Integration of daytime radiative cooling and solar heating for year-round energy saving in buildings

The heating and cooling energy consumption of buildings accounts for about 15% of national total energy consumption in the United States. In response to this challenge, many promising technologies with minimum carbon footprint have been proposed. However, most of the approaches are static and monofunctional, which can only reduce building energy consumption in certain conditions and climate zones. Here, we demonstrate a dual-mode device with electrostatically-controlled thermal contact conductance, which can achieve up to 71.6 W/m2 of cooling power density and up to 643.4 W/m2 of heating power density (over 93% of solar energy utilized) because of the suppression of thermal contact resistance and the engineering of surface morphology and optical property. Building energy simulation shows our dual-mode device, if widely deployed in the United States, can save 19.2% heating and cooling energy, which is 1.7 times higher than cooling-only and 2.2 times higher than heating-only approaches.

My comments regarding the paper are provided in the following sections: About the Physical apparatus -I do not pose sufficient expertise in materials engineering to understand and critique the methods, results and discussion surrounding the physical performance of individual panels. I trust this will be captured by other reviewers. However, I do not perceive the technical description and analysis of the panel itself to be the core proposed novelty of the paper, nor the means by which the authors demonstrate the proposed impact of their innovation.
About the climate analysis undertaken -I feel the main overarching disadvantage of the manuscript is an apparent oversimplification, or misrepresentation, of data and results that aim to assess the viability of the proposed technology for heating and cooling across the US. This begins early in the manuscript in the presentation of a classification of cities with regards to 'heat management monotony'. I'm not familiar with such as term, but the analysis summarised in Figure 1a is counter-intuitive and seems to disagree with supplemental data provided by the authors in Figure S4. For example, from Figure 1a, Seattle appears to be a more cooling dominant city than San Francisco, but clearly from Figure S4, San Francisco very clearly possesses a larger and more persistent cooling load than Seattle, and Seattle conversely possesses a much larger heating load. The authors should consider reverting to more widely-understood metrics for heating/cooling classification of cities such as Heating Degree Days and Cooling Degree Days. About the predictive model of radiant cooling -The model assumes that the main intermediary between the panel and outer space is the atmosphere. This is true, but it would appear that the chosen model assumes a clear sky exists at all times and so transmissivity is only affected by the vertical water column. As it is the intent of this paper to examine the applicability of the radiant cooling panel across the continental US on an annual basis, this assumption cannot hold. Accounting for the effect of cloud cover is challenging, but it is necessary to do so. Not only do clouds change the radiant interaction between the panel and the edge of the atmosphere, depending on their altitude and temperature, they may be a source of radiation in their own right.
Spatial obstructions and view factors -The model assumes that the effective roof area that can be covered by the proposed radiant panel system may be 60%. No further information is given regarding the effective sky view factor of rooftop systems. This surely cannot be 1 due to the diversity of likely obstructions that typical rooftops face, from surrounding vegetation to surrounding buildings.
Other viable interactions between the rooftop and the sun.
-During summer daytime periods, it would appear that there is still an argument to prefer utilisation of roof area for solar PV generation. Electricity generation can serve multiple uses, from building to (increasingly) transport energy services. Modern solar PV systems may generate well over 150 W/m2 in electricity. Under the same form of system modelling as applied in this work, one could envisage this solar PV electricity serving a heat pump-cooling system with a COP in excess of 3. From an energy consumption mitigation standpoint, this would seem to be a far more effective solution for reducing current electricity usage for space cooling across the United States. On the other hand, an understated advantage of the proposed solution is that it can operate under its 'cooling' mode during both daytime and nighttime. This does pose questions, however. Would the system operate at times of night where an immediate indoor cooling demand does not exist, and therefore it would be valuable to store chilled water? If so, how would such a system perform, particularly with respect to exergy? In general, the underdeveloped nature of the energy model, from unclear assumptions made regarding building typologies (is an 1660 m2 office building meant to be representative of the US building stock?), to unclear interactions between the radiant cooling panel and dynamic weather as discussed, would suggest more thoughtful analysis or explanation of this part of the provided work is needed.
Ultimately, I would propose that, if building energy simulation is to be relied on to forecast the viability of the proposed radiant heating / cooling system to entire building stocks, greater consultation of prior research from the building simulation is needed, as is adoption of existing best practises with regards to thermal energy systems modelling.

