Floating Solar Panels: Solar Installations on Water

Developed by a Franco-Israeli partnership,* this innovative solar power technology introduces a new paradigm in energy production. Solar power plays a dominant role in the world-wide effort to reduce greenhouse gases, it is considered a clean energy and is an efficient source of electricity. Yet several obstacles have been undermining the expansion of this sector and many of its actors are looking for a new approach towards the markets.

A win-win Situation

Soon after the design phase was over, at the end of March 2010, the fabrication of a prototype began and the team is now aiming to launch the implementation phase in September 2011. The tests will take place at Cadarache, in the South East of France, the site having a privileged position on the French electric grid and being close to a local hydro-electric facility providing the water surface to be used for the installation of the system. It will operate on-site during a period of nine months, while assessing the system's performances and productivity through seasonal changes and various water levels. The research team members believe that by June 2012, they will have all the information required to allow the technology's entry on the market.

As even leading photovoltaic companies struggle to find land on which to install solar power plants, the project team identified the almost untouched potential of solar installations on water. The water basins, on which the plants could be built, are not natural reserves, tourists' resorts or open sea; rather they are industrial water basins already in use for other purposes. By that, it is assured that the new solar plants will not have a negative impact on natural landscapes."It's a win-win situation," declares Dr. Kassel,"since there are many water reservoirs with energy, industrial or agricultural uses that are open for energy production use."

After solving the question of space, the team also took on the problem of cost."It sounds magical to combine sun and water to produce electricity, but we also have to prove that it carries a financial logic for the long run," explains Dr. Kassel. The developers were able to reduce the costs linked to the implementation of the technology by two means. First they reduced the quantity of solar cells used thanks to a sun energy concentration system based on mirrors, while keeping steady the amount of power produced.

Made of modules

Secondly, the team used a creative cooling system using the water on which the solar panels are floating. Thanks to this efficient cooling method, the photovoltaic system can use silicon solar cells, which tend to experience problems linked to overheating and need to be cooled down in order to allow the system to work correctly, unlike standard type more expensive cells. The particular type of solar cell used also allows a higher efficiency than the standard ones, achieving both reliability and cost reduction.

Still for the purpose of making the technology efficient and ready to market, the system is designed in such way that on a solar platform it is possible to assemble as many identical modules as needed for the power rating desired. Each module produces a standard amount of 200 kiloWatt electricity, and more power can be achieved by simply adding more modules to the plant.

The team also worked on the environmental impact of the technology. It works in fact as a breathing surface through which oxygen can penetrate to the water. This feature ensures that sufficient oxygen will maintain the underwater life of plants and animals. Dr. Kassel adds:"One of the implementation phase's goals is to closely monitor the possible effects of this new technology on the environment with the help of specialists" and"a preliminary check shows no detrimental environmental impact on water quality, flora or fauna. Our choices of materials were always made with this concern in mind."

*The project results from a collaboration between Solaris Synergy from Israel and the EDF Group from France. EUREKA provided the supporting platform which allowed to enhance both companies' partnership. After receiving the"EUREKA label" the project, called AQUASUN, found also support from the Israeli Ministry of Industry, Trade and Labor.

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New Stretchable Solar Cells Will Power Artificial Electronic 'Super Skin'

Super skin, indeed.

"With artificial skin, we can basically incorporate any function we desire," said Bao, a professor of chemical engineering."That is why I call our skin 'super skin.' It is much more than what we think of as normal skin."

The foundation for the artificial skin is a flexible organic transistor, made with flexible polymers and carbon-based materials. To allow touch sensing, the transistor contains a thin, highly elastic rubber layer, molded into a grid of tiny inverted pyramids. When pressed, this layer changes thickness, which changes the current flow through the transistor. The sensors have from several hundred thousand to 25 million pyramids per square centimeter, corresponding to the desired level of sensitivity.

To sense a particular biological molecule, the surface of the transistor has to be coated with another molecule to which the first one will bind when it comes into contact. The coating layer only needs to be a nanometer or two thick.

"Depending on what kind of material we put on the sensors and how we modify the semiconducting material in the transistor, we can adjust the sensors to sense chemicals or biological material," she said.

