Efficient low-cost system for producing power at night

Researchers have designed an off-grid, low-cost modular energy source that can efficiently produce power at night. The system uses commercially available technology and could eventually help meet the need for nighttime lighting in urban areas or provide lighting in developing countries.

Although solar power brings many benefits, its use depends heavily on the distribution of sunlight, which can be limited in many locations and is completely unavailable at night. Systems that store energy produced during the day are typically expensive, thus driving up the cost of using solar power.

To find a less-expensive alternative, researchers led by Shanhui Fan from Stanford University looked to radiative cooling. This approach uses the temperature difference resulting from heat absorbed from the surrounding air and the radiant cooling effect of cold space to generate electricity.

In The Optical Society (OSA) journal Optics Express, the researchers theoretically demonstrate an optimized radiative cooling approach that can generate 2.2 Watts per square meter with a rooftop device that doesn’t require a battery or any external energy. This is about 120 times the amount of energy that has been experimentally demonstrated and enough to power modular sensors such as ones used in security or environmental applications.

“We are working to develop high-performance, sustainable lighting generation that can provide everyone — including those in developing and rural areas — access to reliable and sustainable low cost lighting energy sources,” said Lingling Fan, first author of the paper. “A modular energy source could also power off-grid sensors used in a variety of applications and be used to convert waste heat from automobiles into usable power.”

Maximizing power generation

One of the most efficient ways to generate electricity using radiative cooling is to use a thermoelectric power generator. These devices use thermoelectric materials to generate power by converting the temperature differences between a heat source and the device’s cool side, or radiative cooler, into an electric voltage.

In the new work, the researchers optimized each step of thermoelectric power generation to maximize nighttime power generation from a device that would be used on a rooftop. They improved the energy harvesting so that more heat flows into the system from the surrounding air and incorporate new commercially available thermoelectric materials that enhance how well that energy is used by the device. They also calculated that a thermoelectric power generator covering one square meter of a rooftop could achieve the best trade-off between heat loss and thermoelectric conversion.

“One of the most important innovations was designing a selective emitter that is attached to the cool side of the device,” said Wei Li, a member of the research team. “This optimizes the radiative cooling process so that the power generator can more efficiently get rid of excessive heat.”

The researchers demonstrated the new approach by using computer modeling to simulate a system with realistic physical parameters. The models reproduced previous experimental results faithfully and revealed that the optimized system designed by the researchers could come close to what has been calculated as the maximum efficiency using thermoelectric conversion.

In addition to carrying out experiments, the researchers are also examining optimal designs for operating the system during the day, in addition to nighttime, which could expand the practical applications of the system.

This work is supported by the U.S. Department of Energy under Grant No. DE-FG02-07ER46426.

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Materials provided by The Optical Society. Note: Content may be edited for style and length.

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X-rays reveal in situ crystal growth of lead-free perovskite solar panel materials

Lead-based perovskites are very promising materials for the production of solar panels. They efficiently turn light into electricity but they also present some major drawbacks: the most efficient materials are not very stable, while lead is a toxic element. University of Groningen scientists are studying alternatives to lead-based perovskites. Two factors that significantly affect the efficiency of these solar cells are the ability to form thin films and the structure of the materials in the solar cells. Therefore, it is very important to investigate in situ how lead-free perovskite crystals form and how the crystal structure affects the functioning of the solar cells. The results of the study were published in the journal Advanced Functional Materials on 31 March.

Photovoltaic cells that are based on hybrid perovskites were first introduced in 2009 and rapidly became almost as efficient as standard silicon solar cells. These materials have a very distinctive crystal structure, known as the perovskite structure. In an idealized cubic unit cell, anions form an octahedron around a central cation, while the corners of the cube are occupied by other, larger cations. Different ions can be used to create different perovskites.

