Comparison of Short and Long-Term Energy Performance and Decarbonization Potentials between Cogeneration and GSHP Systems under MARKAL Scenarios

Round 1

Reviewer 1 Report

I judged that this manuscript describes an important theme with clear arguments.

Author Response

We appreciate your time to review this manuscript and your agreement.

 

Reviewer 2 Report

The paper compares energetic and environmental performance of CHP and GSHP systems to provide energy for an office building as the case study. It adopts simulation method (using TRNSYS) for analysing short-term and long-term performance of these systems under UK MARKAL scenarios. It tackles an important topic that fits within the scope of the journal, however, it needs to be improved following the below comments/questions.

 

General comments:

1.      The abbreviations should normally be defined at the first time that they are mentioned, which are in abstract.

2.      At the last sentence of abstract, it is written ‘GSHP system is more promising and recommended compared with cogeneration system in both short- and long-term periods in terms of only decarbonization potentials’. What do you mean by ‘only decarbonization potential’? I know you explained it (though not very clear) at the body of the text but it needs to be clear in the abstract as well. You should briefly mention what is the other aspect that GSHP may NOT (perhaps) be more promising than cogeneration system.

3.      In the Introduction (line 76 to 82), you mentioned ‘there is still much research related to GSHPs and cogeneration systems, and the majority of them are focused on the technical feasibility, energy saving and economic analysis. Although some studies also mentioned carbon emission-related analysis or results, they all focused on short-term carbon emission reduction rather than comprehensive and long-term carbon emission analysis’ without bringing references for this claims. Please provide more literature to support these claims. Specifically for the second part that you said ‘they all focused on short-term carbon emission reduction’. Because by a simple search, you can find some studies that analyse the carbon emissions of these systems in long-term. For example, the study by Subramanyam et al., 2017 (Greenhouse gas emissions mitigation potential in the commercial and institutional sector) have done such an analysis for period of 2013–2050. This may question the novelty of your work. Please clarify and more explicitly mention the main contribution of your work.

4.      The methodology section (though it is clear) is written like a normal report rather than a scientific paper. Particularly, sections 2.1 and 2.2 are sometimes explained in too much details.

5.      In Section 2.3, the first paragraph is well-supported and appropriate. However, the reasons brought for privileging TRNSYS to other software is not sufficient. For instance, the  reasons such as ‘the built-in components/modules in TRNSYS can be easily and directly used, or new components/modules can be developed according to user own needs by the programming software (e.g., C, C++, Python and Fortran). It can also be connected to other software’ are valid for many other competitors of TRNSYS (such as EnergyPlus). Please bring a reason that distinguished between TRNSYS and other software that made you to choose TRNSYS.

6.      Section 4.2.1 is too long and a little bit confusing. Please summarize it in a more understandable way. It seemed to me at the first instance that you are explaining your results while then I realised those are just numbers from literature (yes?). Moreover, did you calculate/simulate the energy demand of the case study building or you just relied on the benchmarking values that explained in this section? If not calculated, please justify why you did not calculate and just used the benchmark values.

7.      Section 4.3: Would you explain why in scenario 5 there is a lower CO2 emission reduction comparing to other scenarios?

8.      Section 4.3 (line 366): How did you calculate ‘The energy related CO2 emissions of GSHP and CHP systems’? Did you just manually multiply energy use of them to the carbon intensity of gas and electricity? Or TRNSYS could help you with that?

9.      The results can be occasionally supported by credible references from literature.

10.  Conclusions section: Please bring these two below paragraphs as a united paragraph because they are practically explaining one fact.

‘Under all the MARKAL scenarios, GSHP system performed extremely better than cogeneration system in terms of carbon reduction in both periods.’

‘Compared with cogeneration system, GSHP system can save 47.8%-84.4% carbon emission in short-term period, while GSHP system can achieve a maximum of 97.5% of carbon emission reduction in long-term period.’

