solar power

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Silicon plus perovskite solar reaches 34 percent efficiency

green, green energy, materials science, perovskites, photovoltaics, Science, solar power, Sustainability / /
Solar panels with green foliage behind them, and a diagram of a chemical's structure in the foreground.

Enlarge / Some solar panels, along with a diagram of a perovskite’s crystal structure.

As the price of silicon panels has continued to come down, we’ve reached the point where they’re a small and shrinking cost of building a solar farm. That means that it might be worth spending more to get a panel that converts more of the incoming sunlight to electricity, since it allows you to get more out of the price paid to get each panel installed. But silicon panels are already pushing up against physical limits on efficiency. Which means our best chance for a major boost in panel efficiency may be to combine silicon with an additional photovoltaic material.

Right now, most of the focus is on pairing silicon with a class of materials called perovskites. Perovskite crystals can be layered on top of silicon, creating a panel with two materials that absorb different areas of the spectrum—plus, perovskites can be made from relatively cheap raw materials. Unfortunately, it has been difficult to make perovskites that are both high-efficiency and last for the decades that the silicon portion will.

Lots of labs are attempting to change that, though. And two of them reported some progress this week, including a perovskite/silicon system that achieved 34 percent efficiency.

Boosting perovskite stability

Perovskites are an entire class of materials that all form the same crystal structure. So, there is plenty of flexibility when it comes to the raw materials being used. Perovskite-based photovoltaics are typically formed by what’s called solution processing, in which all the raw materials are dissolved in a liquid that’s then layered on top of the panel-to-be, allowing perovskite crystals to form across its entire surface. Which is great, except that this process tends to form multiple crystals with different orientations on a single surface, decreasing performance.

Adding to the problems, perovskites are also not especially stable. They’re usually made of a combination of positively and negatively charged ions, and these have to be present in the right ratios to form a perovskite. However, some of these individual ions can diffuse over time, disrupting the crystal structure. Harvesting solar energy, which involves the material absorbing lots of energy, makes matters worse by heating the material, which increases the rate of diffusion.

Combined, these factors sap the efficiency of perovskite solar cells and mean that none lasts nearly as long as a sheet of silicon. The new works tackle these issues from two very different directions.

The first of the new papers tackles stability by using the flexibility of perovskites to incorporate various ions. The researchers started by using a technique called density functional theory to model how different molecules would behave when placed into a spot normally occupied by a positively charged ion. And the modeling got them excited about a molecule called tetrahydrotriazinium, which has a six-atom ring composed of alternating carbon and nitrogen atoms. The regular placement of nitrogens around the ring allows it to form regular interactions with neighboring atoms in the crystal structure.

Tetrahydrotriazinium has a neutral charge when only two of the nitrogens have hydrogens attached to them. But it typically grabs a charged hydrogen (effectively, a proton) out of solution, giving it a net positive charge. This leaves each of its three nitrogens associated with a hydrogen and allows the positive charge to be distributed among them. That makes this interaction incredibly strong, meaning that the hydrogens are extremely unlikely to drift off, which also stabilizes the crystal structure.

So, this should make perovskites much, much more stable. The only problem? Tetrahydrotriazinium tends to react with lots of other chemicals, so it’s difficult to provide as a raw material for the perovskite-forming solution.

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Researchers build ultralight drone that flies with onboard solar

drone, materials science, Physics, robotics, Science, solar power / /

Where does it go? It goes up! —

Bizarre design uses a solar-powered motor that’s optimized for weight.

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Image of a metallic object composed from top to bottom of a propeller, a large cylinder with metallic panels, a stalk, and a flat slab with solar panels and electronics.

Enlarge / The CoulombFly doing its thing.

On Wednesday, researchers reported that they had developed a drone they’re calling the CoulombFly, which is capable of self-powered hovering for as long as the Sun is shining. The drone, which is shaped like no aerial vehicle you’ve ever seen before, combines solar cells, a voltage converter, and an electrostatic motor to drive a helicopter-like propeller—with all components having been optimized for a balance of efficiency and light weight.

