Archive for the ‘solar power’ Category

Unconventional thinking could boost solar cells

Thursday, February 26th, 2009

Unconventional thinking could boost solar cells

There is an old saying about not rocking the boat; but when it comes to hiking the capabilities of solar energy, the opposite is true.

Unconventional solar cell materials, much less costly than silicon and other semiconductors in use today, could substantially reduce the cost of solar photovoltaics, according to a new study from the Energy and Resources Group and the Department of Chemistry at the University of California, Berkeley, and the Lawrence Berkeley National Laboratory (LBNL).

These materials, some of which are highly abundant, could expand the potential for solar cells to become a significant source of low-carbon energy across the world, according to the study authors.

The analysis examines the two most pressing challenges to large-scale deployment of solar photovoltaics as the world moves toward a carbon neutral future: cost per kilowatt hour and total resource abundance.

The UC Berkeley study evaluated 23 promising semiconducting materials and discovered 12 are abundant enough to meet or exceed annual worldwide energy demand. Of those 12, nine have a significant raw material cost reduction over traditional crystalline silicon, the most widely used photovoltaic material in mass production.

The work provides potential for research into novel solar cell types just at a time when the U.S. Department of Energy and other agencies plan to expand the use of clean energy, said Daniel Kammen, UC Berkeley professor of energy and resources and director of the Renewable and Appropriate Energy Laboratory.

Kammen and colleagues Cyrus Wadia of LBNL and A. Paul Alivisatos of UC Berkeley’s Department of Chemistry embarked on an intensive research project to explore the question if high-impact materials have been overlooked or underdeveloped during the last several decades of solar cell research.

“The reason we started looking at new materials is because people often assume solar will be the dominant energy source of the future,” said Wadia, a post-doctoral researcher who spearheaded the research. “Because the sun is the Earth’s most reliable and plentiful resource, solar definitely has that potential, but current solar technology may not get us there in a timeframe that is meaningful, if at all. It’s important to be optimistic, but when considering the practicalities of a solar-dominated energy system, we must turn our attention back to basic science research if we are to solve the problem.”

The most popular solar materials in use today are silicon and thin films made of CdTe (cadmium telluride) and CIGS (copper indium gallium selenide). While these materials have helped elevate solar to a major player in renewable energy markets, they still suffer from manufacturing challenges. Silicon is expensive to process and mass produce. Furthermore, it has become increasingly difficult to mine enough silicon to meet ever-growing consumer demand.

Thin films, while significantly less costly than silicon and easier to mass produce, would rapidly deplete our natural resources if these technologies were to scale to terawatt hours of annual manufacturing production. A terawatt hour is a billion kilowatt hours.

“We believe in a portfolio of technologies and therefore continue to support the commercial development of all photovoltaic technologies,” Kammen said. “Yet, what we’ve found is that some leading thin films may be difficult to scale as high as global electricity consumption.”

“It’s not to say that these materials won’t play a significant role,” Wadia added, “but rather, if our objective is to supply the majority of electricity in this way, we must quickly consider alternative materials that are Earth-abundant, non-toxic and cheap. These are the materials that can get us to our goals more rapidly.”

The team identified a large material extraction cost (cents/watt) gap between leading thin film materials and a number of unconventional solar cell candidates, including iron pyrite, copper sulfide, and copper oxide. They showed iron pyrite is several orders of magnitude better than any alternative on important metrics of both cost and abundance. In the report, the team referenced advances in nanoscale science to argue they could offset the modest efficiency losses of unconventional solar cell materials by the potential for scaling up while saving significantly on materials costs.

Finding an affordable electricity supply is essential for meeting basic human needs, Kammen said, yet 30% of the world’s population remains without reliable or sufficient electrical energy. Scientific forecasts predict to meet the world’s energy demands by 2050, global carbon emissions would have to grow to levels of irreversible consequences.

“As the U.S. envisions a clean energy future consistent with the vision outlined by President Obama, it is exciting that the range of promising solar cell materials is expanding, ideally just as a national renewable energy strategy takes shape,” said Kammen, who is co-director of the Berkeley Institute of the Environment and UC Berkeley’s Class of 1935 Distinguished Chair of Energy.

For related information, go to www.isa.org/manufacturing_automation.