 

Specific comments:

1.      Line 206: It is mentioned ‘TRNSYS simulation have been validated by…’. Do you mean the validation mentioned in section 4.2.1?! If so, you should cross reference within the text when you mention it.

2.      Line 211-213: This sentence is unclear: ‘while its model is for to achieve dynamic energy optimization for simultaneous the energy system total cost and carbon emissions mitigation by 80% by 2050 compared with the levels in 1990’. Please reword it.

3.      Line 217: If Fig. 3 and Table 7 are directly adopted from [51] and [53], it is better to mentioned these references in the caption of the Fig and Table, rather than in the text.

4.      Table 7 should include 8 plans. Why it only has 6 plans?

5.      Line 224-225: What do you mean by ‘technical systems’? Do you mean energy (or heat) generation and distribution systems? Also when you say ‘The technical system of this paper(better to say study) include the space heating distribution system, GSHP unit, and CHP system, and weather data’, you are mentioning the mechanical/thermal systems. So, ‘weather data’ is outside this category. Maybe better to reword this sentence.

6.      Line 249-251: It is written ‘The GSHP system is comprised of heat source, water-to-water heat pump, weekly profiles, circulation pumps, thermal storage tank and controllers’. When you are mentioning physical components of a system, ‘weekly profiles’ should be outside this category. (same comment applied to line 266)

7.      Line 254: Are the vertical boreholes (including U-tubes) already existing in the site and being used? Because you did not mentioned if there are physically there.

8.      Section 3.3: The same as above, is the CHP system an existing system or it is only a theoretical assumption? Please clarify.

9.      Fig 9: Please clarify if you talk about power or energy. You mention the unit ‘kWh’ that means energy while you wrote power input in the caption and text. (same thing applied to Fig 10)

10.  Fig 9: This is the ‘accumulative’ energy input and generated?  

11.  Line 361: It is not clear to me why you concluded that ‘it is difficult to simply compare the advantages and disadvantages of the two systems in terms of energy performance’. Considering heat & electricity consumption/generation of/from each system (CHP and GSHP), it should be possible to elaborate their advantages and disadvantages and make a conclusion (this conclusion can also be added to the Conclusions section of the paper that is not very rich now). If not possible to conclude, bring more logical reason for it.

12.  Table 13 is not very clear. What does the ‘star’ mean in the table? Why you put stars below GSHP but blow CHP is empty? If this means which one performs better, you should briefly clarify it.

13.  Appendix C: For my curiosity, why CO2 emission due to gas consumption of CHP will keep increasing in the future following the MARKAL scenarios? Gas is not always gas?

Author Response

Paper ID: Sustainability-2112062

Response to Reviewer 2 Comments

 

We thank you very much for your constructive suggestions and precious comments concerning our manuscript entitled “Comparison of short and long-term energy performance and decarbonization potentials between cogeneration and GSHP systems under MARKAL scenarios”. These comments are all valuable and very helpful for revising and improving our paper. We have studied comments carefully and have made corrections in our paper highlighted in blue, which we hope meet with your approval.

 

The responses to your comments are as follows:

 

 

Point 1: The abbreviations should normally be defined at the first time that they are mentioned, which are in abstract.

Response 1: We have followed your recommendation, and revised it (Page 1, Lines 12-14)

 

Point 2: At the last sentence of abstract, it is written ‘GSHP system is more promising and recommended compared with cogeneration system in both short- and long-term periods in terms of only decarbonization potentials’. What do you mean by ‘only decarbonization potential’? I know you explained it (though not very clear) at the body of the text but it needs to be clear in the abstract as well. You should briefly mention what is the other aspect that GSHP may NOT (perhaps) be more promising than cogeneration system 

Response 2: We agree with you that comparison between GSHP and CHP or other HVAC technologies can be studeid further, and we do want to do that in our latter papers. However, in this manuscript, our focus is on comparing the carbon emission and decarbonization potentials between GSHP and CHP systems so that we can give related researchers a reference, the long-term and short-term operating performance of these two systems only considering carbon emissions. Carbon neutrality and carbon peaking are currently super hot issues in the field of global energy and the environment, so our research also caters to and solves hot issues.