Before people get excited about buying one, the list of caveats is extensive. There’s no onboard control hardware, and the drone isn’t capable of directed flight anyway, meaning it would drift on the breeze if ever set loose outdoors. Lots of the components appear quite fragile, as well. However, the design can be miniaturized, and the researchers built a version that weighs only 9 milligrams.

Built around a motor

One key to this development was the researchers’ recognition that most drones use electromagnetic motors, which involve lots of metal coils that add significant weight to any system. So, the team behind the work decided to focus on developing a lightweight electrostatic motor. These rely on charge attraction and repulsion to power the motor, as opposed to magnetic interactions.

The motor the researchers developed is quite large relative to the size of the drone. It consists of an inner ring of stationary charged plates called the stator. These plates are composed of a thin carbon-fiber plate covered in aluminum foil. When in operation, neighboring plates have opposite charges. A ring of 64 rotating plates surrounds that.

The motor starts operating when the plates in the outer ring are charged. Since one of the nearby plates on the stator will be guaranteed to have the opposite charge, the pull will start the rotating ring turning. When the plates of the stator and rotor reach their closest approach, thin wires will make contact, allowing charges to transfer between them. This ensures that the stator and rotor plates now have the same charge, converting the attraction to a repulsion. This keeps the rotor moving, and guarantees that the rotor’s plate now has the opposite charge from the next stator plate down the line.

These systems typically require very little in the way of amperage to operate. But they do require a large voltage difference between the plates (something we’ll come back to).

When hooked up to a 10-centimeter, eight-bladed propeller, the system could produce a maximum lift of 5.8 grams. This gave the researchers clear weight targets when designing the remaining components.

Ready to hover

The solar power cells were made of a thin film of gallium arsenide, which is far more expensive than other photovoltaic materials, but offers a higher efficiency (30 percent conversion compared to numbers that are typically in the mid-20s). This tends to provide the opposite of what the system needs: reasonable current at a relatively low voltage. So, the system also needed a high-voltage power converter.

Here, the researchers sacrificed efficiency for low weight, arranging a bunch of voltage converters in series to create a system that weighs just 1.13 grams, but steps the voltage up from 4.5 V all the way to 9.0 kV. But it does so with a power conversion efficiency of just 24 percent.

The resulting CoulombFly is dominated by the large cylindrical motor, which is topped by the propeller. Suspended below that is a platform with the solar cells on one side, balanced out by the long, thin power converter on the other.

Meet the CoulombFly.

To test their system, the researchers simply opened a window on a sunny day in Beijing. Starting at noon, the drone took off and hovered for over an hour, and all indications are that it would have continued to do so for as long as the sunlight provided enough power.

The total system required just over half a watt of power to stay aloft. Given a total mass of 4 grams, that works out to a lift-to-power efficiency of 7.6 grams per watt. But a lot of that power is lost during the voltage conversion. If you focus on the motor alone, it only requires 0.14 watts, giving it a lift-to-power efficiency of over 30 grams per watt.

The researchers provide a long list of things they could do to optimize the design, including increasing the motor’s torque and propeller’s lift, placing the solar cells on structural components, and boosting the efficiency of the voltage converter. But one thing they don’t have to optimize is the vehicle’s size since they already built a miniaturized version that’s only 8 millimeters high and weighs just 9 milligrams but is able to generate a milliwatt of power that turns its propeller at over 15,000 rpm.

Again, all this is done without any onboard control circuitry or the hardware needed to move the machine anywhere—they’re basically flying these in cages to keep them from wandering off on the breeze. But there seems to be enough leeway in the weight that some additional hardware should be possible, especially if they manage some of the potential optimizations they mentioned.

Nature, 2024. DOI: 10.1038/s41586-024-07609-4  (About DOIs).

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will-space-based-solar-power-ever-make-sense?

Will space-based solar power ever make sense?

