Research on Energy-Saving Control Strategies for Single-Effect Absorption Refrigeration Systems
Round 1
Reviewer 1 Report
Comments and Suggestions for Authors- What were the specific criteria used to select ammonia water and lithium bromide water as the working fluid pairs for this study?
- How do the selected working fluids (ammonia water and lithium bromide water) compare in terms of environmental impact and safety standards?
- What were the key assumptions made during the thermodynamic analysis and how might they impact the model's accuracy?
- How does the system handle disturbances during the transition period between stable states, and what measures are in place to mitigate any adverse effects?
- It will add depth to the article to include some studies on absorption and cooling systems related to different heat pumps and pulsating heat pipes in particular. Please mention both of these studies.
Characterization of polypropylene pulsating heat stripes: Effects of orientation, heat transfer fluid, and loop geometry
https://doi.org/10.1016/j.applthermaleng.2020.116304
Physical principles and state-of-the-art of modeling of the pulsating heat pipe: A review
https://doi.org/10.1016/j.applthermaleng.2021.117111
- How was the simulation duration (3000 s) determined, and does it adequately capture the system's dynamic behavior?
- How do the experimental results compare with the simulation results, and what discrepancies, if any, were observed?
- How scalable are the proposed control strategies for larger or more complex absorption refrigeration systems?
- What are the potential areas for future research based on the findings of this study?
Author Response
Comment 1:What were the specific criteria used to select ammonia water and lithium bromide water as the working fluid pairs for this study?
Response 1:Thank you for pointing this out. We agree with this comment. Ammonia-water and lithium bromide-water solutions have been widely applied in absorption refrigeration. The former uses ammonia as the refrigerant and water as the absorbent, and its cooling effect can reach sub-zero temperatures, making it mainly suitable for low-temperature systems. The latter uses water as the refrigerant and lithium bromide as the absorbent. It can only be used for cooling above zero degrees, making it widely applicable to air conditioning systems.
When the ammonia-water solution is used as the working pair, due to the relatively small difference in boiling points between ammonia and water, a distillation device must be installed at the generator outlet to separate the refrigerant from the absorbent. This prevents water vapor from entering the condenser along with ammonia vapor, which would otherwise reduce the refrigeration efficiency of the unit. As a result, the structure of the ammonia-water absorption refrigeration system is relatively complex.
In contrast, when lithium bromide solution is used as the working pair, since the boiling point of lithium bromide is much higher than that of water, lithium bromide does not evaporate within the entire operating temperature range of the lithium bromide absorption refrigeration system. The vapor at the generator outlet is almost pure refrigerant steam, with only trace amounts of non-condensable gases, so no distillation device is needed, resulting in a relatively simple system structure.
At the same evaporation temperature, the performance of a lithium bromide absorption refrigeration system is superior to that of an ammonia-water system. However, lithium bromide solutions have the disadvantage of being highly corrosive to metals in the presence of oxygen, and there is also a risk of crystallization if the heat source temperature is too high.
Comment 2:How do the selected working fluids (ammonia water and lithium bromide water) compare in terms of environmental impact and safety standards?
Response 2:Thank you for pointing this out. We agree with this comment. The first question has already provided a detailed explanation of the two working pairs. The focus of this paper is the study of control strategies, and it does not delve into the environmental impact and safety standards of the working pairs.
Comment 3:What were the key assumptions made during the thermodynamic analysis and how might they impact the model's accuracy?
Response 3:Thank you for pointing this out. We agree with this comment. An absorption refrigeration system transfers heat from a low-temperature environment to a high-temperature environment under the influence of a driving heat source, with the solution pump consuming only a small amount of electrical energy to drive the solution circulation. The thermodynamic analysis of the absorption refrigeration system involves the application of the law of mass conservation, the component conservation equation, the first law of thermodynamics, and the second law of thermodynamics.
