Night sky effect
October 11, 2019•770 words
This TED talk from Aaswath Raman really puzzled me and made so optimistic and hopeful for our planet's future.
Issue
The alarming starting point is the vicious circle of cooling we are currently stucked in:
- We cool:
- To live and sleep comfortably in places where the heat can become unbearable.
- To keep our food longer
- To operate data centers
- The warmer it gets, the more we need to cool.
- Back to --> 1: vicious feedback loop.
Cooling energy counts to 17% of global electrical use and 8% of greenhouse gas (GHG) emissions. Demand may increase up to 6 times by 2050. Cooling systems may become the biggest contributors to GHG and electricity "consumers".
Solution recipe: Night Sky Effect = target atmosphere transmission window to benefit from space coldness and build a negative heat balance sheet thanks to thermal radiation.
Night Sky Effect is a natural phenomenon. This is how ice is made at night in the desert even though the air is warmer than 0° C.
How does it work?
1. Fourier's Law: heat follows negative temperature gradient
q =-k∇T
This law says that local heat flux densityqis equal to the product of thermal conductivitykand the negative local temperature gradient∇T.
In layman's terms it says that heat "flows" towards colder places. This law explains us a first thing: because the sky/space is cooler than the Earth. The heat "flows" from the Earth to Space.
What does "flowing" actually means?
2. Thermal radiation: heat "flows" as light
When matter particles get warmer, they start moving very fast at microscopic level (thermal motion). By doing so they also generate electromagnetic radiation = light.
This phenomenon can be visualized with night vision/thermal googles. Another evidence of this phenomenon is when a material changes color when it gets hot, like a piece a coal that gets orange/red.
So we have heat that flows from hot places (Earth) to cooler places (Space).
3. Absorption & Greenhouse Effect
Unfortunately, in the same way particles can emit energy as electric radiation, they can also "absorb" light and generate heat back!
Especially we say that some gases in the atmosphere "absorb" some of the heat that the Earth tries to "radiate" towards Space: we call these gases Green House Gas (GHG) and this effect the greenhouse effect.
4. Infrared Window
Luckily not all that heat is absorbed and reflected back! Otherwise the Earth would be much warmer than it is.
Light is made up of a range of different wavelengths: the electromagnetic spectrum. There are infrared, ultraviolet, X-Rays...
The atmosphere doesn't react the same way to all wavelengths. Especially the wavelengths between ~[8 μm, 30 μm] aren't reflected back: this is called the infrared transmission window.
So if an object emits its heat within that specific transmission window, we guarantee that its heat will go completely through the atmosphere: the object will get cooler! This how we can turn water into ice at night in the desert.
Why don't we already make use of this phenomenon to cool everything?! Because it is called NIGHT Sky Effect.
During the day, the sun heats all the objects (Earth) that we may want to cool so much that the overall heat balance-sheet gets positive again. The night sky effect is not strong enough to counter balance heating from sunlight during the day.
5. Nanophotonics
Wouldn't be cool to benefit from the Night Sky cooling Effect during the day?!
For this we need to "target" the infrared transmission window of the atmosphere.
Thanks to nanophotonics it is actually possible to design materials that radiate their heat precisely at the wavelengths that are best let out by the atmosphere.
It is like engineering a heat mirror: something that gets cooler when they receive sunlight! Or very counterintuitively, something that get cooler when it gets out of the shadow!
Applications
Manufacturing techniques to build these materials already exist. This is also what Aaswath Raman explains in his TED talk.
Cooling panels
Such materials can be used to build "cooling panels" that are placed in sunlight. It can already increases the efficiency of cooling systems by 12%. In the future, cooling systems may require no electricity at all!
Integration with solar cells.
Solar cells get less efficient when they get hotter. So by integrating such materials into solar panels, we can improve their efficiency.
Heat engines: "generate light from cold darkness of space".
One can imagine using the temperature delta between Earth and Space to generate electricity! Or generating electricity when solar panels can't work.