Response to reviewer #1
Radiative cooling is an emerging technology for energy sustainability. This work presents a dualmode device for energy saving in HAVC systems. An electrostatically-controlled thermal contact film is introduced for better thermal conductance. The solar heating/radiative cooling mode could be adaptively switched to different environment to save energy. The general idea is interesting and well presented. However, I have concerns at the following points: Our reply: We thank the reviewer very much for his/her positive report and constructive suggestions. In the following sections, we would address the questions point by point. 1. The absorption spectrum of the heating material in the mid-infrared regime is missing. By suppressing the infrared emission, is it possible for the heating mode to obtain a higher temperature for more efficient heating?
Our reply: Thank you for pointing out this omission. We have added it to the revised Fig. 3g (see Fig. R1 below as well). The top PDMS layer is visibly-transparent and infrared-emissive for radiative cooling. For heating mode, Cu is the electrode for supplying static charge, and the Cu/Zn is the plasmonic selective absorber. The thickness of PDMS, Cu/Zn, Ag, Cu, and PI film are 110 µm, 1 µm, 300 nm, 300 nm, and 25 µm, respectively. b, Photo of the dual-mode material shows the different visible appearance of the heating/cooling parts c, SEM image of the heating material.
d, XPS spectrum of copper particle on heating material. e and f, Visible, near-IR, and mid-IR reflectance spectra of cooling materials of different thicknesses. g, Absorptance/emittance of dual-mode material. Solar spectrum (yellow shaded area), and atmospheric transmittance window (green shaded area) are plotted for references. h, Reflectance of heating and cooling material before/after 100 times rolling testing. The inset is the photo of the sample under testing.
2. The outdoor test is confusing. From the schematic of the setup shown in Fig. 4b, it seems that the system did not even implement the pre-mentioned dual-mode device into the test. Instead, only a copper plate was used in those tests. How did the authors realize the mode-switching using the copper plate? It is necessary to identify the functionality (both heating mode and cooling mode) of the proposed device in the field tests.

Our reply:
Thank you very much for your constructive suggestion. In fact, as shown in Fig.4 c and d, the switching process between heating and cooling can be achieved by motors or manually (see Fig.   4a).To make these points clear, we have corrected the schematic (Fig. 4b)   4. In the last line of page 16, the authors claimed 51.4 GJ can be saved by the proposed dual-mode device. It is unclear how did this number was calculated. Details are required to justify whether this estimation is convincing.

Our reply:
We appreciate the comments. Because Reviewer#3 also commented on the building energy saving calculation and suggested refining the calculation by taking more practical complexities into account, we combined both reviewers' comments and redid the calculation with a more detailed method explanation. We collaborate with Prof. Gang Tan (Department of Civil & Architectural Engineering, University of Wyoming, USA) for his expertise in building energy engineering can help us improve the scientific rigorousness in the energy saving calculation. The details are listed below and also in Supplementary Information Note 3 (Page 11-16).
System cooling-mode energy saving: Potentially, there are quite many application methods for the proposed dual-mode radiation heating and radiative cooling materials in buildings. We demonstrate an example application of integrating the material with building envelopes to provide space heating and cooling energy using heat exchangers. At this system level application, a comprehensive integrated analysis of the proposed device and the subject building is needed, which creates hourly performance simulation for the 16 cities throughout a whole year. In addition, in order to evaluate the energy savings of the dualmode device under real application condition, the typical meteorological year (TMY3) (National Pcooling power = P'rad -P'atm (1) where ɛfilm and Tfilm are the emissivity and surface temperature of the film, and A is the area. The ɛatm is given by 5 , ɛatm = ɛatm,c (1-0.78CF) + 0.38CF 0.95 RH 0.17 (4) ɛatm,c = 0.618 + 0.056 (5) Where ɛatm,c is the effective sky emissivity under clear skies, CF is the cloud fraction, RH is the ambient relative humidity, and Pw is the ambient water vapor partial pressure, Td is the dew point, P0 = 610.94 Pa, cT = 17.625, and T0 = 243.04 ℃. The hourly values of these weather-related parameters can be obtained from TMY3 weather data.
