Performance Optimization and Economic Evaluation of CO2 Heat Pump Heating System Coupled with Thermal Energy Storage
Round 1
Reviewer 1 Report
Please check and correct the following:
Line 50 „Lorentzen in 1990[18],“ should be „Lorentzen in 1990[17],“;
Line 68 „Yu et al. [24]“ should be „Binbin et al. [24]“
Line 197 „by N.H.S. Tay et al. 36“ please change to „by Tay et al. [36]“
Line 265 „can be seen the reference [6].“ Please check and change to „can be seen in the reference [6].“
Please check all other references!
line 147 „12,14- Water pump“ pump number 14 is not indicated on figure 1 – please check
Line 237 eqn (20) – please explain the exponent „n“
Line 268 „. the detailed components“ change to „. The detailed components“
Line 288 and forward there is used symbol „IC“ and in Nomenclature is stated „CI“ –please explain
Line 418 please check and change „10/kg“ to „10RMB/kg“
Please include in Nomenclature symbols „NTU, k, RPCM, R, Rmax“ that are used in equations (7) – (15); also „OTC, PC, …) etc.; please check all other symbols!
Some symbols are stated in Nomenclature („UsAs“, „ρs“ ) but they are not used in the text – please check and correct;
Please go through the model once again thoroughly!
Fig. 5 and Fig. 12 can be hardly seen; Is it necessary to use both figures - they are almost the same!
Author Response
Dear Editors and Reviewers:
Thank you very much for the review comments on our manuscript entitled "Performance optimization and economic evaluation of CO2 heat pump heating system coupled with thermal energy storage" (sustainability-1473431). We found that all comments are very valuable and helpful to improve the quality of our paper. We have tried our best to accommodate all comments and have made corrections/revisions which we hope meet the requirement. The revised part was marked in yellow in the updated manuscript. Our point-to-point responses to the review comments are as follows.
Reviewer #1
Please check and correct the following:
- Line 50 „Lorentzen in 1990[18],“ should be „Lorentzen in 1990[17],“;
Response: It has been corrected in the updated version (In line 50, marked in yellow).
- Line 68 „Yu et al. [24]“ should be „Binbin et al. [24]“
Response: Sorry for our careless, it has been corrected in the updated version (In line 72, marked in yellow).
- Line 197 „by N.H.S. Tay et al. 36“ please change to „by Tay et al. [36]“
Response: It has been corrected in the updated version (In line 203, marked in yellow).
- Line 265 „can be seen the reference [6].“ Please check and change to „can be seen in the reference [6].“ Please check all other references!
Response: It has been corrected in the updated version (In line 271, marked in yellow). And the other references have been checked.
- line 147 „12,14- Water pump“ pump number 14 is not indicated on figure 1 – please check
Response: Number 14 has been deleted in the updated version (In line 152, marked in yellow).
- Line 237 eqn (20) – please explain the exponent „n“
Response: It has been added in the manuscript (In line 562, marked in yellow).
- Line 268 „. the detailed components“ change to „. The detailed components“
Response: It has been corrected in the updated version (In line 274, marked in yellow).
- Line 288 and forward there is used symbol „IC“ and in Nomenclature is stated „CI“ –please explain.
Response: IC is the abbreviation of the initial cost (IC), it has been explained in the manuscript (In line 293, marked in yellow).
- Line 418 please check and change „10/kg“ to „10RMB/kg“
Response: It has been corrected in the updated version (In line 436, marked in yellow).
- Please include in Nomenclature symbols „NTU, k, RPCM, R, Rmax“ that are used in equations (7) – (15); also „OTC, PC, …) etc.; please check all other symbols!
Response: The Nomenclature symbols have been added in the manuscript and all the symbols have been checked.
- Some symbols are stated in Nomenclature („UsAs“, „ρs“ ) but they are not used in the text – please check and correct;
Please go through the model once again thoroughly!
Response: Sorry, the symbols have been check and corrected in the manuscript.
- 5 and Fig. 12 can be hardly seen; Is it necessary to use both figures - they are almost the same!
Response: Figs. 5 and 12 have been improved in the updated version. The difference of the both figures is the Fig. 12 is added TRNOPT components, which is used for optimized the system. Also, the difference is marked in the Fig. 12.
Fig. 5 CO2 ASHP heating system model coupled with TES
Fig. 12 TRNSYS optimization model of the coupled heating system
Special thanks to you for your constructive comments.