During the operation of the absorption refrigeration system, various components of the unit continuously exchange heat, such as the heat exchange between the shell-side solution and the tube-side heat source in the generator, the shell-side refrigerant and the tube-side cooling water in the condenser, the shell-side refrigerant and the tube-side chilled water in the evaporator, the shell-side solution and the tube-side cooling water in the absorber, and the heat transfer between the hot and cold solutions on both sides of the solution heat exchanger. This section has already been covered in Section 2.3 on page 4 of the paper.
Comment 4:How does the system handle disturbances during the transition period between stable states, and what measures are in place to mitigate any adverse effects?
Response 4:Thank you for pointing this out. We agree with this comment. In the actual operation of the system, external parameters, system load, and other factors may fluctuate, and the system is susceptible to various disturbances. The MIMO (Multiple-Input Multiple-Output) delay-free model-free control algorithm was experimentally applied to a hot-water-driven single-effect lithium bromide absorption refrigeration experimental platform. The system's heat source water was jointly supplied by hot water boilers to ensure a stable temperature of the heat source water as much as possible. A chilled water tank was used to simulate the system load, with an electric heater inside the tank balancing the cooling capacity of the system, thereby maintaining a relatively stable return water temperature of the chilled water. This approach minimizes unnecessary disturbances.
Comment 5:It will add depth to the article to include some studies on absorption and cooling systems related to different heat pumps and pulsating heat pipes in particular. Please mention both of these studies.Characterization of polypropylene pulsating heat stripes: Effects of orientation, heat transfer fluid, and loop geometry.Physical principles and state-of-the-art of modeling of the pulsating heat pipe: A review.
Response 5:Thank you for pointing this out. We agree with this comment. The research findings from the two references, Characterization of Polypropylene Pulsating Heat Stripes: Effects of Orientation, Heat Transfer Fluid, and Loop Geometry and Physical Principles and State-of-the-Art of Modeling of the Pulsating Heat Pipe: A Review, have been added to Section 2.1 on page 4 of the paper.
Comment 6:How was the simulation duration (3000 s) determined, and does it adequately capture the system's dynamic behavior?
Response 6:Thank you for pointing this out. We agree with this comment. In the simulation experiments under different controllers, the system reached a steady state at around 2000 seconds. To allow for further observation, the simulation duration was uniformly set to 3000 seconds, ensuring that the system's dynamic response could be fully captured within this time frame.
Comment 7:How do the experimental results compare with the simulation results, and what discrepancies, if any, were observed?
Response 7:Thank you for pointing this out. We agree with this comment. In the description of Section 4.3 on page 22 of the paper, the experimental results and simulation outcomes were analyzed, revealing that the experimental results were generally consistent with the trends observed in the simulation studies. This alignment serves to validate the effectiveness of the control strategy.
Comment 8:How scalable are the proposed control strategies for larger or more complex absorption refrigeration systems?
Response 8:Thank you for pointing this out. We agree with this comment.In larger systems that involve more control variables and controlled variables, MFAC (Model-Free Adaptive Control), like PID control strategies, has the advantage of not relying on the mathematical model of the controlled object. Instead, it uses a data-driven approach to design the controller, making it broadly applicable and highly practical.
Comment 9:What are the potential areas for future research based on the findings of this study?
Response 9:Thank you for pointing this out. We agree with this comment. In the future, it may be applied to double-effect absorption refrigeration systems or compression refrigeration systems.
Author Response File: 
Author Response.docx
Reviewer 2 Report
Comments and Suggestions for Authors
This study proposes a dual-loop energy-saving control strategy for single-effect hot water lithium bromide absorption refrigeration systems. Simulations demonstrate that this strategy effectively reduces system heat consumption by approximately 20%, decreases power consumption by about 10%, and improves the system SCOP by approximately 19.3%.
It is recommended that the following comments be evaluated by the authors;
*** The similarity index of the study is quite high (34%), and it is crucial to reduce this percentage.
1) In the manuscript, the innovative aspect of the article should be emphasized more stronger.