With heat transfer medium such as water flowing in the heat/cold exchangers or collectors, the cold water will possess varied temperatures with environmental weather changes. Lower temperature water can be directly used for space cooling through indoor systems such as radiant cooling ceilings, which commonly adopts a fluid temperature of 13-18 ℃ (Fernandez, N., Wang, W., Alvine, K., Katipamula, S. Energy savings potential of radiative cooling technologies. PNNL-24904, (2015).) in order to avoid surface condensation. In contrast, when the water temperature is higher than this range, the radiative cooling cold water can be supplied to air conditioner side and cool the condenser side to achieve higher efficiency (details seen in the next section). In other words, the cooling power is to directly cool the building spaces when the temperature is below 18 ℃, and to cool the air conditioner condenser when the temperature is above 18 ℃. Therefore, when the temperature is below 18 ℃, that particular hour's Pcooling saving is calculated by, System cooling power analysis for air-conditioner unit We choose to model the energy saving by considering the case of retrofitting a traditional aircooled vapor-compression air-conditioner unit with the radiative cooling device using water as the heat transfer fluid, as shown in Fig. R3a. The thermodynamic cycles are shown in Fig. R3b. The blue line represents the traditional air-cooled AC and the red line represents the AC coupled with radiatively-cooled, below-ambient-temperature water. Points 1-4 represent saturated vapor at low pressure, compressed vapor, saturated liquid at high pressure, liquid after expansion valve. The radiative cooling system provides additional cooling and reduces the condenser temperature, and the new thermodynamic cycle follows points 1'-4', which is more efficient than 1-4. By comparing the coefficient of performance (COP) before and after installing the radiative cooler, the energy saving can be calculated. Figure R3 Modeling cooling system-level energy savings. a, Scheme for air-cooled AC coupled with radiative-cooling fluid panels. b, Thermodynamic cycle diagram of the air-cooled AC with (red) and without (blue) radiative cooling.
For the air-cooled vapor-compression AC, the basic thermodynamic equations (eq.8 -eq.13) are listed below to demonstrate its performance during the "compression-condensation-expansionevaporation" loop. Pload is the heat removed from the building for maintaining the comfortable room temperature (22°C) per unit time, which is calculated from EnergyPlus. dm/dt is the mass flow rate of refrigerant. h1, h2, h3, h4 are the enthalpies for 4 points in the thermodynamic cycle. Pr is the power consumption of the fan calculated by the fan affinity law. Hence, the COP of the traditional AC can be calculated by dividing Pload with Ptotal.
Then we added radiatively-cooled water panel to the system to enhance the efficiency. For the cooling panels, thermodynamic equations (eq.14-eq.18) were given below. Note pcool is cooling power density (W/m 2 ) and Pcool is the cooling power (W). To account for the negative correlation between cooling power density and sub-ambient temperature drop (ΔTcool) due to the hemispherical ambient thermal radiance, the cooling power density is subtracted by 4.23*ΔTcool, which is based on Eriksson, T. S., and C. G. Granqvist's research (Eriksson, T. S., and C. G. Granqvist. "Radiative cooling computed for model atmospheres." Applied Optics, 21, 4381-4388 (1982).). S is the effective roof area that could be utilized, which is assumed to be 60% of the model building rooftop area.
dmwater/dt is the water mass flow rate inside the tube. Cwater is the heat capacity of water. hw(vwater) is the overall heat transfer coefficient of water approximated with the Dittus-Boelter equation for pipe flow. Twaterin and Twaterout are the temperature for water flowing in and out of the radiative cooling surface-plate heat exchanger, respectively. Ppump(vwater) is input water pump power, which is also calculated by the fan affinity law. The new COP could, therefore, be calculated by dividing Pload by P'total.
r air airout airin water waterout waterin Finally, the cooling energy saved by using cooling materials can be demonstrated by: where E is the cooling electricity consumption with traditional AC, calculated by EnergyPlus Heating-mode energy saving: For heating energy saving, eq.19 and 20 were used to analyze the device performance. Pheating is the radiative heating power of the device. I is the global horizontal solar radiation obtained from TMY3 weather data. S is the effective roof area that could be utilized, which is assumed to be 60% Through the above cooling model and heating model, the cooling and heating energy saving of each city per hour can be calculated. In dual-mode calculation, we can choose to operate in the mode that generates the maximum energy saving in that specific hour. That is, if the cooling saving is larger than heating saving, then we use cooling mode in this hour. Otherwise, heating mode would be taken into consideration.
