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All issues Volume 3 / No 6 (2024) Natl Sci Open, 3 6 (2024) 20240019 Full HTML
Issue
Natl Sci Open
Volume 3, Number 6, 2024
Special Topic: Key Materials for Carbon Neutrality
Article Number 20240019
Number of page(s) 14
Section Materials Science
DOI https://doi.org/10.1360/nso/20240019
Published online 19 August 2024

© The Author(s) 2024. Published by Science Press and EDP Sciences.

Licence Creative CommonsThis is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

INTRODUCTION

The balanced radiative heat transfer among the Earth, the Sun (heat source), and the space (heat sink) endows our planet with a human-friendly but still volatile thermal environment, featuring the frigid nighttime of winter seasons and burning daytime of summer seasons. Thus, various heating/cooling technologies, powered by traditional energy, have been developed to realize a desired and stable temperature out of a volatile thermal environment. It is estimated that heating and cooling in buildings alone account for around a quarter of global energy consumption and one-fifth of energy-related greenhouse gas emissions [1]. Therefore, an alternatively sustainable pathway to regulate temperature out of volatile real environments is worth exploring.

Taking advantage of the fast developments in both passive solar heating and radiative cooling, there have been great efforts and progress in utilizing these two sustainable universe heat/cold resources for temperature regulation [2–20]. Smart materials/devices, with dynamically tailorable optical properties, for example, are also being further developed to mitigate the risk of overheating/overcooling [21–33].

However, the temporal mismatch between the power demand and passive power supply fundamentally poses a huge challenge to realize pure passive temperature stabilization (as summarized in Figure S1). As typical examples of the temporal mismatch, in winter, the Sun provides high heating power in the daytime, while the demand for heating is mostly needed during the nighttime (top panel of Figure 1A and Figure S2). Similarly, in summer, the space generally offers a higher cooling power at night, while cooling is more desirable during the daytime (top panel of Figure 1B and Figure S3) [34–39].

thumbnail Figure 1

Schematic illustration of the design concept of the PTR. (A and B) Top panel: trends show the time-dependent power supply-demand relationships for the solar heat in winter and the cold energy from space (radiative cooling) in summer, respectively. The bottom panel shows the corresponding temperature trends by simply utilizing the heat/cold energy from the Sun/space (with a mismatched power supply-demand relationship), which largely deviates from the desired stable scenario. By timely transferring the solar heat or space cold energy (as directed by the dark lines), it is feasible to establish a balanced energy process both for winter and summer and realize the desired condition with a stable temperature. (C and D) Schematics show the working mechanism of the proposed PTR in winter and summer, respectively, to approach the highly desired stable temperature. In winter, the daytime sunlight is effectively captured by the solar heating layer (1) and is stored in the thermal storage layer (2). During the cold night, the heat within the thermal storage layer is released via the heat emitter (3). In summer, the device is flipped. The nighttime and daytime cold energy from the space with minimized thermal load from the sun is harvested by the cooling layer (3) and stored in the thermal storage layer (2). During the hot daytime, the stored cold energy is released via the cold emitter (1).

Even when considering only the daytime or nighttime, once the temperature is continuously above/below the trigger threshold, most conventional dynamic materials inevitably remain in a single cooling/heating mode and lose the function of dynamic regulation. Therefore, it is clear that enabling passive temperature regulation requires three essential components of passive heating, passive cooling and thermal storage. So far there has been no demonstration of a continuous and purely passive temperature regulation out of the real volatile environment (Figure S1).