 

Point 3: In the Introduction (line 76 to 82), you mentioned ‘there is still much research related to GSHPs and cogeneration systems, and the majority of them are focused on the technical feasibility, energy saving and economic analysis. Although some studies also mentioned carbon emission-related analysis or results, they all focused on short-term carbon emission reduction rather than comprehensive and long-term carbon emission analysis’ without bringing references for this claims. Please provide more literature to support these claims. Specifically for the second part that you said ‘they all focused on short-term carbon emission reduction’. Because by a simple search, you can find some studies that analyse the carbon emissions of these systems in long-term. For example, the study by Subramanyam et al., 2017 (Greenhouse gas emissions mitigation potential in the commercial and institutional sector) have done such an analysis for period of 2013–2050. This may question the novelty of your work. Please clarify and more explicitly mention the main contribution of your work.

Response 3: Thanks for the useful comments. We are sorry that we didn’t clearly.express clearly. Actually, we have added the paper Subramanyam et al., 2017 (Greenhouse gas emissions mitigation potential in the commercial and institutional sector) you mentioned into our manuscript. In addition, we also did the literarure review again, and found some papers on long-term carbon emission performance, but most of them were not focused GSHP and CHP system, and some of them focused more on the energy use and cost savings instead of the carbon emission. In our manuscript, we attached to the super-hot spot ‘carbon peaking and carbon neutrality’ and only concerns about the short-and-long-term carbon performance and emission reduction of building HVAC systems under UK electricity decarbonization plans. We think this is the biggest difference between our manuscript and existing papers. And also this is our innovation. Page 2, Lines 61-74 & 79-91, blue highlights

 

 

Point 4: The methodology section (though it is clear) is written like a normal report rather than a scientific paper. Particularly, sections 2.1 and 2.2 are sometimes explained in too much details.

Response 4: Thanks for your comments. We agree with you that the methology is a little bit rededuant. Our original intention is to describe the methods we use and the various parameters of the case buildings we study in a very comprehensive way, so that other researchers can reproduce our research to the greatest extent. The reason why we provide a lot of information about methods, buildings , the description of the details of the system is to reflect the authenticity and integrity of our research to the greatest extent. Of course, we have also listened to some of your suggestions and deleted a small part of the medethology description.

 

Point 5: . In Section 2.3, the first paragraph is well-supported and appropriate. However, the reasons brought for privileging TRNSYS to other software is not sufficient. For instance, the  reasons such as ‘the built-in components/modules in TRNSYS can be easily and directly used, or new components/modules can be developed according to user own needs by the programming software (e.g., C, C++, Python and Fortran). It can also be connected to other software’ are valid for many other competitors of TRNSYS (such as EnergyPlus). Please bring a reason that distinguished between TRNSYS and other software that made you to choose TRNSYS 

Response 5:. Thank you for the comments. We agree with you that many simulation software (e.g., TRNSYS, EnergyPlus, IDA ICE) can be used for GSHP and CHP system modelling. But we are more familiar with TRNSYS, and this the in-built components/modules make it easier to use and perform. In addition, we have found many related papers on studying energy performance of GSHP and CHP by utilizing TRNSYS (Pages 5&6, Lines 166-189). Thus, we adopted TRNSYS in this study. Here, we just want to show the characteristics and functions of TRNSYS instead of comparing it with other software. As we think TRNSYS is simpler for us, but we cannot say we use it because we are familiar with it in academic paper. Actually, many simulation software can be used for the GSHP and CHP system simulation.