We tried our best to improve the manuscript and made some additional changes/improvements in the manuscript. These changes will not influence the content and framework of the paper. Here we did not list these changes but marked them in yellow in the revised paper.
We appreciate for Editors/Reviewers’ warm work earnestly, and hope that the correction will meet with approval.
Once again, thank you very much for your good comments and suggestions.
Sincerely,
Zhihua Wang, Yujia Zhang FenghaoWang, Guichen Li and Kaiwen Xu
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The paper shows interesting approach to the coupling of the heat pump with the thermal energy storage. I think that the authors provide the paper that can be published. The only comment is to try to provide the figures with the consistent, high resolution.
Author Response
Dear Editors and Reviewers:
Thank you very much for the review comments on our manuscript entitled "Performance optimization and economic evaluation of CO2 heat pump heating system coupled with thermal energy storage" (sustainability-1473431). We found that all comments are very valuable and helpful to improve the quality of our paper. We have tried our best to accommodate all comments and have made corrections/revisions which we hope meet the requirement. The revised part was marked in yellow in the updated manuscript. Our point-to-point responses to the review comments are as follows.
Reviewer #2
The paper shows interesting approach to the coupling of the heat pump with the thermal energy storage. I think that the authors provide the paper that can be published. The only comment is to try to provide the figures with the consistent, high resolution.
Response: Thank you for your comments, the figures have been improved in the updated version.
Special thanks to you for your constructive comments.
We tried our best to improve the manuscript and made some additional changes/improvements in the manuscript. These changes will not influence the content and framework of the paper. Here we did not list these changes but marked them in yellow in the revised paper.
We appreciate for Editors/Reviewers’ warm work earnestly, and hope that the correction will meet with approval.
Once again, thank you very much for your good comments and suggestions.
Sincerely,
Zhihua Wang, Yujia Zhang FenghaoWang, Guichen Li and Kaiwen Xu
Author Response File: Author Response.pdf
Reviewer 3 Report
The article is generally well written and reader-friendly, and the topic is interesting.
The abstract is straightforward but should be shortened of at least 10 words to comply with the rules of the journal, and acronyms should be avoided in it, to improve readability. Also “TES” among keywords could be provided in its non-abbreviated form as “Thermal Energy Storage”. Some other acronyms, as for example “CFC” and “HFC” (line 39) and “PCM” (line 185) are not stated before use in the text.
The Introduction deeply examines the current state of the art for Air source heat pumps and the potential of CO2 as refrigerant for those applications, but it presents lots of lumped references which are not properly analyzed in the text. Please take the time to comment one by one the quoted reference (see for instance line 63), also to better explain what is the novelty of your work.
Quality of Figure 1 must be improved.
Concerning the Methodology, the system, the building model and the mathematical model for the CO2 ASHP are well described. The description of the TES model (Section 3.3), which results in one of the main pieces of novelty of the paper, could benefit from additional details concerning the calculations of the main parameters of the TES, especially in the last part (from line 238) which misses adequate referencing.
Reference 36 at line 197 should be put into square brackets.
The given value of the relative error of 10 % (line 250) seems not to be in accordance with Figure 3, where the range of relative error appears to between 30 % and –1 %. Moreover, the formulation of the relative error in Equation 24 should give the absolute value of a number, while Figure 3 shows a relative error in the range of negative values.
Comments to Figures 4 and 5 are completely missing. Quality of Figure 5 must be improved.
Section 4. Results and discussion starts with the economic evaluation of the system under analysis, even though this part could fit well at the end of Section 3. For all figures with more than one subplot, please use a single caption for the overall figure, and not separate captions for subplot (a), (b), ...
The optimization process and the subsequent discussion are well explained and conclusions are drawn in a succinct but effective way.
Author Response
Dear Editors and Reviewers:
Thank you very much for the review comments on our manuscript entitled "Performance optimization and economic evaluation of CO2 heat pump heating system coupled with thermal energy storage" (sustainability-1473431). We found that all comments are very valuable and helpful to improve the quality of our paper. We have tried our best to accommodate all comments and have made corrections/revisions which we hope meet the requirement. The revised part was marked in yellow in the updated manuscript. Our point-to-point responses to the review comments are as follows.
Reviewer #3
The article is generally well written and reader-friendly, and the topic is interesting.
The abstract is straightforward but should be shortened of at least 10 words to comply with the rules of the journal, and acronyms should be avoided in it, to improve readability. Also “TES” among keywords could be provided in its non-abbreviated form as “Thermal Energy Storage”. Some other acronyms, as for example “CFC” and “HFC” (line 39) and “PCM” (line 185) are not stated before use in the text.
