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Abstract
Passive power generation has recently stimulated interest in thermoelectric generators (TEGs) using the radiative cooling mechanism. However, the limited and unstable temperature difference across the TEGs significantly degrades the output performance. In this study, an ultra-broadband solar absorber with a planar film structure is introduced as the hot side of the TEG to increase the temperature difference by utilizing solar heating. This device not only enhances the generation of electrical power but also realizes all-day uninterrupted electrical output due to the stable temperature difference between the cold and hot sides of the TEG. Outdoor experiments show the self-powered TEG obtains maximum temperature differences of 12.67 °C, 1.06 °C, and 5.08 °C during sunny daytime, clear nighttime, and cloudy daytime, respectively, and generates output voltages of 166.2 mV, 14.7 mV, and 95 mV, respectively. Simultaneously, the corresponding output powers of 879.25 mW/m2, 3.85 mW/m2, and 287.27 mW/m2 are produced, achieving 24-hour uninterrupted passive power generation. These findings propose a novel strategy to combine solar heating and outer space cooling by a selective absorber/emitter to generate all-day continuous electricity for unsupervised small devices.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
As the environmental pollution and energy shortage issues raged higher, it is critical to find a new type of sustainable energy to reduce the excessive fossil fuel dependence [1–4]. In recent decades, the development of renewable natural sources, including solar, hydroelectric, marine, geothermal, and wind, has made a positive contribution to reducing the energy crisis [5,6]. However, these technologies are limited by time and space [7–9]. Green energy harvesting for uninterrupted power supply, unrestricted by time and space, will promote worldwide energy source accessibility, but remains a formidable challenge. Recently, thermoelectric generators (TEGs) coupled with radiative cooling emitters (RCEs) have been extensively studied as a new environmentally friendly method for power supply [10–14]. These devices use high emissivity RCEs as the cold source and ambient air as the heat end of a TEG, respectively. The spontaneous temperature difference between the two ends of the TEG is utilized to generate electrical power directly [15,16].
A variety of radiative cooling materials, such as randomly distributed particle materials [17–19], porous materials [20,21], polymer-coated materials [22,23], and organic-inorganic composite materials [24,25], can be used as the cold end of a TEG. Among them, polymer materials have received significant attention due to their infrared radiation characteristics, low preparation cost, large-scale manufacturing, mature technology [26–28]. For example, Khan et al. [23] proposed a flexible TEG that integrated a micro-thin poly(vinylidene fluoride-co-hexafluoropropylene) radiative cooled heat sink, achieving a high thermal power output of 12.48 µW/cm2. Liu et al. [29] developed a RCE-TEG device based on the polyethylene aerogel and simulated the maximum power output of 0.92 W/m2 using the Carnot limited.
However, relying solely on the limited and unstable temperature between the RCEs cold source and ambient air heat source will degrade the power output of the TEG device [29]. Moreover, the cooling performance of RCEs is significantly affected by environmental and weather conditions. For example, during periods of strong sunlight, the temperature of the RCE will rise due to solar radiation. More importantly, in humid environment, excessive water vapor in the atmosphere can narrow the transparent window of the atmosphere, reducing the net cooling power, resulting in reduced power generation or even failure to generate electricity [30]. Therefore, the existing TEG device, directly using the RCEs and ambient air as the cold and heat ends, respectively, cannot produce uninterrupted and stable voltage output throughout the day.
To achieve all-day uninterrupted power generation, researchers have attempted to utilize solar heating. Solar absorbers (SAs) can attain higher temperature than the ambient air due to their high absorptivity in solar spectrum [31,32]. Incident sunlight can be converted into heat energy to a greater extent, increasing the temperature of the hot end. In recent years, numerous metamaterials absorber, such as electric ring resonator [33], multilayered hyperbolic metamaterials [34,35], and tungsten cross resonators [36], have been designed to achieve higher absorptivity throughout the entire solar spectral range. However, most of these absorbers require a relatively complex fabrication process, making them expensive as the heat end of a TEG.