In this work, we demonstrate a passive temperature regulator (PTR) with a triple-layer sandwich design, including heating, storage, and cooling layers, which passively stabilizes temperature both in frigid winters and sweltering summers. Specifically, during the cold winter season, the top solar heating layer effectively captures sunlight and transmits the thermalized sunlight to the underlying thermal storage layer (Figure 1C), rather than causing a further temperature rise in the daytime. In the nighttime, the stored extra heat is released via radiation from the bottom heat emitter and convection (see Note S1 and Figure S4 for the detailed analysis), serving as an additional heat source to stabilize the temperature. During the hot summer season, the device is flipped to perform the opposite function of radiative cooling (Figure 1D). In the nighttime, the cold energy from radiative cooling of the top cooling layer is harvested and stored in the thermal storage layer, which also avoids the problem of continuous temperature drop with respect to the ambient. In the daytime, the cold energy is released via radiation from the bottom cold emitter and convection, as a heat sink to enable a stable temperature. The radiative cooling material also possesses high reflectivity for sunlight, thus protecting the stored cold energy from loss. Therefore, the PTR, endowed with the integrated multi-functions of harvesting heat/cold energy, storing the surplus energy, and releasing it during the deficit period, timely addresses the temporal mismatch between power supply and demand (as indicated by the black lines in Figure 1A and 1B) and realizes continuous and passive temperature regulation not only in day-night but also seasonal cycles (the dashed red lines in Figure 1A and 1B).

RESULTS AND DISCUSSION

Design and characterizations of the PTR

The requirements for PTR to work both in hot and cold seasons pose great challenges when designing PTR. First, the solar absorber layer in winter is also the cold emitter in the summer. To enable effective sunlight capture in winter, the heating layer should possess high absorptivity at 0.28 to 2.5 μm. While, in summer, the effective radiative release of cold energy requires the integrated heating-storage layer also as a cold emitter to possess a high mid-infrared (MIR, 2.5 to 17 μm) emissivity. Second, the radiative cooler in summer is also the heat emitter in winter. To realize high-performing radiative cooling in summer, the cooling layer ought to be effectively reflective at 0.28 to 2.5 μm and simultaneously emissive across the MIR wavebands (especially the atmosphere transparency window at 8–13 μm). The effective radiative release of heat in winter requires the cooling layer also as a heat emitter to have a high MIR emissivity. Third, for the thermal storage layer, it should be designed to have not only a large thermal capacity and conductivity but also a tailorable working temperature to enable effective and fast energy storage-release cycles in multi-scenarios (see Note S2 and Figure S5 for more details).

As a demonstration of the PTR, we developed a proof-of-concept device with hierarchical copper (Cu) foam coupled with paraffin as the integrated heating-storage layer, and a cellulose acetate butyrate-hexagonal boron nitride (CAB-hBN) hybrid film as the cooling layer (Figure 2A). The hierarchical Cu foam and CAB-hBN hybrid film are thermally integrated via thermally conductive silicone grease and Cu film. The entire PTR is flexible, allowing for easy customization to meet specific shape requirements in practical applications. More fabrication details of this PTR are shown in the Methods section. It is worth noting that the fabrication process, which mainly involves solution chemical treatment and electrospinning, can be easily scaled up for practical applications.

thumbnail Figure 2

Characterizations of the PTR. (A) The photograph (left) and cross-sectional schematic (right) of a PTR, which is mainly composed of the hierarchical Cu foam with paraffin and CAB-hBN hybrid film. The thermally conductive silicone grease and Cu film are utilized for thermal integration. (B) Schematics of the microcosmic designs of the hierarchical Cu foam. (C) Phase change point and enthalpy of the paraffin with different numbers of carbon atoms. Large enthalpy of over 220 kJ kg−1 with customized phase change temperature ensures efficient and broad ranges of heat/cold energy storage. The inset is an example that shows the molecular structure of hexadecane with sixteen carbon atoms. (D) Schematic illustration of the CAB-hBN hybrid film. (E) Optical spectra of the hierarchical Cu foam and CAB-hBN hybrid film. The solar spectrum, atmosphere transmission spectrum, and black-body radiation at 288 K to 308 K are presented for reference. (F) Temperature curves show the opposite temperature responses of the hierarchical Cu foam and CAB-hBN hybrid film of the PTR. (G) Temperature profiles depict that adjustable heating temperatures from 19 to 31°C are realized via changing the chain length of paraffin.

