Integration of Back-Up Heaters in Retrofit Heat Pump Systems: Which to Choose, Where to Place, and How to Control?

Round 1

Reviewer 1 Report

This study investigated the integration of back-up heaters in retrofit heat pump systems. The research topic is interesting and the structure of the paper is relatively complete. However, I have the following major concerns.

1.    The abstract should be well organized, the main novelty, methodology, objective, significance and results of the study should be clearly clarified. They are not clearly and logically described in the abstract.

2.    In the design of HP heating system, auxiliary heater is considered based on the equilibrium point temperature as illustrated in ASHREA handbook. Does the author consider it?

3.    What’s the meaning of ordinate in Fig. 5, 7, 8?

4.    How are the models built and how are the models solved? The authors are suggested to give detailed description.

Author Response

Dear Reviewer, 

First, we thank you very much for your time and effort put into reviewing our paper. According to the comments and suggestions, we revised our manuscript. We revised our English as suggested. Regarding the general review (score of “Can be improved”), we want to emphasize that two other reviewers do not suggest improvement in any of the listed categories. We hope that, including your comments and the comments by other reviewers, the manuscript is sufficiently improved.

Regarding your four specific comments, we provide four dedicated answers:

1. Thank you for your comment. We revised the abstract accordingly.
2. In our case studies, we use building performance simulations that are parametrized to represent building cubature and material according to Germany. Since buildings in Germany differ compared to e.g., USA, we use European standards to design our energy system. As the ASHRAE handbook focuses on North American households, comparing the design approaches does not improve the quality of our investigation, as no building envelopes and demands typically differ.
3. Thank you for this comment. The Figure’s caption already describes the meaning of the x-Axis. The y-Axis contains the different scenarios of the full factorial design. We added this information in the Figure caption as well. It should be clear now.
4. Thank you for your interest in the underlying modeling and solving strategies. All models in use are open-source and available in the git-repo for the corresponding contribution, as well as in other open-source Modelica libraries. Based on your comment, we empathized the use of Modelica and further stated solver settings. If you want to use or test our models, you can just download and execute them in Dymola following this link: https://github.com/RWTH-EBC/BackupHeaterIntegration

We hope that these changes meet your expectations.

Reviewer 2 Report

Well written, easy to understand, diagrams were clear and readable.

Author Response

Dear Reviewer,
we thank you very much for your time and effort put into reviewing our paper. 

Reviewer 3 Report

I was impressed, by the tremendous amount of work, and tremendous amount of material to read and analyze. Additionally, access to files on GitHub. A really very thorough analysis and collection of articles on buildings and energy.

I’ve some comments:

 The Abstract is a bit too long.

Line 35  - The gravity systems work quite well even at 50^oC (usually they start at 40^oC)

Line 47 – double dot “investment. [9,14,15].”

Figure 1 – It is “BH-HP” , and I think it should be “BH-AP” as it is in the description.

Line 321 – But you should mark, that these are the maximum temperatures when the outside temperature is on a minimal level for calculations. During the heating season, there are few such days with a minimum outside temperature. As it results from the multi-year measurements, most days are with average temperature - North Central Europe is 0 or -1 ^oC.

Line 333 – “As retrofit heat pump system are important for multiple locations,…” I think it should be “As retrofit heat pump systems are important for multiple locations,” or “As retrofit heat pump system is important for multiple locations,”.

Line 448 – Something is missing here:” … values below 73K h count as comfortable.”

Lines 466, 468 – the same

 References:

[6] please mark it [in German]

[12] “SEITE NICHT VERFÜGBAR” and please mart it [in German]

[13] please mark it [in German]

[19] please mark it [in German], but it is “Dessau-Roßlau, September 2016” not 2022??

[20] please mark it [in German]. 

 

Please check the Manuscript for spelling errors and punctuation.

Author Response

Dear Reviewer, 

Thank you for your detailed review and your time associated with it. We adjusted our manuscript based on your comments regarding spelling errors, styling, references, and the length of the abstract. We also included your comments regarding gravity-heating systems. We stated that the nominal radiator temperatures are just the maximum and occur infrequently.

We hope that these changes meet your expectations.

Round 2

Reviewer 1 Report

The manuscript has been revised carefully, and it is suitable to be accepted.