Response: The abstract has been shorted, and abbreviations have been carefully revised in the manuscript (In Line 11~13, Line 22, Line 38, marked in yellow).
The Introduction deeply examines the current state of the art for Air source heat pumps and the potential of CO2 as refrigerant for those applications, but it presents lots of lumped references which are not properly analyzed in the text. Please take the time to comment one by one the quoted reference (see for instance line 63), also to better explain what is the novelty of your work.
Response: It has been modified in the manuscript. The quoted references (line 63) are to present the current methods, including internal heat exchange, subcooling, expansion device, thermal energy storage, two or multi-stage compression, parallel compression, and cascade system etc., to decrease the throttle loss and improve the performance of CO2 ASHP. The detailed comment of these methods are reported in the next paragraph.
The novelty of this study is to decrease the return water temperature as low as possible using thermal energy storage to improve the performance of a transcritical CO2 air source heat pump unit for space heating, and the model of the studied system is constructed by TRNSYS and verified by experimental data. Secondly, the dynamic characteristics of the coupled heating system are investigated under the typical day. Thirdly, the performance of the studied system is optimized and its economy is analyzed by life cycle cost. This research is helpful of improving the application of CO2 ASHP unit in cold areas.
Quality of Figure 1 must be improved.
Response: Figure 1 has been improved in the updated version.
Concerning the Methodology, the system, the building model and the mathematical model for the CO2 ASHP are well described. The description of the TES model (Section 3.3), which results in one of the main pieces of novelty of the paper, could benefit from additional details concerning the calculations of the main parameters of the TES, especially in the last part (from line 238) which misses adequate referencing.
Response: The main objective of the present work is to establish a simulation model, optimize the system performance and analyze its economy by life cycle cost. The main parameters of the TES can be seen the following references. Also it has been presented in the manuscript. Thank you!
[1]Tay, N.H.S.,. Belusko, M, Bruno, F., An effectiveness-NTU technique for characterising tube-in-tank phase change thermal energy storage systems. Appl. Energy 91 (2012) 309-319.
[2]Wang, Z., Wang, F., Li, G., et al. Experimental investigation on thermal characteristics of transcritical CO2 heat pump unit combined with thermal energy storage for residential heating, Appl. Therm. Eng. 16525 (2020) 114505.
Reference 36 at line 197 should be put into square brackets.
Response: It has been added in the manuscript.
The given value of the relative error of 10 % (line 250) seems not to be in accordance with Figure 3, where the range of relative error appears to between 30 % and –1 %. Moreover, the formulation of the relative error in Equation 24 should give the absolute value of a number, while Figure 3 shows a relative error in the range of negative values.
Response: As the reviewer’s comment, the range of relative error appears to between 30 % and –1 %. The simulated results are relatively different from the measured value at the beginning of the time, which is about 30%. The possible reason for the difference is that the assumption adopted by the mathematical model for the convenience of calculation has a certain influence on heat transfer of PCM at the beginning of the time. In addition, the water supply temperature has a small range of changes in the experimental process, there are also some errors. However, the relative error is controlled within ±10% after 30 minutes. In a word, the results showed that the trend of the simulation results is consistent with the measured results (Fig. 3), and proving that the mathematical model is in good agreement. Also, the equation 24 has been corrected in the manuscript (In lines 254-263, marked in yellow).
Comments to Figures 4 and 5 are completely missing. Quality of Figure 5 must be improved.
Response: Figure 4 is the control logic of the TRNSYS simulation model of the studied system. From this figure, it could be easy to understand the control strategy for reader. The TRNSYS model (Figure 5) is set up according to the schematic diagrams of the studied system (Figure 1). It has been described in Section 2.
Section 4. Results and discussion starts with the economic evaluation of the system under analysis, even though this part could fit well at the end of Section 3. For all figures with more than one subplot, please use a single caption for the overall figure, and not separate captions for subplot (a), (b), ...
The optimization process and the subsequent discussion are well explained and conclusions are drawn in a succinct but effective way.
Response: It has been corrected in the manuscript.
Special thanks to you for your constructive comments.
We tried our best to improve the manuscript and made some additional changes/improvements in the manuscript. These changes will not influence the content and framework of the paper. Here we did not list these changes but marked them in yellow in the revised paper.