Spin coating

The best results in solar cells have been obtained using perovskites with lead as the central cation. As this metal is toxic, tin-based alternatives have been developed, for example, formamidinium tin iodide (FASnI3). This is a promising material; however, it lacks the stability of some of the lead-based materials. Attempts have been made to mix the 3D FASnI3 crystals with layered materials, containing the organic cation phenylethylammonium (PEA). ‘My colleague, Professor Maria Loi, and her research team showed that adding a small amount of this PEA produces a more stable and efficient material,’ says Assistant Professor Giuseppe Portale. ‘However, adding a lot of it reduces the photovoltaic efficiency.’

That is where Portale comes in. Perovskites have been studied for a long time by Professor of Photophysics and Optoelectronics Maria Loi, while Portale developed an X-ray diffraction technique that allows him to study the rapid formation of thin films in real-time during spin-coating from solution. On a laboratory scale, the perovskite films are generally made by spin coating, a process in which a precursor solution is delivered onto a fast-spinning substrate. Crystals grow as the solvent evaporates. At the beamline BM26B-DUBBLE at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France, Portale investigated what happens during the tin-perovskite film formation.


‘Our initial idea, which was based on ex situ investigations, was that the oriented crystals grow from the substrate surface upwards,’ Portale explains. However, the in situ results showed the opposite: crystals start to grow at the air/solution interface. During his experiments, he used 3D FASnI3 with the addition of different amounts of the 2D PEASnI4. In the pure 3D perovskite, crystals started to form at the surface but also in the bulk of the solution. However, adding a small amount of the 2D material suppressed bulk crystallization and the crystals only grew from the interface.

‘PEA molecules play an active role in the precursor solution of the perovskites, stabilizing the growth of oriented 3D-like crystals through coordination at the crystal’s edges. Moreover, PEA molecules prevent nucleation in the bulk phase, so crystal growth only takes place at the air/solvent interface,’ Portale explains. The resulting films are composed of aligned 3D-like perovskite crystals and a minimal amount of 2D-like perovskite, located at the bottom of the film. The addition of low concentrations of the 2D material produces a stable and efficient photovoltaic material, while the efficiency drops dramatically at high concentrations of this 2D material.


The experiments by Portale and Loi can explain this observation: ‘The 2D-like perovskite is located at the substrate/film interface. Increasing the content of the 2D material to above a certain amount causes the formation of an extended 2D-like organic layer that acts as an insulator, with detrimental effect for the device’s efficiency.’ The conclusion of the study is that the formation of this insulating layer must be prevented to achieve a highly efficient and stable tin-based perovskite. ‘The next step is to realize this, for example by playing with solvents, temperature or specific perovskite/substrate interactions that can break up the formation of this thick insulating layer.’

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Oblique electrostatic inject-deposited TiO2 film for efficient perovskite solar cells

The need to efficiently harvest solar energy for a more sustainable future is increasingly becoming accepted across the globe. A new family of solar cells based on perovskites — materials with a particular crystal structure — is now competing with conventional silicon materials to satisfy the demand in this area. Perovskite solar cells (PSCs) are continually being optimized to fulfill their commercial potential, and a team led by researchers from Kanazawa University has now reported a new and simple oblique electrostatic inkjet (OEI) approach to deposit a titanium oxide (TiO2) compact layer on FTO-pattern substrates without the need for a vacuum environment as an electron transport layer (ETL) for enhancing the efficiency of PSCs. The findings are published in Scientific Reports.

The PSCs comprise a stack of different component layers that all have a specific role. The ETL, which is often composed of TiO2, enables the transport of electrons — which carry charge — to the electrodes, while blocking the transport of holes — which can recombine with electrons to prevent their flow. Establishing a complete TiO2 layer with the correct thickness, which is uniform and free of flaws, is therefore critical to producing efficient solar cells.

Many of the numerous TiO2 deposition techniques reported to date have associated limitations, such as poor coverage or reproducibility, or being unsuitable for scale-up. They can also require challenging preparation conditions such as a vacuum. The researchers report a simple, low-cost OEI-method that achieves a compact layer without requiring a vacuum.

“Our technique can produce uniform electron transport layers whose thickness can be varied by controlling the deposition time.” Study lead author Assistant Professor Dr. Md. Shahiduzzaman explains. “Solar cells made using our approach had power-conversion efficiencies of up to 13.19%, which, given the other advantages of our technique, is very promising for scale-up and commercialization.”

