Since the Stone Age, mankind was spending energy during the dark night, either for heat or light. What if one could turn this upside down & create energy out of the darkness? Wouldn’t that be a game changer? Recently a test was conducted on the rooftop of Stanford University on generating electricity in the darkness. There is a long journey of research behind it, which we will discuss in this article. The research work which also resulted in a startup bringing this work to the market.
In the start of the research  the aim was to develop a product or solution which can increase the efficiency of the existing and upcoming cooling system. According to International energy agency the number of air-conditioning units installed globally is set to jump from about 1.6 billion today to 5.6 billion by the year 2050, almost 250% change in the current scenario. This increase in number of air conditioning units is going to come in developing countries, mostly in Asia and Africa. So, this would have a significant impact on the emissions caused by newly installed air conditioning units. This simply means as your fridge makes the room temperature warmer, and hence it justifies the statement “coolers makes the planet hotter.”
What’s the solution to temperature rise and how it can be controlled? Answer is Radiative cooling.
What is Radiative cooling?
Radiative cooling is how a body emits its radiation to the surrounding or we can also say how earth transfers its temperature to space. How radiative cooling will be a game changer is a very interesting point to understand. Imagine one of our conventional cooling or refrigeration systems, which takes in electricity and provides us the desired output of cooling or refrigeration need. However, when it is serving the purpose of cooling or refrigerating, it radiates some amount of heat to its surroundings which in greater numbers could add to global warming. So, here we are talking about a system which will boost the efficiency of refrigeration and the cooling system.
How does this technology works?
Skycool systems, a California based startup, has been successful in inventing a multijunction wafer which can provide temperature lower than the ambient air temperature under the direct sunlight. For an example, if the temperature outside is 35°C and you place the radiative cooling wafer outside, so on the surface of the wafer you will find the temperature to be 4-7 °C less. These are just the testing values. As per the claims, this temperature difference can be maintained as much as 42°C. So what is so unique about this system? The uniqueness of this system is that “it does not get heated up under the direct sunlight”. Hence, there is no chance of emitting radiation to the atmosphere. 
This multi layered wafer (1.8 microns thick) is made up of Silicon, Titanium, Silver, Silicon dioxide, Hafnium dioxide and one more layer of Silicon dioxide. 
Solar PV cells produce energy by being exposed to direct sunlight, while high temperature reduces their power efficiency. This wafer can be applied on top of the solar cells, and hence will increase the power output of the solar cell by maintaining cell temperature below the atmospheric temperature, that too passively.  Results of this innovation will be very interesting to see, as it would give a big boost to the Solar PV modules manufacturing industry.
On the other hand, one of the exciting outcomes of this multijunction wafer is the generation of electricity in the darkness. This electricity generation takes place in a modern version of a thermoelectric generator which harnesses the cold of space through radiative cooling to generate electricity. Unlike traditional thermoelectric generators, this system is made from a 200 mm aluminum disk which is painted in black color and connected to a commercial thermoelectric generator. This, then couples the cold side of the thermoelectric generator to a sky-facing surface that radiates heat to the cold of space and has its warm side heated by the surrounding air, enabling electricity generation at night. The electricity generated by this is 25 mW/m² and can be more than 0.5 w/m² by using existing commodity components.
Advantages & Disadvantages
This multi junction wafer is a 3 in 1 solution for the problems of modern community, which can be summarized as follows:
- It can boost the efficiency of the existing air conditioning and refrigeration system. How? By minimizing the hot air emissions of the systems.
- it can boost the efficiency of the solar cells by applying this wafer on top of the solar cells which would result in maintaining the cell temperature below the atmospheric temperature.
- It can generate low power electricity in a modern version of a thermoelectric generator which harnesses the cold of space through radiative cooling to generate electricity.
Elements and compounds used (Si, Ti, Ag, SiO2, and HfO2) for the fabrication of this wafer are used extensively in the semiconductor market and is a positive point for the manufacturing in large scale. Also, this off the shelf thermoelectric generator can be used for research work in remote places like polar region, where it can be used to provide power to low power sensor to record values for long periods of time. It would be interesting to know its application in study of space.
Few commercial aspects which need to be kept in mind are:
- This product will be an add-on in the existing system (refrigeration/cooling or solar cells) which comes with a cost, so it would be interesting to see how it can be made affordable for the customers.
- Generating electricity in the darkness, which comes with its own challenges like low power output as compared to solar panels which can generate hundred watts of power for the same size. With its existing power output it can be used to power lower LEDs and sensors, but it cannot be directly used in off grid systems because of its low power. Hence, there is a long way for the improvement of this system to achieve a commercially viable results.
Idea of employing radiative cooling for these existing energy equipment seems to be very promising and holds the true potential for providing the necessary boost in the efficiency of the system to fight against climate change. A lot of effort needs to be made for research and development and to make it commercially viable.
-  Generating Light from Darkness, Aaswath P. Raman, Wei Li, Shanhui Fan – URL: https://www.cell.com/joule/pdf/S2542-4351(19)30412-X.pdf
-  TED Talks, How we can turn the cold of outer space into a renewable resource | Aaswath Raman, URL: https://www.youtube.com/watch?v=7a5NyUITbyk