2) In the introduction section, the literature review should be expanded and the recent studies should be added.
3) The last paragraph of the introduction should be rewritten to clarify the innovation in this work and the aim of the study.
4) In order to increase the understandability of the study, thermal analyzes of the absorption refrigeration system should also be given in the study.
5) Are there any thermal assumptions in the analyzes performed within the scope of the study? If available, it should be given in the study.
6) In Figure 11, both graphs should have the same dimensions.
7) Conclusion section should be revised and slightly improved, ensuring that conclusions are supported by numerical and net results.
8) The manuscript is generally well written, but there are some spelling and grammatical errors. So, the manuscript should be revised for these errors.
Comments on the Quality of English Language
The manuscript is generally well written, but there are some spelling and grammatical errors. So, the manuscript should be revised for these errors.
Author Response
Comment 1:In the manuscript, the innovative aspect of the article should be emphasized more stronger.
Response 1:Thank you for pointing this out. We agree with this comment. This study applied a model-free adaptive control algorithm to the system's control. Through experimental data and mechanistic analysis, cooling water and heat source water were identified as the control variables, while generator temperature and chilled water outlet temperature were identified as the controlled variables. Based on this, both SISO and MIMO model-free control algorithms with time-delay components were implemented. Simulations comparing MFAC, improved MFAC, and traditional PID control demonstrated that the dual-loop energy-saving control strategy effectively reduced system heat consumption by approximately 20%, decreased power consumption by about 10%, and enhanced the system's SCOP by approximately 19.3%.
Comment 2:In the introduction section, the literature review should be expanded and the recent studies should be added.
Response 2:Thank you for pointing this out. We agree with this comment. In the introduction section on page 2 of the paper, parts of the latest literature from recent years have been added in sections 1.2 and 1.3, corresponding to references [30-32] in this paper.
Comment 3:The last paragraph of the introduction should be rewritten to clarify the innovation in this work and the aim of the study.
Response 3:Thank you for pointing this out. We agree with this comment. The final paragraph of the introduction on page 3 of the paper has been rewritten to emphasize the innovation and research objectives of this study.
Comment 4:In order to increase the understandability of the study, thermal analyzes of the absorption refrigeration system should also be given in the study.
Response 4:Thank you for pointing this out. We agree with this comment. An absorption refrigeration system transfers heat from a low-temperature environment to a high-temperature environment under the influence of a driving heat source, with the entire system consuming only a small amount of electrical energy to drive the solution circulation via the solution pump. The thermodynamic analysis of the absorption refrigeration system involves the application of the law of mass conservation, the component conservation equations, the first law of thermodynamics, and the second law of thermodynamics. During the operation of the absorption refrigeration system, various components continuously exchange heat, such as the heat transfer between the shell-side solution and the tube-side heat source in the generator, the shell-side refrigerant and the tube-side cooling water in the condenser, the shell-side refrigerant and the tube-side chilled water in the evaporator, the shell-side solution and the tube-side cooling water in the absorber, as well as the heat transfer between the hot and cold solutions on both sides of the solution heat exchanger. The thermodynamic analysis and assumptions have been provided in the newly added Section 2.3, following Section 2.2 on page 4 of the paper.
Comment 5:Are there any thermal assumptions in the analyzes performed within the scope of the study? If available, it should be given in the study.
Response 5:Thank you for pointing this out. We agree with this comment.The thermodynamic analysis and assumptions have been provided in the newly inserted Section 2.3 on page 4 of the paper.
Comment 6:In Figure 11, both graphs should have the same dimensions.
Response 6:Thank you for pointing this out. We agree with this comment. The dimensions of Figure 11 on page 21 of the paper have been adjusted to be consistent.
Comment 7:Conclusion section should be revised and slightly improved, ensuring that conclusions are supported by numerical and net results.
Response 7:Thank you for pointing this out. We agree with this comment. The conclusions section on page 24 of the paper has been revised and improved.