As an alternative solution to overcome this problem, multilayer film structures with simple and non-nano patterning have been used. Ding et al. [37] proposed a multilayered stack consisting of thin SiO2 and Ti films on the Au coated substrate, which achieved a maximum absorptivity of 95% in the visible light (400–900 nm). Liao et al. [38] simulated an absorber with four pairs of W-CaF2 films on a Au substrate without using lithography during fabrication, which displayed an absorptivity exceeding 90% throughout the wavelength range of 400–1639 nm. Li et al. [39] prepared an 11-layered Al-SiO2 films structure and the absorptivity reached up to 98% from 350 to 1400 nm. However, the absorptivity performance of these multilayer planar films structures is significantly lower than those of patterned subwavelength structures needing sophisticated fabrication procedures. This is due to the former's narrow and weak solar absorptivity spectrum [40–42].
To the best of our knowledge, if multiple resonances can be simultaneously superimposed at different solar spectrum wavelengths, the performance of solar heat absorptivity will be considerably improved. However, this method has rarely been reported for planar film structures. In this work, an ultra-broadband solar absorber (UBSA) based on the metal-dielectric-metal (MDM) multilayer planar films structure is first introduced as the hot side of the TEG. The UBSA is not only fabricated with simple procedure, but also can achieved higher absorptivity performance than that of complex patterned structures [43]. Therefore, adding the simple and low-cost UBSA to the hot side of the TEG will undoubtedly produce a larger and more stable temperature difference, resulting in continuous electricity generation.
In this paper, a self-powered TEG was proposed for 24-hour uninterrupted electricity generation by simultaneous harvesting of solar heating and radiative cooling. The cold and hot sources of TEG are composed of a polymethylmethacrylate (PMMA)-based radiative cooling emitter (P-RCE) and an UBSA based on Fe, Ti, SiO2, and MgF2 planar films, which were designed using the finite difference time domain (FDTD) method. The experimental results show that the P-RCE can maintain an extremely high emissivity (>97.94%) in the atmospheric transparent window (ATW, 8–13 µm) to radiate excess heat to the external space. Additionally, it can reflect 89.80% of the solar energy. Thus, the P-RCE achieves a maximum temperature drop of 2.14 °C during the daytime and 6.68 °C during the nighttime, respectively. Meanwhile, the UBSA with polarization-independent and angle insensitive properties, exhibits an average absorptivity of 95.86% covering the entire solar spectrum (300–2500 nm). Furthermore, the operating temperature of the UBSA film with simple heat preservation can rise to 100.88°C, demonstrating its outstanding solar thermal conversion properties. Due to the addition of UBSA to the hot side of the TEG, the temperature difference between the two sides of the TEG is uplifted to 12.67 °C on a sunny daytime, generating output voltages of 166.2 mV. Finally, the power output of TEG in different climatic environments have also been investigated experimentally and the performance of 24-hour uninterrupted power output was also confirmed.
2. Methods
2.1 Materials
Ag pellets (99.99% purity), Fe pellets (99.95% purity), Ti pellets (99.995% purity), SiO2 pellets (99.999% purity), MgF2 pellets (99.99% purity) were purchased from Zhongnuo Advanced Material (Beijing) Technology Co., Ltd. The PMMA film was purchased from Xintao Optoelectronics (Anhui) Technology Co., Ltd. Silicon wafers with a thickness of 500 µm were purchased from Tebow (Harbin) Technology Co., Ltd. Acrylic adhesive was purchased from Extraordinary Power Co., Ltd. Polyimide foils with a thickness of 25 µm were provided by DuPont. TEG (TEC1-12703) was purchased from Zave (Zhejiang) Co., Ltd.
2.2 Sample fabrication
P-RCE: Firstly, a 200-nm Ag layer was deposited on a silicon wafer (4 × 4 cm) by electron beam evaporation (DZS500 system, Sky Technology Development Co., Ltd. Chinese Academy of Sciences). Then, the 200-µm thick PMMA was pasted on the Ag layer with acrylic adhesive and cured by UV irradiation. Finally, the bubbles between the PMMA and Ag layer were removed using the squeegee method.
UBSA: 100 nm Fe, 65 nm SiO2, 15 nm Ti, 90 nm SiO2, 10 nm Ti, 100 nm SiO2, 5 nm Ti and 110 nm MgF2 were sequentially deposited on a four-inch silicon wafer using electron beam evaporation. Thus, the sample fabrication was completed.
