Scientists Use ‘Thermal Trap Effect’ To Capture Solar Energy At Extreme Temperatures

In the global fight against climate change, decarbonizing electricity and transportation has been at the forefront. However, there’s an often-overlooked sector that accounts for a staggering 50 percent of the world’s energy consumption: industrial heat. The majority of this heat is generated by burning fossil fuels, contributing significantly to greenhouse gas emissions. But what if we could harness the power of the sun to provide this heat cleanly and efficiently? A new study by researchers at ETH Zurich in Switzerland has brought us one step closer to this goal.

The team has experimentally demonstrated a phenomenon known as the “thermal trap effect” at temperatures exceeding 1,832 degrees Fahrenheit. This is a significant milestone, as many industrial processes, such as the production of cement and metals, require heat at these high temperatures.

“To tackle climate change, we need to decarbonize energy in general,” says corresponding study author Emiliano Casati, from ETH Zurich. “People tend to only think about electricity as energy, but in fact, about half of the energy is used in the form of heat.” 

Thermal-trapping device reaching 1,050 degrees Celsius
Thermal-trapping device reaching 1,050 degrees Celsius. (CREDIT: Device/Casati et al.)

The thermal trap effect is a clever way of using certain materials to capture and retain solar energy. Some semi-transparent materials, like quartz and water, allow visible light from the sun to pass through them easily but strongly absorb the infrared radiation emitted by hot surfaces. This means that when these materials are exposed to concentrated sunlight, they can reach higher temperatures inside than on their surface.

In their experiments, researchers attached a quartz rod to an opaque absorber plate. When exposed to the equivalent of 135 times the intensity of normal sunlight, the absorber plate reached a scorching 1,922 degrees Fahrenheit, while the quartz rod’s front face remained at a relatively cool 842 degrees Fahrenheit. This significant temperature difference demonstrates the thermal trap effect in action.

“Previous research has only managed to demonstrate the thermal-trap effect up to 170 degrees Celsius (338°F),” explains Casati. “Our research showed that solar thermal trapping works not just at low temperatures, but well above 1,000 degrees Celsius. This is crucial to show its potential for real-world industrial applications.”

The implications of this discovery are far-reaching. By incorporating the thermal trap effect into the design of solar receivers, we can achieve higher operating temperatures and thermal efficiencies compared to conventional receivers. This could make solar-driven industrial processes more viable and cost-effective, accelerating the transition away from fossil fuels.

To further explore the potential of this technology, researchers developed a detailed 3D heat transfer model, which they validated against their experimental data. This model allowed them to simulate the performance of solar receivers utilizing the thermal trap effect under various conditions.

The results were promising. Solar receivers with thermal trapping could achieve target temperatures with higher efficiency and/or lower concentration ratios (the intensity of the focused sunlight) compared to conventional receivers. This is crucial because higher concentration ratios typically require more expensive solar collectors and can introduce technical challenges related to heat transfer and material durability.

For example, at a concentration ratio of 500, a conventional solar receiver would have a thermal efficiency of around 40 percent. By adding a 300-millimeter thick layer of quartz, the thermal efficiency could be increased to an impressive 70 percent. To reach similar performance without thermal trapping, the concentration ratio would need to be doubled to 1,000, which would significantly increase costs.

While there are still some engineering challenges to overcome, such as minimizing reflective losses at the air-quartz interface, the potential benefits of this technology are clear. By enabling more efficient and cost-effective solar-driven industrial processes, thermal trapping could play a key role in decarbonizing the heat sector and mitigating climate change.

“Energy issue is a cornerstone to the survival of our society,” concludes Casati. “Solar energy is readily available, and the technology is already here. To really motivate industry adoption, we need to demonstrate the economic viability and advantages of this technology at scale.”

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