Ultrathin, Lightweight Solar Panels

A race is on in solar engineering to create almost impossibly-thin, flexible solar panels. Engineers imagine them used in mobile applications, from self-powered wearable devices and sensors to lightweight aircraft and electric vehicles. Against that backdrop, researchers at Stanford University have achieved record efficiencies in a promising group of photovoltaic materials. Chief among the benefits of these transition metal dichalcogenides – or TMDs – is that they absorb ultrahigh levels of the sunlight that strikes their surface compared to other solar materials.

Transition metal dichalcogenide solar cells on a flexible polyimide substrate

Imagine an autonomous drone that powers itself with a solar array atop its wing that is 15 times thinner than a piece of paper,” said Koosha Nassiri Nazif, a doctoral scholar in electrical engineering at Stanford and co-lead author of a study published in the Dec. 9 edition of Nature Communications. “That is the promise of TMDs.”

The search for new materials is necessary because the reigning king of solar materials, silicon, is much too heavy, bulky and rigid for applications where flexibility, lightweight and high power are preeminent, such as wearable devices and sensors or aerospace and electric vehicles.

Silicon makes up 95 percent of the solar market today, but it’s far from perfect. We need new materials that are light, bendable and, frankly, more eco-friendly,” said Krishna Saraswat, a professor of electrical engineering and senior author of the paper. While TMDs hold great promise, research experiments to date have struggled to turn more than 2 percent of the sunlight they absorb into electricity. For silicon solar panels, that number is closing in on 30 percent. To be used widely, TMDs will have to close that gap.

The new Stanford prototype achieves 5.1 percent power conversion efficiency, but the authors project they could practically reach 27 percent efficiency upon optical and electrical optimizations. That figure would be on par with the best solar panels on the market today, silicon included.

Moreover, the prototype realized a 100-times greater power-to-weight ratio of any TMDs yet developed. That ratio is important for mobile applications, like drones, electric vehicles and the ability to charge expeditionary equipment on the move. When looking at the specific power – a measure of electrical power output per unit weight of the solar cell – the prototype produced 4.4 watts per gram, a figure competitive with other current-day thin-film solar cells, including other experimental prototypes. “We think we can increase this crucial ratio another ten times through optimization,” Saraswat said, adding that they estimate the practical limit of their TMD cells to be a remarkable 46 watts per gram.”

Source: https://news.stanford.edu/

How to Predict Stress at Atomic Scale

The amount of stress a material can withstand before it cracks is critical information when designing aircraft, spacecraft, and other structures. Aerospace engineers at the University of Illinois Urbana-Champaign used machine learning for the first time to predict stress in copper at the atomic scale.

According to Huck Beng Chew and his doctoral student Yue Cui, materials, such as copper, are very different at these very small scales.

Left: Machine learning based on artificial neural networks as constitutive laws for atomic stress predictions. Right: Quantifying the local stress state of grain boundaries from atomic coordinate information

Metals are typically polycrystalline in that they contain many grains,” Chew said. “Each grain is a single crystal structure where all the atoms are arranged neatly and very orderly.  But the atomic structure of the boundary where these grains meet can be very complex and tend to have very high stresses.”

These grain boundary stresses are responsible for the fracture and fatigue properties of the metal, but until now, such detailed atomic-scale stress measurements were confined to molecular dynamics simulation models. Using data-driven approaches based on machine learning enables the study to quantify, for the first time, the grain boundary stresses in actual metal specimens imaged by electron microscopy.

“We used molecular dynamics simulations of copper grain boundaries to train our machine learning algorithm to recognize the arrangements of the atoms along the boundaries and identify patterns in the stress distributions within different grain boundary structures,” Cui said. Eventually, the algorithm was able to predict very accurately the grain boundary stresses from both simulation and experimental image data with atomic-level resolution.

We tested the accuracy of the machine learning algorithm with lots of different grain boundary structures until we were confident that the approach was reliable,” Cui explained. The task was more challenging than they imagined, and they had to include physics-based constraints in their algorithms to achieve accurate predictions with limited training data.

When you train the machine learning algorithm on specific grain boundaries, you will get extremely high accuracy in the stress predictions of these same boundaries,” Chew said, “but the more important question is, can the algorithm then predict the stress state of a new boundary that it has never seen before?” For Chew , the answer is yes, and very well in fact.

Source: https://aerospace.illinois.edu/

Thin Heat Shield For Superfast Aircraft

The world of aerospace increasingly relies on carbon fiber reinforced polymer composites to build the structures of satellites, rockets and jet aircraft. But the life of those materials is limited by how they handle heat.