The technique is based on the deposition of positively charged droplets that are attracted to a negatively charged surface. Previous reports using the same electrostatic approach achieved lower power-conversion efficiencies because the droplets formed a stack on the surface as a result of gravity. Introducing an oblique angle into the process — spraying the TiO2 precursor at 45° to the surface — eliminated the effect of gravity, leading to the deposition of a more uniform layer.

“An optimum ETL deposition method must offer a number of properties to result in a high efficiency solar cell,” Dr. Shahiduzzaman explains. “The ability to control the layer thickness and achieve a uniform, reproducible layer at low cost, without the need for a vacuum, provides a unique package of advantages that has not been reported to date. We hope that these properties will lead to effective and commercially relevant scale-up that will contribute to the drive towards cleaner energy worldwide.”

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Researchers design an improved pathway to carbon-neutral plastics

Researchers from U of T Engineering and Caltech have designed a new and improved system for efficiently converting CO2, water, and renewable energy into ethylene — the precursor to a wide range of plastic products, from medical devices to synthetic fabrics — under neutral conditions. The device has the potential to offer a carbon-neutral pathway to a commonly used chemical while enhancing storage of waste carbon and excess renewable energy.

“CO2 has low economic value, which reduces the incentive to capture it before it enters the atmosphere,” says Professor Ted Sargent, the U of T Engineering lead on the project. “Converting it into ethylene, one of the most widely-used industrial chemicals in the world, transforms the economics. Renewable ethylene provides a route to displacing the fossil fuels that are currently the primary feedstock for this chemical.”

Last year, Sargent and his team published a paper in Science describing how they used an electrolyzer — a device that uses electricity to drive a chemical reaction — to convert CO2 into ethylene with record efficiency. In this system, the three reactants, CO2 gas, water and electricity, all come together on the surface of a copper-based catalyst.

Though the device was a breakthrough for the team, there was still room for improvement. The latest version, described in a paper published today in Nature, further modifies the catalyst in order to enhance the system’s performance and lower its operating cost.

“One of the challenges with this reaction is that while some of the CO2 is converted into ethylene, most of it turns into side products, especially carbonate, which dissolves on the liquid side of the electrolyzer,” says post-doctoral fellow Fengwang Li, lead author of the new paper. “This undesired loss increases the cost of ensuing product separation and purification.”

In the latest work, Sargent’s team partnered with Caltech chemistry professors Jonas C. Peters and Theodor Agapie. Their published research on a class of molecules known as arylpyridiniums suggested that adding them to the catalyst could favour the production of ethylene over other side products.

Using theoretical calculations and experiments, the two teams sifted through more than a dozen different kinds of arylpyridiniums before selecting one. Sure enough, adding a thin layer of this molecule to the copper catalyst surface significantly increased the selectivity of the reaction for ethylene. It also led to another benefit: lowering the working reaction pH from basic to neutral.

“The previous system required the water side of the reaction to be at high pH, very basic conditions,” says Li. “But the reaction of the CO2 with caustic soda in the water lowers the pH, so we would’ve had to continuously add chemicals to keep the pH up. The new system works just as well under neutral conditions, so we can eliminate that additional cost, as well as loss of CO2 in the form of carbonate.”

The improved catalyst also lasted longer than the previous version, remaining stable for nearly 200 hours of operation. Another enhancement — increasing the area of the catalyst surface by a factor of five — gave the teams a taste of the challenges that will need to be overcome in order to scale production up to industrial levels.

While the prototype is still a long way from commercialization, the overall concept offers a promising way to address several key challenges in sustainability. It eliminates the need to extract more oil to make plastics and other consumer goods based on ethylene, and it turns waste CO2 into a feedstock, adding a new incentive to invest in carbon capture.

Li also points out that such a system could be powered by intermittent renewable sources, such as wind or solar power. Currently, there is often a mismatch between the amount of electricity produced by these systems and consumer demand. By storing excess electricity in the form of ethylene, the system offers a way to smooth out those peaks and valleys.

