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Temperature-dependent dual-mode thermal management device with net zero energy for year-round energy saving - Nature Communications
Results .
Concept of zero-energy intelligent dual-mode device .
As shown in Fig.? 1a , dual-mode thermal management device consists of three functional layers, which are in order as follows: radiative cooling layer, temperature-sensitive actuating layer and solar heating layer. The essence of zero-energy dual-mode radiative thermal management strategy is based on the transform of required different high-selectivity spectral characteristics in the temperature control system (Fig.? 1b ). When heating mode is required, the radiative cooling layer is coiled automatically to maximum the uncovered solar heating layer. Due to the high solar absorptivity and the low infrared emissivity of solar heating layer, most of the solar radiation is absorbed and converted into heat, and the heat loss from infrared radiation is suppressed to a minimum. For cooling mode, the automatic unfolded radiative cooling layer completely covers the solar heating layer, where high solar reflection of radiative cooling layer on the sunlight reduces solar absorption as much as possible, thereby avoiding increase of internal energy from solar radiation. Meanwhile, the high mid-infrared emission in the specific wavelength range (8–13?μm) directly transfers heat through the transparent atmospheric window into outer space by full power thermal radiation, reducing undesired input infrared radiation from air and surrounding environment. The steady-state temperature of dual-mode device is determined by the thermal balance relationship among four key components: the absorbed solar radiation from the sun ( P sun ), the emitted heat by the device ( P device ), the absorbed heat radiation from the atmosphere ( P atm ), and the parasitic heat ( P parasitic ) characterized by a heat-transfer coefficient ( h c ) (Eq. ( 1 ) and Supplementary Note? 1 ) 14 . The net heat flux ( P net ) is a function of the temperature of the device ( T device ).
$${P}_{net}={P}_{sun}+{P}_{atm}-{P}_{device}-{P}_{parasitic}$$
(1)
$${P}_{parasitic}={A}_{device}{h}_{c}({T}_{device}-{T}_{amb})$$
(2)
Here, we fixed the ambient temperature ( T amb ) to be 25?°C, and used the universal global solar spectrum (ASTM G173) and the typical atmospheric window (US standard 1976). When the net heat flux is zero, the steady-state temperature of the device is reached, and the thermal management power (negative represents cooling, positive represents heating) is the intersection corresponding to the temperature of the device equal to that of the ambient (Fig.? 1c ). The former is sensitive to parasitic heat. Taking cooling mode as an example, the steady-state temperature of the device is gradually close to the ambient temperature (from I to II) with increase of heat-transfer coefficient (from 0 to 10?W?m ?2 K ?1 ). Different from steady-state temperature, radiative cooling power is independent from parasitic heat (III). This analysis is also suitable for heating mode.
The auto-switching mechanism is based on the spontaneous morphological adjustment of the dual-mode device responding to the ambient temperature change (Fig.? 1a ). The length of the actuating layer is sensitive to temperature, but the length of the radiative cooling layer is almost unchanged under the same conditions. When it is hot, the actuating layer shrinks. To eliminate the internal stress at the interface between the radiative cooling layer and actuating layer, the radiative cooling layer gradually unfolds until completely covering the solar heating layer for cooling. When it is cold, the actuating layer responds in the opposite way to expose the solar heating layer as much as possible. More importantly, the stimulus triggering the switch of thermal management modes is temperature, which is the physical quantity that determines the requirements of thermal management. This means that the dual-mode device is intelligent and can select an appropriate mode according to the ambient temperature, without any external energy consumption during the whole switching process.
We summarize that the successful realization of an intelligent and zero-energy dual-mode thermal management device requires three typical characteristics (Fig.? 2 ): (a) The device should have high-selectivity electromagnetic spectrum in both heating and cooling modes to obtain dual-mode high thermal management performance. (b) The device has the ability to switch between heating and cooling modes by using the change in its own physical-chemical properties. This is a key factor to realize zero-energy thermal management. (c) The reversible auto-switch of thermal management mode should be triggered by temperature. Combining these three characteristics together would not only give the dual-mode device “intelligence” to choose an appropriate mode by perceiving the ambient automatically with zero-energy input, but also lead?to high efficiency in both heating and cooling modes for our dual-mode thermal management device.
Fig. 2: Feature comparison of the typical dual-mode thermal management devices. Three criteria of the dual-mode device: dual-mode thermal management (solar heating/radiative cooling), reversible auto-switch, energy consumption for switch.
