Solar energy

The hybrid photovoltaic cell has a claimed efficiency of 21.7 percent, already better than the 10 to 20 percent of standard polycrystalline silicon solar cells currently in use(Credit: Onur Ergen/UC Berkeley)

http://newatlas.com/perovskite-solar-pv-grahene-aerogel/46346/

Scientists working at the University of California, Berkeley (UC Berkeley), and Lawrence Berkeley National Laboratory (LBNL) have created a hybrid photovoltaic cell from multiple layers of different perovskite materials that has a claimed a peak efficiency of 26 percent. It’s said that the cell can easily be sprayed onto flexible surfaces to make bendable, high-efficiency solar panels.

A hybrid organic-inorganic conglomerate, perovskite is used in solar cells to capture light in a similar way to common silicon-based solar cells by converting incoming photon energy into electrical current. Unlike rigid silicon semiconductor materials that require a great deal of expensive processing and manipulation to turn them into solar cells, however, perovskite photovoltaic devices are said to be cheaper and easier to make, in addition to being much more flexible.

The new UC berkeley/LBNL device is also very efficient thanks to a sandwich of two types of perovskite separated by a single-atom thick layer of hexagonal boron nitride (sometimes referred to as “White graphene”) with each perovskite slice designed as a graded bandgap layer (put simply, of low resistance and high gain) able to absorb different wavelengths of light. This combination effectively creates a photovoltaic cell able to collect and convert energy across most of the light spectrum.

“This is realizing a graded bandgap solar cell in a relatively easy-to-control and easy-to-manipulate system,” said Alex Zettl, a UC Berkeley professor of physics. “The nice thing about this is that it combines two very valuable features – the graded bandgap, a known approach, with perovskite, a relatively new but known material with surprisingly high efficiencies – to get the best of both worlds.”

In detail, the perovskite materials are made of methyl and ammonia organic molecules, with one containing tin and iodine and designed to absorb infrared light in the 1 electron volt (eV) range, and the other consisting of lead and iodine doped with bromine that absorbs amber photons of energy at 2 eV. A single-atom layer of boron nitride then provides an intermediate junction to operate in tandem and create electricity from across the light band.

This entire layered combination is then stabilized mechanically by placing it on top of a lightweight graphene aerogel to enhance the formation of fine-grained perovskite crystals as well as serve as a moisture barrier to stop the water-soluble perovskites falling to pieces. Lastly, the whole conglomeration has a gold electrode attached to the underside, along with a gallium nitride layer added to the uppermost part that gathers up the electrons generated when the cell is exposed to light. And all this with an active layer just 400 nanometers thick.

“Our architecture is a bit like building a quality automobile roadway,” said Zettl. “The graphene aerogel acts like the firm, crushed rock bottom layer or foundation, the two perovskite layers are like finer gravel and sand layers deposited on top of that, with the hexagonal boron nitride layer acting like a thin-sheet membrane between the gravel and sand that keeps the sand from diffusing into or mixing too much with the finer gravel. The gallium nitride layer serves as the top asphalt layer.”

With a standard operating efficiency of around 21.7 percent, the new wide spectrum hybrid perovskite cell is already better than the 10 to 20 percent efficiency of standard polycrystalline silicon solar cells currently in use in a host of commercial equipment and household solar systems. Even the best silicon solar cells made today are lucky to get over 25 percent efficiency, and are complex and expensive to produce.

“We have set the record now for different parameters of perovskite solar cells, including the efficiency,” said Zettl. “The efficiency is higher than any other perovskite cell – 21.7 percent – which is a phenomenal number, considering we are at the beginning of optimizing this.”

“Our theoretical efficiency calculations should be much, much higher and easier to reach than for single-bandgap solar cells because we can maximize coverage of the solar spectrum,” added Onur Ergen, a UC Berkeley physics graduate student.

The possibility exists to add further layers of hexagonal boron nitride-separated perovskite to help increase efficiencies even further, but the researchers believe that the thin new material may be efficient enough, and certainly sufficient for producing acceptable efficiencies for commercial production.

“People have had this idea of easy-to-make, roll-to-roll photovoltaics, where you pull plastic off a roll, spray on the solar material, and roll it back up,” said Zettl. “With this new material, we are in the regime of roll-to-roll mass production; it’s really almost like spray painting.”