The hierarchical Cu foam coupled with paraffin is carefully designed to simultaneously meet the optical and thermal requirements as an integrated heating-storage layer. Micrometer-scale porous Cu foam with high porosity (~95%, Note S3) is chosen as the building block of the integrated heating-storage layer, as it serves as a good thermal conductive skeleton when coupled with phase change media for faster thermal storage. To effectively trap light for realizing ultra-high solar absorbance, bi-level Cu-based nanowires are introduced onto the surface of Cu foam via two successive chemical etching processes (Figure 2B and Figures S6–8, see Methods for more details). Organic chains of 2, 3, 6, 7, 10, 11-hexahydroxytriphenylene are also grafted on the second-level nanowires, triggering molecular bond vibrations for high emission in MIR waveband [40,41]. As a result, the hierarchical Cu foam as the heating layer side exhibits a high solar absorptivity of 0.99 and simultaneous high MIR emissivity of 0.89.

Besides, the bi-level nanostructures of the Cu foam also enrich the thermal network of the integrated heating-storage layer, which is beneficial for thermal storage performance. Paraffin is used as the phase change media for thermal storage due to its tunable working temperature and high phase change enthalpy of greater than 220 kJ kg−1 (Figure 2C and Figure S9). As paraffin is coupled with the hierarchical Cu skeleton, the thermal conductivity can be exponentially enhanced, reaching 3.35 W m−1 K−1, 13-fold higher than that of the intrinsic value of paraffin (Figure S10). Our calculations in Note S4 show that such a thermal conductivity is adequate for heat/cold energy storage (which is also experimentally demonstrated later). It is expected that the thermal storage layer can be further tailored for faster response and higher capacity with more elaborate designs of the phase change molecules and thermally conductive networks based on fundamental physics and chemistry in the future [42–44].

The CAB-hBN hybrid film is designed as the cooling layer of the PTR to have a strong emission at MIR wavelengths and a high reflection at the sunlight waveband (Figure 2D). The CAB is chosen as the matrix material because it can be derived from the renewable polymer of plant cellulose and has excellent optical weather resistance [45]. From an optical perspective, it naturally possesses partially high MIR absorption induced by molecular vibration at 5.7, 8.1, 9.6, and 16.6 μm (Figure S11). To further enhance the MIR emission at the atmospheric transparency window (8–13 μm), the hBN nanoparticles with more emission modes at 7.2 and 12.4 μm are designed to be embedded in the nanowires. In the sunlight waveband, we design the CAB-hBN hybrid film to have a large number of pores with a broad diameter distribution on the scale of hundreds of nanometers to enable effective reflection of incident light. We developed the CAB-hBN co-spinning method to prepare the CAB-hBN hybrid film as the cooling layer (Figures S12 and S13, see Methods for more details). It is found that the as-prepared CAB-hBN hybrid film exhibits a stable reflectivity of 0.97 at the sunlight wavelength range of 0.28–2.5 μm, effective emissivity of 0.92 within the atmosphere transparency window, and 0.89 across the whole MIR waveband (Figure 2E and Figures S14 and S15). Therefore, a high radiative cooling power of 95 W m−2 and a cooling temperature of 10°C were observed (Figures S16 and S17).

We next experimentally examine the temperature responses of the heating and cooling layers, as well as the tunability of the thermal storage layer. It is found that the temperature of the heating layer side (hierarchical Cu foam) reaches up to 66°C at 12:00, which is 39°C higher than that of the ambient (27°C, Figure 2F). In contrast, the temperature of the cooling side (CAB-hBN hybrid film) is merely 19°C, which is 8°C below the ambient temperature. The temperature responses of heating and cooling layers suggest that strong heating and cooling effects can be provided by passive solar heating and radiative cooling in one device. The working temperature of the thermal storage layer can also be tuned from 19 to 31°C by changing the energy storage media from hexadecane to nonadecane, which is verified via outdoor solar heating tests and shown as the platforms of heat release curves in Figure 2G.