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Design/selection of the materials/layers for heating/cooling mode .
For dual-mode thermal management, an important aspect of the functional parts in the device is to achieve different electromagnetic spectrums with high selectivity required for solar heating and radiative cooling. Figure? 3a illustrates the structure of a dual-mode thermal management device. Here, we introduced an aluminum plate coated with nano-chromium oxide powders (nano-Cr black Al plate) in the design of the dual-mode thermal management device for solar heating layer. The uniformly distributed nano-chromium oxide powders act as an absorbent and mirror agent to ensure high solar absorption and low infrared emission (inset in Fig.? 3a ). Due to plasmon resonances, the sunlight undergoes non-radiative damping in chromium oxide powders, and is further high-efficiency transformed into heat 12 .
Fig. 3: Structure and spectral characteristics of high performance dual-mode thermal management device. a Structural illustration of dual-mode thermal management device. Nano-Cr black Al plate is the solar collector with an electromagnetic spectrum close to ideal for solar heating. The functional layer for radiative cooling in RC tape is composed of DOP-modified PMP matrix and TiO 2 NPs fillers. The adhesive layer ensures the integration of the interface between the RC tape and temperature-sensitive actuator during complex and repeated deformation process. A piece of narrow VHB tape, used as the only joint part between solar heating and radiative cooling layers, reserves the maximum effective area for dual-mode thermal management. The inset of SEM image shows that nano-chromium oxide powders are uniformly distributed on the aluminum plate. b Optical images of dual-mode device in heating and cooling modes. c Cross-sectional view of a light field (magnitude of normalized electric field component of light) around a rutile TiO 2 sphere with different diameters ( d ). The wavelength of incident light is 475?nm, corresponding to the maximum energy density of solar radiation (ASTM G173). Electric field of the incident light and wave vector of the incident light are represented symbolically by E and k, respectively. d Simulated scattering cross-section spectra of TiO 2 spheres with different diameters in PMP matrix. e Absorptivity/Emissivity ( α / ε ) of dual-mode thermal management device in heating and cooling modes, respectively.
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The radiative cooling in the dual-mode thermal management device is mainly achieved by home-made stretchable radiative cooling tape (RC tape) with excellent performance. The functional layer for radiative cooling in RC tape is made of dioctyl phthalate (DOP)-modified poly(4-methyl-1-pentene) (PMP) encapsulating rutile titanium dioxide nanoparticles (TiO 2 NPs) (Supplementary Fig.? 1 ). PMP is an excellent solar transparent polymer with a wavelength-independent refractive index of 1.46 from visible to near-infrared range (Supplementary Fig.? 2 ), while the refractive index of rutile TiO 2 NPs is much higher (>2.39) than that of PMP 32 . The large difference of refractive index is a condition required for multiple scattering and internal reflection in the composite matrix. As corroborated by finite difference time domain (FDTD) simulation, the smaller TiO 2 NPs are more capable of redirecting incident light (Fig.? 3c ). On the other hand, the scattering center wavelength shows a red-shift trend with the increase in diameter of TiO 2 NPs (Fig.? 3d ). As scattering center with high refractive index, TiO 2 NPs with broad size distribution are able to produce the required scattering wavelength range covering the entire solar radiation, because of the collective effect of multiple Mie resonances (Fig.? 3d and Supplementary Fig.? 3 ). In addition, large amounts of infrared absorption peaks from various characteristics bonds in DOP-modified PMP, TiO 2 NPs, adhesive, and even shape memory polymer (materials for temperature-sensitive actuating layer), provide enough infrared radiation for transferring heat into outer space (Supplementary Fig.? 4 ). The optimized RC tape can reflect >90% of solar radiation and have high absorptivity/emissivity of ~96% in the mid-infrared atmospheric window (8–13?μm) (Supplementary Fig.? 4 ).
Nano-Cr black coated Al plate is black to absorb sunlight, and RC tape is glossy-white to reflect sunlight. Benefited from this, the device shows a drastic difference in visual appearance between heating and cooling modes (Fig.? 3b ). As shown in Fig.? 3e , the device in heating mode can absorb ~91% of solar radiation and there is almost no mid-infrared emission (~8%) in the wavelength range of 8–13?μm. Such a huge difference in the spectral characteristics of the device in the two modes lays the foundation for the zero-energy intelligent dual-mode thermal management device (Supplementary Fig.? 5 ).
