Thermoelectric Generator Using Space Cold Source

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Thermoelectric Generator Using Space Cold Source Zhilin Xia, Zhenfei Zhang, Zhenghua Meng, Liyun Ding, and Zhongquan Yu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b10981 • Publication Date (Web): 27 Aug 2019 Downloaded from pubs.acs.org on August 28, 2019

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ACS Applied Materials & Interfaces

Thermoelectric Generator Using Space Cold Source Zhilin Xia1*, Zhenfei Zhang1, Zhenghua Meng2*, Liyun Ding3, Zhongquan Yu1 1State

Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, Wuhan, Hubei 430070, PR China. 2School of Automotive Engineering, Wuhan University of Technology, Wuhan, Hubei 430070, PR China. 3National Engineering Laboratory for Fiber Optic Sensing Technology, Wuhan University of Technology, Wuhan, 430070, China.

ABSTRACT: Most of the renewable and sustainable natural energy are distributed uneven on the earth in time and space. Here we have proposed a new kind of thermoelectric generator, which can use the temperature difference caused by passive cooling via the atmospheric window. This generator can continuously output electric energy at anywhere 24-hours a day independent on the existence of any natural or manmade energy resource. A test generator with two couples of n-p thermoelectric legs has been prepared. The created average temperature difference is 4.4K and average voltage is 1.78mV in a whole day. This design paves a path to the pollution-free and sustainable power generation which is not restricted by time, space and not consuming any existing energy resource. KEYWORDS: thermoelectric generator, space cold source, radiative cooling, Bi2Te3, Sb2Te3, silica film 1. INTRODUCTION Accompany with the increase of world’s population and the development of the technology, the energy consumption increases quickly and causes the environment degradation. Currently commonly used fossil energy resources including oil, coal and natural gas were formed when prehistoric plants and animals died and were gradually buried by layers of rock over millions of years1. These energy resources are non-renewable and environmentally unfriendly. As the alternatives of fossil, energy resources such as tide, solar, wind, heat, acoustic and mechanical energy are renewable and sustainable

ACS Paragon Plus Environment

2-9.

However, most of these

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natural energy resources are distributed uneven on the earth in time and space. For instance, the solar energy is unavailable at the night, and the tidal energy only can be obtained along the coastal. Space cold source is even distributed in time and space. Passive radiative cooling is a method to use the space cold. It enables objects on earth to reduce their temperature, and even cool to below the ambient temperature. Radiative cooling works anytime and anywhere without consuming energy, and it has been studied theoretically and experimentally for decades 10-11. Sub-ambient cooling experiments were mostly achieved at night 12-14. Until recently, daytime radiative cooling has been realized by Fan15. Since then, various materials and surface structures were adopted for obtaining sub-ambient temperature 16-19. Besides the improvement of radiative cooling capacity, the application of the radiative cooling has also been developed 20-23.

Fan and his team have achieved energy conservation of an air conditioning system using

the cooled fluid. This system saves energy consumption with the cooperation of the coolers’ heat radiation effect 20. Erzhen Mu achieved an average voltage output of 0.18mv throughout the day using radiative cooling. This voltage is generated by a TEG consisting of 46,000 P-N modules in series24. Here, a new kind of thermoelectric generator has been proposed to use the radiative cooling caused temperature difference. This power generation can output electric energy anytime and anywhere independent on the existence of any natural or manmade energy resource. A test generator contains two couples of n-p thermoelectric legs has been prepared. At the both ends of its thermoelectric legs, the created average temperature difference is about 5.35K (night), 3.45K (daytime) and consequently generates 2.19mV, 1.36mV DC voltage, respectively. 2. RESULTS AND DISCUSSION The designed thermoelectric generator contains three main parts: the radiative cooler, the heat sink and the thermoelectric legs (Figure 1). When the generator is put outdoor, the radiative cooler emits heat to the outer space continuously and its temperature decreases below


