Control Strategy for Inverter Air Conditioners under Demand Response
Round 1
Reviewer 1 Report
1. The authors have made an interesting attempt to write a paper addressing a research on control strategy of inverter air conditioners under demand response.
2. Introduction: From my point of view, introduction is not well focused. In a research paper, it is expected that introduction section briefly explains the starting background and, even more important, the originality (novelty) and relevancy of the study is well established. Once this is done, hypothesis and objectives of the study need to be addressed, as well as a brief justification of the conducted methodology. It is my belief that, in this case, authors do not put effort enough (or any effort) in highlighting the relevancy and (specially) the novelty of the study. Consequently, both major aspects are compromised. I strongly recommend that authors clearly explain all these aspects (including hypothesis and objectives) in order to add scientific rigor to the manuscript.
3. Discussion: It is my opinion that a separate discussion section would help the reader to understand the study. However, the main issue arises from the lack of comparison with state-of-the-art studies. From my point of view, even though many publications are listed in the literature review, it has no justification not conducting a comparison with main previous works already published. The lack of such a comparison highly compromises the significance of the paper, so I strongly recommend authors to conduct as much and suitable comparisons as needed to solve this issue.
4. Shouldn’t equation (14) be 2 different equations?
5. A more detailed result analysis section could be added.
6. What can the authors do to validate their results?
7. Figure 5 shows its tracking effect. But it can hardly be seen since the reader has to squint their eyes to extract any useful information out of this Figure.
8. Figures 3 to 8 should be enhanced. There are no limitations of page numbers, so they can be augmented. As they’re now, the axis labels and the content of these figures are small.
9. The conclusion is short and incomplete. The obtained results should be briefly and clearly mentioned through the support of numerical data. What were the most sounding quantifiable findings of this study? The conclusion is intended to help the reader understand why your research should matter to them after they have finished reading the paper. A conclusion is not merely a summary of the main topics covered or a re-statement of your research problem, but a synthesis of key points and, if applicable, where you recommend new areas for future research.
10. The citation section is quite short, please add the following citations:
· https://doi.org/10.1109/EEEIC.2017.7977766
· https://doi.org/10.3390/en10122133
· https://doi.org/10.3390/en12040671
· https://doi.org/10.1109/INTLEC.2017.8214142
11. This paper seems to be fairly crafted, yet with some limitations. Thus, the authors need to improve in order to have an acceptable paper for Processes.
Author Response
Point 1: The authors have made an interesting attempt to write a paper addressing a research on control strategy of inverter air conditioners under demand response.

Response 1: Thank you very much for your kind comments.
Point 2:Introduction: From my point of view, introduction is not well focused. In a research paper, it is expected that introduction section briefly explains the starting background and, even more important, the originality (novelty) and relevancy of the study is well established. Once this is done, hypothesis and objectives of the study need to be addressed, as well as a brief justification of the conducted methodology. It is my belief that, in this case, authors do not put effort enough (or any effort) in highlighting the relevancy and (specially) the novelty of the study. Consequently, both major aspects are compromised. I strongly recommend that authors clearly explain all these aspects (including hypothesis and objectives) in order to add scientific rigor to the manuscript.
Response 2:Thank you very much for your valuable comments. We have updated the Introduction section comprehensively to incorporate the points you raised.
First of all, a new paragraph has been added at the very beginning of the Introduction section to briefly explain the starting background of the paper. The newly added paragraph is as follows:
‘Renewable energy sources such as wind and solar power have a high degree of unpredictability and time-variation [1]. The increasing penetration of renewable power generation in electric power systems make it even more costly and difficult for power systems to maintain the balance between supply and demand. Furthermore, the conventional generators are increasingly difficult to meet the needs of system regulation [2]. Demand response technology is one of the core technologies of smart grid, which can effectively restrain the random fluctuation of power flow, alleviate the tension between supply and demand, improve the efficiency of system operation, and promote energy saving and emission reduction [3]. Therefore, it is becoming increasingly important to utilize the flexibility at the demand side of power systems.’