Esaving,dual = MAX (Esaving,cool, Esaving,heat) Therefore, by arranging all cooling and heating energy saving in each hour of each city, we could get the annul energy saving in the U.S. in heating-only, cooling-only, and dual-mode approaches.
This can be found in Page 11-16 of the revised Supporting information.
5. The required voltage to induce the high-thermal contact is in kV level, which seems too high for civil applications. Besides, the duration after charge removal in fig 2d is too short for an HVAC application that usually last for months. Therefore, I have major concern on the practicability at this point.
Our reply: We appreciate the reviewer's comments. Even though the voltage looks high, the current is only about 0.07 mA, which is a safe current for the human body (David W. Smith, Preventing Electrical Shock, The Texas A&M University system, https://cdn-ext.agnet.tamu.edu/wpcontent/uploads/2019/06/E-221_-Preventing-Electrical-Shock.pdf). In addition, the working voltage of ionizer air purifiers, a common household appliance, can also reach 5 kV without the concern of electrical shock (http://www.air-purifier-power.com/ionizer-air-purifier.html).
Accordingly, as long as proper management and control are implemented, we believe our highvoltage, low-power device would be safe for civil applications.
We have added it in Page 8 of the revised manuscript.
According to reviewer's suggestions, we have tested the electrostatic effect lifetime for one week.
The results can be seen in revised Fig. 2d (see Fig. R4 below as well). While this static electricity may not last for several months, we believe this concern can be solved by periodic "charging" in practical applications. The time for each "charging" is completed within only a few seconds.
We have added it in Page 8 of the revised manuscript. Our reply: We thank the reviewers for pointing this out. We have added more discussion about heating material in revised Note 2 (see below).
For heating material, on the copper film, a layer of zinc film of 1 µm thick was electrodeposited   Response to reviewer #2 The authors propose a dual-mode device and demonstrate that it is capable of switching between radiative cooling and solar heating for year-long energy savings both in the heating and cooling conditions. Designed actuators will physically rotate the cooling/heating films along a track system to switch between two different films. The work focuses on reducing the interfacial thermal resistance between the functioning layer and the substrate surface by applying a high voltage of 2 kV. The overall energy savings are estimated and compared with heating-only or cooling-only devices. A dual-mode device is beneficial to switch between the radiative cooling mode or solar heating mode at different seasons. However, we believe that the work is an incremental advance and does not have the novelty that is suitable for publication in Nature Communications. In addition, there are several concerns that need to be addressed.

Our reply:
We thank the reviewer for his/her comments. Indeed, there were many pioneering works in the fields of nighttime radiative cooling and solar heating that laid the foundation of the present work.
However, they are all static or quasi-dynamic devices (see Table R1), which cannot completely solve the dynamic heating and cooling demand problem effectively, especially in the daytime. We are grateful for the inspirations and the shared knowledge. Still, we believe our work contains a good amount of elements of novelty. To better convey our ideas, we summarize our key accomplishments as follows: 1. This is the first experimental demonstration of the solar heating and daytime radiative cooling active dual-function building envelope. We have conducted extensive literature research to support this statement, which is compiled in Supporting Information Table 4 (see Table R1 as well). First of all, despite its significant potential to enhance the building energy efficiency, the concept of switchable or dual-mode building rooftops are still in an early stage. Among these papers, none of them truly accomplished both solar heating and daytime radiative cooling. This is because of one fundamental limit -solar heating and daytime radiative cooling have exactly the opposite required optical properties. Based on this fundamental limit, it can be concluded that the materials for these two modes must be physically or chemically changed. For adapting to various complicated scenarios, such tuning should be active rather than passive. Therefore, from the materials science point of view, there are only two feasible approaches: electrochromism and mechanochromism. This paper chooses mechanochromism for its compatibility with existing building technologies and thus a higher chance of creating real social impacts. It was not a trivial task, but we eventually solved it using the electrostatic effect, as described below.
2. This is the first paper to achieve reversible thermal contact for mechanochromic heating/cooling. We realized that, in practice, all the tuning of material's optical properties would be in vain if the heating/cooling energy cannot be transported to the buildings or the heat exchangers. In mechanochromism that relies on frequent movement or shapechanging to tune the optical properties, this means the thermal contact needs to be tunable as well. To the best of our knowledge, this is the first paper to employ the concept of reversible electrostatic forces to achieve large-scale thermal contact switching for net-zero buildings.