Comment 8:The manuscript is generally well written, but there are some spelling and grammatical errors.
Response 8:Thank you for pointing this out. We agree with this comment. We have requested assistance from a professional English editor at MDPI to correct spelling and grammatical errors throughout the entire document.
Author Response File: Author Response.docx
Reviewer 3 Report
Comments and Suggestions for AuthorsThe reviewed article discusses the topic of "Research on Energy-Saving Control Strategies for Single-Effect Absorption Refrigeration Systems". In the opinion of the reviewer the article is interesting however, there are some remarks that require comments from the authors.
1) The article does not follow the MDPI template. The lack of line numbers on the left makes it difficult to clearly indicate errors!!!
2) In section 1.2: "K.L. Cézara et al. [14] successfully developed a closed-loop PID control system, based on the Arduino platform..." Does this mean that no one has implemented the so-called feedback loop control before?
3) In chapter 1.3 the authors present intelligent control. Then, in the next chapter 1.4 they state "Existing control schemes for absorption refrigeration systems have certain limitation." What then of the control techniques given in the previous chapter, i.e. 1.3? Will the techniques in chapter 1.3 solve the problem? There is no negation of the techniques in chapter 1.3.
4) Figure 1 is illegible and of poor quality. Authors should provide a high-quality figure and clearly describe it in a figure caption or refer directly in the text to individual elements of the system.
5) Section 1.4 "Most advanced control algorithms depend highly on precise mathematical models of the controlled ob-ject, and the complex operating mechanisms of absorption refrigeration systems make it difficult to establish accurate system models." while in section 2.4 the author uses a high-order model.Why?
6) The results of the comparison with the PID controller are highly controversial. The authors did not provide any methods of selecting the PID controller and what structure they used. It is clear from the adopted parameters that no other method was used. In view of the above, I kindly ask you to compare the proposed method with the following controllers and methods:
PI
- ZN – Ziegler–Nichols tuning formul
- CHR – Chien–Hrones–Reswick tuning formula
- IMC – Internal Model Control tuning formula
PID
- ZN – Ziegler–Nichols tuning formul
- CHR – Chien–Hrones–Reswick tuning formula
- IMC – Internal Model Control tuning formula
7) In relation to the previous remark, there is no analysis regarding the robustness of the proposed solution in relation to the PID controller. The analysis regarding the robustness should be treated as a separate chapter.
8) The sentences throughout the article are definitely too long and the article requires English-language steps.
9) The figures are of poor quality. They need to be corrected.
Comments on the Quality of English Language
The article requires thorough improvement.
Author Response
Comment 1:The article does not follow the MDPI template. The lack of line numbers on the left makes it difficult to clearly indicate errors!!!
Response 1:Thank you for pointing this out. We agree with this comment. We have once again sought specialized assistance from MDPI for formatting issues. The article's format has been further corrected, but no additional recommendations were provided by the editors regarding line numbers.
Comment 2:In section 1.2: "K.L. Cézara et al. [14] successfully developed a closed-loop PID control system, based on the Arduino platform..." Does this mean that no one has implemented the so-called feedback loop control before?
Response 2:Thank you for pointing this out. We agree with this comment. Feedback closed-loop control, as a fundamental control structure, has been previously implemented by researchers. The reference here pertains to KL Cézara and colleagues, who provided a detailed study based on an Arduino controller. This approach involves certain innovations and adjustments, significantly reducing costs compared to PLC controllers in terms of controller deployment.
Comment 3:In chapter 1.3 the authors present intelligent control. Then, in the next chapter 1.4 they state "Existing control schemes for absorption refrigeration systems have certain limitation." What then of the control techniques given in the previous chapter, i.e. 1.3? Will the techniques in chapter 1.3 solve the problem? There is no negation of the techniques in chapter 1.3.