2.3 Sample characterization
The reflectivity of the P-RCE and UBSA in the wavelength range of 300–2500 nm were measured by a UV-Vis-NIR spectrophotometer (Cary-5000, Agilent) with an integrating sphere using a barium sulfate white plate as a baseline. The infrared emissivity of the P-RCE and UBSA between 2.5–25 µm were measured using a Fourier transform infrared spectrometer (Nicolet iS50, Thermo Scientific) and a silver mirror as a baseline. A field emission electron microscope (Sigma 300, Zeiss, Germany) was used to acquire a scanning electron microscopy (SEM) image of UBSA. Global solar irradiation was measured using a solar radiation sensor (RS-RA-N01-AL, Renke). The temperatures of the P-RCE, UBSA and ambient air were measured using K-type thermocouples (NI-T325, Uni-TrendU). The open circuit voltage of the TEG is measured by a Keithley 2400 multimeter (Tektronix, USA).
2.4 Principle of self-powered TEG
Figure 1 shows the schematic diagram of proposed self-powered TEG, where UBSA and P-RCE serve as the heat and cold sources of the TEG, respectively. In this device, UBSA heats up by absorbing the energy of solar radiation, while P-RCE cools down by radiating heat into outer space [44,45]. To achieve best performance, both need to face the sky without being obstructed. In addition, the net heating power of the UBSA is significantly greater than the net cooling power of the P-RCE under normal sunlight intensity. Therefore, the RCE is placed on top of the TEG in order to generate the maximum cooling temperature. The UBSA is positioned beneath the TEG, and its area is greater than that of the TEG. When sunlight hits the entire device, the unshaded parts of UBSA absorb the sun’s energy to heat up, while the top P-RCE begins to cool. As a result, this divice will generate a high temperature difference between the two sides of the TEG.
Fig. 1. Schematic diagram of continuous self-powered TEG based on P-RCE and UBSA, and the energy exchange in the working state. During a typical day, the P-RCE acts as the cool side of the TEG and radiates more heat to outer space (PRrad), while absorbing less solar radiation (PRsun), atmospheric heat radiation (PRatm), and undergoing heat convection and conduction with the outside environment (PRcond + conv). On the other hand, the UBSA acts as the hot side of the TEG and absorbs more solar radiation (PUsun), atmospheric heat radiation (PUatm), and radiates less heat to outer space (PUrad), and exchanges heat through convection and conduction mechanisms (PUcond + conv). This temperature difference generates a large heat conduction power density (Pcond), part of which is converted into electrical energy (Pele) by the TEG.
In addition, it can be further inferred from Fig. 1 that when the thermal radiation power (${P_{Rrad}}$) of the P-RCE was stronger than the sum of the absorptivity power of P-RCE to solar radiation (${P_{Rsun}}$) and atmospheric heat radiation (${P_{Ratm}}$), the P-RCE began to cool spontaneously, generating a cooling temperat ${P_{Rsun}}$ure difference (${T_R}$). With the increase of ${T_R}$, the non-radiative heat exchange (${P_{Rcond + conv}}$), including heat convection and heat conduction, will inevitably take place between P-RCE and ambient air, which hinders the cooling of P-RCE [46].
When the heat exchange achieve rebalance, the power balance equation of P-RCE is shown in Eq. (1):
Similarly, the UBSA absored solar radiation (${P_{Usun}}$), and leads to a heat temperature difference (${T_U}$). Its power balance formula is shown in Eq. (2):
where ${P_{Uatm}}$ is atmospheric heat radiation absorbed by UBSA. ${P_{Urad}}$ is the thermal radiation power of UBSA. ${P_{Ucond + conv}}$ is the non-radiative heat exchange between the UBSA and ambient air.In our device, UBSA and P-RCE were placed at the hot and cold sides of the TEG, respectively. Therefore, a large heat conduction power density (${P_{cond}}$) was generated due to the high temperature difference between the hot and cold ends of the TEG. Part of the energy of ${P_{cond}}$ will be converted to electricity ${P_{ele}}$ through the TEG finally to achieve thermoelectricity generation [47]. Further details on the calculation of heating and cooling power are provided in the Supplement 1.
