Advertisements

MIT, Stanford researchers develop new kind of tandem solar cell

perovskiteResearchers at MIT and Stanford University have developed a new kind of solar cell that combines two different layers of sunlight-absorbing material in order to harvest a broader range of the sun’s energy. The development could lead to photovoltaic cells that are more efficient than those currently used in solar-power installations, the researchers say.

The new cell uses a layer of silicon — which forms the basis for most of today’s solar panels — but adds a semi-transparent layer of a material called perovskite, which can absorb higher-energy particles of light. Unlike an earlier “tandem” solar cell reported by members of the same team earlier this year — in which the two layers were physically stacked, but each had its own separate electrical connections — the new version has both layers connected together as a single device that needs only one control circuit.

The new findings are reported in the journal Applied Physics Letters by MIT graduate student Jonathan Mailoa; associate professor of mechanical engineering Tonio Buonassisi; Colin Bailie and Michael McGehee at Stanford; and four others.

“Different layers absorb different portions of the sunlight,” Mailoa explains. In the earlier tandem solar cell, the two layers of photovoltaic material could be operated independently of each other and required their own wiring and control circuits, allowing each cell to be tuned independently for optimal performance.

By contrast, the new combined version should be much simpler to make and install, Mailoa says. “It has advantages in terms of simplicity, because it looks and operates just like a single silicon cell,” he says, with only a single electrical control circuit needed.

One tradeoff is that the current produced is limited by the capacity of the lesser of the two layers. Electrical current, Buonassisi explains, can be thought of as analogous to the volume of water passing through a pipe, which is limited by the diameter of the pipe: If you connect two lengths of pipe of different diameters, one after the other, “the amount of water is limited by the narrowest pipe,” he says. Combining two solar cell layers in series has the same limiting effect on current.

To address that limitation, the team aims to match the current output of the two layers as precisely as possible. In this proof-of-concept solar cell, this means the total power output is about the same as that of conventional solar cells; the team is now working to optimize that output.

Perovskites have been studied for potential electronic uses including solar cells, but this is the first time they have been successfully paired with silicon cells in this configuration, a feat that posed numerous technical challenges. Now the team is focusing on increasing the power efficiency — the percentage of sunlight’s energy that gets converted to electricity — that is possible from the combined cell. In this initial version, the efficiency is 13.7 percent, but the researchers say they have identified low-cost ways of improving this to about 30 percent — a substantial improvement over today’s commercial silicon-based solar cells — and they say this technology could ultimately achieve a power efficiency of more than 35 percent.

They will also explore how to easily manufacture the new type of device, but Buonassisi says that should be relatively straightforward, since the materials lend themselves to being made through methods very similar to conventional silicon-cell manufacturing.

One hurdle is making the material durable enough to be commercially viable: The perovskite material degrades quickly in open air, so it either needs to be modified to improve its inherent durability or encapsulated to prevent exposure to air — without adding significantly to manufacturing costs and without degrading performance.

This exact formulation may not turn out to be the most advantageous for better solar cells, Buonassisi says, but is one of several pathways worth exploring. “Our job at this point is to provide options to the world,” he says. “The market will select among them.”

“I think this work is very significant,” says Martin Green, a professor at the University of New South Wales, in Australia, who was not connected with this research. “The work is important in establishing a proof-of-concept and will stimulate higher efficiencies with this approach. … It’s an excellent starting point for further work in this area.”

The research team also included Eric Johlin PhD ’14 and postdoc Austin Akey at MIT, and Eric Hoke and William Nguyen of Stanford. It was supported by the Bay Area Photovoltaic Consortium and the U.S. Department of Energy.

 

Advertisements

Researchers use nanotechnology to increase solar power efficiency

brian-korgelResearchers at UT are working to improve the efficiency of solar panels, which could lead to lower energy costs in Texas, according to Brian Korgel, chemical engineering professor.

Korgel spoke at the UT Energy Symposium on Thursday about increasing access to solar and nanotechnology in Texas.

Korgel’s team is replacing silicon slabs in solar panels with cadmium telluride ink, a new synthetic material made of crystals, because the material is smaller and the crystals absorb sunlight better.

 

New family of light-converting materials points to cheaper, more efficient solar power and LEDs

perovskite: cheaper more efficient solar?
A pure perovskite crystal, orange in colour, mounted on a cryostat.

Engineers have shone new light on an emerging family of solar-absorbing materials that could clear the way for cheaper and more efficient solar panels and LEDs.

The materials, called perovskites, are particularly good at absorbing visible light, but had never been thoroughly studied in their purest form: as perfect single crystals.

Using a new technique, researchers grew large, pure perovskite crystals and studied how electrons move through the material as light is converted to electricity.