3. To make a broader impact, we estimate the energy saving of dual-mode (daytime radiative cooling and solar heating) device for the entire U.S. climate zones. We pointed out the potential need for dual-mode building envelope by noting the needs for both heating and cooling during one year (the revised manuscript Fig. 1 and Fig. R1 below).
After experimentally demonstrating the dual-mode device, we then calculated the building energy consumption and saving for different climate zones and cities. The results not only support our assumption but provide a guideline for future employment of the dual-mode device. We envision these data will be an essential piece of information to encourage the readers of Nature Communications from a variety of disciplines to contribute to the field of energy-efficiency buildings and sustainability.  . In addition, the working voltage of ionizer air purifiers, a common household appliance, can also reach 5 kV without the concern of electrical shock (http://www.air-purifier-power.com/ionizerair-purifier.html). Accordingly, as long as proper management and control are implemented, we believe our high-voltage, low-power device would be safe for civil applications.
We have added it in Page 8 of the revised manuscript.

1.Economic analysis
The cost analysis for this study was based on the post-1980 medium office model defined by the  Table R2, the 10-years cost is about $608. The price evaluation of heating material and cooling material refers to similar products in the market (see Table R2).
Besides the equipment and material cost, labor fee is another aspect to consider for cost analysis.
Based on the life expectancy in Table R2, the labor fee mainly comes from the initial assembling and annual change of electrostatic generator. As shown in 2. The work does not address some of the reliability issues when dirt or water gets into the interface between the PI film and substrate, or inside the track system. If there is dirt accumulating on the substrate beneath the films under windy conditions, the high voltage may not be sufficient to reduce the interfacial thermal resistance. Furthermore, water or humidity is also known to reduce the effect of static electricity. These are especially important when the films are exposed to the ambient air.
Our reply: Thank you very much for your constructive suggestion. The analysis of reliability issues is listed below.

Analysis of the potential impact of dirt
It is expected that the interfacial thermal resistance could be impacted when dirt gets into the interface between the PI film and the substrate. When manufacturing the proposed dual-mode heating and cooling device, it is critical to package the film, the underneath substrate, and the rolling components into a tightly sealed modular system to highly reduce the dirt penetration risk.
In addition, operation maintenance architectures such as brush cleaners installed at the separation edge of the heating/cooling film will intermittently clean the surface of the underneath substrate and help remove dirt that may appear between PI film and substrate.
Analysis of the potential impact of humidity As shown Fig. R2, the effect of humidity on interfacial thermal resistance is also demonstrated.
The result shows that the good thermal contact can maintain for two days when the relative humidity is in the range of 40% to 60%. Even in the rare case of > 95% humidity, a good thermal contact can still last for more than one day. In practical applications, periodic "charging" will be performed to ensure continuous and good thermal contact. Figure R2. The thermal contact over time at different humidity.
We have added it in Page 7 of the revised Supporting information.
Finally, the authors are very grateful to you for investing your valuable time in reviewing our manuscript again

Response to reviewer #3
This paper presented the technical apparatus and performance of a novel dual-mode radiant heating and cooling panel which can, most critically, reject heat to the ambient sky even during sunny daytime conditions. The authors provide a validation of experimental data, followed by calibration of an empirical model, followed by the use of this model (coupled with a building energy model) to assess the potential cooling and heating savings offered by the technology across the United States.
My comments regarding the paper are provided in the following sections: Our reply: We thank the reviewer for his/her evaluation. The point-by-point response on the detailed comments is as follows.
(1) About the Physical apparatus -I do not pose sufficient expertise in materials engineering to understand and critique the methods, results and discussion surrounding the physical performance of individual panels. I trust this will be captured by other reviewers. However, I do not perceive the technical description and analysis of the panel itself to be the core proposed novelty of the paper, nor the means by which the authors demonstrate the proposed impact of their innovation.

Our reply:
We thank the reviewer for his/her comments. Indeed, there were many pioneering works in the fields of nighttime radiative cooling and solar heating that laid the foundation of the present work.