Response 3:Thank you for pointing this out. We agree with this comment. Our research group is currently investigating technologies related to intelligent control, such as DDQN, and therefore cannot immediately dismiss the practical performance of the technology described in Section 1.3.
Comment 4:Figure 1 is illegible and of poor quality. Authors should provide a high-quality figure and clearly describe it in a figure caption or refer directly in the text to individual elements of the system.
Response 4:Thank you for pointing this out. We agree with this comment. Figure 1 on page 4 of the paper has been redrawn to enhance clarity. Descriptions of the various components in the paper are provided in detail in Section 2.1 on page 4.
Comment 5:Section 1.4 "Most advanced control algorithms depend highly on precise mathematical models of the controlled ob-ject, and the complex operating mechanisms of absorption refrigeration systems make it difficult to establish accurate system models." while in section 2.4 the author uses a high-order model.Why?
Response 5:Thank you for pointing this out. We agree with this comment. The control strategy employed in this paper is the Model-Free Adaptive Controller (MFAC). The primary feature of this controller is its data-driven approach, which does not rely on a specific mathematical model of the controlled object during the design phase. Therefore, the research on this control strategy only utilizes real-time data generated by the controlled object. In actual system operation, external parameters, system load, and other factors may vary and are susceptible to various disturbances. Consequently, this paper references the absorption refrigeration mechanism model created in reference [23] on page 6 for simulation purposes, to conduct related control strategy research and result analysis.
Comment 6:The results of the comparison with the PID controller are highly controversial. The authors did not provide any methods of selecting the PID controller and what structure they used. It is clear from the adopted parameters that no other method was used. In view of the above, I kindly ask you to compare the proposed method with the following controllers and methods:
Response 6:Thank you for pointing this out. We agree with this comment. This study on control strategies first analyzed the effects of hot water mass flow rate and cooling water mass flow rate on the chilled water outlet temperature. Through experimental data analysis of open-loop characteristics, it was determined that the SISO control strategy uses the hot water mass flow rate as the control variable and the chilled water outlet temperature as the controlled variable. The MIMO control strategy involves controlling the generator temperature with the heat source water flow rate and the chilled water outlet temperature with the cooling water flow rate.
Based on this, the focus of this paper is on the Model-Free Adaptive Control (MFAC) algorithm. The MFAC was modified to address the characteristics of significant time delays in the controlled object. Simulations were conducted using PID control, classic MFAC control, and time-delay MFAC control in the refrigeration system. Therefore, this paper does not address the structure and parameter tuning methods of PID controllers in detail but employs the standard PID block in SIMULINK for parameter tuning using the trial-and-error method, comparing and analyzing it against other control strategies.
Comment 7:In relation to the previous remark, there is no analysis regarding the robustness of the proposed solution in relation to the PID controller. The analysis regarding the robustness should be treated as a separate chapter.
Response 7:Thank you for pointing this out. We agree with this comment. The stability proof of the improved MFAC control algorithm is discussed at the end of Section 3.2 on page 19 of the paper. It is noted that the robustness proof for MIMO time-delay systems can be extended from the results in references [25] and [26]. Due to space constraints, this part has been omitted in the present paper.
Comment 8:The sentences throughout the article are definitely too long and the article requires English-language steps.
Response 8:Thank you for pointing this out. We agree with this comment. We have sought assistance from MDPI's professional English editors to correct spelling and grammatical errors throughout the entire document.
Comment 9:The figures are of poor quality. They need to be corrected.
Response 9:Thank you for pointing this out. We agree with this comment. We have improved the image quality as much as possible, including enhancements in clarity and adjustments to dimensions.
Author Response File: Author Response.docx
Round 2
Reviewer 2 Report
Comments and Suggestions for AuthorsThe authors have made the necessary arrangements, taking into account the reviewers suggestions. I find it appropriate to publish the article in its current form.
Reviewer 3 Report
Comments and Suggestions for AuthorsThe answers to my questions are satisfactory. The article can be published in its current form.