Led by Professor Ted Sargent of The Edward S. Rogers Sr. Department of Electrical & Computer Engineering at the University of Toronto and Professor Osman Bakr of the King Abdullah University of Science and Technology (KAUST), the team used a combination of laser-based techniques to measure selected properties of the perovskite crystals. By tracking down the rapid motion of electrons in the material, they have been able to determine the diffusion length—how far electrons can travel without getting trapped by imperfections in the material—as well as mobility—how fast the electrons can move through the material. Their work was published this week in the journal Science.

“Our work identifies the bar for the ultimate solar energy-harvesting potential of perovskites,” says Riccardo Comin, a post-doctoral fellow with the Sargent Group. “With these materials it’s been a race to try to get record efficiencies, and our results indicate that progress is slated to continue without slowing down..”

In recent years, perovskite efficiency has soared to certified efficiencies of just over 20 per cent, beginning to approach the present-day performance of commercial-grade silicon-based solar panels mounted in Spanish deserts and on Californian roofs.
“In their efficiency, perovskites are closely approaching conventional materials that have already been commercialized,” says Valerio Adinolfi, a PhD candidate in the Sargent Group and co-first author on the paper. “They have the potential to offer further progress on reducing the cost of solar electricity in light of their convenient manufacturability from a liquid chemical precursor.”

The study has obvious implications for green energy, but may also enable innovations in lighting. Think of a solar panel made of perovskite crystals as a fancy slab of glass: light hits the crystal surface and gets absorbed, exciting electrons in the material. Those electrons travel easily through the crystal to electrical contacts on its underside, where they are collected in the form of electric current. Now imagine the sequence in reverse—power the slab with electricity, inject electrons, and release energy as light. A more efficient electricity-to-light conversion means perovskites could open new frontiers for energy-efficient LEDs.

Parallel work in the Sargent Group focuses on improving nano-engineered solar-absorbing particles called colloidal quantum dots. “Perovskites are great visible-light harvesters, and quantum dots are great for infrared,” says Professor Sargent. “The materials are highly complementary in solar energy harvesting in view of the sun’s broad visible and infrared power spectrum.”

“In future, we will explore the opportunities for stacking together complementary absorbent materials,” says Dr. Comin. “There are very promising prospects for combining perovskite work and quantum dot work for further boosting the efficiency.”

 

graphene: a solar breakthrough?

Graphene Could Double Electricity Generated From Solar

graphene: a solar breakthrough?The amount of sunlight that hits the Earth every 40 minutes is enough to meet global energy demands for an entire year. The trick, of course, is harnessing it and converting it into useful electricity. A new study has revealed that tweaking graphene allows it to generate two electrons for every photon of light it receives. This could double the amount of electricity currently converted in photovoltaic devices. Marco Grioni from École Polytechnique Fédérale de Lausanne (EPFL) in Switzerland is one of the senior authors on the paper, which was published in Nano Letters.

Graphene is a monolayer of carbon atoms arranged in a honeycomb pattern. It is incredibly light, flexible, exponentially stronger than steel, and capable of conducting electricity even better than copper. In order to make it useful in photovoltaic devices, the researchers needed to have a better idea of graphene’s mechanism for converting light into electricity. This process takes only a femto-second (10-15 sec), which is too quick to easily study.

Laser processing technology enhances the ability of solar cells to harvest more sunlight

laser-enhanced-solarLaser processing produces deep ripples in silicon over a wide area — something that could enhance solar cell efficiency.

A*STAR scientists have produced a uniform nanoscale ripple pattern over a wide area on a silicon surface by scanning a femtosecond laser beam across it. Given that a rippled surface is much less reflective than a smooth surface, this simple innovation could enhance the efficiency of solar cells by boosting their ability to harvest more sunlight1.

The use of lasers to produce periodic surface structures is currently an area of intense research. Laser processing has the important advantage that it heats only the surface of a material, leaving underlying structures unaffected. However, many laser processing methods are limited: they can process only small areas and shallow ripples.

 

Selling Solar Homes

Homebuyers pay more for solar PV homes, says Berkeley Lab report

Selling Solar HomesA multi-institutional research team of scientists led by the U.S. Department of Energy’s Lawrence Berkley Laboratory (Berkeley Lab), in partnership with Sandia National Laboratories, universities, and appraisers found that home buyers consistently have been willing to pay more for homes with host-owned solar photovoltaic (PV) energy systems —averaging about $4 per watt of PV installed—across various states, housing and PV markets, and home types. This equates to a premium of about $15,000 for a typical PV system. The team analyzed almost 22,000 sales of homes, almost 4,000 of which contained PV systems in eight states from 1999 to 2013—producing the most authoritative estimates to date of price premiums for U.S. homes with PV systems. FINALCover_010915

“Previous studies on PV home premiums have been limited in size and scope,” says Ben Hoen, the lead author of the new report. “We more than doubled the number of PV home sales analyzed, examined a number of states outside of California, and captured the market during the recent housing boom, bust, and recovery.”