After that, the relevance of the study is established, through revealing the importance of demand response from air conditioning loads especially inverter air conditioning loads, which is the research topic of this paper. In the paper, relevant paragraphs have been revised and a new paragraph has been added. The revised and newly added paragraphs are as follows:
‘Besides the many other flexible loads such as pool pumps [1] [9] [10], air-conditioning loads are an important type of demand response resources with great potential. According to incomplete statistics, air-conditioning loads accounts for 30% to 40% of peak load in summer [11] in China, and this number in some other cities such as Beijing reaches 52%, with an increasing trend year by year. Buildings with inverter air-conditioners can act as virtual energy storage (VES) because of their thermal energy storage capability and the fact that human body has no obvious response to the temperature change within a certain range. [12] proved through experiments that the demand response of air-conditioners is effective. Under the premise of ensuring the users’ basic comfort [13], air-conditioners can be equivalent to a load resource with flexible dispatch capability, which have huge potential [14]. Effective management of air conditioning loads during peak period through reasonable control means is conducive to reducing power shortage and optimizing power consumption mode.
There have been a number of studies conducted regarding demand response from air conditioners at different levels, including household level [15] [16] and aggregation level. Both simulation and experimental studies [12] have been conducted. The air conditioners considered include split air conditioners and central air conditioners [17]. This paper focuses on the load reduction services from aggregated split inverter air conditioners.
Most existing studies regarding the demand response from air-conditioners focus on constant-speed air-conditioners. However, in recent years, inverter air-conditioners are favored by users for its advantages of energy saving and comfort. With the guidance of national energy saving and emission reduction policy in many countries like China, the number of inverter air-conditioners keeps growing, accounting for a significant share of total air-conditioners installed. Therefore, it is necessary to study their participation in demand response [18].’
Then the novelty of the study is highlighted by comparing the study with the existing ones. In the revised version of the paper, more existing studies are reviewed, and further divided into two categories. After describing each category of existing studies, several sentences are added to highlight the novelty of the study in comparison with the existing ones. The updated paragraphs regarding literature review and novelty declaration are as follows:
‘In this context, some preliminary research has been conducted regarding the demand response from inverter air-conditioners, which could be further classified into two categories. The first category of studies is to explore the building thermodynamic model of inverter air-conditioners. The Equivalent thermal parameter (ETP) model has been widely used as the basis for further control. In [19], based on the first-order ETP model, the VES model of a building with air-conditioners is established considering comfort level, and a series of VES parameters such as rated capacity, rated power and charging and discharging time are derived. However, in [19], the electric power of air conditioners are just taken as a constant power input, lacking in-depth analysis of the electrical parameters of air conditioners, so it cannot describe the influence of the state change of air-conditioning loads on the power grid. [20] proposed a high-precision mode reflecting the different operating conditions of electric water heaters, but due to the limitation of calculation, the model is only suitable for small-scale regulation, and cannot be applied to power grid cluster dispatch. Compared to the existing studies, such as [19, 20], this paper establishes a VES model for inverter air-conditioning loads, which is more conducive to practical application. Two parts are involved in the VES model, including the electrical parameters part based on the operating characteristics of inverter air-conditioning loads and the thermal parameters part based on the equivalent thermal parameter model.
The second category of studies is to explore the load reduction from inverter air-conditioners. [21] studied the direct load control to achieve the purpose of the load reduction, but has not given a control strategy of air-conditioner load. [22, 23] proposed direct load control based on state-priority queue control method, but its stability heavily depends on the diversity of load states, which is easy to be destroyed. Ninagawa et al established neural network models based on practical operational data of inverter air-conditioners for simulating their load reduction response [24]. The practical communication environment was also emulated. Compared to these studies [21-24], the study has the following novel contributions: 1) virtual state-of-charge (VSOC) is defined for reflecting the energy storage level of a VES, based on which a VSOC-priority strategy is proposed to control inverter air conditioners to provide demand response services; 2) the electric power of inverter air conditioners is controlled at the level where the corresponding heating output exactly compensates the heat loss, so that the indoor temperature will not go beyond the limit during control; 3) the impact of the shape and magnitude of load reduction targets and the length of communication time steps on the control performance is investigated.’
Finally, the hypothesis and objectives of the paper are presented and summarized in a new paragraph added to the end of the revised version of the paper.