2.5 Outdoor measurement
The radiative cooling, heat absorptivity, and thermoelectric power generation experiments were performed on the rooftop of a building at Jimei University in Xiamen, China (24°34'55.36''N, 118°05'21.77''E). A sample container was created to reduce the thermal convection and thermal conduction from the environment in the setup. Therefore, the container consisted of an acrylic box (size: 15 × 15 × 10 cm) with an open top, surrounded by aluminum foil, and its top covered with a low-density polyethylene (PE) film (∼ 30 µm-thick). To reduce heat transfer from the ground, the container was placed on a thermal barrier. The test sample was placed inside the container. K-type thermocouples were utilized to measure the ambient and sample temperatures.
3. Results and discussion
3.1 Radiative cooling emitter based on PMMA
Figure 2(a) depicts the three-dimensional structure (left) and cross-section (right) of the RCE based on PMMA film. The thickness of PMMA was optimized using the FDTD method and the calculated results are presented in Fig. 2(b). It can be seen that the infrared emissivity of P-RCE increased in the wavelength range from 6 to 15 µm when the thickness of PMMA gradually increased. When the PMMA thickness exceeded 200 µm, the infrared emissivity of the cooler almost stops changing, especially in the ATW. In addition, an increased thickness in the PMMA layer will lead to a high absorptivity of solar radiation, which reduces the radiative cooling performance [48]. In order to achieve the high infrared emissivity of P-RCE and the reflectivity of solar radiation, the final 200-µm PMMA film was determined as the radiation layer of P-RCE.
Fig. 2. Schematic diagram and performance of P-RCE. (a) Three-dimensional structure (left) and cross-section (right) of P-RCE. (b) The emissivity of P-RCE with different PMMA thicknesses. (c) Photograph of P-RCE. (d) Reflectivity of P-RCE (red solid line) and solar spectral irradiance (orange solid region) from 300–2500 nm at AM 1.5. (e) Infrared emissivity (red solid line) and the atmospheric transmittance spectrum (crayon blue region) of P-RCE. (f, g) Practical radiative cooling performance of P-RCE in a sunny daytime and a clear nighttime, respectively. (h) Net cooling power of P-RCE under different conditions. The atmospheric temperature is set to be 300 K, and the solar intensity is 1000 W/m2. hc is the convective heat-transfer coefficient.
Figure 2(c) is the photo of the fabricated P-RCE. The surface of the P-RCE sample was smooth and uniform to ensure its radiation capacity. Figure 2(d) depicts the measured reflectivity of P-RCE (red solid line) in the wavelength range from 300 to 2500 nm and the AM1.5 global tilted solar spectrum is used as the background (orange region). The results show the P-RCE has an average reflectivity of 89.80% in considered wavelength due to the bottom Ag reflection layer. This ensures that as little heat as possible is absorbed by direct sunlight. Figure 2(e) plots the measured infrared emissivity of P-RCE (red solid line) and the atmospheric transmittance spectrum is used as the background (crayon blue region). The average infrared emissivity of 97.94% in the ATM demonstrates the efficient thermal radiation ability.
To explore the practical performance of P-RCE, we performed outdoor cooling tests on sunny daytime and clear nighttime and the results are shown in Fig. 2(f) and 2(g), respectively. During the daytime, P-RCE was found to achieve a maximum temperature drop of 2.14 °C below the ambient air temperature. Simultaneously, a maximum drop of 6.68 °C below the ambient air temperature on a clear nighttime was realized. These results show that P-RCE showed an outstanding cooling capacity and could be applicable to the cold side of TEG to generate electricity. Moreover, the theoretical net cooling power of P-RCE under different conditions was also calculated, as shown in Fig. 2(h). ${h_c}$ is the convective heat-transfer coefficient. The results show that the net cooling power of P-RCE is affected by the conduction, convection and temperature of P-RCE.
It is worth noting that the temperature drops of 2.14 °C and 6.68 °C obtained by P-RCE during a sunny daytime and a clear nighttime, respectively, were performed under stringent thermal insulation. That is to say, it was necessary to ensure that the ambient air temperature tended to be stable to minimize non-radiative heat exchange. However, this is highly difficult. In particular, as the P-RCE is treated as the cold source of TEG, the temperature of the ambient air naturally cannot remain constant and will inevitably result in the generation of non-radiative heat exchange, thereby reducing the cooling capacity of P-RCE [48]. Therefore, it is necessary to increase the temperature difference by other means to obtain a greater amount of electricity.