Performance in cold winter

We evaluate the outdoor performance of the PTR in winter for stabilizing the temperature. Control experiments with the setups schematically shown in Figure 3A (see Figure S18 for the corresponding photograph) were performed. A PTR (with hexadecane as an example of phase change media), and a conventional solar absorber (with an average solar absorptivity of 0.98; see Figure S19 for the spectrum) are set as controls. The two samples were mounted on the roof of the campus (in Nanjing, China; 32° N, 118° E) for realizing passive solar heating. A scroll MIR reflective film (like an electric roller shutter, as schematically shown in Figure S20) was adopted to reduce the upward radiative heat loss during the night of the heating test.

thumbnail Figure 3

Performance of the PTR in the winter of 2021. (A) A schematic of the experimental setup. A PTR with the hierarchical Cu foam side facing the sky and a conventional solar absorber are set as controls. The foam, polypropylene (PP) box, and polyethylene (PE) film are used to minimize heat loss to the surroundings. (B) A whole day temperature profile of the samples. Shaded area: 18 ± 3°C. Our PTR realizes a near-flat temperature curve, highly desirable in practical applications. In contrast, the temperature of the conventional solar absorber severely fluctuates. (C) Nighttime heating power enabled by the PTR. (D) Cycling performance of the PTR. The temperature profiles from 0:00 to 8:00 every day are presented. The temperatures of the conventional solar absorber and ambient are also shown for comparison.

In Figure 3B, we record the temperature profiles of the two samples. It is found that the temperature of the PTR stays steadily at 18 ± 3°C for 96% of the test period (as shown by the shaded area) with average sunlight intensity of 385 W m−2 (daytime) and an ambient temperature varying from 7 to 14°C (daylong). This suggests that the PTR is capable of effectively capturing sunlight and storing the extra thermal energy during the day, and releasing the stored thermal energy at night, therefore, enabling a constant temperature even without any artificial energy input. In sharp contrast, the temperature of the conventional solar absorber violently fluctuates with the sunlight: in the daytime, its temperature increases quickly with the intensity of sunlight, reaching even around 50°C; while at the nighttime without energy input, the temperature drops along with the ambient temperature, even to about 7°C. Thus, for the conventional solar absorber, only 7% of the test time maintains the targeted temperature (18 ± 3°C). Monitoring the heating power of the PTR during nighttime is also pivotal to evaluate its potential for energy savings. As shown in Figure 3C, under an average ambient temperature of 6°C, the heating power of the PTR is measured to be 70 W m−2 (for maintaining 18°C), which is already comparable with the heating load index recommended for residences [46].

The cycling stability of the PTR is also important for practical applications. A 15-day continuous measurement (Figure 3D) reveals that our PTR possesses good cycling stability. It is found that its temperature remains around 18°C at night with an average ambient temperature of 8°C (from 0:00 to 8:00), a highly desirable condition with a stable temperature. In contrast, the temperature of the conventional solar absorber fluctuates greatly (between 4 and 14°C) with the ambient temperature at night, still requiring extra heat energy supply to maintain a stable temperature.

Performance in hot summer

We also evaluate the outdoor performance of the PTR in the summer period. Firstly, we theoretically study its thermal process on summer nights, when the cold energy from radiative cooling is stored in the thermal storage layer. In this thermal model, we mount a 10 cm (length) × 10 cm (width) × 3 cm (height) PTR with the CAB-hBN hybrid film facing upwards on a thermal insulation foam. The governing heat transfer equation, incorporating radiative cooling and thermal conduction processes, is applied to the PTR coupled with the foam to determine the temperature distribution and evolution (see Note S5 and Figures S21 and S22 for more details of the thermal model). As shown in Figure 4A, the average temperature profile of the PTR rapidly stabilizes at the phase change point of hexadecane and gradually decreases to a temperature lower than the phase change point in 7.5 hours (e.g., from 9:00 PM to 4:30 AM the next day). Therefore, the PTR stabilizes the temperature at night in summer. The temperature field at the eighth hour visually revalidates the complete phase change of the PTR by radiative cooling (inset of Figure 4A), suggesting its great potential as a heat sink in the daytime of summer.