More than half a million U.S. homes had PV as of 2014, and the number is growing rapidly. The growth in home PV systems means that the real estate industry will need reliable methods to value these homes appropriately. Further, having greater certainty in those methods will likely facilitate additional growth in the residential PV market.

Berkeley Lab's Ben Hoen (Credit: Roy Kaltschmidt)

Hoen is a researcher in the Environmental Energy Technologies Division of Berkeley Lab, who collaborated with researchers from Adomatis Appraisal Services, Real Property Analytics/Texas A&M University, University of California at San Diego, San Diego State University, and Sandia National Laboratories.

The study also found only a small and non-statistically significant difference between PV premiums for new and existing homes. Additional findings include the existence of a PV “green cache” (home buyers paying a certain amount for a PV system of any size and incrementally more as system size increases) and an apparent sharp depreciation rate for the PV premium in home sales transactions as those PV systems age. The study also finds that market premiums are statistically similar to those estimated using the income and cost approaches, methods familiar to appraisers. This similarity to standard appraisal practices further bolsters the report’s usefulness to real estate professionals and markets.

“As PV systems become more and more common on U.S. homes, it will be increasingly important to value them accurately, using a variety of methods,” says co-author Sandra Adomatis, an appraiser who helped develop the Appraisal Institute’s Green Addendum and who has written and spoken extensively on valuing green features. She noted, “Our findings should provide greater confidence that PV adds a quantifiable premium to a wide variety of homes in California and beyond.”

The research was supported by funding from the U.S. Department of Energy SunShot Initiative. The SunShot Initiative is a collaborative national effort that aggressively drives innovation to make solar energy fully cost-competitive with traditional energy sources before the end of the decade. Through SunShot, DOE supports efforts by private companies, universities, and national laboratories to drive down the cost of solar electricity to $0.06 per kilowatt-hour. Learn more at energy.gov/sunshot.

Chip-Making Tools Could Cut Solar Power Costs

Ultra-Efficient Solar Cells
A wafer bearing 500 tiny solar cells, made by Soitec, has produced a new world record.

Soitec, a French manufacturing company, says it has used techniques designed for making microprocessors to produce solar cells with a record-setting efficiency of 46 percent, converting more than twice as much sunlight into electricity as conventional cells.

Although the cells are more complicated to produce, using established manufacturing techniques promises to keep production costs down.

Ordinary solar cells use one semiconductor to convert sunlight into electricity. The cells made by Soitec have four semiconductors, each designed to target a different part of the solar spectrum. Soitec produced its first four-semiconductor cell about a year ago. Since then, it’s been improving efficiencies rapidly, and it looks on track to be the first company to hit the long-awaited milestone of 50 percent efficiency.

 

perovskite solar PV

Will New Technologies Give Critical Boost to Solar Power?

perovskite solar PVPromising new technologies, including more efficient photovoltaic cells that can harvest energy across the light spectrum, have the potential to dramatically increase solar power generation in the next two decades. But major hurdles remain.

Today, despite recent progress, solar power accounts for about one percent of the world’s energy mix. Yet the International Energy Agency (IEA) says that solar energy, most of it generated by decentralized “rooftop” photovoltaic systems, could well become the world’s single biggest source of electricity by mid-century.

So how do we get from here to there?

Engineer Reimagines Solar Energy With Stick-On Panels

stick-on solar
These peel-and-stick solar cells could revolutionize the way we harness the immense energy of the sun.

Editor’s Note: Xiaolin Zheng is one of National Geographic’s 2014 Emerging Explorers, part of a program that honors tomorrow’s visionaries—those making discoveries, making a difference, and inspiring people to care about the planet.

The catalyst for Xiaolin Zheng’s groundbreaking work in solar energy began with an offhand comment her father made years ago at her parents’ apartment, a 13-story complex in the northeast China city of Anshan.

“In China, the rooftops of many buildings are packed with solar energy devices,” says Zheng. “One day my father mentioned how great it would be if a building’s entire surface could be used for solar power, not just the roof, but also walls and windows.”

An invention from Zheng’s research team at Stanford University might someday make that possible. They have created a type of solar cell that is thin, flexible, and adhesive—a solar sticker, in effect, that could help power everything from buildings to airplanes.