“What’s great about this CO2-to-ethylene conversion system is that you don’t need to choose between capturing and recycling CO2 emissions versus trying to prevent them from occurring in the first place by displacing the used fossil fuels,” says Li. “We can do both at the same time.”

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New material captures carbon dioxide and converts it into useful chemicals

A new material that can selectively capture carbon dioxide (CO2) molecules and efficiently convert them into useful organic materials has been developed by researchers at Kyoto University, along with colleagues at the University of Tokyo and Jiangsu Normal University in China. They describe the material in the journal Nature Communications.

Human consumption of fossil fuels has resulted in rising global CO2 emissions, leading to serious problems associated with global warming and climate change. One possible way to counteract this is to capture and sequester carbon from the atmosphere, but current methods are highly energy intensive. The low reactivity of CO2 makes it difficult to capture and convert it efficiently.

“We have successfully designed a porous material which has a high affinity towards CO2 molecules and can quickly and effectively convert it into useful organic materials,” says Ken-ichi Otake, Kyoto University materials chemist from the Institute for Integrated Cell-Material Sciences (iCeMS).

The material is a porous coordination polymer (PCP, also known as MOF; metal-organic framework), a framework consisting of zinc metal ions. The researchers tested their material using X-ray structural analysis and found that it can selectively capture only CO2 molecules with ten times more efficiency than other PCPs.

The material has an organic component with a propeller-like molecular structure, and as CO2 molecules approach the structure, they rotate and rearrange to permit C02 trapping, resulting in slight changes to the molecular channels within the PCP — this allows it to act as molecular sieve that can recognize molecules by size and shape. The PCP is also recyclable; the efficiency of the catalyst did not decrease even after 10 reaction cycles.

“One of the greenest approaches to carbon capture is to recycle the carbon dioxide into high-value chemicals, such as cyclic carbonates which can be used in petrochemicals and pharmaceuticals,” says Susumu Kitagawa, materials chemist at Kyoto University.

After capturing the carbon, the converted material can be used to make polyurethane, a material with a wide variety of applications including clothing, domestic appliances and packaging.

This work highlights the potential of porous coordination polymers for trapping carbon dioxide and converting into useful materials, opening up an avenue for future research into carbon capture materials.

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ANYbotics Announces a Production-Ready Version of the ANYmal Quadruped Robot

If you ignore toys and novelties, robots are generally designed to do one of two things: perform more efficiently than humans can, or to tackle jobs that humans won’t — or can’t — do. The former group consists largely of industrial robots that can handle repetitive tasks quickly and with high precision. The latter category, however, is where things start to get really interesting. That’s the domain of quadruped robots, like those made famous by Boston Dynamics. Now ANYbotics is throwing their hat into the commercial ring with the new ANYmal C quadruped robot.

As the name suggests, ANYmal C isn’t the first robot that ANYbotics has designed. But unlike earlier models, ANYmal C is polished and ready for production. It is roughly the size of an average dog, weighs in at 50kg, and moves around on four powerful legs. Thanks to a host of sensors, including lidar, it can map its environment in three dimensions and navigate autonomously. It’s capable of moving at one meter per second, can climb smooth slopes up to 20 degrees and stairs up to 45 degrees, and can squeeze through relatively narrow passages. ANYmal C can run for up to two hours on a single battery charge, and then autonomously charge itself back up at a special cone-shaped docking station.

ANYmal C is intended primarily to perform inspections in environments that are unsuitable, or simply uncomfortable, for humans. It’s IP67 rated, which means it is completely dust proof and can even handle being submerged under a meter of water for up to an hour. It can carry up to 10kg, and can be equipped with a wide range of sensors and data-gathering equipment. That makes ANYmal C ideal for inspection work in industrial areas. For example, the robot could easily navigate through a potentially hazardous mine to determine if it’s safe for humans to enter. Only time will tell if there is really a commercial market for a robot like this, but it certainly looks promising.

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Author: Cameron Coward

IEEE Spectrum

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