Bao's team has successfully demonstrated the concept by detecting a certain kind of DNA. The researchers are now working on extending the technique to detect proteins, which could prove useful for medical diagnostics purposes.

"For any particular disease, there are usually one or more specific proteins associated with it -- called biomarkers -- that are akin to a 'smoking gun,' and detecting those protein biomarkers will allow us to diagnose the disease," Bao said.

The same approach would allow the sensors to detect chemicals, she said. By adjusting aspects of the transistor structure, the super skin can detect chemical substances in either vapor or liquid environments.

Regardless of what the sensors are detecting, they have to transmit electronic signals to get their data to the processing center, whether it is a human brain or a computer.

Having the sensors run on the sun's energy makes generating the needed power simpler than using batteries or hooking up to the electrical grid, allowing the sensors to be lighter and more mobile. And having solar cells that are stretchable opens up other applications.

A recent research paper by Bao, describing the stretchable solar cells, will appear in an upcoming issue ofAdvanced Materials. The paper details the ability of the cells to be stretched in one direction, but she said her group has since demonstrated that the cells can be designed to stretch along two axes.

The cells have a wavy microstructure that extends like an accordion when stretched. A liquid metal electrode conforms to the wavy surface of the device in both its relaxed and stretched states.

"One of the applications where stretchable solar cells would be useful is in fabrics for uniforms and other clothes," said Darren Lipomi, a graduate student in chemical engineering in Bao's lab and lead author of the paper.

"There are parts of the body, at the elbow for example, where movement stretches the skin and clothes," he said."A device that was only flexible, not stretchable, would crack if bonded to parts of machines or of the body that extend when moved." Stretchability would be useful in bonding solar cells to curved surfaces without cracking or wrinkling, such as the exteriors of cars, lenses and architectural elements.

The solar cells continue to generate electricity while they are stretched out, producing a continuous flow of electricity for data transmission from the sensors.

Bao said she sees the super skin as much more than a super mimic of human skin; it could allow robots or other devices to perform functions beyond what human skin can do.

"You can imagine a robot hand that can be used to touch some liquid and detect certain markers or a certain protein that is associated with some kind of disease and the robot will be able to effectively say, 'Oh, this person has that disease,'" she said."Or the robot might touch the sweat from somebody and be able to say, 'Oh, this person is drunk.'"

Finally, Bao has figured out how to replace the materials used in earlier versions of the transistor with biodegradable materials. Now, not only will the super skin be more versatile and powerful, it will also be more eco-friendly.

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High-Efficiency Photovoltaic Cells Developed

The cells developed by the UPC researchers have surpassed the 15% barrier -- the average efficiency of the most common photovoltaic cells. Specifically, a conversion efficiency (of incident light to electric power) of 20.5% has been achieved, which means the energy produced per unit of area can be increased by one third.

For example, thanks to the high efficiency of this new cell type, only 4.8 m² of photovoltaic panels would be needed to meet one family's annual energy needs (an average of about 4 kWh per day). This compares to an area of 6.5 m² for traditional cells.

The cells are made of crystalline silicon and work in a simple way, much as conventional cells do. The light captured by the cells generates charges that are drawn off at the panel contacts and transformed into an electric current."The goal is to generate a lot of charges that don't get lost -- that make it to the contacts," says Alcubilla, a member of the research group. Finally, after the light from the sun has been converted into electric current, it is fed into the power grid for domestic and industrial use.

The key to the success of the project was therefore to minimize losses, and by pursuing this approach the UPC researchers have managed to produce the most efficient silicon cells in Spain."We've done a lot of work on the conception and development of new materials and structures, and on the technology needed to optimize the entire process and achieve high levels of efficiency," says Alcubilla. The next step is to develop procedures that facilitate large-scale production.

The result achieved in this research (which has involved 38 trials since 2002) is comparable to those obtained in other research projects carried out in countries that are taking the lead in the field of photovoltaic energy. The maximum efficiency obtained for cells of this type is 24.7%, a record set by an Australian group at the University of New South Wales.

