Passive Self‐Sustained Thermoelectric Devices Powering the 24 h Wireless Transmission via Radiation‐Cooling and Selective Photothermal Conversion

Abstract The rapid development of the Internet of Things has triggered a huge demand for self‐sustained technology that can provide a continuous electricity supply for low‐power electronics. Here, a self‐sustained power supply solution is demonstrated that can produce a 24 h continuous and unipolar electricity output based on thermoelectric devices by harvesting the environmental temperature difference, which is ingeniously established utilizing radiation cooling and selective photothermal conversion. The developed prototype system can stably maintain a large temperature difference of about 1.8 K for a full day despite the real‐time changes in environmental temperature and solar radiation, thereby driving continuous electricity output using the built‐in thermoelectric device. Specifically, the large output voltage of >102 mV and the power density of >4.4 mW m−2 could be achieved for a full day, which are outstanding among the 24 h self‐sustained thermoelectric devices and far higher than the start‐up values of the wireless temperature sensor and also the light‐emitting diode, enabling the 24 h remote data transmission and lighting, respectively. This work highlights the application prospects of self‐sustained thermoelectric devices for low‐power electronics.


Note S1. Working principle for the PSS-TE devices
First, the internal mechanism of the radiation-cooling effect is introduced.At night, the net radiant cooling capacity Pnet of the radiation cooling film can be expressed as the difference between the energy radiated outward (Prad) and the energy received from the environment (Pen): ( 1 where Ac is the area of radiation cooling film, Ibb is the spectral intensity, ε is the material emissivity, and εatm is the atmospheric emissivity.It can be seen from the above formula that when ε(λ,θ) [1-εatm(λ,θ)] is large, a large Pnet can be obtained.That is, to maximally exert the role of the radiation-cooling effect, the emission spectrum of the material should match the atmospheric transmission spectrum.
In the daytime, it is necessary to consider the solar radiation.In this condition, the Pnet is: It can be seen that the emission spectrum of the material not only needs to match with the atmospheric window but also should have low emissivity in the solar spectral range in daytime conditions.
Based on these, the theoretical model of the PRC-TE device was further established.As shown in Fig. S1, solar irradiation energy is used as the energy source at the hot end of the TE device, and the cold end emits heat through the radiation-cooling effect.The energy balance at both ends is shown in the following formula: The Pnet,c of the cold end can be expressed as: The Pnet,h of hot end can be expressed as: The current I is:  Table S1.Dimensional parameters and material properties involved in the simulation process.

Parameters Values Source
The , )(, ) −   ∫  cos  ∫  ∞ 0   (  , )(, )  (, ) =   ∫  cos ∫  ∞ 0   (  , )(, )[1 −   (, )] ( w h i l e T c = T en) 7 ) where Ah are the areas of the hot end, Th and Tc are the temperatures of the hot and cold end, respectively, and the S, K, and R are the Seebeck coefficient, the thermal conductance, and the electrical resistance of the TE device.When the internal resistance r of the external equipment is equal to the internal resistance R of the thermoelectric device, the maximum output power can be obtained.Th and Tc, as well as the open-circuit valtage Voc and the maximum output power Pmax of the device, can be solved by combining the Equations.(3-7) and substituting the spectral characteristics, thermoelectric parameters, dimensions, and environmental factors of specific materials.

Figure S1 .
Figure S1.The schematic diagram of the energy conversion of the PSS-TE device.

Figure S2 .
Figure S2.COMSOL simulation analysis.A) The mapped Pmax under different εLW of the radiation-cooling film and εSW of the selective photothermal film.B) The mapped Pmax under different solar radiation density.

Figure S5 .
Figure S5.The spectral performance of the graphene and black chrome films.A) Transmission spectrum.B) Reflection spectrum.

Figure S6 .
Figure S6.The solar radiation intensity throughout the day for the test.

Figure S7 .
Figure S7.The test diagrams for the PSS-TE device supplying power to wireless temperature sensors.

Figure S8 .
Figure S8.The test diagrams for the PSS-TE device supplying power to the LED.