A team of FAMU-FSU College of Engineering researchers from Florida State University’s High-Performance Materials Institute (HPMI) is developing a design for a heat shield that better protects those extremely fast machines. Their work will be published in the November edition of Carbon .

Right now, our flight systems are becoming more and more high-speed, even going into hypersonic systems, which are five times the speed of sound,” said Professor Richard Liang, director of HPMI. “When you have speeds that high, there’s more heat on a surface. Therefore, we need a much better thermal protection system.”

The team used carbon nanotubes, which are linked hexagons of carbon atoms in the shape of a cylinder, to build the heat shields. Sheets of those nanotubes are also known as “buckypaper,” a material with incredible abilities to conduct heat and electricity that has been a focus of study at HPMI. By soaking the buckypaper in a resin made of a compound called phenol, the researchers were able to create a lightweight, flexible material that is also durable enough to potentially protect the body of a rocket or jet from the intense heat it faces while flying.

Existing heat shields are often very thick compared to the base they protect, said Ayou Hao, a research faculty member at HPMI. This design lets engineers build a very thin shield, like a sort of skin that protects the aircraft and helps support its structure. After building heat shields of varying thicknesses, the researchers put them to the test.

One test involved applying a flame to the samples to see how they prevented heat from reaching the carbon fiber layer they were meant to protect. After that, the researchers bent the samples to see how strong they remained. They found the samples with sheets of buckypaper were better than control samples at dispersing heat and keeping it from reaching the base layer. They also stayed strong and flexible compared to control samples made without protective layers of nanotubes.

That flexibility is a helpful quality. The nanotubes are less vulnerable to cracking at high temperatures compared to ceramics, a typical heat shield material. They’re also lightweight, which is helpful for engineers who want to reduce the weight of anything on an aircraft that doesn’t help the way it flies.

How To Reduce Fuel Burn By 60 Percent In Future Planes

Boeing Co unveiled a speedier and higher-flying version of a concept plane on Tuesday aimed at sharply reducing fuel use thanks to its elongated ultra-light wings. The so-called Transonic Truss-Braced Wing (TTBW) aircraft boasts a 170-foot (52 meter) wingspan that sits atop the fuselage and is braced from underneath by a truss in a design reminiscent of biplanes from the early years of aviation. The world’s largest planemaker and U.S. space agency NASA have been studying the concept plane for nearly a decade as part of the Subsonic Ultra Green Aircraft Research program. Boeing unveiled a reconfigured model or prototype and artist’s rendering at an aerospace conference in San Diego.

Chicago-based Boeing said it tweaked the plane’s designs with an optimized truss and a modified wing sweep that allow it to fly at speeds of Mach .8, or about 600 miles (965 km) per hour, slightly faster than previous designs but on par with current passenger jetliners. Boeing said the jet ideally would reduce fuel burn by 60 percent compared to an aircraft in 2005, but said it did not have final data to compare the fuel savings to present-day aircraft.

Source: https://www.reuters.com/

Electric Powered Flight Ten Times Less Expensive

Consumer passenger flight could be the next industry that’s transformed by electric powertrains, and Seattle’s Zunum Aero wants to be at the forefront of that change. The Seattle-based company, which is backed by Boeing’s HorizonX fund and Jet Blue’s Technology Ventures, has a plan to change the fundamental economics of regional flight, and shift the economics of air travel on a path towards eventual fully electric flight.

The first Zunum aircraft is designed for regional service, with seating for 12 passengers and a delivery window starting in 2022. The economics are potentially game-changing, with operating expenses of around $260 per hour for the aircraft. With a max cruise stepped of 340 miles per hour (547 km/h) in the air, a take-off distance of 2,200 feet (671 meters), a total hybrid-electric range of 700 miles (1127 km), which it hopes to scale to over 1,000 (1610 km) )in time and 80 percent lower noise and emissions vs. traditional regional planes, Zunum is position its airplane as the perfect way to light up under-utilized regional airports across the U.S., providing affordable and efficient commuter flights where economic realities have made running regular service impractical.

In the past, very intentionally, we were quiet about operating costs, because it’s just shockingly low what you can get with an electric. So that you can get an aircraft of a size that could never compete with an airliner that can get you below commercial fares,” Zunum Aero CEO Ashish Kumar told in an interview. He put the cost per seat operating expenses at around 8 cents per mile. “That’s about one-tenth the operating cost of a business jet per hour,” he said.

Source: zunun.aero