However, they are all static or quasi-dynamic devices (see Table R1), which cannot completely solve the dynamic heating and cooling demand problem effectively, especially in the daytime. We are grateful for the inspirations and the shared knowledge. Still, we believe our work contains a good amount of elements of novelty. To better convey our ideas, we summarize our key accomplishments as follows: 1. This is the first experimental demonstration of the solar heating and daytime radiative cooling dual-function active building envelope. We have conducted extensive literature research to support this statement, which is compiled in Supporting Information Table 4 (see Table R1 as well). First of all, despite its significant potential to enhance the building energy efficiency, the concept of switchable or dual-mode building rooftops are still in an early stage. Among these papers, none of them truly accomplished both solar heating and daytime radiative cooling. This is because of one fundamental limit -solar heating and daytime radiative cooling have exactly the opposite required optical properties. Based on this fundamental limit, it can be concluded that the materials for these two modes must be physically or chemically changed. For adapting to various complicated scenarios, such tuning should be active rather than passive. Therefore, from the materials science point of view, there are only two feasible approaches: electrochromism and mechanochromism. This paper chooses mechanochromism for its compatibility with existing building technologies and thus a higher chance of creating real social impacts. It was not a trivial task, but we eventually solved it using the electrostatic effect, as described below.
2. This is the first paper to achieve reversible thermal contact for mechanochromic heating/cooling. We realized that, in practice, all the tuning of material's optical properties would be in vain if the heating/cooling energy cannot be transported to the buildings or the heat exchangers. In mechanochromism that relies on frequent movement or shapechanging to tune the optical properties, this means the thermal contact needs to be tunable as well. To the best of our knowledge, this is the first paper to employ the concept of reversible electrostatic forces to achieve large-scale thermal contact switching for net-zero buildings.
3. To make a broader impact, we estimate the energy saving of dual-mode (daytime radiative cooling and solar heating) device for the entire U.S. climate zones. We pointed out the potential need for dual-mode building envelope by noting the needs for both heating and cooling during one year (the revised manuscript Fig. 1 and Fig. R1 below).
After experimentally demonstrating the dual-mode device, we then calculated the building energy consumption and saving for different climate zones and cities. The results not only support our assumption but provide a guideline for future employment of the dual-mode device. Admittedly, the calculation process can be further improved in the future. Still, we aimed to present these data to encourage the readers of Nature Communications from a variety of disciplines to contribute to the field of energy-efficiency buildings and sustainability. References for Table R1. (2) About the climate analysis undertaken -I feel the main overarching disadvantage of the manuscript is an apparent oversimplification, or misrepresentation, of data and results that aim to assess the viability of the proposed technology for heating and cooling across the US. This begins early in the manuscript in the presentation of a classification of cities with regards to 'heat management monotony'. I'm not familiar with such as term, but the analysis summarised in Figure 1a is counter-intuitive and seems to disagree with supplemental data provided by the authors in Figure S4. For example, from Figure 1a, Seattle appears to be a more cooling dominant city than San Francisco, but clearly from Figure S4, San Francisco very clearly possesses a larger and more persistent cooling load than Seattle, and Seattle conversely possesses a much larger heating load. The authors should consider reverting to more widely-understood metrics for heating/cooling classification of cities such as Heating Degree Days and Cooling Degree Days.
Our reply: We thank the reviewers for the suggestion. We have removed the previous expression and used the Heating Degree Days and Cooling Degree Days to describe the requirement of heating and cooling of building in the revised manuscript, Fig.1. (see Fig. R1 below as well). We revised the manuscript accordingly as follows: "…heating degree days and cooling degree days can commonly and quantitively describe the heating and cooling demands of buildings 5 . Fig. 1a shows the annual heating and cooling degree days of 16 U.S. cities that represent the 16 climate zones. It can be found most cities need both heating and cooling in the whole year. Taking Durham, North Carolina as an example, the cooling consumption predominates from May to October, and the rest 6 months are heating-dominant (Fig.   1b)." This can be found in page 2 of the manuscript.
(3) About the predictive model of radiant cooling -The model assumes that the main intermediary between the panel and outer space is the atmosphere. This is true, but it would appear that the chosen model assumes a clear sky exists at all times and so transmissivity is only affected by the vertical water column. As it is the intent of this paper to examine the applicability of the radiant cooling panel across the continental US on an annual basis, this assumption cannot hold. Accounting for the effect of cloud cover is challenging, but it is necessary to do so. Not only do clouds change the radiant interaction between the panel and the edge of the atmosphere, depending on their altitude and temperature, they may be a source of radiation in their own right.
