The concept of night sky cooling or radiative cooling is accepted as a very important natural phenomenon by meteorologists and climate scientists. The night sky cooling system operates with the exact opposite concept of solar panels. Unlike the solar collectors, a night sky cooling system would lose heat to the outer atmosphere instead of collecting the heat of the sun.
Have you ever heard about making ice in desert climate? As impossible as it might sound, this was actually done by ancient Iranian population, who used structures called “Yakhchal”, also known as Ice House. Currently, you can find ruins of many ice house structures in many parts of Iran and similar structures are also evident throughout the Middle East and all the way to Asian counties like China. So how did the Iranians and other ancient civilizations operate this ice house structure?
The ancient Iranians would pour water in the ice pool right after sunset. There was no use of any technology but purely atmospheric conditions. The water in the pool would freeze even though the temperature of the air was above the freezing mark around five degrees Celsius. The ice was then collected during early morning and stored in the Ice House for use during the summer months. If you have seen frost on grass during a clear night with above freezing air temperature, you have witnessed something very similar to what the ancient Iranians were leveraging for their ice house. So how does this actually happen? Evaporation could play a role in this process but it will not cause the water to freeze, there is another process at play that cooled down the water.
The concept of sky cooling is very similar to a very basic concept of cooling any hot object. For instance, if you leave a very hot coffee outside, the heat from the coffee will slowly flow somewhere cooler, in other words the air surrounding the coffee mug. In the case of the Iranian ice pool, the heat of the water is actually flowing to the cold of the space.
To understand how the cold of space could cool something down, we have to look at the concept of thermal radiation. Most of the materials that we know send out heat in form of light. You are sending sending out heat in form of infrared light to your surroundings. You have probably seen footage from thermal cameras, which show images that can visualize the amount heat in and around the objects. In case of the water pool, it is sending the heat up towards the atmosphere, which absorbs some of the heat and sends it back. The heating up of the atmosphere is also responsible for the greenhouse effect leading to climate change. However, what is more interesting is that our atmosphere doesn’t absorb all of heat and if it did then it would make our planet much warmer than it actually is. The atmosphere has this thing called the transmission window, which operates between specific wavelength from eight to thirteen microns. The transmission window allows some of the heat to escape in form of the infrared light. The heat escapes to a colder place near the top of the atmosphere and all the way out to the space, which is almost as cold as minus 270 degrees Celsius (or around 450 degrees Fahrenheit). Overall, the pool of water starts to freeze because it is able to send more heat out than the atmosphere sends back to it, which allows the pool to cool down below the temperature of its surroundings. This in a nut shell is known as radiative cooling or night-sky cooling.
The concept of radiative cooling has been explored before however there wasn’t any success around it because of the one big glaring issue, the Sun. This is why it was also called the night-sky cooling because the surface needs to be face the sky in order to send the heat out in the atmosphere. During the day, the heat of our Sun completely reverses the cooling effect and render the concept of radiative cooling useless for any useful applications during a hot summer day.
Looking at this problem, the researchers in nanophotonics and metamaterials at Stanford, have developed a way to make this work during the day as well. The researchers at Stanford have created a structure with materials at very small length scale that are smaller than wavelength of light itself. The material is forty times thinner than a human hair and it serves two main functions. First the nanomaterial is able to send out heat between the transition window of the atmosphere. Secondly, it serves a mirror and avoids the absorbance of heat from the sunlight. How do you know that this material even works? When the researchers put the material in direct sunlight on rooftop, it actually became colder within few minutes. The material will actually get colder when it is in direct sunlight. The data from these experiments show that the material is able to stay colder than the air temperature by almost five degrees Celsius or nine degrees Fahrenheit.
The Applications
The materials that were used to create this structure are actually available at large scale volume, which will make it very useful for real life applications. The obvious benefit of this technology is that we will be able to save energy during operation of heavy cooling systems like the refrigeration and air-conditioning systems. The worldwide usage of electricity is impacted greatly by cooling systems as they account for almost seventeen percent of total electricity usage. The electricity is used by cooling systems in big data centers (by Tech giants like Google), refrigerators in supermarkets, and air-conditioners in your household. These cooling systems also contribute towards the global greenhouse gas emissions as they account for eight percent of global emissions. Due to increasing demand of cooling systems in Asian and African countries, it is forecasted that the energy usage of cooling systems will grow significantly in the future (six fold by year 2050). With warmer climate conditions, the usage of cooling systems becomes a life necessity to ensure well-being and productivity of people. This creates a never ending feelback loop where the cooling systems that keep us cool during the warmer weather will also contribute towards the warmer climate with the greenhouse gas emissions.
With the skycooling technology, the researchers at SkyCool systems have developed fluid cooling panels that look like solar panels but with a completely opposite functionality. The cooling panels will actually cool the water using the nanomaterial that sends heat out instead of absorbing it. The cooling panels can be incorporate with condenser of a cooling system to improve efficiency. Recent field experiences in California have shown that the cooling panels can improve the efficiency of the cooling system by almost twelve percent.
In the future, this skycooling technology has the potential to be integrated with cooling systems of buildings to increase cooling efficiencies and reduce energy usage by almost two-thirds. With better engineering, we could design systems that do not rely on electricity at all, as the cooling panels have the potential to maintain temperatures of around forty-two degrees Celsius below the atmospheric temperature. In addition, the the nanomaterials can also be used with solar panels as they become very inefficient as they get hot under the sun. The integration of microstructures that maintain a low operating temperature of solar panels can also increase the efficiency in many ways.
The sky cooling technology holds the potential to change the cooling systems on earth by utilizing the coldness of space to manage the thermal radiation. The temperature difference between the earth’s atmosphere and space could be utilized to generate electricity using a heat engine. This skycooling technology could allow us to change the flow of energy and heat, which could generate light from darkness?
Source: SkyCool
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