Researchers have created the first heat-resistant emitters that could break new ground in terms of solar energy.
"The novel component is designed to convert heat from the sun into infrared light, which can [then] be absorbed by solar cells to make electricity -- a technology known as thermophotovoltaics," a Stanford University news release stated.
The problem was that past thermal emitters would fall apart at 2,200 degrees Fahrenheit; this new model can remain intact at temperatures as high as 2,500 Fahrenheit.
"This is a record performance in terms of thermal stability and a major advance for the field of thermophotovoltaics," Shanhui Fan, a professor of electrical engineering at Stanford University, said in the statement.
Most solar cells contain silicon semiconductors that convert sunlight directly into electrical energy. The silicone semiconductors can only respond to infrared light. This causes the cells to waste an exceptional amount of energy as high-energy light waves become excess heat and low-energy light simply flows through the solar panel.
"In theory, conventional single-junction solar cells can only achieve an efficiency level of about 34 percent, but in practice they don't achieve that," study co-author Paul Braun, a professor of materials science at Illinois, said. "That's because they throw away the majority of the sun's energy."
Thermophotovoltaic systems have an extra step between when the sunlight is absorbed, and when it is sent to the solar cell. The system contains an emitter that converts the heat to infrared light.
"Essentially, we tailor the light to shorter wavelengths that are ideal for driving a solar cell," Fan said. "That raises the theoretical efficiency of the cell to 80 percent, which is quite remarkable."
Thermophotovoltaic systems have not yet been able to achieve an efficiency level above eight percent. This has to do with a poor performance from that extra intermediate piece, which is usually made of a material called tungsten.
"Our thermal emitters have a complex, three-dimensional nanostructure that has to withstand temperatures above 1,800 F (1,000 C) to be practical," Braun said. "In fact, the hotter the better."
In past experiments, the emitter was destroyed when exposed to a temperature of 1,800 Fahrenheit. The team tried coating the tungsten emitter in a "nanolayer of a ceramic material called hafnium dioxide."
Once coated, the device was able to remain stable in 1,800-degree heat for up to 12 hours, and in 2,500 for one hour.
"These results are unprecedented," former Illinois graduate student Kevin Arpin, lead author of the study, said. "We demonstrated for the first time that ceramics could help advance thermophotovoltaics as well other areas of research, including energy harvesting from waste heat, high-temperature catalysis and electrochemical energy storage."
The team will work to see if other types of ceramic are more effective in protecting the solar cell.
"We've demonstrated that the tailoring of optical properties at high temperatures is possible," Braun said. "Hafnium and tungsten are abundant, low-cost materials, and the process used to make these heat-resistant emitters is well established. Hopefully these results will motivate the thermophotovoltaics community to take another look at ceramics and other classes of materials that haven't been considered."