thumbnail Figure 4

Performances of the PTR in the summer of 2022. (A) Simulated temperature profiles over time of the PTR at night. The device fast reaches a steady temperature and completes the phase change in 7.5 hours. Inset: the temperature field of the PTR at the eighth hour. The stored cold energy from radiative cooling can serve as an efficient heat sink during the daytime. (B) A schematic shows the test setups. The PTR is flipped with the CAB-hBN hybrid film facing the sky. A conventional radiative cooler is adopted for comparison. (C) Temperature comparisons between the PTR and a conventional radiative cooler. (D) Comparisons in cooling temperature (the difference between ambient temperature and temperatures of samples). Compared with a conventional radiative cooler, the PTR realizes a much larger cooling temperature, suggesting a better capacity in controlling the daytime temperature within the desired range. (E) Environmental conditions of the tests in (C and D). (F and G) Comparisons in cooling power and the corresponding environmental conditions, respectively. The temperature of the heating source is set as 22°C for both samples to compare the cooling power. (H) For areas with a large temperature difference between day and night, the PTR realizes much better (stabler) temperature regulation than the conventional radiative cooler. Color box: target temperature range (18 ± 3°C). The insert photograph shows the cloudy weather during the daytime tests. The PTR also weakens the dependence on the weather, a bottleneck of conventional radiative cooling that requires a clear day for excellent cooling performance.

Subsequently, we experimentally test the performance of the PTR in stabilizing the daytime temperature during hot summer, when a huge amount of energy is consumed for cooling. Based on the above theoretical analysis, we placed the PTR in a refrigerator with a set point of 16°C to simulate the night thermal process. In the daytime, the PTR, which is flipped with the CAB-hBN hybrid film facing the sky, and a conventional radiative cooler (consisting of the CAB-hBN hybrid film) were together mounted on the roof of the campus for comparison (Figure 4B).

In Figure 4C, we plot the temperatures of the PTR and the conventional radiative cooler under exactly the same conditions (the sky has few clouds during the tests as shown in Figure S23, not very ideal for radiative cooling). It is clear that the radiative cooler realizes effective cooling with a sub-ambient temperature. In comparison, the PTR even reaches a much lower temperature due to the heat sink from the night. In Figure 4D, the cooling temperature (difference between ambient temperature and that of the sample) is presented to further quantitatively evaluate the different cooling performances. It is found that the mean cooling temperature of the PTR reaches up to 15°C over the hottest period during the day (with an ambient temperature of 36°C (Figure 4D), sunlight power of 600 W m−2, and relative humidity of 15% (Figure 4E)). Such a cooling effect surpasses that of most previous radiative coolers tested under ideal weather conditions. In comparison, the radiative cooler with a sample temperature of 30°C only achieves a cooling temperature of mere 5°C.

Cooling power, as another important figure of merit to evaluate the performance of the PTR during hot daytime of summer, is also carefully evaluated. We compare the cooling power of the two samples at a constant 22°C. As the temperature of the radiative cooler is 10°C higher than the set point of 22°C (Figure S24), no cooling power is observed (Figure 4F). In contrast, the mean cooling power of the PTR reaches 200 W m−2, under 650 W m−2 of sunlight radiation and relative humidity of 15% (Figure 4G). The above cooling temperature and cooling power tests together verify that the PTR shows much better potential than conventional radiative cooling materials in controlling the temperature during daytime of summer within an ideal range.

The PTR also exhibits great advantages in temperature management for areas with a large temperature difference between day and night, where conventional radiative cooling may lead to a negative effect on energy saving due to overcooling [17]. As shown in Figure 4H, the conventional radiative cooler realizes a 5°C lower temperature than that of the ambient at night, when the ambient temperature is 10–20°C, which is already so cold that it sometimes requires additional heating. In contrast, the PTR is capable of stabilizing the temperature around the target temperature (phase change temperature, 18°C in this case), because cold energy from radiative cooling and ambient is stored via the phase change process during nighttime. This part of cold energy offers an excellent heat sink to support cooling for hot daytime. In the hot daytime, even on a cloudy day as shown by the inset photograph in Figure 4H, we observe 12°C sub-ambient temperature on average from the PTR under a mean ambient temperature of 33°C. It is very difficult for the conventional radiative cooler to realize an effective sub-ambient temperature on such a cloudy day. The result suggests that, as a heat sink during hot daytime, the PTR not only has better cooling capacity in stabilizing the temperature but also has weaker dependence on weather than that of the conventional radiative cooling, which has a strong requirement for a clear sky for high-performing cooling.