Design of automatic actuation material/layer .
To fully realize such an intelligent and automatic dual-mode thermal management device, there must be an auto-switching mechanism applied to the device. This is achieved with a temperature-triggered intelligent auto-switch using a temperature-sensitive layer with reversible shape memory sandwiched between the heating and cooling layers. The core mechanism of this actuation is to minimize the internal stress at the interface between the radiative cooling layer and the actuating layer, during reversible shape evolution of the actuating layer with temperature. Herein, two-way shape memory polymer (2?W SMP) is the key material for realization of temperature-triggered intelligent switch, which can be synthesized easily by a one-step esterification reaction of three monomers (polytetrahydrofuran (PTHF), polycaprolactone (PCL), and hexamethylene diisocyanate (HDI)) on a catalyst (dibutyltin dilaurate (DBTDL)) with almost 100% yield (Supplementary Fig.? 6 ). The appearance of typical urethane group in the reaction product confirms successful synthesis of polyurethane prepolymer (Supplementary Fig.? 7 ). The reaction product is then transferred directly to a stainless-steel petri dish to fully evaporate solvent at room temperature to get the required as-prepared 2?W SMP film for further preparation of the actuating layer later.
The temperature-triggered reversible shape memory performance is achieved after a programming process (Supplementary Fig.? 8 ). During the heating-cooling cycles, there is a spontaneous and reversible length-shifting between shrinkage and elongation as expected, which is caused by the reversible melting-crystallization process of partial segments in the polymer (Fig.? 4a ). Remarkably, programmed 2?W SMP in stretching direction shrinks when heated and expands when cooled. A tight laminate could be formed by attaching a piece of same-sized RC tape to the programmed 2?W SMP at the shrinking state. Thanks to the huge difference of length along programming direction between RC tape and programmed 2?W SMP caused by the abnormal shrinkage behavior of the programmed 2?W SMP, the laminate could bend to RC tape side when cooled. As shown in Fig.? 4b , the coiled laminate gradually unfolds until it is completely flat as the temperature increases. Notably, the bending angle starts to reduce slowly in the heating process. Once the temperature is higher than triggering temperature, the bending angle decreases sharply. This sharp angle change is determined by the melting of partial crystalline structure in programmed 2?W SMP (Fig.? 4a ). This ensures that RC tape-2W SMP laminate keeps in coiled state at low temperature when heating is needed and unfolded state at high temperature when cooling is needed without excessive bending to programmed 2?W SMP side, achieving the designed automatic and temperature-triggered switching. A hysteresis of bending angle exists during a heating-cooling cycle, which is from the difference between melting temperature and crystallization temperature of programmed 2?W SMP. The triggering temperature of RC tape-2W SMP laminate could be adjusted by the molecular weight (M w ) of PCL monomers, according to the requirements of practical scenario (Supplementary Figs.? 9 and 10 ). For PCL with M w ?=?10,000, the triggering temperature is in the range of 23–24?°C, around the comfortable temperature zone for human living (Fig.? 4b ). In addition, RC tape-2W SMP laminate exhibits excellent cyclability during repeated heating-cooling process, indicating good stability in long-term operation (Supplementary Movie? 1 and Supplementary Fig.? 11 ).
Fig. 4: Reversible shape memory performance. a X-ray diffraction spectrums of programmed 2?W SMP in heating and cooling modes, respectively. b Bending deformation performance of RC tape-2W SMP laminate as a function of temperature of heating plate. The molecular weight of PCL monomer is 10,000. The inset of optical images shows that RC tape-2W SMP is in coiled state at low temperature and unfolded state at high temperature. c Reversible bending deformation of RC tape-2W SMP film array in the dual-mode device as a function of the number of cycles between heating and cooling modes.
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Then, several RC tape-2W SMP laminates are placed side by side and bonded together to form a large-sized film exactly covering the nano-Cr black Al plate (Supplementary Note? 2 and Supplementary Fig.? 21 ). A piece of narrow VHB tape, as the only joint part between solar heating and radiative cooling layers, reserves the maximum effective area for dual-mode thermal management. We also demonstrated the robustness of dual-mode device repeatedly switching between heating and cooling modes (Fig.? 4c ). Briefly, the reversible shape transformation of RC tape-2W SMP laminate realizes the maximum percentage of active area in dual-mode device, which is conductive to achieving the best effect of thermal management in heating and cooling modes, respectively. And the temperature-sensitive trigger mechanism makes it possible for dual-mode devices to intelligently and freely switch between two thermal management modes without any external energy consumption (Supplementary Movie? 2 ).