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Energizing Solar Cells

Thursday, February 12th, 2009

Energizing solar cells

Carrier multiplication—when a photon creates multiple electrons—is a real phenomenon in tiny semiconductor crystals and not a false observation born of extraneous effects that mimic carrier multiplication. While that sounds complicated, what it really boils down to is there is a possibility that solar cells can create more than one unit of energy per photon.

Questions about the ability to increase the energy output of solar cells have prompted researchers at Los Alamos National Laboratory to reassess carrier multiplication in extremely small semiconductor particles.

When a conventional solar cell absorbs a photon of light, it frees an electron to generate an electrical current. Energy in excess of the amount needed to promote an electron into a conducting state is lost as heat to atomic vibrations (phonons) in the material lattice. Through carrier multiplication, excess energy can transfer to another electron instead of the material lattice, freeing it to generate electrical current, which would yield a more efficient solar cell.

Los Alamos Researcher Victor Klimov and colleagues have shown nanocrystals of certain semiconductor materials can generate more than one electron after absorbing a photon. This is partly due to strengthened interactions between electrons squeezed together within the confines of the nanoscale particles.

In 2004, Los Alamos researchers Richard Schaller and Klimov reported the first observations of strong carrier multiplication in nanosized crystals of lead selenide resulting in up to two electron-hole pairs per absorbed photon. A year later, Arthur Nozik and coworkers at the National Renewable Energy Laboratory reproduced these results. Eventually, researchers observed spectroscopic signatures of carrier multiplication in nanocrystals of various compositions, including silicon.

This is not without controversy, as some studies described low or negligible carrier multiplication efficiencies, which seemed to run contrary to earlier findings. To sort out these discrepancies, Los Alamos researchers analyzed factors that could have led to a spread in the reported carrier multiplication results. These factors included variations between samples, differences in detection techniques, and effects mimicking the signatures of carrier multiplication in spectroscopic measurements.

To analyze how a particular detection technique might affect an outcome, John McGuire, a postdoctoral researcher on Klimov’s team, investigated carrier multiplication using two different spectroscopic techniques—transient absorption and time-resolved photoluminescence. The results obtained by these two methods were in remarkable agreement, indicating the use of different detection techniques is unlikely to explain discrepancies highlighted by other researchers. Further, although these measurements revealed some sample-to-sample variation in carrier multiplication yields, these variations were much smaller than the spread in reported data.

After ruling out these two potential causes of discrepancies, the researchers focused on effects that could mimic carrier multiplication. One such effect is photoionization of nanocrystals.

“When a nanocrystal absorbs a high-energy photon, an electron can acquire enough energy to escape the material,” Klimov said. “This leaves behind a charged nanocrystal, which contains a positive ‘hole.’ Photogeneration of another electron by a second photon results in a two-hole, one-electron state, reminiscent of one produced by carrier multiplication, which can lead to false positives,” he said.

To evaluate the influence of photoionization, the Los Alamos researchers conducted back-to-back studies of static and stirred solutions of nanocrystals. Stirring removes charged nanocrystals from the measured region of the sample. Therefore, when they subject crystals to light, the stirring eliminates the possibility that charged nanocrystals will absorb a second photon. While stirring of some samples did not affect the results of the measurements, other samples showed a significant difference in the apparent carrier multiplication yields measured under static and stirred conditions. Since most previous studies occurred on static samples, these results suggest discrepancies noted by other researchers arise at least in part from uncontrolled photoionization, which stirring seeks to eliminate.

The Los Alamos researchers re-evaluated carrier multiplication efficiencies with suppressed photoionization. The results are encouraging, they said.

While the newly measured electron yields are lower than previously reported, the efficiency of carrier multiplication is still greater than in bulk solids. Specifically, the energetic onset and the energy required to generate an extra electron in nanocrystals are about half of those in bulk solids.

These results indicate significant promise for nanosized crystals as efficient harvesters of solar radiation. “Researchers still have a lot of work to do,” Klimov said.

“One important challenge is to figure out how to design a material in which the energetic cost to create an extra electron can approach the limit defined by a semiconductor band gap,” he said. “Such a material could raise the fundamental power conversion limit of a solar cell from 31% to above 40%.”