Taking advantage of the temperature regulation, it is found that the temperature of the PTR stays around the targeted temperature range (18 ± 3°C) for most of the time (around 71% of the test period, as outlined by the red color box in Figure 4H) under a real and volatile ambient temperature that fluctuates from 10°C to 40°C. In the future, by further tailoring the energy storage media within the thermal storage layer to better match with local weather/climate, it is expected that a more ideal condition with a very stable temperature will be achievable. However, for the conventional radiative cooler with fundamental restrictions, its temperature seriously fluctuates across a broad range of 30°C, with only 15% of the test period within the target temperature range (18 ± 3°C). As such, a large amount of additional heat/cold energy is required to offset such a volatile temperature change.

Modeled temperature stabilization and energy-saving performance

The temperature variations of the PTR, a radiative cooler, and a solar absorber in winter (October) and summer (May) of Nanjing over three continuous days are modeled using EnergyPlus. A simplified cubic model with dimensions of 2 m (length) × 2 m (width) × 2 m (height) is designed (see Note S6 and Figure S25 for details). The top surface is covered by the above three control samples, while the other five surfaces are wrapped in low thermal conductivity foam. Besides the passive heating/cooling provided by the three control samples, an ideal-loads-air-system connected to district cooling and heating sources is used to maintain a stable temperature and calculate heating and cooling energy consumption. As shown in Figure 5A and 5B, the temperature of the PTR stays very stable, which is in stark contrast to that of the solar absorber and radiative cooler. It is calculated that the temperature variation of the PTR in the winter/summer is mere 1.2/0.6°C, much smaller compared to 10.4/11.2, 27.5/43.9, and 12.9/13.9°C for a radiative cooler, a solar absorber, and the ambient, respectively (Figure S26).

thumbnail Figure 5

Modeled temperature stabilization and evaluation of energy savings. (A and B) Modeled temperature profiles of the PTR, a radiative cooler, and a solar absorber in winter and summer in Nanjing, respectively. The ambient temperature is also presented. Compared with the radiative cooler and solar absorber, the PTR shows better performance in temperature stabilization. (C and D) Monthly and annual energy consumption for stabilizing the temperature (21/24°C in winter/summer) in Nanjing, respectively. (E) The PTR is calculated to realize relative energy savings of 39% and 57%, in comparison with a radiative cooler and a solar absorber, respectively. (F) Relative energy savings enabled by the PTR for the cities located in different climate zones.

We also compare the monthly and annual energy consumptions of the PTR and controls (solar absorber and radiative cooler) in realizing a desired stable temperature (which is set as 21/24°C in the winter/summer). From the monthly energy consumption curves in Figure 5C, it is found that the PTR always consumes the least energy to reach the temperature setpoint. The PTR demonstrates better energy-saving performance in winter (January to April and October to December) than that in summer (May to September). This is mainly ascribed to the following two reasons: the heat power of sunlight is several times greater than that of the cold energy from radiative cooling; besides, the ambient temperature during the summer in Nanjing (generally 30–40°C in the daytime) is too high compared with the phase change point of the PTR of 18°C. We also estimate the annual cumulative energy consumptions in Figure 5D, which are 3.08, 5.09, and 7.15 GJ for the PTR, radiative cooler, and solar absorber, respectively. Therefore, compared with the radiative cooler and solar absorber, the PTR reduces relative energy consumption by 39% and 57% respectively, for achieving temperature stabilization (Figure 5E).

To demonstrate the global energy saving potential of the PTR, we also calculate its annual energy savings compared to the radiative cooler and solar absorber in 31 cities located in different climate zones, from the equatorial to the arctic types (Figure 5F). The PTR is calculated to have a relative energy saving of 10%–47% compared with the radiative cooler, and 13%–95% compared with the solar absorber. As most modeled cities are located in the sub-tropical and temperate climate zones, which are also the densely populated regions of the world, the average energy consumption reduction of 30% (compared with the radiative cooler) and 56% (compared with the solar absorber) generally reflects global energy-saving potential of the PTR.