The results were recently published in the journal Nature Materials.

http://insights.globalspec.com/article/1732/clear-solar-panels-could-offer-energetic-window-retrofit

Engineers from Michigan State University (MSU) are designing transparent solar panels that could be retrofit to existing glass-covered buildings to generate electric power.

Traditional opaque solar panels such as silicon soak up much of the sun’s light, including visible light, and convert it to energy. A transparent panel allows visible light to shine through and making light that is invisible to the human eye—such as ultraviolet and infrared—do the work.

By making the solar panels transparent, MSU materials scientist and chemical engineer Richard Lunt and his team are creating the potential for them to cover existing windows.

However, making the panels clear is a challenge. So the team came up with ways to layer patterns onto the cell in a way that makes them uniformly transparent. The transparent solar cell under development incorporates thin coatings of organic and inorganic nanostructure materials that selectively harvest the parts of the solar radiation spectrum that are not visible to the eye.

“We actually used a variety of different stencils to pattern our devices,” Lunt says. Each active material has its own pattern. After every layer, the researchers put down a new stencil and in this way build complex structures, he adds.

Team member Margaret Young is testing whether the same process can be used on thin plastic.

“This is much lighter and much more flexible, so instead of rebuilding windows, we could just put this over an existing window,” she says.

Lunt says that he expects that in the next 20 years, this type of technology will be deployed extensively—turning cities and landscapes into solar harvesting systems, surfaces and solar farms without the aesthetic issues that today’s opaque solar panels create.

The Sunflower has a massive total efficiency of around 80%, thanks to very clever tech.

http://arstechnica.com/science/2015/08/the-solar-sunflower-harnessing-the-power-of-5000-suns/

Behold, the Solar Sunflower! Here they are trying a bunch of different reflector materials, which is why the segments all look slightly different. Some of the reflectors are covered up to protect them from the elements, or to stop them from frying a nearby engineer.

High on a hill was a lonely sunflower. Not a normal sunflower, mind you; that would hardly be very notable. This sunflower is a solar sunflower that combines both photovoltaic solar power and concentrated solar thermal power in one neat, aesthetic package that has a massive total efficiency of around 80 percent.

The Solar Sunflower, a Swiss invention developed by Airlight Energy, Dsolar (a subsidiary of Airlight), and IBM Research in Zurich, uses something called HCPVT to generate electricity and hot water from solar power. HCPVT is a clumsy acronym that stands for “highly efficient concentrated photovoltaic/thermal.” In short, it has reflectors that concentrate the sun—”to about 5,000 suns,” Gianluca Ambrosetti, Airlight’s head of research told me—and then some highly efficient photovoltaic cells that are capable of converting that concentrated solar energy into electricity, without melting in the process. Airlight/Dsolar are behind the Sunflower’s reflectors and superstructure, and the photovoltaics are provided by IBM.

The two constituent technologies of the Solar Sunflower—concentrated solar thermal power and photovoltaic solar power—are both very well known and understood at this point, and not at all exciting. What’s special about the Sunflower, however, is that it combines both of the technologies together in a novel fashion to attain much higher total efficiency. Bear with me, as this will take a little bit of explaining.

The reflectors are simply slightly curved, mirrored panels. Airlight has tried a variety of different reflector materials, from glass to mylar, but it looks like they have finally settled on aluminium foil, which isn’t prohibitively expensive and has very high reflectance. Aluminium foil does need additional material to protect it from the elements, though, as it’s very flimsy. The Sunflower has six “petals,” each consisting of six reflectors. At the focal point of the 36 reflectors there are six collectors, one for each block of six reflectors.

The collectors are where most of the magic occurs. To begin with, each collector has an array of gallium-arsenide (GaAs) photovoltaic cells. GaAs is much more efficient at converting sunlight into electricity (38 percent in this case, versus about 20 percent for silicon), but it’s much, much more expensive. With the Sunflower, though, space is at a premium: the sunlight is only focused on a very small region, so you need to use the absolute best cells available. The GaAs array in each collector only measures a few square centimetres, and yet it can produce about 2 kilowatts of electricity (so, one Sunflower generates about 12kW of electricity in total).

Photovoltaic cells, like most semiconductors, become less efficient as they get hotter. The GaAs cells used by the Sunflower have a max operating temperature of around 105°C. The problem is, when you focus the power of 5,000 suns on a single point, things get a lot hotter than 105°C. During one test, Airlight told me that they used the reflectors to melt a hole in a lump of iron (which has a melting point of 1538°C); during another test, the reflectors were misaligned and “we had molten aluminium dripping everywhere.”