3.2 Ultra-broadband solar absorber
With the above consideration, a UBSA with multilayer planar films structure was introduced as the hot side of the TEG to increase the temperature difference by utilizing solar heating. Figure 3(a) depicts a 3D schematic diagram (left) and a cross-sectional diagram (right) of the UBSA. The thickness of each layer is: h1 = 110 nm, h2 = 5 nm, h3 = 100 nm, h4 = 10 nm, h5 = 90 nm, h6 = 15 nm, and h7 = 65 nm. Figure 3(b) is the photograph of the fabricated UBSA. Figure 3(c) is the cross-section SEM image of UBSA. It shows that the thickness of the UBSA film deposited on the Si substrate is uniform and flat, with clear layers. The red solid line in Fig. 3(d) indicates the measured absorptivity spectrum of UBSA at 300–2500 nm. The result shows that the UBSA has an average absorptivity of 95.86% in the wavelength range of 300–2500 nm, demonstrating its extraordinary solar thermal conversion property. For comparison, the simulated absorptivity curve (blue solid line) using the FDTD method is also provided. There was relatively consistent compared with the simulated result. The difference is mainly attributed to thickness and refractive index in the experiment, which may change slightly from those used in the simulation. Figure 3(e) depicts the emissivity (red solid line) of UBSA in the infrared region. It can be seen that the absorber has a low infrared emissivity, and can maintain a high temperature due to reduced heat radiation. In addition, the polarization-independent and angle insensitive properties of the UBSA were also confirmed, as detailed in Figs. S1(e-g) of Supplement 1.
Fig. 3. Schematic diagram and performance of UBSA. (a) 3D schematic diagram (left) and cross-sectional diagram (right) of UBSA. (b) Photograph of UBSA. (c) The cross-section SEM of the UBSA. (d) Measured absorptivity spectrum (red solid line), simulated absorptivity spectrum (blue solid line), and solar spectral irradiance (orange solid region) of UBSA from 300-2500 nm at AM1.5. (e) Infrared emissivity (red solid line) and atmospheric transmittance spectrum (crayon blue region) of UBSA. (f) Energy current density vector distribution of the UBSA. (g) Practical absorbing performance of UBSA at sunny daytime. (h) Results of the net heating power of UBSA under different conditions. The atmospheric temperature is set to 300 K and the solar intensity is 1000 W/m2. hc is the convective heat-transfer coefficient.
To elucidate the physical mechanism of UBSA with high absorptivity, the energy current density vector distribution of the model was analyzed at wavelengths of 300–2500 nm, as shown in Fig. 3(f). It can be seen that most of the power absorbed by UBSA was associated with 5-nm-thick Ti, while a small amount of energy was contained in 10-nm-thick Ti, and little energy was observed for 15-nm-thick Ti. This is mainly because when the incident light irradiates the UBSA, the light first penetrates the lossless dielectric layer of MgF2, and most of the incident light's energy is absorbed by the 5-nm-thick lossy Ti layer. Then, the rest of the energy enters the lossless SiO2 or MgF2 dielectric layer, and will be absorbed by the 10nm-thick lossy metal Ti film or reflected. In this way, most of the energy is absorbed by the lossy Ti metal layers, resulting in high absorptivity. In addition, the strong magnetic field distribution of UBSA can further explain the high absorptivity, as shown in Fig. S1(b) of Supplement 1.
To explore the practical absorbing performance of UBSA, we performed outdoor test during the sunny daytime and the temperature change was recorded in Fig. 3(g). At noon, UBSA reached a maximum temperature of 100.88 °C and the maximum temperature difference from the ambient air temperature was 68.38 °C, showing an excellent ability to absorb solar energy. Figure 3(h) depicts the net heating power of UBSA under different conditions. ${h_c}$ is the convective heat-transfer coefficient. The results also show that the net heating power of UBSA is affected by the conduction, convection and temperature of UBSA. Specifically, at room temperature (about 300 K), the net heating power of UBSA was 106.11% greater than an ideal blackbody. The operating temperature of UBSA was usually quite hotter compared to room temperature. However, with the increase in UBSA temperature, the thermal radiation loss increased as well. When the operating temperature was 400 K, the net heating power of UBSA was only 57.76% of the ideal blackbody. Although the net heating power of the UBSA is reduced, it can still act as the hot side of the TEG to increase the temperature difference for power generation.