‘This paper investigates how large amounts of inverter air conditioners can be modelled and controlled to provide demand response services for power systems. It is assumed that power utility companies will recruit customers with inverter air conditioners installed, and further install necessary communication and control devices to aggregate the inverter air conditioners and enable the control strategy proposed in this paper. Specifically, in this paper, inverter air conditioners are modelled as VES, including electrical parameters part and thermal parameters part. Based on the VES model, a VSOC-priority strategy is proposed to control the inverter air conditioners for providing demand response services. Finally, the performance of the proposed control strategy is verified, with the impact of the shape and magnitude of load reduction targets and the length of communication time steps assessed.’
Point 3:Discussion: It is my opinion that a separate discussion section would help the reader to understand the study. However, the main issue arises from the lack of comparison with state-of-the-art studies. From my point of view, even though many publications are listed in the literature review, it has no justification not conducting a comparison with main previous works already published. The lack of such a comparison highly compromises the significance of the paper, so I strongly recommend authors to conduct as much and suitable comparisons as needed to solve this issue.
Response 3:Thank you very much for your valuable comments. In the Introduction section, we comprehensively updated the literature review part and added discussions regarding the comparison of the study with the existing ones. The updated paragraphs are presented as follows:
‘In this context, some preliminary research has been conducted regarding the demand response from inverter air-conditioners, which could be further classified into two categories. The first category of studies is to explore the building thermodynamic model of inverter air-conditioners. The Equivalent thermal parameter (ETP) model has been widely used as the basis for further control. In [19], based on the first-order ETP model, the VES model of a building with air-conditioners is established considering comfort level, and a series of VES parameters such as rated capacity, rated power and charging and discharging time are derived. However, in [19], the electric power of air conditioners are just taken as a constant power input, lacking in-depth analysis of the electrical parameters of air conditioners, so it cannot describe the influence of the state change of air-conditioning loads on the power grid. [20] proposed a high-precision mode reflecting the different operating conditions of electric water heaters, but due to the limitation of calculation, the model is only suitable for small-scale regulation, and cannot be applied to power grid cluster dispatch. Compared to the existing studies, such as [19, 20], this paper establishes a VES model for inverter air-conditioning loads, which is more conducive to practical application. Two parts are involved in the VES model, including the electrical parameters part based on the operating characteristics of inverter air-conditioning loads and the thermal parameters part based on the equivalent thermal parameter model.
The second category of studies is to explore the load reduction from inverter air-conditioners. [21] studied the direct load control to achieve the purpose of the load reduction, but has not given a control strategy of air-conditioner load. [22, 23] proposed direct load control based on state-priority queue control method, but its stability heavily depends on the diversity of load states, which is easy to be destroyed. Ninagawa et al established neural network models based on practical operational data of inverter air-conditioners for simulating their load reduction response [24]. The practical communication environment was also emulated. Compared to these studies [21-24], the study has the following novel contributions: 1) virtual state-of-charge (VSOC) is defined for reflecting the energy storage level of a VES, based on which a VSOC-priority strategy is proposed to control inverter air conditioners to provide demand response services; 2) the electric power of inverter air conditioners is controlled at the level where the corresponding heating output exactly compensates the heat loss, so that the indoor temperature will not go beyond the limit during control; 3) the impact of the shape and magnitude of load reduction targets and the length of communication time steps on the control performance is investigated.’
Point 4:Shouldn’t equation (14) be 2 different equations?
Response 4:Thank you very much for your comments. Equation(13) and (14) do consist of two differentequations. I've taken them apart. Thank you for your reminder.
Point 5:A more detailed result analysis section could be added.
Response 5:Thank you very much for your valuable comments. The simulation results and analysis section have been comprehensive improved. More detailed analysis and discussions have been made across the whole section. The revised and newly added parts have been highlighted in red in the revised version of the paper.
Point 6:What can the authors do to validate their results?
Response 6:Thank you very much for your valuable comments. The performance of the proposed control strategy and the impact of various factors have been validated through computer simulation using MATLAB software. The simulation results and the corresponding analysis have been comprehensively updated, which can be found in the revised version of the paper.
