, KND2045, with a particular focus on the challenges of integrating variable renewable energy sources (RES) alongside various storage technologies, employing a well-researched and technically rigorous approach. The authors project future scenarios for 2023, 2030, and 2045, emphasizing practicality, cost, and system efficiency, utilizing real-time high-density energy production and demand data from 2023 and 2024. The manuscript can be accepted for the publication in sustainability after addressing the following major comments. 

1. Has the simulator created for this research experienced peer review or validation by outside experts? If so, kindly list references or findings.
2. Please benchmark your tool against another acknowledged model (such as PyPSA or TIMES) with a sensitivity analysis.
3. Could the writers strike a balance between appreciating any reasonable or feasible aim of KND2045 and criticizing it?
4. As possible complements to storage, kindly include additional debate on demand-side flexibility, grid enhancements, and distributed energy models.
5. In the 2045 horizon, is the zero energy exchange assumption with European neighbors reasonable? Either justify this assumption or run a sensitivity study with limited import/export capability.
6. The authors presuming €100/kWh and a 10-year battery lifespan Is this line of consistency with the most recent 2030–2045 projections? To account for uncertainty, kindly add a range of battery cost/lifespan forecasts.

 

Author Response

Thank you very much for your thoughtful and challenging comments and questions.   
These questions together with those of the other reviewers have motivated to add several new sections. 

Your specific questions are answered as follows:

1. The simulator tool is currently being peer-reviewed, but the process is not yet finished. Additionally, we have done the following to make the tool transparent:
   1. We have created an Appendix to the paper where the tool is described in Detail
   2. The Tool is written in HTML and JavaScript and therefore completely open source. The code is readable with every web browser's source code functionality
   3. The whole JavaScript code is not more than 650 lines of code, and the 3 critical functions are identified in the paper. Each of them has less than 5 lines, which implement equations of the paper.
2. To be honest, this seems to us asking a bit too much. From a mathematical point of view, we are talking about weighted additions, subtractions and Min/Max operations (which are described in the paper), otherwise only administrative operations of data (reading, renaming, concatenation, display as table, chart, or histogram). Looking at articles where PyPsa or Times is used, authors discuss how a mathematical expression describing e.g. the charging of a battery is implemented in one or the other framework.  In a generic language like JavaScript, the equation translates more or less directly into an expression.  The methods you propose, seem to apply to "closed" untransparent frameworks, but here we have plain readable text, as we described above. 
3. We appreciate this remark, because it helped to transform the "uneasiness" about Agora and KND2045 into a scientific concept, which we now clearly express in the revised paper. What we see mostly among those advocating the energy transition are projects or papers trying to "prove" that it somehow works or to advocate for an aspect of the concept. But progress in science is made by questioning and trying to make the concept fail and find a counter example.  Only then it justified to make optimistic assumptions (what we partly choose and partly have to do) or use an arbitrarily chosen data set. 
4.  Following your advice, we added a subsection about demand-side management focused on Germany.  We can show, that by substitution with battery storage, an upper bound for the potential of demand-side management can be found. We also added a discussion about grid enhancement within Europe and even beyond, but came to the conclusion to drop it under the current conditions.  For the purpose of our investigation, we deliberately use the copperplate approximation. If the concept fails under this over-optimistic condition, it will definitively fail with a distributed energy model. 
5. We dropped the zero energy exchange assumption and added a configuration field for an upper bound of import and export power. But in the light of current experiences with European neighbors, in our scenarios we stick to the relatively small but guaranteed limit of 5 GW for both export and import. The simulation tool obviously allows other choices. You may create different scenarios (you may even download the parameter configuration)
6. The price of battery capacity is one of the many free parameters that are hard to predict, but can be set in the initial configuration.   In the context of the paper, costs are not a key criterion. But you are right, in a future version of the tool and possible in a future publication, we will have to deal with price ranges. 

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

No more comments!

Reviewer 3 Report

Comments and Suggestions for Authors

The authors have thoroughly addressed all of my previous comments and concerns. I recommend accepting the manuscript for publication