Thermal management performance of dual-mode device .
To estimate working efficiency of this dual-mode device for both heating and cooling modes, a Joule heating-based measurement system is designed to monitor heat flux (Supplementary Fig.? 18 ). The Peltier device combined with a fan is used as a stable cold source in the system. An indoor experiment has been carried out first with the solar simulator (AM 1.5) before field test outdoors. The solar heating power and radiative cooling power of the dual-mode device is tested for five cycles (Supplementary Fig.? 19c ). For solar heating, the average heat flux of dual-mode device achieves 933.6?±?13.7?W?m ?2 , which is almost consistent with the theoretical value of dual-mode device in heating mode, approximately equal to 94% of solar radiation (ASTM G173) (Fig.? 3e ). Similarly, for radiative cooling, the average heat flux is ?94.4?±?42.8?W?m ?2 , which is ~55% of theoretical value in cooling mode. Both a certain difference and fluctuation may be from insufficient thermal contact between RC tape-2W SMP film and nano-Cr black Al plate. More details about theoretical model calculation are described in Supplementary Note? 3 . Furthermore, the dual-mode device spontaneously switches between heating and cooling modes by responding to the temperature, without external energy consumption. After repeated switching, whether it is in heating mode or cooling mode, the thermal management performance has no obvious degradation.
Further, we performed a daily field experiment in the real-world scenarios outdoors to test its truly practical thermal management performance in a real environment (located on the campus of Nankai University in Tianjin (38.99?N, 117.34E), China) (Supplementary Note? 5 ). Two same systems are set in parallel for comparison (Supplementary Fig.? 18 ). One copper (Cu) plate is covered by our dual-mode device, and the other is covered by a same-sized aluminum (Al) foil as a control group, because its solar absorption and infrared emission are close to zero (Supplementary Fig.? 13 ). The heater in the system for dual-mode device is connected to a constant current source, and the other is connected to a feed-back control program to maintain the temperature of Al foil the same as that of the dual-mode device (Supplementary Fig.? 23 ). Shown in Fig.? 5a are the three heat flux curves recorded for the solar radiation, dual-mode device in heating and cooling modes, respectively. The solar heating power continues to increase and achieves close to 958.7?W?m ?2 with stronger and stronger solar radiation, where the real-time solar-thermal conversion efficiency always remains around 91%. In addition, the average radiative cooling power around noon reaches 126.0?W?m ?2 under the normal-incidence solar radiation >850?W?m ?2 . Considering the reduced ambient thermal radiation and the inevitable heat convection and conduction (Supplementary Note? 4 ), the measuring heating flux data of dual-mode devices in both heating and cooling modes outdoors matches well with the indoor experimental results. These results demonstrate that our dual-mode device could achieve rather high-efficiency thermal management performance repeatedly in both solar heating and radiative cooling modes, and automatically switch between them according to the temperature. During the whole process, including working and switching, zero external energy is required. The dual-mode device is feasible to work in the real world throughout different seasons of the entire year. As far as we know, the design of this dual-mode thermal management device with these features combined together, including two thermal management modes, zero-energy consumption, and intelligent and free switching, has not been reported in the literature (Supplementary Table? 1 ).
Fig. 5: Thermal management performance of dual-mode device. a Continuous time-resolved solar heating power (red line) and radiative cooling power (blue line) measured in field test. The solar-thermal conversion efficiency ( η solar-thermal ) fluctuates around ~91% (red dash line) according to real-time solar radiation (orange line). b Modeled monthly all energy saving of dual-mode device in heating (red) and cooling (blue) modes in Tianjin for 1?year and year-round energy saving (green). The critical temperature for dividing heating and cooling modes is assumed as 17?°C, which is approximately equal to the average temperature of Beijing in spring and autumn. Heating mode: January–April, October–December. Cooling mode: May–September. c , d The effects of solar absorptivity ( α solar ) and infrared emissivity ( ε infrared ) on ( c ) heating energy saving in January and ( d ) cooling energy saving in July in Tianjin. Solar absorptivity with the corresponding infrared emissivity of the dual-mode device (star) is compared with those of temperature-responding device (doped-vanadium dioxide (VO 2 ), square; hydrogel, circle; phase-changing polymer, triangle; other materials, hexagon) in the literature. e Modeled energy-saving (radius of circle) map for some cities with dual-mode device in heating mode (red circle) or cooling mode (blue circle) in January. f Real-time temperature difference (Δ T ?=? T sample ??? T Cu plate ) of dual-mode device ( T sample , black line) compared with 200-μm-thick Cu plate ( T Cu plate , blue dash line) under solar radiation ( I sun , orange line). As Joule heating power is repeated to be on-off, the dual-mode device switches between cooling mode and heating mode by perceiving temperature.