Our reply: Thank you very much for your constructive suggestion. According to Reviewer's suggestions, we have considered the cloud cover and recalculated the model (see Fig. 5 in the revised manuscript, see Fig. R2 below as well). The more details see below.
Potentially, there are quite many application methods for the proposed dual-mode radiation heating and radiative cooling materials in buildings. We demonstrate an example application of integrating the material with building envelopes to provide space heating and cooling energy using heat exchangers. At this system level application, a comprehensive integrated analysis of the proposed device and the subject building is needed, which creates hourly performance simulation for the 16 cities throughout a whole year. In addition, in order to evaluate the energy savings of the dualmode device under real application condition, the typical meteorological year (TMY3) (National https://rredc.nrel.gov/solar/old_data/nsrdb/1991-2005/tmy3/.) weather data are used, and the impacts of the humidity and the clouds on cooling capability are evaluated. Therefore, the following calculation algorithms (eq.1 -eq.6) are selected to estimate the cooling power with effective atmospheric emissivity (ɛatm): Pcooling power = P'rad -P'atm (1) where ɛfilm and Tfilm are the emissivity and surface temperature of the film, and A is the area. The Where ɛatm,c is the effective sky emissivity under clear skies, CF is the cloud fraction, RH is the ambient relative humidity, and Pw is the ambient water vapor partial pressure, Td is the dew point, This can be found in Page 11 of the revised Supporting information. of the heating and cooling energy), which is 1.7 times higher than cooling-only (138 GJ) and 2.2 times higher than heating-only (106 GJ) devices.
(4) Spatial obstructions and view factors -The model assumes that the effective roof area that can be covered by the proposed radiant panel system may be 60%. No further information is given regarding the effective sky view factor of rooftop systems. This surely cannot be 1 due to the diversity of likely obstructions that typical rooftops face, from surrounding vegetation to surrounding buildings.
Our reply: We appreciate the reviewer's comments. Here, we assume the sky view factor is one based on two reasons. First, our simulation aimed to provide a prospect for dual-mode application on a large scale. Accounting for the real effect of sky view factor requires case-by-case analysis of buildings and the surrounding landscape, which can be an excellent research project but also, in our opinion, Our reply: We appreciate the reviewer's comments. We think the relationship between our devices and solar cells is not a competitive relationship, but a compatible relationship. For example, a project we are working on is to combine heating material, cooling material, and solar cells to achieve the triplemode system (see Fig. R3 In this response, I appreciate the authors' major efforts to address most of my concerns. However, I still have two comments before my final recommendation.
1. In their response to my 6th comments, the authors added more detailed discussion about the heating material. In this section, they claimed that "As the size of clusters increases, both near-field coupling and the volume of light-matter interactions increase, which promotes broadband absorption in the solar spectrum." I agree that a strong absorption peak can be obtained by light coupling of nanoparticles. However, the increased cluster size only affects the wavelength of resonance. I do not see how the size of cluster promotes broadband absorption.
In addition, in the cited article at the end of this section (i.e. Advanced Materials 29, 1702156, (2017)), the explanation for broadband absorption was stated as "A broader absorption (i.e., lowered reflectance) extending into the infrared wavelengths due to resonance peaks is also seen with increasing d, (Figure 3b) although simulations indicate that h has a stronger effect." One can clearly see from its Fig. 3b that the difference size (d) of cluster does not significantly change the absorption bandwidth. Instead, the thickness (h) broadens the absorption. Therefore, I do not think the discussion is convincing in this revised section. Clarification at this point is required.
2. As though I do agree the novelty of this work using an electrostatically-controlled thermal contact for switchable thermal regulation, it is NOT the first demonstration of the dual functioning design for solar heating and radiative cooling, which is stated in the authors' response to reviewer 2. The paper used a mechanical system to switch between radiative cooling film and absorbing film to alternate between cooling and heating mode. Static voltage of 2kV is used to ensure a good thermal contact of the film to the substrate. In this revision, the authors addressed the concern about the safety of the device, the reliability with dirt, and elaborated more details of the field test device. The revision addressed all of our concerns. We would like to recommend the publication.