3.3 Continuous self-powered TEG
Figure 4(a) is a photograph of the self-powered TEG depicting how the device uses UBSA as the heat source and P-RCE as the cold source. The working mechanism of the TEG is provided in the Supplement 1. The experimental setup is shown in Fig. 4(b). A K-type thermocouple in the acrylic box on the left was used to measure the ambient air temperature, while two K-type thermocouples in the right box measured the temperature of the RCE and UBSA, respectively. An almost transparent PE film completely surrounded the whole experimental device to reduce the non-radiative heat exchange caused by thermal convection and heat conduction.
Fig. 4. Performances of the self-powered TEG combining UBSA and P-RCE. (a) Photograph of the self-powered TEG. (b) Side view of outdoor test setup. The illustration at the upper right corner shows the top view of the test setup. (c) Output voltage of setup without RCE (group 1), without UBSA (group 2), and output voltage of setup with RCE and UBSA (group 3) on a sunny day. (d-f) For group3, temperature curves of the UBSA and P-RCE measured on a sunny daytime, a clear nighttime, and a cloudy daytime, respectively.
To verify the superiority of the proposed self-powered TEG (group 3), two control groups (group 1 and group 2) were set up wherein the P-RCE or UBSA were removed, respectively. As shown in Fig. 4(c), group 1, containing only UBSA, obtained the maximum voltage of 67.8 mV at noon but could not work at night. Group 2, containing only P-RCE, achieved a maximum output voltage of 5.3 mV on a clear night, but it cannot work in humidity weather, such as a cloudy day. This is because the cloud blocks the ATW, which makes the P-RCE unable to perform cooling. Group 3 achieved a maximum voltage of 166.2 mV since it had both UBSA and P-RCE to realize a largest temperature difference at both sides of the TEG for electricity generation.
For group 3, temperature curves for UBSA and P-RCE were measured during the initial 6 hours of a sunny daytime, a clear nighttime, and a cloudy daytime, respectively, as shown in Figs. 4(d-f). On a sunny daytime, the UBSA as the hot side of the TEG reached a maximum temperature of 100.24 °C at noon. With direct sunlight, the heating power of UBSA was markedly higher than the cooling of P-RCE. So, heat was conducted from the hot side of TEG to the cold side, raising the temperature of P-RCE to 91.92 °C. Fortunately, P-RCE can spontaneously cool down. Finally, a maximum temperature difference of 12.67 °C and an average temperature difference of 8.87 °C were achieved between the cold and hot sides of the TEG, as shown in Fig. 4(d). On a clear nighttime, UBSA cannot be heated since there is no sunlight, and its temperature is approximately the same as the ambient air temperature. Without solar energy input, P-RCE's net cooling power increases further relative to daytime, resulting in a decrease in the temperature of the cold end of the TEG. Nevertheless, the self-powered TEG only achieved a maximum temperature difference of 1.06 °C and an average temperature difference of 0.91 °C, as shown in Fig. 4(e). On a cloudy daytime, the ATW was hidden by clouds, preventing P-RCE from achieving effective radiative cooling. However, UBSA can still raise the temperature to some extent by absorbing ambient light. Finally, the self-powered TEG achieved a maximum temperature difference of 5.08 °C and an average temperature difference of 2.06 °C, as shown in Fig. 4(f). As a result, our device can maintain a spontaneous temperature difference at work in all hours of sunny and cloudy weather.
Figure 5(a) depicts the voltage output of the self-powered TEG based on P-RCE and UBSA monitored for 24 hours on clear and cloudy days. On a clear daytime, the device realized a peak voltage output of 166.2 mV. Correspondingly, the maximum output voltage of 14.7 mV was achieved on a clear nighttime. In addition, a maximum voltage of 95 mV was also achieved on a cloudy daytime. These results show its potential ability to generate all-day electricity in different periods and weathers. The corresponding output powers are also calculated and the calculation method is provided in the Supplement 1. The area of the UBSA is 25π cm2. By dividing the test results by the area of the UBSA, the device's power output per unit area can be obtained and normalized, as shown in Fig. 5(b). The results show that the self-powered TEG achieved normalized maximum output powers of 879.25 mW/m2, 3.85 mW/m2, and 287.27 mW/m2 during clear daytime, clear nighttime, and cloudy daytime.
