Based on a strategy of the synergy of materials and micro-nano structures to enhance the light-trapping effect and photothermal conversion performance31,32, we polymerized the photothermal material-polyaniline (PANI) in-situ on a commercial nanoporous anodic aluminum oxide (AAO) with vertical channels (Figure S1-S3). The PANI nanoparticles completely cover the surface of the AAO template and are randomly distributed on the inner walls of the vertical holes as shown in Fig. 1a. When light falls onto the PANI@AAO, it will be reflected and gradually absorbed by the structure. This AAO vertical channels with large aspect ratio and abundant rough PANI nanoparticles attached form optical traps to capture light efficiently, and thus is an ideal solar absorber (SA) with a near-perfect black surface in a wide spectral range (from 300 ~ 2500 nm) (Fig. 1b). What is more, the PANI@AAO converts the solar energy into heat owing to the lattice vibrations and localized surface plasmon resonance (LSPR) effect. Under one sun illumination (1000 W/m2), the surface temperature of the PANI@AAO exhibits a significant increase and eventually reaches an equilibrium value of 82°C. As the intensity of sunlight increases, the surface of the sample can reach a higher equilibrium value which shows the potential to obtain a higher temperature by using concentrator optics (Fig. 1c).
The net heating power of the SA for the outdoor performance (under the standard AM1.5 sunlight) under different environmental parameters and various weather conditions was simulated based on a theoretical thermal measurement model as shown in Fig. 1d (more details are given in Figure S4). The heat exchange process between the SA and environment has been taken into account in the theoretical simulation. A maximum net heating power of 830 W/m2 can be achieved at room temperature (298 K). With the increase in temperature, the collective effect of nonradiative heating (conductive and convective heating) increases, leading to a decrease of the net heating power.
Passive radiative cooling emitters (RCE) can shed the heat of ground objects to the ultra-cold space by taking advantage of the thermal radiation through the atmospheric transparency window from 7 ~ 14 µm, thereby cooling the objects and its surrounding environment33,34. In this work, we fabricated a typical RCE (PDMS backed with a 100-nm-thick silver thin film, the silver reflect solar irradiation as a metallic mirror) on a silicon wafer (Fig. 2a-b). The optical properties of the device were investigated in the standard AM1.5 solar spectrum and the long wave infrared (LWIR) atmospheric transparency window. The complete emissivity/absorptivity spectrum across an ultrabroadband wavelength range in the solar and the mid-infrared regions of the RCE is shown in Fig. 2c. The RCE shows minimal absorption from 300 nm to 2.5 µµm, where the solar spectrum is located, and possesses a nearly saturated emissivity of 92% in the LWIR atmospheric transparency window (7 ~ 14 µm). The real-time temperature tracking of the air and the RCE that exposed to sky were measured to investigate the actual cooling capability during 24 h in Shanghai, China. The RCE can achieve a sub-ambient temperature drop of ~ 6℃, especially at nighttime, showing a steady and sustained cooling effect as shown in Fig. 2d. However, considering the complexity of the daytime environment under direct sunlight, the measured cooling performance of the RCE was fluctuated.
Theoretically, the terrestrial radiative cooling effect comes from the imbalance of radiative heat flow between the sky-facing object and cold space, and the atmospheric transparency window providing a major channel for ground objects to radiate heat into the cold space with low losses. Figure 2e shows the net cooling power of the RCE during daytime and nighttime for various values of hc (nonradiative heat coefficient) that ranged from 0 to 7 W/m2/K (more details are given in Figure S5). Obviously, the RCE could achieve a maximum cooling power in excess of 120 W/m2 making a theoretically possibility for nighttime operation as hc→0 (the solid line in Fig. 2e). The degree of solar absorption by the RCE has an inversely adverse implication on its achievable cooling powers. What is more, when the surface temperature of the RCE is above or close to ambient temperature, the emitter cooler has a stronger sub-ambient cooling effect.
