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By this time, we are all probably aware of how solar panels work-light particles from the sun is absorbed in the solar panel cells in order to generate direct current that will later on be converted to usable alternating current thus producing power for a household or an establishment.  And being able to utilize a renewable source of energy, solar panels are extremely prized, especially in off-grid locations.  However, there is one obvious disadvantage-it does not work during the day.  This means unproductive night hours for solar panels.

Just recently, the combined effort of scientists from the University of California Los Angeles and Stanford University have produced a nighttime generator, which uses the principle of radiative cooling, as they report in an issue of Joule.  This means that the device uses the temperature gradient between the Earth and space. 

During the day, the Earth absorbs heat from the sun.  At night, this heat is released by the Earth back into space.  This process sometimes makes the ground colder than the air, which explains why frost and dew form over the grass or on leaves over night.  This temperature difference between the air and any object exposed to the sky-that is, when there is no sunlight-is used to generate electricity.

Proving this technique did not take much investment.  All the researchers needed were a shoe box sized polystyrene unit covered in aluminum foil, a metal disk that was painted black, and the key component, a commercially bought thermoelectric generator that coupled the disk and the block.  All of these materials cost the researchers only about 30 dollars.  The image below shows the setup of this nighttime generator.

The thermoelectric generator produces electricity when a temperature difference is present between the two sides of the generator.  The upper side of the generator that faces the sky is attached to an aluminum plate sealed underneath a transparent material.  It is also surrounded with insulation to stop the process of heat transfer.  This plate is then kept cooler than the surrounding air as it dispels any heat it absorbs in the form of infrared radiation.  At the same time, the bottom side of the generator is attached to an aluminum plate that is exposed to the warm ambient air.  This would cause for the top plate to be cooler than the bottom during the night.  A figure below shows this mechanism.

To evaluate the effectiveness of their device, the researchers used it to light up an LED bulb.  The generator was able to produce about 25 milliwatts per square meter of the device.  Further research would determine what improvements need to be done in the design to increase this generation rate.

The researchers claim that when this device is scaled up, it could provide enough energy for lighting and supplying power for electronic devices and that this kind of clean energy is done in a completely passive way.