 

Point 6: Section 4.2.1 is too long and a little bit confusing. Please summarize it in a more understandable way. It seemed to me at the first instance that you are explaining your results while then I realised those are just numbers from literature (yes?). Moreover, did you calculate/simulate the energy demand of the case study building or you just relied on the benchmarking values that explained in this section? If not calculated, please justify why you did not calculate and just used the benchmark values. 

Response 6: Thank you for the useful comments. In this section, we want to show that our results meent the regulations, and want to show the accuracy and compliance of our simulation results compared with different regulations (e.g., Energy Consumption Guide 19, CIBSE TM46. We don’t think this part is rededuant because this part can find out whether the simulation results meet the specification requirements. In addition, we have followed your comments and deleted some unnecessary descriptions in this section. Page 13&14, Lines 313-325.

 

Point 7: Section 4.3: Would you explain why in scenario 5 there is a lower CO2 emission reduction comparing to other scenarios?

Response 7: Thanks for the questions. As you can see from Table 7. Scenario 5 represents reducing the carbon emission by adjusting the electricity resilience (demand side management), while its goals are to decrease the energy demand by minimum 1.2% per year, limiting the proportion of single energy below 40% in primary energy mix, constraining the expected unserved energy level, and supplementing power sector models to better explain intermittency. Compared with other scenarios, Scenario 5 is more comprehensive to achieve carbon emission in the future.

 

Point 8: Section 4.3 (line 366): How did you calculate ‘The energy related CO2 emissions of GSHP and CHP systems’? Did you just manually multiply energy use of them to the carbon intensity of gas and electricity? Or TRNSYS could help you with that?

Response 8: In this paper, we used Microsoft to do the calculations of energy related CO2 emissions of GSHP and CHP systems.

 

Point 9: The results can be occasionally supported by credible references from literature.

Response 9:. Yes, we agree with you that there are some studies have shown that GSHP system can achieve carbon emission. However, this article is not only to express this result, but more importantly, to express that GSHP has more potential in carbon emission reduction than CHP systems in both/ short-and-long-terms. Thus, we did not use existing reference to support the results.

 

Point 10: Conclusions section: Please bring these two below paragraphs as a united paragraph because they are practically explaining one fact

Response 10:.Thanks for the comments, we have revised it according to your recommendations. Page 17, Lines 418-420.

 

Point 11: . Line 206: It is mentioned ‘TRNSYS simulation have been validated by…’. Do you mean the validation mentioned in section 4.2.1?! If so, you should cross reference within the text when you mention it

Response 11: Here, we want to introduce the UK MARKAL scenarios instead of validation of simulation. Thus, you are right we place this sentense in the wrong place. Following your advice, we have deleted the sentense in this paragraph. Page 7, Line 215.

 

Point 12: Line 211-213: This sentence is unclear: ‘while its model is for to achieve dynamic energy optimization for simultaneous the energy system total cost and carbon emissions mitigation by 80% by 2050 compared with the levels in 1990’. Please reword it

Response 12: Sorry for the mistake, and we have revised this sentense according to your useful comments. Page 7, Lines 218-220

 

Point 13: Line 217: If Fig. 3 and Table 7 are directly adopted from [51] and [53], it is better to mentioned these references in the caption of the Fig and Table, rather than in the text.

Response 13: We agree with you, and have added the corresponding references for Table 7 and Fig. 3. Page 7, Lines 226, Page 8, Line 227.

 

Point 14: Table 7 should include 8 plans. Why it only has 6 plans?

Response 14: we have added the left two plans’ description in Table 7 according to your comments. Page 8. 

 

Point 15: . Line 224-225: What do you mean by ‘technical systems’? Do you mean energy (or heat) generation and distribution systems? Also when you say ‘The technical system of this paper(better to say study) include the space heating distribution system, GSHP unit, and CHP system, and weather data’, you are mentioning the mechanical/thermal systems. So, ‘weather data’ is outside this category. Maybe better to reword this sentence.?

