Passive Radiative Cooling : Stay Cool Without Electriciy

In a hot summer day we use lot of the cooling devices such as the electric fan, cooler, air conditioner (AC). all are consuming a large amount of electric energy and are become reason for increasing the electricity bills.these devices provide the instant cooling. but what happen if i says that we use such system that provide cooling for a long period of time at free of cost after its installation, you will says this is impossible  according to basic principle of thermodynamics the work done take place in removing heat from the body and this work done by the external electrical source by mean of the cooling devices.  so we need energy for cooling.

but this is possible to remove heating from a body without doing some extra work on it by the virtue of the property that the heat energy is transferred from hotter body to colder body. removal of heat from atmosphere by this method called the passive radiative cooling. 

before understing the radiative cooling process let understand how earth maintain their temperature. the earth is surrounded by the atmosphere, this atmosphere work like a blanket which allow to enter the sun light black body in it but not allow to pass the heat energy of earth, this is the reason for increasing temperature of earth. in summer more energy enter in the earth and trap there hence temperature increase highly.

in 1960s the scientist found something interesting about the transparency of atmosphere, they found that the radiation of wavelength between 8 to 13 micron (micrometre) can passed by the atmosphere to cold space. this is called the window of atmosphere. this transmission of radiation of selective wavelength by the atmosphere is called passive radiative cooling.

Passive radiative cooling

in this process e need the kind of material that absorbs the heat from the atmosphere and then emits this radiation in the form of the infrared radiation of wavelength 8 to 10 micron. this can happen as according to plank's law every physical body absorb and emits the radiation and this body is called the black-body and the radiation emitted by the black body is called the black body radiation.
this type of material can be achieved by the different method, the community of scientist are hardly doing their job to find such a materials the can use for radiative cooling in day time with more sufficiently. this kind of material formed by the that material which capable to reflects the most part of sun's radiation energy and also be a good absorber that absorb heating radiation and radiate it with the radiation of wlength of 8 to 13 micrometre. some substance like  HfO₂and SiO₂ that can reflect approx 97% of sunlight that fall on it.
there are design of the radiative cooling material one the most popular design are in following photo.

transparent radiative cooling film

the above material consist the transparent film of plastic and silvered surface for reflecting the light and glass bead for continuous emission of heat and infrared radiation. scientist of University of Colorado discover this material, they found that it is able to maintain the temperature 10 degree lower then around it.
the main conclusion of this topic is that passive radiative cooling is best way of cooling without consuming electricity it can also maintain the temperature of earth and can become the solution of the Global Worming.

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