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Wind energy gets boost

Thursday, January 8th, 2009

of the cleanest sources of energy touted these days is wind power. After all, it is a clean and inexhaustible energy source found pretty much anywhere around the globe.

But the problem is the wind is intermittent, and so the power output of wind farms can be variable.

That is where Asghar Abedini, Goran Mandic, and Adel Nasiri at the Department of Electrical Engineering and Computer Science, Power Electronics and Motor Drives Laboratory, University of Wisconsin-Milwaukee, come in. They created a plan to solve the issue of the electricity grid being susceptible to changes in wind speed.

Proposed measures to smooth these power fluctuations usually involve the installation of units of batteries or capacitors to store electricity on good days and release their energy on slow days or at times when wind speeds are too high for system stability.

Technology to smooth the power supply and prevent blackouts due to the tripping of safety switches when electricity frequency deviates wildly is also essential.

Despites its deficiencies, a report from the U.S. Department of Energy said installed wind energy capacity could reach 300 gigawatts by 2030 to meet a fifth of the U.S. electricity demand.

The researchers created a novel control method that can mitigate power fluctuations using the inertia of the wind turbine’s rotor as an energy storage component.

Simply put, they have created a braking control algorithm that adjusts the rotor speed so when incoming wind power is greater than the average power, the rotor can speed up so it can store the excess energy as kinetic energy rather than generating electricity. This energy then releases when the wind power falls below average.

This approach precludes the need for external energy storage facilities such as capacitors and the additional infrastructure and engineering they entail, team members said.

Their method also captures wind energy more effectively and so improves the overall efficiency of wind farming potentially reducing the number of turbines required at any given site.

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New nano approach sheds new light on solar energy

Thursday, January 8th, 2009

Generating electricity from sunlight at a modest cost remains a challenge.

With that in mind, a new approach to solar cells—lacing them with nanoscopic metal particles—has the potential to hike the ability of solar cells to harvest light efficiently.

Like plants, solar cells turn light into energy. Plants do this inside vegetable matter, while solar cells do it in a semiconductor crystal doped with extra atoms. Current solar cells cannot convert all the incoming light into usable energy because some of the light can escape back out of the cell into the air. Additionally, sunlight comes in a variety of colors, and the cell might be more efficient at converting bluish light while being less efficient at converting reddish light.

The nanoparticle approach seeks to remedy these problems, said Kylie Catchpole and Albert Polman, authors of a paper on the subject.

The key is creating a tiny electrical disturbance called a “surface Plasmon,” they said. When light strikes a piece of metal, it can set up waves in the surface of the metal. These waves of electrons then move about like ripples on the surface of a pond. If the metal is in the form of a tiny particle, the incoming light can make the particle vibrate, thus effectively scattering the light. If the light is at certain “resonant” colors, the scattering process is particularly strong.

When a thin coating of nanoscopic (a billionth of a meter in size) metal particles go on a solar cell, interesting things happen, Catchpole and Polman said. First of all, the use of nanoparticles causes the incoming sunlight to scatter more fully, keeping more of the light inside the solar cell. Second, varying the size and material of the particles allows researchers to improve light capture at otherwise poorly-performing colors.

In their work, carried out at the FOM Institute for Atomic and Molecular Physics in The Netherlands, Catchpole and Polman showed light capture for long-wavelength (reddish) light could improve by a factor of more than ten. Previously, Catchpole and co-workers at the University of New South Wales showed overall light-gathering efficiency for solar cells using metallic nanoparticles can improve by 30%.

“I think we are about three years from seeing plasmons in photovoltaic generation,” said Catchpole, who has now started a new group studying surface plasmons at the Australian National University. “An important point about plasmonic solar cells is that they are applicable to any kind of solar cell.” This includes the standard silicon or newer thin-film types.

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Energy harvesting sensors bridge safety gap

Thursday, December 18th, 2008

Bridge sensors allow for monitoring its safety, but the problem is when you have to change the batteries on the hundreds of sensors across the span.

That is where energy harvesting comes in. An energy-harvesting radio could transmit important data like stress measurements on a bridge without needing a change of batteries, said engineers at Kansas State University, who are working with a semiconductor manufacturer to implement the idea.