Considering that the proposed control strategy involves a large number of customers and large amount of power shortage of public power networks, it is not very practical for the authors to conduct the validation through experiments in real life at the moment. Simulation results can basically validate the performance of the proposed method, as did in many existing studies in this area, and validation through practical experiments can be a future work to be further conduced.
Point 7:Figure 5 shows its tracking effect. But it can hardly be seen since the reader has to squint their eyes to extract any useful information out of this Figure.
Response 7:Thank you very much for your comments. Figure 5 has been further augmented for better reading. Actually, in Figure 5, the response curve is almost totally coincident with the target curve, showing that the proposed VSOC-priority control strategy basically achieves no error tracking.
Point 8:Figures 3 to 8 should be enhanced. There are no limitations of page numbers, so they can be augmented. As they’re now, the axis labels and the content of these figures are small.
Response 8:Thank you very much for your valuable comments. After the revision, all the figures have been augmented, including the axis label and the content of these figures.
Point 9:The conclusion is short and incomplete. The obtained results should be briefly and clearly mentioned through the support of numerical data. What were the most sounding quantifiable findings of this study? The conclusion is intended to help the reader understand why your research should matter to them after they have finished reading the paper. A conclusion is not merely a summary of the main topics covered or a re-statement of your research problem, but a synthesis of key points and, if applicable, where you recommend new areas for future research.
Response 9:Thank you very much for your valuable comments. The Conclusion section has been comprehensively improved, which incorporates the points you kind suggested. The updated Conclusion section is as follows:
‘This paper investigated how to model and control inverter air conditioners to provide demand response services for electric power systems. In terms of modelling, based on the ETP model, a complete VES model of inverter air conditioners is established, which can reflect the practical electro-thermal characteristics. The model is divided into electrical parameters and thermal parameters parts, reflecting the impact of inverter air-conditioning loads on the power grid. The model is further discretized to reduce communications traffic. In terms of control, a virtual state of charge priority based control method was proposed, where the electric power of inverter air conditioners is controlled at the level where the corresponding heating output exactly compensates the heat loss, so that the indoor temperature will not go beyond the limit during control.’
Simulation results verified the models established and the control method proposed, and also assessed the impact of various factors. The key findings are presented as follows:
1) The control strategy can track the load reduction target smoothly for a long time (120 min in the simulation) and make the inverter air conditioners gradually stabilize to a certain state (within 40 min in the simulation).
2) The control strategy can track the load reduction target accurately with different shapes (constant and sinusoidal targets in the simulation).
3) The control strategy can track the load reduction target accurately when it is within the air conditioner population’s capability (below 50 kW in the simulation), but will have higher errors if the target is beyond the population’s capability (above 50 kW in the simulation).
4) Shorter communication time steps (2 min in the simulation) will result in less power fluctuation and less violation of VSOC limits, compared to those of longer time steps (5 min in the simulation).
Future research topics may include the cost-effectiveness analysis of the control scheme proposed, and the remuneration mechanism for compensating the customers who participate in the proposed demand response program. If having the conditions in place, the proposed control strategy can be tested in real-life systems for further validation.’
Point 10:The citation section is quite short, please add the following citations:
· https://doi.org/10.1109/EEEIC.2017.7977766
· https://doi.org/10.3390/en10122133
· https://doi.org/10.3390/en12040671
· https://doi.org/10.1109/INTLEC.2017.8214142
Response 10:Thank you very much for your valuable comments. The suggested papers are very important. All of them have been discussed and cited in the relevant places of the Introduction section of the revised version of the paper.
Point 11:This paper seems to be fairly crafted, yet with some limitations. Thus, the authors need to improve in order to have an acceptable paper for Processes.
Response 11:Thank you very much for your valuable time and very helpful comments. We have significantly improved the paper based on your valuable comments and hope that the paper could be an acceptable one for Processes.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The paper described scheduling of of ACs as thermal inertia to provide energy to the grid in case of a shortage.
The authors should take the following into consideration:
1- Please clearly describe the intellectual merit of your paper comparing to other works using AC scheduling to hedge against uncertainty in the electricity market.
2- Please cite the following highly related research materials
Ancillary service to the grid using intelligent deferrable loads, in IEEE Transactions on Automatic Control
Reserves from controllable swimming pool pumps: Reliability assessment and operational planning in HICCS 2018.