Energy, renewable energy, Science, solar power, space-based solar / /
Artist's depiction of an astronaut servicing solar panels against the black background of space.

Is space-based solar power a costly, risky pipe dream? Or is it a viable way to combat climate change? Although beaming solar power from space to Earth could ultimately involve transmitting gigawatts, the process could be made surprisingly safe and cost-effective, according to experts from Space Solar, the European Space Agency, and the University of Glasgow.

But we’re going to need to move well beyond demonstration hardware and solve a number of engineering challenges if we want to develop that potential.

Designing space-based solar

Beaming solar energy from space is not new; telecommunications satellites have been sending microwave signals generated by solar power back to Earth since the 1960s. But sending useful amounts of power is a different matter entirely.

“The idea [has] been around for just over a century,” said Nicol Caplin, deep space exploration scientist at the ESA, on a Physics World podcast. “The original concepts were indeed sci-fi. It’s sort of rooted in science fiction, but then, since then, there’s been a trend of interest coming and going.”

Researchers are scoping out multiple designs for space-based solar power. Matteo Ceriotti, senior lecturer in space systems engineering at the University of Glasgow, wrote in The Conversation that many designs have been proposed.

The Solaris initiative is exploring two possible technologies, according to Sanjay Vijendran, lead for the Solaris initiative at the ESA: one that involves beaming microwaves from a station in geostationary orbit down to a receiver on Earth and another that involves using immense mirrors in a lower orbit to reflect sunlight down onto solar farms. He said he thinks that both of these solutions are potentially valuable. Microwave technology has drawn wider interest and was the main focus of these interviews. It has enormous potential, although high-frequency radio waves can also be used.

“You really have a source of 24/7 clean power from space,” Vijendran said. The power can be transmitted regardless of weather conditions because of the frequency of the microwaves.

“A 1-gigawatt power plant in space would be comparable to the top five solar farms on earth. A power plant with a capacity of 1 gigawatt could power around 875,000 households for one year,” said Andrew Glester, host of the Physics World podcast.

But we’re not ready to deploy anything like this. “It will be a big engineering challenge,” Caplin said. There are a number of physical hurdles involved in successfully building a solar power station in space.

Using microwave technology, the solar array for an orbiting power station that generates a gigawatt of power would have to be over 1 square kilometer in size, according to a Nature article by senior reporter Elizabeth Gibney. “That’s more than 100 times the size of the International Space Station, which took a decade to build.” It would also need to be assembled robotically, since the orbiting facility would be uncrewed.

The solar cells would need to be resilient to space radiation and debris. They would also need to be efficient and lightweight, with a power-to-weight ratio 50 times more than the typical silicon solar cell, Gibney wrote. Keeping the cost of these cells down is another factor that engineers have to take into consideration. Reducing the losses during power transmission is another challenge, Gibney wrote. The energy conversion rate needs to be improved to 10–15 percent, according to the ESA. This would require technical advances.

Space Solar is working on a satellite design called CASSIOPeiA, which Physics World describes as looking “like a spiral staircase, with the photovoltaic panels being the ‘treads’ and the microwave transmitters—rod-shaped dipoles—being the ‘risers.’” It has a helical shape with no moving parts.

“Our system’s comprised of hundreds of thousands of the same dinner-plate-sized power modules. Each module has the PV which converts the sun’s energy into DC electricity,” said Sam Adlen, CEO of Space Solar.

“That DC power then drives electronics to transmit the power… down toward Earth from dipole antennas. That power up in space is converted to [microwaves] and beamed down in a coherent beam down to the Earth where it’s received by a rectifying antenna, reconverted into electricity, and input to the grid.”

Adlen said that robotics technologies for space applications, such as in-orbit assembly, are advancing rapidly.

Ceriotti wrote that SPS-ALPHA, another design, has a large solar-collector structure that includes many heliostats, which are modular small reflectors that can be moved individually. These concentrate sunlight onto separate power-generating modules, after which it’s transmitted back to Earth by yet another module.

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