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'Tall Order' Sunlight-to-Hydrogen System Works, Neutron Analysis Confirms

Photosynthesis, the natural process carried out by plants, algae and some bacterial species, converts sunlight energy into chemical energy and sustains much of the life on earth. Researchers have long sought inspiration from photosynthesis to develop new materials to harness the sun's energy for electricity and fuel production.

In a step toward synthetic solar conversion systems, the ORNL researchers have demonstrated and confirmed with small-angle neutron scattering analysis that light harvesting complex II (LHC-II) proteins can self-assemble with polymers into a synthetic membrane structure and produce hydrogen.

The researchers envision energy-producing photoconversion systems similar to photovoltaic cells that generate hydrogen fuel, comparable to the way plants and other photosynthetic organisms convert light to energy.

"Making a, self-repairing synthetic photoconversion system is a pretty tall order. The ability to control structure and order in these materials for self-repair is of interest because, as the system degrades, it loses its effectiveness," ORNL researcher Hugh O'Neill, of the lab's Center for Structural Molecular Biology, said.

"This is the first example of a protein altering the phase behavior of a synthetic polymer that we have found in the literature. This finding could be exploited for the introduction of self-repair mechanisms in future solar conversion systems," he said.

Small angle neutron scattering analysis performed at ORNL's High Flux Isotope Reactor (HFIR) showed that the LHC-II, when introduced into a liquid environment that contained polymers, interacted with polymers to form lamellar sheets similar to those found in natural photosynthetic membranes.

The ability of LHC-II to force the assembly of structural polymers into an ordered, layered state -- instead of languishing in an ineffectual mush -- could make possible the development of biohybrid photoconversion systems. These systems would consist of high surface area, light-collecting panes that use the proteins combined with a catalyst such as platinum to convert the sunlight into hydrogen, which could be used for fuel.

The research builds on previous ORNL investigations into the energy-conversion capabilities of platinized photosystem I complexes -- and how synthetic systems based on plant biochemistry can become part of the solution to the global energy challenge.

"We're building on the photosynthesis research to explore the development of self-assembly in biohybrid systems. The neutron studies give us direct evidence that this is occurring," O'Neill said.

The researchers confirmed the proteins' structural behavior through analysis with HFIR's Bio-SANS, a small-angle neutron scattering instrument specifically designed for analysis of biomolecular materials.

"Cold source" neutrons, in which energy is removed by passing them through cryogenically chilled hydrogen, are ideal for studying the molecular structures of biological tissue and polymers.

The LHC-II protein for the experiment was derived from a simple source: spinach procured from a local produce section, then processed to separate the LHC-II proteins from other cellular components. Eventually, the protein could be synthetically produced and optimized to respond to light.

O'Neill said the primary role of the LHC-II protein is as a solar collector, absorbing sunlight and transferring it to the photosynthetic reaction centers, maximizing their output."However, this study shows that LHC-II can also carry out electron transfer reactions, a role not known to occur in vivo," he said.

The research team, which came from various laboratory organizations including its Chemical Sciences Division, Neutron Scattering Sciences Division, the Center for Structural Molecular Biology and the Center for Nanophase Materials Sciences, consisted of O'Neill, William T. Heller, and Kunlun Hong, all of ORNL; Dimitry Smolensky of the University of Tennessee; and Mateus Cardoso, a former postdoctoral researcher at ORNL now of the Laboratio Nacional de Luz Sincrotron in Brazil.

"That's one of the nice things about working at a national laboratory. Expertise is available from a variety of organizations," O'Neill said.

The work, published in the journalEnergy& Environmental Science, was supported with Laboratory-Directed Research and Development funding. HFIR is supported by the DOE Office of Science.

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World Can Be Powered by Alternative Energy, Using Today's Technology, in 20-40 Years, Experts Say

According to a new study coauthored by Stanford researcher Mark Z. Jacobson, we could accomplish all that by converting the world to clean, renewable energy sources and forgoing fossil fuels.

"Based on our findings, there are no technological or economic barriers to converting the entire world to clean, renewable energy sources," said Jacobson, a professor of civil and environmental engineering."It is a question of whether we have the societal and political will."