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Referred to historically meteorological data, we calculated monthly produced heat and cold of dual-mode device in heating and cooling modes, respectively, to quantitatively predict the potential impact of dual-mode device on energy saving (Supplementary Note? 6 ). With the periodic change of relative position between the earth and the sun, the heating and cooling capacities of the dual-mode device in different months show a certain regularity. Taking Tianjin, a typical continental-monsoon-climate city, as an example, the total solar radiation and average temperature increase first and then decrease together in 1?year (Fig.? 5b and Supplementary Table? 2 ). Even in colder winter, the dual-mode device is still able to produce considerable heat (>0.15?GJ m ?2 ), thanks to its high solar-thermal conversion efficiency, although the total solar radiation is very low. The cooling capacity is mainly determined by temperature, less affected by the solar radiation. The peak reaches 0.24?GJ m ?2 in July and August, just corresponding to the hot summer. The year-round accumulated energy saving exceeds 2.9?GJ m ?2 in prediction. The maximum energy saving for heating in January will happen at α solar ?=?100% and ε infrared ?=?0%, and that for cooling in July occurs at α solar ?=?0% and ε infrared ?=?100% (Fig.? 5c, d ). It agrees well with our proposed two ideal high-selectivity electromagnetic spectrums (Fig.? 1b ). Compared with temperature-responding thermal management devices (including windows and coatings) reported in the literature 30 , 33 , 34 , 35 , 36 , 37 , 38 , our dual-mode device could reach 91% of solar absorptivity and 8% of infrared emissivity for heating, and 90% of solar reflectivity and 97% of infrared emissivity for cooling, which is very close to the ideal electromagnetic spectrums. This great improvement of spectral selectivity puts our device in a different operational space and sets a new mark for dual-mode radiative thermal management. Some cities are selected to represent typical terrestrial climatic zones around the world (Supplementary Fig.? 25 and Supplementary Table? 3 ). It can be seen that the dual-mode device has significant effects of energy saving in almost all climate zones, whether in heating mode or cooling mode. We assumed that the dividing temperature between heating and cooling modes is 17?°C, which is approximately equal to the average temperature of Beijing in spring and autumn. The corresponding energy-saving map is shown in Fig.? 5e . In January, the weather is cold in most areas north of the Tropic of Cancer, and the dual-mode device works in heating mode. In general, the closer to the Tropic of Cancer, the more energy for heating can be saved from solar-thermal conversion of dual-mode device. It is consistent with the change of solar radiation as a function of the latitude. In contrast, the weather, in most areas located in the south of the Tropic of Cancer, is warm or even hot in January. Dual-mode device in cooling mode achieves good effect of energy saving for cooling, especially in the area near the Tropic of Capricorn, where it is in summer. The above analysis describes the great potential of the dual-mode device in terms of global thermal management and energy saving.
A real-time demonstration of the high-performance temperature control by the dual-mode device outdoors is shown in Fig.? 5f . With alternative applying and removing of a constant Joule heating power, the dual-mode device spontaneously switches between cooling mode and heating mode by perceiving temperature (Supplementary Fig.? 27 ). A bare Cu plate with an almost invariable electromagnetic spectrum is used as a control group. As expected, the Cu plate covered by the dual-mode device in heating mode is obviously ~6?K higher than the bare one under the solar radiation, when it is cool. And when it is hot, a temperature reduction close to 15?K is realized by the dual-mode device in cooling mode. Even at dark night, the dual-mode device could also preserve heat due to the low infrared emission in heating mode, and still efficiently produces cooling in cooling mode (Supplementary Fig.? 28 ). A total of ~21?K reduction of temperature fluctuation strongly and visually shows the ability to control temperature for the dual-mode device. .
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