How then do you stop your collectors turning into very expensive puddles of molten metal?

What a complete Solar Sunflower installation would look like, providing heat and electricity to a nearby house/office/factory.

Cooling with hot water? Cool.

The answer is a very clever cooling system, borrowed from one of IBM’s specialities: supercomputers. In high-performance computing (HPC) installations, heat is one of the limiting factors on how much processing power you can squeeze into a given volume. Computer chips are tiny; you could physically cram hundreds of them into a 1U server chassis, if you so wished. Cooling them, though, is another matter entirely. You can use fancy heat pipes, or perhaps immerse the whole thing in liquid, but it only gets you so far.

Over the last few years, IBM has been working on advanced methods of liquid cooling, primarily to boost compute density, but also to reduce the amount of waste heat (getting rid of it increases efficiency and reduces costs). In a conventional liquid cooling setup, there’s a water block (a lump of metal with some channels for liquid to flow through), a pump, and a radiator. The liquid is usually water, but it could be something else, like Fluorinert or mineral oil. Heat is transferred from the computer chip to the liquid, and then carried to the radiator and released into the atmosphere. This is inefficient for two reasons: there’s a limit to how much heat can be “picked up” as the fluid passes through the block; and the heat being radiated into the atmosphere is wasted.

IBM solves both of these problems with its hot-water cooling technology. First, instead of the hot water passing through a radiator and venting the thermal energy out into the atmosphere, the hot water is simply used as hot water: to heat homes, or to drive industrial processes, such as desalination, pasteurisation, drying, cooking, etc. IBM already has an example of this in Aquasar, a supercomputer at ETH in Zurich, where the hot water is used to heat university buildings.

Second, to increase heat transfer from the chip to the water, IBM has replaced the dumb ol’ water block with a piece of silicon with microfluidic channels. This piece of silicon, which is then stuck to the back of the computer chip like a tiny water block, has thousands of tiny channels that bring the water to within just a few microns of those pesky heat-generating transistors. This massively increases the amount of heat that can be dissipated, plus all of those discrete channels do a lot better job of dealing with chip hot spots (small regions that are more active than others) than the handful of giant channels in a conventional water block.

Okay, Seb, get back to the sunflowers…

The Solar Sunflower uses this exact same cooling technology—but instead of computer chips, those microfluidic slices of silicon are stuck to the backside of those gallium-arsenide photovoltaic cells. The cooling system ensures that the GaAs efficiently converts photons into electrons, while at the same time whisking away the thermal energy of 5,000 suns. It’s pretty cool, to be honest. Or hot. Or something.

The end result is a device that produces about 12kW of electricity, along with 21kW of thermal energy (with water temperatures up to 90°C). Neither Airlight or IBM would reveal the exact pricing of a single Sunflower, but the fully installed cost will likely be in the tens-of-thousands-of-pounds range—and that’s just the first caveat of many.

For a start, concentrated solar power only works with direct sunlight: the reflectors need to be pointed directly at the sun, and anything less than totally clear skies will significantly reduce power generation. The Sunflower has some control software that automatically tracks the sun, but IBM gave me a distinct “no comment” when I petulantly probed them about their ability to rid the world of clouds.

Second, there’s the lack of energy density: the Sunflower is very efficient, but it still only produces 12kW of electricity. That’s enough to power maybe three or four homes—during the few hours of the day that the sun is visible, anyway. You would need a large field of these things to power a town—and again, you’d need some kind of energy storage solution to get through the evenings, winters, and periods of inclementousness.

Perhaps the largest problem, though, is cost. When Airlight and IBM started work on the Solar Sunflower, the cost of bog-standard silicon solar cells was about £1 ($1.60) per watt. Over the last few years, as China has ramped up production, the cost has dropped precipitously to about 25p (40 cents) per watt—plus the efficiency of silicon PVs has improved, too. With its GaAs cells, fancy plumbing, control systems and motors, the giant lump of concrete, and the time it takes to construct the whole thing, the Solar Sunflower simply can’t compete with hectares of boring-ass silicon photovoltaics.

What a completed Solar Sunflower installation might look like in the future. Maybe. (This is a computer render.)