He and Mark Delucchi, of the University of California-Davis, have written a two-part paper inEnergy Policyin which they assess the costs, technology and material requirements of converting the planet, using a plan they developed.

The world they envision would run largely on electricity. Their plan calls for using wind, water and solar energy to generate power, with wind and solar power contributing 90 percent of the needed energy.

Geothermal and hydroelectric sources would each contribute about 4 percent in their plan (70 percent of the hydroelectric is already in place), with the remaining 2 percent from wave and tidal power.

Vehicles, ships and trains would be powered by electricity and hydrogen fuel cells. Aircraft would run on liquid hydrogen. Homes would be cooled and warmed with electric heaters -- no more natural gas or coal -- and water would be preheated by the sun.

Commercial processes would be powered by electricity and hydrogen. In all cases, the hydrogen would be produced from electricity. Thus, wind, water and sun would power the world.

The researchers approached the conversion with the goal that by 2030, all new energy generation would come from wind, water and solar, and by 2050, all pre-existing energy production would be converted as well.

"We wanted to quantify what is necessary in order to replace all the current energy infrastructure -- for all purposes -- with a really clean and sustainable energy infrastructure within 20 to 40 years," said Jacobson.

One of the benefits of the plan is that it results in a 30 percent reduction in world energy demand since it involves converting combustion processes to electrical or hydrogen fuel cell processes. Electricity is much more efficient than combustion.

That reduction in the amount of power needed, along with the millions of lives saved by the reduction in air pollution from elimination of fossil fuels, would help keep the costs of the conversion down.

"When you actually account for all the costs to society -- including medical costs -- of the current fuel structure, the costs of our plan are relatively similar to what we have today," Jacobson said.

One of the biggest hurdles with wind and solar energy is that both can be highly variable, which has raised doubts about whether either source is reliable enough to provide"base load" energy, the minimum amount of energy that must be available to customers at any given hour of the day.

Jacobson said that the variability can be overcome.

"The most important thing is to combine renewable energy sources into a bundle," he said."If you combine them as one commodity and use hydroelectric to fill in gaps, it is a lot easier to match demand."

Wind and solar are complementary, Jacobson said, as wind often peaks at night and sunlight peaks during the day. Using hydroelectric power to fill in the gaps, as it does in our current infrastructure, allows demand to be precisely met by supply in most cases. Other renewable sources such as geothermal and tidal power can also be used to supplement the power from wind and solar sources.

"One of the most promising methods of insuring that supply matches demand is using long-distance transmission to connect widely dispersed sites," said Delucchi. Even if conditions are poor for wind or solar energy generation in one area on a given day, a few hundred miles away the winds could be blowing steadily and the sun shining.

"With a system that is 100 percent wind, water and solar, you can't use normal methods for matching supply and demand. You have to have what people call a supergrid, with long-distance transmission and really good management," he said.

Another method of meeting demand could entail building a bigger renewable-energy infrastructure to match peak hourly demand and use the off-hours excess electricity to produce hydrogen for the industrial and transportation sectors.

Using pricing to control peak demands, a tool that is used today, would also help.

Jacobson and Delucchi assessed whether their plan might run into problems with the amounts of material needed to build all the turbines, solar collectors and other devices.

They found that even materials such as platinum and the rare earth metals, the most obvious potential supply bottlenecks, are available in sufficient amounts. And recycling could effectively extend the supply.

"For solar cells there are different materials, but there are so many choices that if one becomes short, you can switch," Jacobson said."Major materials for wind energy are concrete and steel and there is no shortage of those."

Jacobson and Delucchi calculated the number of wind turbines needed to implement their plan, as well as the number of solar plants, rooftop photovoltaic cells, geothermal, hydroelectric, tidal and wave-energy installations.

They found that to power 100 percent of the world for all purposes from wind, water and solar resources, the footprint needed is about 0.4 percent of the world's land (mostly solar footprint) and the spacing between installations is another 0.6 percent of the world's land (mostly wind-turbine spacing), Jacobson said.

One of the criticisms of wind power is that wind farms require large amounts of land, due to the spacing required between the windmills to prevent interference of turbulence from one turbine on another.