Notably, the above primary estimations of the PTR in temperature stabilization and energy saving are based on the optical and thermal properties that are experimentally demonstrated in this work. Future development, for example, designing PTR in both the material and system levels for matching local climate conditions, would enable even better energy-saving performance. Materials design encompasses the energy storage media together with the thermal transfer network of the thermal storage layer and intelligently adaptive optical properties of the heating/cooling layer. System-level designs include heat exchangers and integration with other energy-efficient heating/cooling devices. It is expected that the PTR can be further tailored to suit a wide range of climate/weather conditions and customized requirements for autonomous temperature stabilization.

Beyond Earth, the PTR system offers significant potential for temperature stabilization and energy conservation, making it highly attractive for future outer space explorations. Among the array of life-support systems, a space station with a hospitable thermal environment undoubtedly stands out as a vital component for sustaining long-term extraterrestrial survival. However, the limited energy supply of modern fuels presents a formidable challenge. Therefore, establishing a hospitable thermal environment within the space station with minimized energy consumption is a crucial necessity. Heat energy from stars and cold energy from space are naturally ubiquitous and inexhaustible. Hence, the PTR is also expected to be a promising solution, serving as a building block for space station development and offering passive protection against outer temperature fluctuations.

In conclusion, we propose and experimentally demonstrate a passive thermal regulator which enables continuous and purely passive temperature regulation out of a real volatile environment through day and night, by harvesting, storing, and releasing sustainable solar heat and space cold energy. The field tests demonstrate that the PTR is capable of steadily maintaining at the designed temperature of ~18°C for 71%–96% of the testing time for both summer and winter periods, in contrast to 7%–15% for conventional passive thermal technology of the solar absorber and radiative cooler. Due to such a unique capability of passive temperature regulation, it is calculated that the PTR saves energy by 30%–56%, in comparison to radiative coolers and solar absorbers. It is expected that further advancement in this type of PTR can make impacts in a wide range of fields from infrastructure, transportation, food production to outer space explorations.

Data availability

The original data are available from corresponding authors upon reasonable request.

Acknowledgments

We acknowledge the microfabrication center of the National Laboratory of Solid State Microstructures (NLSSM) for technique support. J.Z. acknowledges the support from the XPLORER PRIZE.

Funding

This work was jointly supported by the National Key Research and Development Program of China (2022YFB3804902 and 2022YFA1404704), the National Natural Science Foundation of China (52322211, 51925204, 52102262, 52003116, 92262305, 52372197 and 52381260325) and the Natural Science Foundation of Jiangsu Province (BK20220035 and BK20200340).

Author contributions

J.L., N.X. and J.Z. conceived this research. J.L., Y.S., Y.J., P.S., L.Z. and B.Z. fabricated the samples and performed the measurements. T.J. and G.T. developed the energy saving model. Z.L. developed the thermal model. J.L., T.J., N.X., G.T. and J.Z. wrote the manuscript. N.X., G.T. and J.Z. supervised this research. All authors read and commented on the manuscript.

Conflict of interest

The authors declare that they have no conflict of interest.

Supplementary information

Supplementary file provided by the authors. Access here

The supporting information is available online at https://doi.org/10.1360/nso/20240019. The supporting materials are published as submitted, without typesetting or editing. The responsibility for scientific accuracy and content remains entirely with the authors.

References

All Figures

thumbnail Figure 1

Schematic illustration of the design concept of the PTR. (A and B) Top panel: trends show the time-dependent power supply-demand relationships for the solar heat in winter and the cold energy from space (radiative cooling) in summer, respectively. The bottom panel shows the corresponding temperature trends by simply utilizing the heat/cold energy from the Sun/space (with a mismatched power supply-demand relationship), which largely deviates from the desired stable scenario. By timely transferring the solar heat or space cold energy (as directed by the dark lines), it is feasible to establish a balanced energy process both for winter and summer and realize the desired condition with a stable temperature. (C and D) Schematics show the working mechanism of the proposed PTR in winter and summer, respectively, to approach the highly desired stable temperature. In winter, the daytime sunlight is effectively captured by the solar heating layer (1) and is stored in the thermal storage layer (2). During the cold night, the heat within the thermal storage layer is released via the heat emitter (3). In summer, the device is flipped. The nighttime and daytime cold energy from the space with minimized thermal load from the sun is harvested by the cooling layer (3) and stored in the thermal storage layer (2). During the hot daytime, the stored cold energy is released via the cold emitter (1).
In the text
thumbnail Figure 2