Response 15: Yes, you are right ‘the technical systems’ are the energy (or heat) generation and distribution systems. We have reword the sentence. In addition, following your advice, we have replaced ‘this paper’ by ‘this study’ throughout the manuscript. Page 8, Lines 228-230.

 

Point 16: Line 249-251: It is written ‘The GSHP system is comprised of heat source, water-to-water heat pump, weekly profiles, circulation pumps, thermal storage tank and controllers’. When you are mentioning physical components of a system, ‘weekly profiles’ should be outside this category. (same comment applied to line 266)

Response 16: Yes, you are right, and we have revised it. Page 9&10, Lines 255-259. Page 10, Lines 271-274.

 

Point 17: Line 254: Are the vertical boreholes (including U-tubes) already existing in the site and being used? Because you did not mentioned if there are physically there

Response 17: We have clarify it in Pages 9&10, Lines 255-259. The GSHP system is an existing system in the studied building, which is comprised of heat source (a vertical U-tube ground heat exchanger), water-to-water heat pump, weekly profiles, circulation pumps, thermal storage tank and controllers.

 

Point 18: Section 3.3: The same as above, is the CHP system an existing system or it is only a theoretical assumption? Please clarify

Response 18: We have clarify it in Page 10, Lines 271-274. The CHP system is only a theoretical assumption in this study, which is comprised of CHP unit, controllers, circulation pump, thermal storage tank, weekly profiles, and fluid mixing/diversion valves.

 

Point 19: Fig 9: This is the ‘accumulative’ energy input and generated? 

Response 19: Yes, you are right, and this figure shows the accumlative energy input and generated.

 

Point 20: Line 361: It is not clear to me why you concluded that ‘it is difficult to simply compare the advantages and disadvantages of the two systems in terms of energy performance’. Considering heat & electricity consumption/generation of/from each system (CHP and GSHP), it should be possible to elaborate their advantages and disadvantages and make a conclusion (this conclusion can also be added to the Conclusions section of the paper that is not very rich now). If not possible to conclude, bring more logical reason for it.

Response 20: In this paper, we just want to compare the performance of carbon emission of these two systems (GSHP and CHP) in both short and long-term, and this is all our research focus. We didn’t mean to comprehensively compare their energy performance, including all the energy use, energy cost, thermal performance, and carbon emission. GSHP and CHP systems. Both systems have their own advantages. Thus, we said ‘it is difficult to simply compare the advantages and disadvantages of the two systems in terms of energy performance’. All in all, we just want to compare their decarbonization potentials in both short-and-long term, and it cannot reach the whole energy performance comparison.

 

Point 21: Table 13 is not very clear. What does the ‘star’ mean in the table? Why you put stars below GSHP but blow CHP is empty? If this means which one performs better, you should briefly clarify it.

Response 21: Thank you for the remind. We did make it unclear. Actually, the ‘☆’ represents better choice. Following you advice, we have added the description of ☆ in Table 14. Page 16

 

Point 22: Appendix C: For my curiosity, why CO2 emission due to gas consumption of CHP will keep increasing in the future following the MARKAL scenarios? Gas is not always gas?

Response 22: Here, the calculation of carbon emissions of GHSP and CHP systems is as follows:

	(1)
		(2)

Where ACE=Annual carbon emission, AEU=Annual electricity use, CI=Carbon intensity, AGU=Annual gas use, EG=Electricity generation. We also added this in Page 12 Lines 375-378, and in Appendix C, Page 19

 

 

 

In the end, we really appreciate it that our paper is rated well by you, and hope that our responses and the revised version could meet with your approval.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

First of all, you have missed one of my comments to be addressed, as follows:

 Fig 9: Please clarify if you talk about power or energy. You mention the unit ‘kWh’ that means energy while you wrote power input in the caption and text. (same thing applied to Fig 10)

Moreover, there are a couple of issues remaining from the comments that have been answered by you. Please see below:

Point 4: You mentioned that you deleted a small part of the methodology description but I did not see any notable difference between the text of the new version and the previous version.