Fig. 5. Electrical performances of the self-powered TEG combining UBSA and P-RCE. (a) Output voltage of the self-powered TEG combining UBSA and P-RCE monitored for 24 h on clear and cloudy days. (b) Normalized electric power of the device.
A comparison of our self-powered TEG with the previous self-generation power devices is shown in Table 1. Using a comparative method that objectively assessed the structure, max output voltage and max output power during different weathers and periods, we find that our self-powered TEG has outstanding performance in generating electricity.
Table 1. Comparison of the proposed device and other studies. (Note: ‘-’ indicates the lack of relevant research)
We further tested the stability and repeatability performance of the proposed device. The test time was selected in five similar clear days and the results are shown in Fig. 6. Although many factors, such as the ambient air temperature, humidity, and wind speed, may have a degree of influence on the voltage output, the maximum output voltages were still found to be stable during the clear daytime and nighttime, which indicates that this device can realized all-day stable uninterrupted generation.
Fig. 6. Stability and repeatability of the self-powered TEG device. Five sets of data were monitored during the clear daytime and clear nighttime to verify the stability and repeatability of the self-powered TEG device.
4. Flexible substrates implementation of P-RCE and UBSA
P-RCE and UBSA can also be prepared on a flexible substrate, as shown in Fig. 7. Flexible P-RCE was realized by depositing 200 nm of Ag by electron beam evaporation directly on a 200-µm-thick PMMA radiation layer thin film, as presented in Fig. 7(a). Similarly, UBSA in Fig. 7(b) was fabricated on a polyimide flexible substrate with a thickness of 25 µm. The illustrations at the top right corner of Fig. 7(a) and Fig. 7(b) indicate their softness, as well as their compatibility. This indicates that the proposed self-powered TEG device has excellent malleability and compatibility, which can be applied in wearable systems.
Fig. 7. Flexible Devices. (a) Flexible P-RCE consists of 200 nm Ag layer and 200-µm-thick PMMA film; (b) Flexible UBSA. UBSA is fabricated on a 25-µm-thick polyimide film substrate. The illustrations in the upper right corner of (a) and (b) reflect softness and compatibility.
5. Conclusion
In this work, we designed and fabricated a self-powered TEG device by simultaneously using solar heat energy and space cold sources for all-day uninterrupted electricity generation. A simple high performance UBSA is first introduced as the hot side of the TEG to increase the temperature difference by utilizing solar heating, and a P-RCE with high emissivity is used as the cold end of TEG. Outdoor experiments showed that the device can keep a spontaneous temperature difference during different weathers. The maximum output voltage of 166.2, 14.7, and 95 mV are generated in the sunny daytime, clear nighttime, and cloudy daytime, respectively. Furthermore, the corresponding output powers of 879.25, 3.85, and 287.27 mW/m2, are achieved, respectively. These results confirm this device can realized all-day uninterrupted generation. In addition, both P-RCE and UBSA can be fabricated using only simple fabrication processes, even onto flexible substrates. The proposed device has many potential applications, such as self-powered outdoor sensors, that need operate uninterrupted during the day and night.
Funding
National Natural Science Foundation of China (62275102); Youth Talent Support Program of Fujian Province (Eyas Plan of Fujian Province) (2021); Science Fund for Distinguished Young Scholars of Fujian Province (2020J06025); Science and Technology Major Project of Fujian Province (2022HZ022019); Innovation Fund for Young Scientists of Xiamen (2020FCX012501010105); Marine and Fishery Development Special Fund of Xiamen (20CZB014HJ03); Youth Talent Support Program of Jimei University (ZR2019002); Xiamen Ocean and Fishery Development Special Fund Project (21CZB013HJ15); Xiamen Key Laboratory of Marine Intelligent Terminal R&D and Application (B18208); Scientific Research Foundation of Jimei University (ZQ2019034).
Acknowledgement
The authors would like to thank the shiyanjia lab (www.shiyanjia.com) for the FTIR test.
Disclosures
The authors declare that there are no conflicts of interest related to this article.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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