Architecture and Algorithms for Privacy Preserving Thermal Inertial Load Management by A Load Serving Entity, in IEEE transactions on Power systems, 2017.
Balancing California's Grid Without Batteries, in CDC 2018
Ancillary service to the grid using intelligent deferrable loads, in IEEE Transactions on Automatic Control
3- Please read through the paper and check for grammar errors. There are many extra long sentences in the paper that makes it hard to read.
4- You should use a period at the end of each figure caption.
Author Response
Point 1: Please clearly describe the intellectual merit of your paper comparing to other works using AC scheduling to hedge against uncertainty in the electricity market.

Response 1:Thank you very much for your valuable comments. We have reviewed more existing studies and clearly described the intellectual merit of our paper compared with state-of-the-art studies. Specifically, the following paragraphs have been updated and added in the Introduction section of the revised paper.
‘In this context, some preliminary research has been conducted regarding the demand response from inverter air-conditioners, which could be further classified into two categories. The first category of studies is to explore the building thermodynamic model of inverter air-conditioners. The Equivalent thermal parameter (ETP) model has been widely used as the basis for further control. In [19], based on the first-order ETP model, the VES model of a building with air-conditioners is established considering comfort level, and a series of VES parameters such as rated capacity, rated power and charging and discharging time are derived. However, in [19], the electric power of air conditioners are just taken as a constant power input, lacking in-depth analysis of the electrical parameters of air conditioners, so it cannot describe the influence of the state change of air-conditioning loads on the power grid. [20] proposed a high-precision mode reflecting the different operating conditions of electric water heaters, but due to the limitation of calculation, the model is only suitable for small-scale regulation, and cannot be applied to power grid cluster dispatch. Compared to the existing studies, such as [19, 20], this paper establishes a VES model for inverter air-conditioning loads, which is more conducive to practical application. Two parts are involved in the VES model, including the electrical parameters part based on the operating characteristics of inverter air-conditioning loads and the thermal parameters part based on the equivalent thermal parameter model.
The second category of studies is to explore the load reduction from inverter air-conditioners. [21] studied the direct load control to achieve the purpose of the load reduction, but has not given a control strategy of air-conditioner load. [22, 23] proposed direct load control based on state-priority queue control method, but its stability heavily depends on the diversity of load states, which is easy to be destroyed. Ninagawa et al established neural network models based on practical operational data of inverter air-conditioners for simulating their load reduction response [24]. The practical communication environment was also emulated. Compared to these studies [21-24], the study has the following novel contributions: 1) virtual state-of-charge (VSOC) is defined for reflecting the energy storage level of a VES, based on which a VSOC-priority strategy is proposed to control inverter air conditioners to provide demand response services; 2) the electric power of inverter air conditioners is controlled at the level where the corresponding heating output exactly compensates the heat loss, so that the indoor temperature will not go beyond the limit during control; 3) the impact of the shape and magnitude of load reduction targets and the length of communication time steps on the control performance is investigated.’
Point 2:Please cite the following highly related research materials
Ancillary service to the grid using intelligent deferrable loads, in IEEE Transactions on Automatic Control
Reserves from controllable swimming pool pumps: Reliability assessment and operational planning in HICCS 2018.
Architecture and Algorithms for Privacy Preserving Thermal Inertial Load Management by A Load Serving Entity, in IEEE transactions on Power systems, 2017.
Balancing California's Grid Without Batteries, in CDC 2018
Response 2:Thank you very much for your valuable comments. These papers are highly related and very important. All of them have been discussed and cited in the relevant places of the Introduction section of the revised version of the paper.
Point 3:Please read through the paper and check for grammar errors. There are many extra long sentences in the paper that makes it hard to read.
Response 3:Thank you very much for your valuable comments. We have carefully checked the paper and tried to revise the grammar errors across the paper. We also tried to re-organize the very long sentences to make it easier to read.
Point 4:You should use a period at the end of each figure caption.
Response 4:Thank you very much for your kind comments. We are very sorry for our negligence. We haveadded a period to the end of each figure caption.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
The authors have addressed all the concerns of the reviewer.
