Inequality and climate change are bound to natural resource utilization. India is one of the world's most populated countries, making renewable energy sources the most suitable. One of the most conventional methods is converting the energy trapped from the sun directly to electrical power through the photovoltaic (PV) cells. The efficiency of a PV cell is determined by the amount of electricity produced, and it is characterized by the wavelength of light that the material can absorb and convert into electrical energy. If the semiconductor’s band gap matches the wavelengths of light shining on the PV cell, then that cell can efficiently make use of all the available energy.

Now the researchers have come up with a new thesis where they can trap the sunlight in the dead of night, and they are called a thermo-radiative diode that converts infrared heat into electricity. 

Structure of Solar Panel that Traps Sunlight in the Dead of Night

The main characteristics of solar panels are that they are straightforward to handle during the day. Now scientists have designed a new type of panel where they are able to trap the sunlight in the absence of sun. The thermo-radiative diode is similar to the technology used for night-vision goggles. It can generate up to 50 watts of power per square meter at night under certain conditions. That is about a quarter of a standard solar panel’s output during the day.

N.J. Ekins-Daukes, Associate Professor in the School of Photovoltaic & Renewable Energy Engineering, says, “In the late 18th and early 19th century, it was discovered that the efficiency of steam engines depended on the temperature difference across the engine, and the field of thermodynamics was born. The same principles apply to solar power the sun provides the hot source and a relatively cool solar panel on the earth’s surface provides a cold absorber. This allows electricity to be produced. However, when we think about the infrared emission from the earth into outer space, it is now the earth that is the comparatively warm body, with the vast void of space being extremely cold. By the same principles of thermodynamics, it is possible to generate electricity from this temperature difference too: the emission of infrared light into space.”

How Does the Thermo-Radiative Diode Work? How is it different from a Normal Solar cell?

As we know, the solar panel absorbs energy from the sunlight and converts it into electrical energy. It uses two different silicon semiconductor layers, Called P- N junctions. The N layer is doped with free-electron donor impurities. In contrast, the P layer is doped with the free-electron acceptor impurities, the spaces for the electrons to fit into, and in the middle, there's a depletion region, where those electrons and electron-accepting holes more or less eliminate one another, creating a barrier that stops all the N side's electrons from diffusing straight through to the P side. 

When the sunlight falls on the solar panel, the thermal energy in the incoming photon is absorbed in the silicon. Suppose an electron is absorbed in the depletion region and receives enough energy to jump the band gap between the two sides. In that case, it can jump out of its hole and be accelerated across to the N side, increasing the voltage potential between the two sides. Connecting the two sides in an external circuit can run the electrons back around to the P side and enable them to do the electrical work. 

Finally, we can conclude that the thermal energy that comes from the photon runs the whole process. It is just a one-way process. Solar radiation heats the earth during the daytime as our planets rotate around the sun. But the earth releases the energy at night in the form of infrared radiation.

The thermo radiative diode basically works inverse to solar cells, accepting thermal energy radiated upward from the earth into a colder area and turning the energy flow across that temperature differential into electrical potential. It's built using some of the same materials used in infrared night vision goggles.

Dr. Michael Nielsen, a lecturer, and researcher at UNSW's School of Photovoltaic and Renewable Energy Engineering, says, “It is truly the inverse of a conventional solar cell in its functionality, But it still uses a semiconductor P-N junction as the core of the device (just run in reverse).The idea that thermodynamically we can produce power via the emission of light rather than absorption can be a stumbling block for many, but much like a solar cell, what we ultimately have here is a heat engine, with the difference being swapping the power converter from the cold side (solar cell being on earth absorbing photons from the sun) to the hot side (thermoradiative diode being on earth emitting photons into the coldness of space).”

Scientists proclaim that whenever there is a flow of energy, they can convert it into different forms. The concept of the emissive energy harvester was proposed back in 2014, recorded as the first-ever made thermo-radiative diode that produced a measurable amount of energy. 

It is noticeable that there’s not much power at this stage. With a temperature differential of just 12.5 °C, the research team managed to measure a peak thermo-radiative electrical power density of 2.26 mW per square meter, with an estimated radiative efficiency of 1.8 percent. 

Prof Ekins-Daukes says, “Right now, the demonstration we have with the thermoradiative diode is relatively very low power. One of the challenges was actually detecting it. But the theory says it is possible for this technology to ultimately produce about 1/10th of the power of a solar cell.”

 

Besides, it is possible to use the same technology to generate more or less glows when observed from the thermal camera. This could include the energy from the industrial waste heat or creating bionic devices that run off the body's heat. 

Prof Ekins-Daukes says, “I think for this to be breakthrough technology, we shouldn't underestimate the need for industries to step in, and really drive it. I'd say there's still about a decade of university research work to be done here. And then it needs industry to pick it up. If industry can see this is a valuable technology for them, then progress can be extremely fast. The miracle of solar power today owes itself to world-renowned researchers like Scientia Professor Martin Green at UNSW, but also to industrialists who have raised large sums of money to scale up manufacturing.”