Characterizations of the PTR. (A) The photograph (left) and cross-sectional schematic (right) of a PTR, which is mainly composed of the hierarchical Cu foam with paraffin and CAB-hBN hybrid film. The thermally conductive silicone grease and Cu film are utilized for thermal integration. (B) Schematics of the microcosmic designs of the hierarchical Cu foam. (C) Phase change point and enthalpy of the paraffin with different numbers of carbon atoms. Large enthalpy of over 220 kJ kg−1 with customized phase change temperature ensures efficient and broad ranges of heat/cold energy storage. The inset is an example that shows the molecular structure of hexadecane with sixteen carbon atoms. (D) Schematic illustration of the CAB-hBN hybrid film. (E) Optical spectra of the hierarchical Cu foam and CAB-hBN hybrid film. The solar spectrum, atmosphere transmission spectrum, and black-body radiation at 288 K to 308 K are presented for reference. (F) Temperature curves show the opposite temperature responses of the hierarchical Cu foam and CAB-hBN hybrid film of the PTR. (G) Temperature profiles depict that adjustable heating temperatures from 19 to 31°C are realized via changing the chain length of paraffin.

In the text
thumbnail Figure 3

Performance of the PTR in the winter of 2021. (A) A schematic of the experimental setup. A PTR with the hierarchical Cu foam side facing the sky and a conventional solar absorber are set as controls. The foam, polypropylene (PP) box, and polyethylene (PE) film are used to minimize heat loss to the surroundings. (B) A whole day temperature profile of the samples. Shaded area: 18 ± 3°C. Our PTR realizes a near-flat temperature curve, highly desirable in practical applications. In contrast, the temperature of the conventional solar absorber severely fluctuates. (C) Nighttime heating power enabled by the PTR. (D) Cycling performance of the PTR. The temperature profiles from 0:00 to 8:00 every day are presented. The temperatures of the conventional solar absorber and ambient are also shown for comparison.
In the text
thumbnail Figure 4

Performances of the PTR in the summer of 2022. (A) Simulated temperature profiles over time of the PTR at night. The device fast reaches a steady temperature and completes the phase change in 7.5 hours. Inset: the temperature field of the PTR at the eighth hour. The stored cold energy from radiative cooling can serve as an efficient heat sink during the daytime. (B) A schematic shows the test setups. The PTR is flipped with the CAB-hBN hybrid film facing the sky. A conventional radiative cooler is adopted for comparison. (C) Temperature comparisons between the PTR and a conventional radiative cooler. (D) Comparisons in cooling temperature (the difference between ambient temperature and temperatures of samples). Compared with a conventional radiative cooler, the PTR realizes a much larger cooling temperature, suggesting a better capacity in controlling the daytime temperature within the desired range. (E) Environmental conditions of the tests in (C and D). (F and G) Comparisons in cooling power and the corresponding environmental conditions, respectively. The temperature of the heating source is set as 22°C for both samples to compare the cooling power. (H) For areas with a large temperature difference between day and night, the PTR realizes much better (stabler) temperature regulation than the conventional radiative cooler. Color box: target temperature range (18 ± 3°C). The insert photograph shows the cloudy weather during the daytime tests. The PTR also weakens the dependence on the weather, a bottleneck of conventional radiative cooling that requires a clear day for excellent cooling performance.
In the text
thumbnail Figure 5

Modeled temperature stabilization and evaluation of energy savings. (A and B) Modeled temperature profiles of the PTR, a radiative cooler, and a solar absorber in winter and summer in Nanjing, respectively. The ambient temperature is also presented. Compared with the radiative cooler and solar absorber, the PTR shows better performance in temperature stabilization. (C and D) Monthly and annual energy consumption for stabilizing the temperature (21/24°C in winter/summer) in Nanjing, respectively. (E) The PTR is calculated to realize relative energy savings of 39% and 57%, in comparison with a radiative cooler and a solar absorber, respectively. (F) Relative energy savings enabled by the PTR for the cities located in different climate zones.

In the text

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