Scientists develop heat-resistant materials that could vastly improve solar cell efficiency

Scientists have created a heat-resistant thermal emitter that could significantly improve the efficiency of solar cells.

The novel component is designed to convert heat from the sun into infrared light, which can than be absorbed by solar cells to make electricity – a technology known as thermophotovoltaics. Unlike earlier prototypes that fell apart at temperatures below 2200 degrees Fahrenheit (1200 degrees Celsius), the new thermal emitter remains stable at temperatures as high as 2500 F (1400 C).

“This is a record performance in terms of thermal stability and a major advance for the field of thermophotovoltaics,” said Shanhui Fan, a professor of electrical engineering at Stanford University. Fan and his colleagues at the University of Illinois-Urbana Champaign (Illinois) and North Carolina State University collaborated on the project. Their results are published in the October 16 edition of the journal Nature Communications.

A typical solar cell has a silicon semiconductor that absorbs sunlight directly and converts it into electrical energy. But silicon semiconductors only respond to infrared light. Higher-energy light waves, including most of the visible light spectrum, are wasted as heat, while lower-energy waves simply pass 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,” said study co-author Paul Braun, a professor of materials science at Illinois. “That’s because they throw away the majority of the sun’s energy.”

Thermophotovoltaic devices are designed to overcome that limitation. Instead of sending sunlight directly to the solar cell, thermophotovoltaic systems have an intermediate component that consists of two parts: an absorber that heats up when exposed to sunlight, and an emitter that converts the heat to infrared light, which is then beamed to the solar cell.

“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.”

So far, thermophotovoltaic systems have only achieved an efficiency level of about 8 percent, Braun noted. The poor performance is largely due to problems with the intermediate component, which is typically made of tungsten – an abundant material also used in conventional light bulbs.

“Our thermal emitters have a complex, three-dimensional nanostructure that has to withstand temperatures above 1800 F (1000 C) to be practical,” Braun explained. “In fact, the hotter the better.”

In previous experiments, however, the 3D structure of the emitter was destroyed at temperatures of around 1800 F (1000 C). To address the problem, Braun and his Illinois colleagues coated tungsten emitters in a nanolayer of a ceramic material called hafnium dioxide.

The results were dramatic. When subjected to temperatures of 1800 F (1000 C), the ceramic-coated emitters retained their structural integrity for more than 12 hours. When heated to 2500 F (1400 C), the samples remained thermally stable for at least an hour.

The ceramic-coated emitters were sent to Fan and his colleagues at Stanford, who confirmed that devices were still capable of producing infrared light waves that are ideal for running solar cells.

“These results are unprecedented,” said former Illinois graduate student Kevin Arpin, lead author of the study. “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.”

Braun and Fan plan to test other ceramic-type materials and determine if the experimental thermal emitters can deliver infrared light to a working 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.”

Problem-Solving Parrots Understand Cause and Effect

Scientists speculate two factors may influence why some animal species are smarter than others: the foraging behavior of a species (for instance, how cognitively demanding it is for the animals to obtain food) and the social complexity of the animals’ society.

A new study looked at problem-solving skills, which reflect animals’ ability to understand and solve a novel situation, and whether they’re related to a species’ social complexity or foraging ecology. Anastasia Krasheninnikova, Stefan Bräger, and Ralf Wanker of the University of Hamburg, Germany, tested four parrot species with different social systems and diets: spectacled parrotlets, green-winged macaws, sulphur-crested cockatoos, and rainbow lorikeets.

“One of the characteristics of complex cognition in animals is the ability to understand causal relationships spontaneously, and one way of testing this is asking the animal to obtain a reward that is out of reach,” says Krasheninnikova. She and her colleagues gave the birds five variations on a string-pulling task, involving strings that varied in their relationship to each other or to a food reward, to see whether the birds really understood the means-end relationship between the string and the food.

The first test was a basic string-pulling task in which the bird must figure out how to pull up a piece of food suspended from a perch by a single piece of string. Almost all the birds of all species solved this test immediately.
In the second task, there were two hanging strings, but only one was attached to a piece of food. If the bird really understood the string as a means to obtain the reward, it should pull only the rewarded string. Most of the birds (more than 75%) were able to solve this test on their first try.
To make sure that the bird really understood the functional relationship between food and string and was not just pulling the string closest to the food reward, the third task used a pair of crossed strings. In this test, pulling the string directly above the food would not result in obtaining the food, while pulling the further string that is actually attached to the food would. The spectacled parrotlets and rainbow lorikeets outperformed the macaws and cockatoos on this test, and only the parrotlets were able to figure out the test when the strings were the same color. Krasheninnikova says this study is the first to document a parrot species solving the crossed-strings task spontaneously.
The fourth task probed the flexibility of the bird’s behavior. The string was longer, so the bird could obtain the food from the ground rather than pulling the string up. Several members of all species adapted their problem-solving strategies by stopping string-pulling behavior and obtaining the food from the ground, but only the parrotlets and lorikeets clearly preferred the alternative strategy.
In the fifth and final task, there were two rewarded strings, but one had a gap between its end and the reward. Solving this task required the bird to understand the mere presence of the reward does not guarantee the reward can be obtained; the food had to be connected to the string to work properly. Parrotlets were the only species to successfully solve task five.

When Krasheninnikova and her colleagues compared their results to the birds’ lifestyles, they found the pattern in performance was best explained by differences in the species’ social structures rather than their diets.
Spectacled parrotlets performed best of the four species tested and they live in what’s known as a fission-fusion society. These birds live in large groups where they form different social subunits that split and merge, providing the opportunity for many different kinds of social interactions. They are also the only one of the four species tested to form crèches where young birds pass through the socialization process.
Green-winged macaws and sulphur-crested cockatoos live in small, stable family groups centered around a breeding pair and their offspring. These species failed tests four and five.
The social organization of rainbow lorikeets falls somewhere between the parrotlets and the macaws and cockatoos — as does their performance on the string-pulling tasks. Lorikeets live in social groups of 10-40 individuals, but do not form subunits such as crèches. They performed better than macaws and cockatoos, but not as well as parrotlets.
While these results support the social complexity hypothesis, the correlation between social structure and cognitive performance is mostly indirect. The reasoning behind the hypothesis is that living in social groups is cognitively demanding. “Individuals have to recognize group members and infer relationships among them,” Krasheninnikova says. “These demands favor the evolution of understanding functional relationships, such as which actions cause which outcomes.” Socially living animals might be able to apply this cause and effect thinking to their physical world as well as their social lives.