TEG is a kind of energy harvester that can convert thermal energy into electrical energy directly based on Seebeck effect, and have been considered as one of the most promising and environmental-friendly power devices for the future. For current commercial TEG, its improvement is limited by the innovation and the development of highly integrated nano/micro-devices which allows for larger series structures can take advantage of small temperature differences. With the development of Micro-Electro-Mechanical System (MEMS) and thin film deposition technology, the design and preparation of large array nano/micro-devices with high integration degree have gradually become a reality. Based on our previous work35, an ultrathin MEMS-TEG on a 1×0.7 cm rectangular silicon wafer at the chip level was fabricated along with further increasing the thickness of TE materials. The MEMS-TEG is composed of 572 P-N (Sb2Te3-Bi2Te3) modules in series, and the size of each P/N leg is 200 µm×200 µm×1 µm (length×width×thickness) (Fig. 3a, the inset of Fig. 3b and Figure S6). The measured open-circuit voltage and output power of the TEG as a function of varied temperature from 40 to 160℃ is shown in Fig. 3b and Figure S7. The open-circuit voltage increases with temperature and reaches a value as high as 8 mV at T ≈ 160°C. Meanwhile, a power output of about 0.4 µW could be achieved when the external load resistance matches the internal resistance of the device of about 70 Ω. The bulk power density of the MEMS-TEG (with a volume of 4.6 E− 11 m3 including electrodes and thermoelectric materials) achieved 5.5 kW/m3 when its hot end temperature is 140 ℃ with an active heat source and the cold end faces the air. Figure 3c shows the stability of the ultrathin TEG under relatively small temperature difference of 1.5℃, indicating the reliability of long-term operation. The wide operating temperature range and efficient power output capability represent the practical application potential of the MEMS-TEG.
Owing to the characteristics of ultra-thin and high integration, the MEMS-TEG is extremely sensitive to small temperature difference. Interestingly, when a hand or ice gets close to the MEMS-TEG, it will send back an obvious voltage signal (Fig. 3d). In a word, both thermal and cold stimuli will create a temperature difference across the MEMS-TEG, resulting in a voltage output (with a typical sensitivity of 10− 4 K based on a total Seebeck coefficient of 200µV/K of one TE pair). Because the ultrathin MEMS-TEG could utilize such a tiny temperature difference, it has great potential in the special fields of self-powered nano-micro devices, environmental energy utilization, industrial waste heat utilization and distributed generation.
By careful integration of the SA and the RCE to the two ends of the TEG, an uninterrupted power generation SA-RCE-TEG that utilizes photo-thermal effect, radiation cooling and thermoelectric effect is realized. As shown in Fig. 4a, the SA converts the solar energy into heat energy in a clear daytime and thus provide a heat source for TEG. Meanwhile, the RCE function as a metallic mirror strongly reflecting the sunlight and selectively radiating the heat into the cold outer space in the form of infrared radiation throughout the day, and thus provided a continuous cold source for TEG. In the whole thermal dynamic process, the TEG acts as an energy converter that provide a channel for directional heat transmission to form a stable temperature difference between the upper and lower surfaces, thus realizing continuous electric power output.
In this hybrid system, during the daytime, SA absorbs sunlight as a heat source and is the main temperature difference generator. But at nighttime, because there is no sun, RCE is the only contributor of temperature difference. More importantly, due to the disappearance of solar radiation, the cooling effect of RCE in nighttime is better than that in the daytime. Figure 4a shows the basic scheme of all-day self-powered energy harvesting system in a building roof, and Fig. 4b is the real-time temperature and output voltage of continuous outdoor test for more than 4 days (more details show in Figure S8-S10). In particularly, because the heating power of the SA is much greater than the cooling power of the RCE, the SA-RCE-TEG has a higher surface temperature and temperature difference in the daytime. What is remarkable is that the SA-RCE-TEG can not only achieve up to a 120 µV output voltage (~ 1.1 W/m3) during daytime, but also produce an 8 µV stable output voltage (~ 5.0 mW/m3) at night, showing the viability of all-day continuous environmental thermal energy harvesting and outdoor electricity generation. However, compared to the single TEG with an active heat source (Thot =80°C, Tcold=25°C, Pout=960 W/m3), the performance of SA-RCE-TEG with similar temperature of environmental energy source (TSA =82°C, TRCE=20°C, Pout=1.1 W/m3) is insufficient. This demonstrates that, with better thermal management and good thermoelectric materials may increase the output power of the next generation of devices. The sustainable output electricity of the SA-RCE-TEG could be collected to power nano-micro electronic devices, especially for the special scenes, such as harsh and remote environment.