 

google NRG

The Strange Thing About Google’s Decision To Stop Renewable Energy Research

google NRG
From left, Rick Needham of Google and David Crane of NRG Energy field questions during 2014 dedication of Ivanpah Solar Electric Generating System, the world’s largest solar thermal plant.

Two senior Google engineers have written a confusing article explaining what they learned after Google stopped its advanced research and development effort into renewable energy technologies in 2011.

The answer they offer — that their effort was not on track to deliver renewable R&D breakthroughs that by themselves would reverse climate change — has always been obvious and thus makes very little sense as a reason for giving up on such an important effort, as we will see.

More likely, Google saw renewable energy prices coming down so quickly as global deployment accelerated that they realized their chances to make money in the R&D arena were much smaller than they thought. And they clearly understood that the real action in advancing renewable energy was in deployment, which Google continued to fund at a far greater level than they ever invested in R&D.

 

Soitec-Fraunhofer ISE multi-junction CPV cell hits world record 46% conversion efficiency

 record-breaking solar efficiency
New world record-breaking solar cell on a 100 mm wafer yielding approximately 500 concentrator solar cell devices. Image: Fraunhofer ISE / Alexander Wekkeli

A Soitec multi-junction solar cell for use in concentrator photovoltaic (CPV) systems has become the company’s latest cell to reach a world record for conversion efficiency at 46%, according to Fraunhofer Institute for Solar Energy (Fraunhofer ISE).

The Germany-headquartered research institute once again collaborated with Soitec on the new cell, along with CEA-Leti, a division of the French research and technology organisation. The three parties were also involved in the most recent record-breaking CPV cell last year.

The efficiency of the new cell has been independently verified under standard test conditions by the Japanese National Institute of Advanced Industrial Science and Technology (AIST). The multi-junction cell’s efficiency was measured at a concentration of 508 suns under a Fresnel concentrator lens. So-called multi-junction cells, based on III-V semiconductor compound materials, allow the materials at each junction to respond to different wavelengths of light.

 

SolarCity installation

Why More Solar Panels Should Be Facing West, Not South

SolarCity installation
Workers for SolarCity installing solar panels in Camarillo, Calif. A study of 110,000 California houses with rooftop solar systems confirmed that a vast majority of the panels are pointed south, which has its drawbacks. Credit J. Emilio Flores for The New York Times

For years, homeowners who bought solar panels were advised to mount them on the roof facing south. That captures the most solar energy over the course of the day, which benefits the homeowner, but does so at hours that are not so helpful for the utility and the grid as a whole.

Mount them to catch the sunlight from the west in the afternoon, and the panels’ production over all would fall, but it would come at hours when the electricity was more valuable.

But that idea is slow to take hold. A new study of 110,000 California houseswith rooftop solar systems confirmed that a vast majority of the panels were pointed south because most of the panel owners were paid by the number of kilowatt-hours the panels produced. Pointing them southward maximizes production over all, but peak production comes at midday, not in late afternoon, when it would be more helpful.

 

cheaper solar power

Solar-Power System Is Easy to Install, and Therefore Much Cheaper

cheaper solar power
Researchers from the Fraunhofer Institute install novel, flexible solar panels with an adhesive backing and quick-connect cables.

Ordinarily, installing and connecting a new array of rooftop solar panels takes days, weeks, or even months because the hardware is complex and various permits are needed. Yesterday, on a frigid day in Charlestown, Massachusetts, researchers completed the process in about an hour.

Homeowners can install the system themselves, by gluing it to a rooftop. The permitting is handled by a combination of electronic sensors and software that communicates with local jurisdictions and utilities.

Installation and permit-related expenses currently account for more than half of the overall cost of a new solar power setup. “By simplifying the system so that it’s like installing an appliance, we envision that the soft cost will be virtually eliminated,” says Christian Hoepfner, director of the Fraunhofer Center for Sustainable Energy Systems, which developed the system. Doing so would lower the cost of a typical residential solar installation from $22,000 to as little as $7,500, he says.

Reconsidering the Rebound Effect

520-reboundThe rebound effect from improving energy efficiency has been widely discussed—from the pages of the New York Times and New Yorker to the halls of policy and to a voluminous academic literature. It’s been known for over a century and, on the surface, is simple to understand. Buy a more fuel-efficient car, drive more. Invent a more efficient bulb, use more light. If efficiency improves, the price of energy services will drop, inducing increased demand for those services. Consumers will respond, producers will respond, and markets will re-equilibrate. All of these responses can lead to reductions in the energy savings expected from improved energy efficiency. And so some question the overall value of energy efficiency, by arguing that it will only lead to more energy use—a case often called “backfire.”