outdoor sensors
With the use of a wavelength-selective emitter to create constant
radiative cooling, thermoelectric devices can generate voltage continuously


Date:
July 7, 2020
Source:
American Institute of Physics
Summary:
Thermoelectric devices, which use the temperature difference between
the top and bottom of the device to generate power, offer some
promise for harnessing naturally occurring energy. Authors tested
one made up of a wavelength-selective emitter that constantly cools
the device during the day using radiative cooling. As a result, the
top of the device is cooler than the bottom, causing a temperature
difference that creates constant voltage through day and night.



FULL STORY ========================================================================== Sensors placed in the environment spend long periods of time outdoors
through all weather conditions, and they must continuously power
themselves in order to collect data. Many, like photovoltaic cells,
use the sun to produce electricity, but powering outdoor sensors at
night is a challenge.


========================================================================== Thermoelectric devices, which use the temperature difference between the
top and bottom of the device to generate power, offer some promise for harnessing naturally occurring energy. But, despite being more efficient
than photovoltaics, many thermoelectric devices flip the sign of their
voltage, meaning the electrical current changes the direction of its
flow, when environmental temperatures change, so the voltage drops to
zero at least twice a day.

"The sign of the thermoelectric device depends on the temperature
difference between the top and bottom of the device," author Satoshi Ishii said. "Cooling can be used to create a temperature difference compared
to the ambient temperature, and if there is a temperature difference, thermoelectric generation is possible." In a study published this
week in Applied Physics Letters, by AIP Publishing, the authors tested
a thermoelectric device made up of a wavelength-selective emitter that constantly cools the device during the day using radiative cooling, the dispersion of thermal energy from the device into the air. As a result,
the top of the device is cooler than the bottom, causing a temperature difference that creates constant voltage through day and night and
various weather conditions.

The authors compared a broadband emitter with a selective emitter,
showing the selective emitter avoids the problem of the voltage dropping
to zero during environmental changes in temperature.

"For the selective emitter, it is best to have emissivity close to unity
in the atmospheric window, approximately 8 to 13 micrometers, where the atmospheric transmittance is high and thermal emission can effectively
radiate into space, which in turn cools the device," Ishii said.

The device they tested is comprised of a 100-nanometer-thick aluminum
film on the bottom of a glass substrate. The authors discovered that
other sources of heat, such as the roof where a sensor might be mounted,
can augment its ability to generate voltage.

"A large temperature difference results in a large thermoelectric
voltage," Ishii said. "Using the heat on the backside of the device
makes the temperature difference between the bottom and top larger, so
heat from behind the device is beneficial for thermoelectric generation."

========================================================================== Story Source: Materials provided by American_Institute_of_Physics. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Satoshi Ishii, Thang Duy Dao, Tadaaki Nagao. Radiative cooling for
continuous thermoelectric power generation in day and night. Applied
Physics Letters, 2020; 117 (1): 013901 DOI: 10.1063/5.0010190 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/07/200707113225.htm

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