We appreciate for Editors/Reviewers’ warm work earnestly, and hope that the correction will meet with approval.
Once again, thank you very much for your good comments and suggestions.
Sincerely,
Zhihua Wang, Yujia Zhang FenghaoWang, Guichen Li and Kaiwen Xu
Author Response File: Author Response.pdf
Reviewer 4 Report
The paper presents consideration of a transcritical CO2 ASHP coupled with thermal energy storage (PCM) for a heating system. In my opinion the present study, although presented neatly and consistency, does not introduce significant scientific contributions to the area of transcritical CO2 ASHP. Incorporation of TES, which is aimed to alleviate the problems of too high return water temperature, is a minor effect. Developed algorithm of optimisation confirms that. Changes of the order of few percentages are merely academic. The increased efficiency of the cycle with incresased amount of PCM material is obvious. At the same time the cost of installation is significantly increased.
In order to improve the paper:
- A better and more convincing discussion, incorporating the above, is requred
- Presentation of the economic indicators is recommended such as NPV, IRR, SPBT
- PLease use the currency such as USD or euro.
Author Response
Dear Editors and Reviewers:
Thank you very much for the review comments on our manuscript entitled "Performance optimization and economic evaluation of CO2 heat pump heating system coupled with thermal energy storage" (sustainability-1473431). We found that all comments are very valuable and helpful to improve the quality of our paper. We have tried our best to accommodate all comments and have made corrections/revisions which we hope meet the requirement. The revised part was marked in yellow in the updated manuscript. Our point-to-point responses to the review comments are as follows.
Reviewer #4
The paper presents consideration of a transcritical CO2 ASHP coupled with thermal energy storage (PCM) for a heating system. In my opinion the present study, although presented neatly and consistency, does not introduce significant scientific contributions to the area of transcritical CO2 ASHP. Incorporation of TES, which is aimed to alleviate the problems of too high return water temperature, is a minor effect. Developed algorithm of optimisation confirms that. Changes of the order of few percentages are merely academic. The increased efficiency of the cycle with incresased amount of PCM material is obvious. At the same time the cost of installation is significantly increased.
In order to improve the paper:
A better and more convincing discussion, incorporating the above, is required.
Response: Thank you very much for your comments.
The coefficient of performance of CO2 heat pump is not very satisfactory due to the higher inlet water temperature at the gas cooler, which causes a large throttle loss when it is used for space heating in winter. It badly blocks the development of CO2 heat pump system. Note that the existing studies focus on the methods of expansion work recovery, internal heat exchange and ejector etc. to reduce its throttle loss and improve its performance. In this work, a heating system of transcritical CO2 ASHP coupled with TES is proposed. Firstly, the work principle of the coupled heating system is presented. Secondly, the model of the studied system is constructed by TRNSYS and verified by experimental data. Thirdly, the dynamic characteristics of the coupled heating system are investigated under the typical day. Finally, some main conclusions are given. This research is helpful of improving the application of CO2 ASHP unit in cold areas.
Presentation of the economic indicators is recommended such as NPV, IRR, SPBT
Response: The main objective of the present work is to establish a simulation model, optimize the system performance and analyze its economy by life cycle cost.
In the next work, a heating system experiment will be constructed, and it will further deal with the system performance, control strategy, the characteristics of heat pump unit, as well as the techno-economic analysis of the system to improve the system efficiency, and redesign, optimize system under different work conditions. After, the system performance and economy using NPV, IRR and SPBT will be compared with some reported investigations. Researches in this respect are going on and will bear significant fruit as expected.
Please use the currency such as USD or euro.
Response: Due to the price of the heat pump unit, PCM etc. is different from country to country, if the currency changes to USD, it may cause some deviation. In the next work, we will use USD as the currency in the beginning of the research. Thanks for your understanding.
Special thanks to you for your constructive comments.
We tried our best to improve the manuscript and made some additional changes/improvements in the manuscript. These changes will not influence the content and framework of the paper. Here we did not list these changes but marked them in yellow in the revised paper.
We appreciate for Editors/Reviewers’ warm work earnestly, and hope that the correction will meet with approval.
Once again, thank you very much for your good comments and suggestions.
Sincerely,
Zhihua Wang, Yujia Zhang FenghaoWang, Guichen Li and Kaiwen Xu
Author Response File: Author Response.pdf
Round 2
Reviewer 3 Report
Good revision, thanks a lot.
Reviewer 4 Report
THanks for the explanations, the paper is much better now
