Eclipsing the Limitations of Solar Energy

On April 8, the moon’s orbit crossed directly between the Earth and the Sun, completely eclipsing the burning star for about four minutes as the two orbs hovered over the U.S., shrouding parts of the world in eerie gray darkness for multiple hours. 

As people donned special glasses and oohed and aahed at the cosmic marvel, power grid workers and electrical engineers worked to keep the power running in the absence of solar energy. 

According to the U.S. Energy Information Administration, the eclipse partially or completely blocked sunlight to solar generation facilities with a combined 91.3 gigawatt-hours of capacity during peak solar generating time. To put it in perspective, one gigawatt-hour could power about 90,000 homes in the U.S. for a month. 

States in a northeast trajectory from Texas to Maine were affected the worst due to their position in the path of totality. During the eclipse, Texas was expected to lose between 90 and 99% of solar power generation, which makes up about 30% of the state’s electricity, and rely on other methods of electricity generation like gas, wind and coal. 

Solar panels' inability to generate electricity when the sun isn’t shining directly on them is an issue that researchers like Professor of Electrical and Computer Engineering Jeremy Munday and Professor of Materials Science and Engineering Marina Leite, both of UC Davis, are tackling in the clean energy sphere are tackling in the clean energy sphere. 

Recently, this research team has proposed a solution that would enable solar panels to produce power when the sun is not present and operate at more than 50% efficiency. Basically, the panels would be able to convert more than 50% of the energy shining on them into usable energy, a significant upgrade from the technology’s current conversion efficiency of about 20%. 

Let There Be Light and Heat

Most solar panels that can be seen on rooftops or in solar farms are made of photovoltaic cells that are made up of semiconductor materials. The cells receive photons, or light particles, from the sun and create an electric current through those materials However, there is a wide spectrum of light emitted by the sun that the panels cannot use efficiently on their own. 

The proposal: create a thermophotovoltaic device with an optical emitter to convert the sun’s heat into a concentrated light spectrum that can then be transformed into usable energy. 

“A traditional photovoltaic cell converts sunlight into electricity, but a large portion of the sun’s spectrum is not efficiently used,” Leite said. “In the case of a thermophotovoltaic device, the conversion is from heat to electricity. The thermal radiation emitted from an object called the optical emitter is absorbed by the conventional photovoltaic device.” 

The optical emitter comprises a heat source — in this instance, a rod of material like tungsten or silicon carbide — that will absorb the heat from the sun. Scott McCormack, an assistant professor of materials science and engineering, provided insight into which materials could withstand the high-heat input. The heat is converted into light, and thin films layered on top of one another shape the emission spectrum. The photovoltaic cells then absorb the light from the optical emitter more efficiently than sunlight. 

After running calculations and simulations that revealed the thermophotovoltaic device as a promising solution to solar power inefficiencies, Leite and her lab fabricated the first round of possible optical emitters in the Center for Nono-MicroManufacturing at UC Davis for testing. Those initial emitters were then heated up to 1,500 degrees Celsius, and light shaped by thin films was emitted. 

The researchers are embarking on a second round of testing newly designed emitters with an engineered photonic filter that reflects heat back to the source and takes only the energy that is needed to create the light spectrum. They are using a carbon source to heat the emitter up to 1,350 degrees Celsius and will investigate the amount of light that reaches the photovoltaic cells and the output power from the cell using current-voltage measurements. 

“When the system is well-engineered, it will allow us to shape the spectrum in a way that minimizes the inefficient aspects of conventional solar cells and maximizes the power of the sun,” said Leite. 

Power Shift 

The filter paired with the optical emitter will also facilitate energy conversion when the sun is down. That’s because when it receives energy from the sun or other heat source, the filter reflects it back to the source and stores the rest of the energy to be used later. This could be done by covering the filter and emitter and uncovering them when energy is needed, much like when a dish of food is covered with foil to keep the heat in when it’s reheated. 

Of course, the ability to generate electricity when the sun is down could drastically increase the effectiveness of solar panels and could be a huge boon to the clean energy field. 

“In California, we have almost 10 times as much installed solar power as we did 10 years ago, which is amazing,” Munday said. “Because our technology uses a local heat source, we can help shift the power from daytime to nighttime, which could have a huge impact on further increasing the usefulness of solar energy.” 

The team is confident this system has the potential for 50% efficiency, which could fundamentally change how solar energy is harvested and stored, and steer society further toward clean energy. 

“Climate change is one of the biggest problems facing humanity today, and finding alternative power conversion technologies that can help address this problem is crucial,” said Munday. “This particular technology is an excellent new option. Together, we are super excited to see where it takes us.” 

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