Reviewer #1
In this response, I appreciate the authors' major efforts to address most of my concerns.
However, I still have two comments before my final recommendation.
Our reply: We thank the reviewer very much for his/her positive report and constructive suggestions. In the following sections, we would address the questions point-by-point.
1. In their response to my 6th comments, the authors added more detailed discussion about the heating material. In this section, they claimed that "As the size of clusters increases, both nearfield coupling and the volume of light-matter interactions increase, which promotes broadband absorption in the solar spectrum." I agree that a strong absorption peak can be obtained by light coupling of nanoparticles. However, the increased cluster size only affects the wavelength of resonance. I do not see how the size of cluster promotes broadband absorption.
In addition, in the cited article at the end of this section (i.e. Advanced Materials 29, 1702156, (2017)), the explanation for broadband absorption was stated as "A broader absorption (i.e., lowered reflectance) extending into the infrared wavelengths due to resonance peaks is also seen with increasing d, (Figure 3b) although simulations indicate that h has a stronger effect." One can clearly see from its Fig. 3b that the difference size (d) of cluster does not significantly change the absorption bandwidth. Instead, the thickness (h) broadens the absorption. Therefore, I do not think the discussion is convincing in this revised section. Clarification at this point is required.
Our reply: Thank you very much for your professional suggestion. As shown in Figure R1, it can be seen that the particle cluster size has a wide distribution, which is the basis of broadband optical absorption. As the reaction time increased, the average size of the particle became larger, and the distribution remained highly diversified, which again resulted in broadband absorption rather than the red-shift behavior. The increase of absorption intensity is explained by the increased total volume of particles, which is corroborated by the elemental analysis showing that Cu content increased from 16wt% to 21wt as the reaction time increased. Therefore, as the reaction time increases, a broadband increase of absorption occurs. In other words, we agree with the reviewer's comment and prediction that larger cluster particle size would result in the resonance wavelength shift, but such phenomenon did not appear in our experiment due to the broad size distribution, which was not captured by optical simulation and theory in the cited article (Advanced Materials 29, 1702156, (2017)). To make these points clear, we have included the new SEM and EDX elemental mapping ( Figure  R1) in Figure S5 and corrected the description in revised supporting information, page 8. We also found and fixed a typo in the previous version regarding the dependence of mid-IR absorption on reaction time. We copy the section below for the reviewer's reference.
"For heating material, on the copper film, a layer of zinc film of 1 µm thick was electrodeposited (voltage: 2 V, anode: zinc metal, electrolyte: 0.25 M ZnSO4(aq)), followed by galvanic replacement reaction with 0.12 mM CuSO4(aq), and the heating material was obtained after deionized water washing and drying. As shown in Fig. S4, it can be found with the increase of reaction time with CuSO4(aq), the absorption of both 300 -2000 nm and 4 -18 µm increased. The observations can be attributed to the size of copper/copper oxide clusters is increased (as shown in Fig. S5). Specifically, the absorption of 300 -2000 nm stems from the localized surface plasmon resonances of the Cu nanoparticles. The wide size distribution of the Cu nanoparticle clusters results in broadband absorption, which is beneficial for solar heating. As the reaction time increases, both near-field coupling and the total volume of light-matter interactions increase, which promotes broadband absorption in the solar spectrum. For 4 -18 µm part, the nanoparticle layer behaves as a lossy effective medium because of the small cluster size compared to thermal radiation wavelength. Therefore, longer reaction time leads to a higher attenuation of light in both solar and mid-IR regimes 3 ." 2. As though I do agree the novelty of this work using an electrostatically-controlled thermal contact for switchable thermal regulation, it is NOT the first demonstration of the dual functioning design for solar heating and radiative cooling, which is stated in the authors' response to reviewer 2. , by simply stretching/compressing a porous PDMS film, the material can also switch between opaque and transparent state, hence obtaining mode switching when incorporated with a black back coating. Thereby, the authors need to better clarify the innovation of the work.
Our reply: We thank the reviewers for pointing this out. After carefully comparing the literature and our result, we believe a more appropriate statement is that our device is the first experimental demonstration of the solar heating with selective absorber and daytime radiative cooling active dual-function building envelope (see Table R1).