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the ambient temperature. At the same time, the copper heat sink keeps temperature similar to that of the ambient due to its high efficient heat convection. The thermoelectric legs connect with the cooler and the heat sink at its two ends and hence a temperature gradient creates between them. Accompany with the heat conduction in the thermoelectric legs, an electric current generates due to the thermoelectric effect. This generator works independent on the existing of any energy resource. In order to improve the performance of the thermoelectric generator, its structure has been carefully designed. Firstly, aerogel felt was used to separate difference parts of the generator. The aerogel felt has low thermal conductivity, and it can reduce the heat transfer both between the radiative cooler and the heat sink, and between the thermoelectric legs and environment. Besides, the aerogel felt contains nano-silica particles, and has a high thermal emissivity. A layer of aluminum foil should be used to cover on the surface of the aerogel felt. This can reduce the radiative heat exchange both between the aerogel felt and the radiative cooler (Figure S6b), and between the aerogel felt and the sky. Secondly, for the radiative cooler that is used to emit heat to the out space, it was separated from the aerogel insulation chamber and only connected to the thermoelectric legs. This makes that two main kinds of heat fluency occur to the cooler: the heat conduction from the thermoelectric legs and the heat radiation to the sky. Thirdly, in order to getting heat from ambient quickly (Figure S6a), the copper heat sink was put on the frame and connects to the thermoelectric legs. Lastly, the key part of the generation, the thermoelectric legs, was put in the aerogel wall to reduce the heat exchange with the ambient. The thermoelectric legs were connected with the radiative cooler and the heat sink at their two ends for using the temperature difference. We have prepared a test generator. The used thermoelectric materials in the test generator are Bi2Te3 and Sb2Te3 films, which were screen-printed on the surface of kraft papers. The kraft paper has low thermal conductivity and it can reduce the heat waste. In order to increase the films electrical properties and thermoelectric figure of merit, the conductive polyurethane (cPU) was used as the binder of the thermoelectric powder, and the dosage of cPU has been optimized. For the Bi2Te3 and Sb2Te3 thermoelectric films, when the cPU content is about



28.6wt% and 16.7wt.% respectively, their ZT value and power factor are both the highest (Figure 2a-b). After screen-printing, the films were pressed to strongly adhere to the substrate. The used pressing pressure and time can also affect the films electrical properties, and have been optimized (Figure. S3-5). In the test generator, for preparing thermoelectric films, the mixing amount of 0.4g cPU in 1.0g Bi2Te3 and 0.2g cPU in 1.0g Sb2Te3 were adopted, and the pressing pressure, temperature and time were 15MPa, 353K and 25mins. The thermoelectric powders are homogeneously dispersed in the prepared films (Figure 2c-d). The electrical resistivity of the prepared Bi2Te3 and Sb2Te3 films are 0.093 and 0.057Ω·cm, the Seebeck coefficients of them are 85.9 and 56.5μV/K, and the thermal conductivities of them are 0.13 and 0.09W/m/K respectively (Table S1). Thereafter, the calculated ZT values of them at 300K are 17.6×10-3 and 19×10-3 (Table S1). In the test generator, the used radiative material for the cooler is silica films, which was screen-printed on a sheet of aluminum foil. The silica film has a nano-sized particle stacked structure (Figure 3a-b). Nanoparticle packed films has very small thermal conductivity and hence small heat convection coefficient. This is help for the film to keep its low temperature. While this also causes an uneven temperature distribution in the film. The uneven distributed temperature is harmful for improving the temperature gradient in the thermoelectric legs. Therefore, we use aluminum foil as the substrate to reduce temperature gradient in the radiative cooler. Silica has an ignorable absorption in the solar radiation band and little solar energy will deposit in the silica films 25. For transmission test, the films substrate was substituted, and a new silica film was printed on a polyethylene substrate with thickness of 0.2mm. This film has high transmission in the solar radiation band, and the particles with diameter of 200nm can efficiently scatter ultraviolet ray in solar (Figure 3c). Besides, the used silica particles has a high thermal emissivity at the wavelength near 9.08μm (Figure 3d), which locates in the atmosphere’s window and the heat radiative band of a body with a temperature near 300K. Thereafter, the prepared silica film can emit its heat to the outer space efficiently and reduce its temperature below that of the ambient. The cooling curve of the cooler is shown in Figure S1. At night, the average temperature drop of the cooler is 7.06K and the maximum temperature drop is 8.9K.The practical application performance of the prepared test generator has been