“This type of radio technology may exist in your house, for instance if you have a temperature sensor outside that radios data to a display inside,” said Bill Kuhn, K-State professor of electrical and computer engineering. “But those devices need to have their batteries changed. This radio doesn’t.”

San Diego-based Peregrine Semiconductor is looking at possible applications for the technology. This could include monitoring stress, temperature, and pressure on bridges and other structures.

Ron Reedy, Peregrine’s chief technical officer, said the concept requires highly integrated, low power radio chips, which K-State and Peregrine demonstrated to NASA’s Jet Propulsion Laboratory.

K-State engineers are looking at the design challenges of a radio system like this. Kuhn and Xiaohu Zhang, a Master’s student in electrical engineering, have been working on the project for a little more than a year. They are creating a demonstration to test how far the signals can travel from the sensors.

Zhang constructed a demonstration board using solar cells from inexpensive calculators to power the radio. The board has capacitors that capture and store the light energy to power the radio without a battery. Although this prototype captures and stores light energy, Kuhn said energy-harvesting radios could get power a number of different ways, including by electrochemical, mechanical, or thermal energy.

The demonstration board Zhang created includes a microprocessor to store data before transmitting via radio. The radio used is the “Mars chip” Kuhn and his team helped develop for NASA. They developed a micro transceiver to use on Mars rovers and scouts.

Kuhn said the energy-harvesting radio they are working on now is an example of a NASA spinoff.

When the stored data is ready for transmission, the radio sends out a data-burst. In Zhang’s model, this happens every five seconds. It may just sound like a “blip,” but that burst contains data a computer can translate into meaningful information, such as telling an engineer the stress or strain on the underside of a bridge. Kuhn said it is kind of like sending a text message from one cell phone to another: After data transmit through the air, the recipient’s cell phone turns that data back into understandable text.

Kuhn and Zhang are working toward perfecting the radio system design. This includes determining which frequencies to use based on how the environment affects radio waves indoors versus outdoors. They also have to look at how noise and other factors may limit the sensitivity of the receiver that is getting the data from all the sensors.

Because these sensors save data in their microprocessors, Kuhn and Zhang are working on timing and wake-up commands that tell the sensors when to send the stored information to the receiver. Through engineering analysis, they are determining tradeoffs between power requirements, data-rate, and transmission range issues.

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Microalgae offer green hope for alternative fuel

Thursday, December 18th, 2008

A green plant may be too green, but if a new process holds true, it may help bring in more green.

In pursuing cleaner energy, unicellular microalgae may work out as a possible source in the pursuit of a viable biofuel, said scientists at the University of California, Berkeley.

By genetically modifying the tiny organisms, the researchers can minimize the number of chlorophyll molecules needed to harvest light without compromising the photosynthesis process in the cells. With this modification, instead of making more sugar molecules, the microalgae could produce hydrogen or hydrocarbons.

Researchers discovered the genetic instructions in the algae genome responsible for deploying 600 chlorophyll molecules in the cell’s light-gathering antennae. Algae can get along with as few as 130 molecules, the researchers said. Basically they divert the normal function of photosynthesis from generating biomass to making products such as lipids, hydrocarbons, and hydrogen.

The algae’s chlorophyll antennae help the organisms compete for sunlight absorption and survive in the wild, where there is limited sunlight, said Tasios Melis, a scientist at Berkeley and an author of a paper on the subject. The problem is that may also be detrimental to the engineering-driven effort of using algae to convert sunlight into biofuel, Melis said.

“Cellular optics” describes the effort to maximize the efficiency of the solar-to-product conversion process, Melis said. Besides getting the algae to convert more sunlight to fuel, another issue that needs addressing is how to configure bio-culture tanks in such a way that sunlight can penetrate the outer layer of algae so lower-down layers can participate in the photo-conversion, too.

Microalgae are ideal because of their high rate of photosynthesis; they are perhaps 10 times more efficient in this than sugarcane, corn, and switchgrass, which researchers say are possible biofuel stocks.

What is the timetable for algae to become a viable fuel?

“Progress is substantial to date, but not enough to make the process commercially competitive with fossil fuels,” Melis said. “Further improvement in the performance of photosynthesis under mass culture conditions, and in the yield of biofuels by the microalgae are needed before a cost parity with traditional fuels can be achieved.”

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