Pint 20: The way you explained this in the text is not convincing because the readers may immediately raise a question and ask: just because it is 'difficult', you neglected it and did a more limited analysis. Specifically because you used the word 'should' (should be compared) in the text. I suggest that you clearly mention other aspects (e.g., energy use, energy cost, thermal performance, and carbon emission) that you wrote in your response to me and justify why it is not possible to assess them in this study. It is also recommended to bring a couple of references and review what other studies might have done to compare GSHP and CHP systems with regards to other aspects (energy use, ...). Or if you did not find any study, you may mention that these aspects also need to be analysed in future.  

Finally, please write the scenario numbers (S1, S2, ...) in Table 7 to avoid confusion.

 

Author Response

Paper ID: Sustainability-2112062

Response to Reviewer 2 Comments

 

We thank you very much for your constructive suggestions and precious comments concerning our manuscript entitled “Comparison of short and long-term energy performance and decarbonization potentials between cogeneration and GSHP systems under UK MARKAL scenarios”. These comments are all valuable and very helpful for revising and improving our paper. We have studied comments carefully and have made corrections in our paper highlighted in blue, which we hope meet with your approval.

 

The responses to your comments are as follows:

 

 

Point 1: Fig 9: Please clarify if you talk about power or energy. You mention the unit ‘kWh’ that means energy while you wrote power input in the caption and text. (same thing applied to Fig 10)

Response: Thank you for the kind remind. We have deleted Fig. 10 as we consider it a lit bit repeatable with Fig. 9 in content. We have replace the power input by electricity input in Fig.9. Sorry for the misleading description.  

 

Point 2: Point 4: You mentioned that you deleted a small part of the methodology description but I did not see any notable difference between the text of the new version and the previous version.

Response: Thank you for the suggestions. we have further and tried our best to streamline the content of section 2 (Methodology part). As you know, the description of methodology part is also important, so we keep most of the description, and just deleted unnecessary part. We have deleted about 10% of the original methodology part, but we cannot mark where deleted as they have been delected. 

 

Point 3: Pint 20: The way you explained this in the text is not convincing because the readers may immediately raise a question and ask: just because it is 'difficult', you neglected it and did a more limited analysis. Specifically because you used the word 'should' (should be compared) in the text. I suggest that you clearly mention other aspects (e.g., energy use, energy cost, thermal performance, and carbon emission) that you wrote in your response to me and justify why it is not possible to assess them in this study. It is also recommended to bring a couple of references and review what other studies might have done to compare GSHP and CHP systems with regards to other aspects (energy use, ...). Or if you did not find any study, you may mention that these aspects also need to be analysed in future.

Response: Thank you for the useful comments. We think that each article has its own focus and innovation. Some articles do consider a comprehensive analysis results (3E or 4E analysis), and their focus is to verify their proposals through comprehensive analysis and advantages of their proposed system. But the focus of our article is to compare the carbon emissions of the two systems we selected, with the aim of finding out which system can achieve carbon peaking and neutrality as soon as possible. In the abstract and introduction part of the article, we have explained twice that the focus and innovation of our article lies in the decarbonization potentials in short and long-terms, not the comprehensive system performance analysis. The difference between our article and other similar articles is that our focus is on carbon, and they may focus on energy, economics, market potential, and carbon emissions, etc. Thus, we agree with you that systematic and comprehensive analysis (3E or 4E) is indeed more convincing in mots of the papers, but not always for some papers with specific research focus. 

 

Point 4: Finally, please write the scenario numbers (S1, S2, ...) in Table 7 to avoid confusion.

Response: Thanks for the suggestion. Actually, Table 7 is aimed to describe the UK MARKAL plans instead of the scenarios of our study. In addition, we have listed the scenario numbers and descriptions of our study in Table 14.

 

In the end, we really appreciate it that our paper is rated well by you, and hope that our responses and the revised version could meet with your approval.