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verified in a partly cloudy day in Kunming (North latitude 25°02'11 ", east longitude 102°42'31"), on 16-17th May, 2019. The used thermoelectric legs have size of 4.0×0.3cm2. The used radiative cooler has size of 5.0×5.0cm2. The thickness of the aerogel frame is 1.0cm. The temperature probes were pasted on the electrodes with 2.0cm distance. The temperature and voltage testing connection diagram is show in Figure 4a. The test generator are shown in Figure 4b. When a generator with two couples of n-p legs was used (Figure 4c), its total resistance is 1.015kΩ. The temperature difference at the two ends of the legs can reach an average of 3.45K in the daytime. The corresponding average generation voltage is 1.36mV. At night, the temperature difference can reach an average of 5.35K. The corresponding average generation voltage is 2.19mV. In a whole day, the average generation voltage is 1.78mV, and the maximum output power can reach 0.78 nW under the condition of an average temperature difference of 4.4K. This voltage is generated independent on the existing of nature or manmade energy. The properties of the designed generator have a lot of improvement space by increasing the temperature difference and the thermoelectric figure of merit. To increase the temperature difference, the temperature of the radiative cooler should be lower. The maximum temperature reduction in experiment is about 42K obtained by Fan 16. If an ideal cooler has been used and better thermal isolation has been adopted, higher temperature reduced of 135K can be obtained (Figure S7). In this case, the corresponding Carnot cycle efficiency is 45.0% at the temperature of 300K. To improve the generation efficiency, the thermoelectric figure of merit should be improved. If it reaches to 4.0, the generation efficiency of a couple of n-p legs would reach to 4.85% and 20.56% when the temperature difference is 35 and 135K respectively (Figure S8). The output power density of our designed generator would reach to 0.9 and 2.84 W/m2 when the temperature difference is 35 and 135K respectively (Figure S9). 3. CONCLUSIONS The proposed thermoelectric generation uses the temperature difference caused by passive radiative cooling. This temperature difference is in essentially derived from the difference between the atmosphere temperature and the space temperature. The always-existing



characteristic of this temperature difference determines that this electric generation is renewable and sustainable. It is also a kind of environmentally friendly energy because it releases nothing but heat radiation wave. This electric generator works not relying the existence of natural or man-made energy resources, such as solar, wind, tide, acoustics and kinetic energy. It can be used at any time, 24-hours a day and prospectively four seasons a year. This generator has a simple structure and the process of preparing thermoelectric films as well as radiative cooler is not complicated. It is easy to apply on a large scale. This generator has a broad application prospect in the regions of ocean, desert, mountain, polar region, et al. In these areas, it is unrealistic to transport the electric power for supporting the electricity instruments. While, using fuel generator, batteries, etc. needs labor cost and fuel supply, using natural energy is limited by time and space.

ASSOCIATED CONTENT Supporting Information The thermoelectric generator preparation and characterization ， the radiative cooler preparation modeling and anticipating, and additional experimental results

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected](Z.X) *E-mail:[email protected](Z.M) ORCID Zhilin Xia: 0000-0003-2457-1783. Author Contributions Z.Xia proposed the idea and conceptual design of thermoelectric generator and performed the


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theoretical work. Z.Xia, Z.Meng, L.Ding and Z.Zhang proposed the strategy for experimental design and analysed the data. Z.Xia and Z.Zhang completed the writing of the paper. Z.Zhang and Z.Yu performed the experimental work. Notes The authors declare no competing financial interests.

ACKNOWLEDGMENTS The research was supported by the Open Research Fund of Key Laboratory of Material for High Power Lasers - Chinese Academy of Sciences.

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Figure 1| The working principle and structure scheme of the designed thermoelectric generator. (a) the top view; (b) the main view.


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Figure 2| The thermoelectric films and its properties. (a) The ZT value and power factor of Bi2Te3 films; (b) The ZT value and power factor of Sb2Te3 films. For all films, 1.0g of Bi2Te3 powders or Sb2Te3 powders was used. (c) The scan electron microscope photographs of the Bi2Te3 films; (d) The scan electron microscope photographs of the Sb2Te3 films.



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Figure 3| The radiative cooler and its properties. (a) Schematic diagram of silica film’s microstructures; (b) The scan electron microscope photographs of the silica film. Here the silica film was printed on a sheet of Aluminium foil with thickness of 0.01mm; (c) The transmission spectrum of the silica films in the solar radiation band. Here the silica films was printed on a polyethylene substrate with thickness of 0.2mm; (d) The absorption spectrum of the used silica particles in the thermal radiation band.


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Figure 4| The generator and its properties through a 24-h day-night cycle. (a) Thin temperature and voltage testing connection diagram; (b) The physical photos of the thermoelectric generator; (c) The evolution of the solar power, voltage and temperature difference in a whole day. The test was performed in a partly cloudy day in Kunming (North latitude 25°02'11 ", east longitude 102°42'31"), on 16-17th May, 2019.



Graphic for manuscript


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Thermoelectric Generator Using Space Cold Source

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