"Most of the land between wind turbines is available for other uses, such as pasture or farming," Jacobson said."The actual footprint required by wind turbines to power half the world's energy is less than the area of Manhattan." If half the wind farms were located offshore, a single Manhattan would suffice.

Jacobson said that about 1 percent of the wind turbines required are already in place, and a lesser percentage for solar power.

"This really involves a large scale transformation," he said."It would require an effort comparable to the Apollo moon project or constructing the interstate highway system."

"But it is possible, without even having to go to new technologies," Jacobson said."We really need to just decide collectively that this is the direction we want to head as a society."

Jacobson is the director of Stanford's Atmosphere/Energy Program and a senior fellow at Stanford's Woods Institute for the Environment and the Precourt Institute for Energy.

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Practical Full-Spectrum Solar Cell Comes Closer

Although full-spectrum solar cells have been made, none yet have been suitable for manufacture at a consumer-friendly price. Now Wladek Walukiewicz, who leads the Solar Energy Materials Research Group in the Materials Sciences Division (MSD) at the U.S. Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab), and his colleagues have demonstrated a solar cell that not only responds to virtually the entire solar spectrum, it can also readily be made using one of the semiconductor industry's most common manufacturing techniques.

The new design promises highly efficient solar cells that are practical to produce. The results are reported in a recent issue ofPhysical Review Letters.

How to make a full-spectrum solar cell

"Since no one material is sensitive to all wavelengths, the underlying principle of a successful full-spectrum solar cell is to combine different semiconductors with different energy gaps," says Walukiewicz.

One way to combine different band gaps is to stack layers of different semiconductors and wire them in series. This is the principle of current high-efficiency solar cell technology that uses three different semiconductor alloys with different energy gaps. In 2002, Walukiewicz and Kin Man Yu of Berkeley Lab's MSD found that by adjusting the amounts of indium and gallium in the same alloy, indium gallium nitride, each different mixture in effect became a different kind of semiconductor that responded to different wavelengths. By stacking several of the crystalline layers, all closely matched but with different indium content, they made a photovoltaic device that was sensitive to the full solar spectrum.

However, says Walukiewicz,"Even when the different layers are well matched, these structures are still complex -- and so is the process of manufacturing them. Another way to make a full-spectrum cell is to make a single alloy with more than one band gap."

In 2004 Walukiewicz and Yu made an alloy of highly mismatched semiconductors based on a common alloy, zinc (plus manganese) and tellurium. By doping this alloy with oxygen, they added a third distinct energy band between the existing two -- thus creating three different band gaps that spanned the solar spectrum. Unfortunately, says Walukiewicz,"to manufacture this alloy is complex and time-consuming, and these solar cells are also expensive to produce in quantity."

The new solar cell material from Walukiewicz and Yu and their colleagues in Berkeley Lab's MSD and RoseStreet Labs Energy, working with Sumika Electronics Materials in Phoenix, Arizona, is another multiband semiconductor made from a highly mismatched alloy. In this case the alloy is gallium arsenide nitride, similar in composition to one of the most familiar semiconductors, gallium arsenide. By replacing some of the arsenic atoms with nitrogen, a third, intermediate energy band is created. The good news is that the alloy can be made by metalorganic chemical vapor deposition (MOCVD), one of the most common methods of fabricating compound semiconductors.

How band gaps work

Band gaps arise because semiconductors are insulators at a temperature of absolute zero but inch closer to conductivity as they warm up. To conduct electricity, some of the electrons normally bound to atoms (those in the valence band) must gain enough energy to flow freely -- that is, move into the conduction band. The band gap is the energy needed to do this.

When an electron moves into the conduction band it leaves behind a"hole" in the valence band, which also carries charge, just as the electrons in the conduction band; holes are positive instead of negative.

A large band gap means high energy, and thus a wide-band-gap material responds only to the more energetic segments of the solar spectrum, such as ultraviolet light. By introducing a third band, intermediate between the valence band and the conduction band, the same basic semiconductor can respond to lower and middle-energy wavelengths as well.

This is because, in a multiband semiconductor, there is a narrow band gap that responds to low energies between the valence band and the intermediate band. Between the intermediate band and the conduction band is another relatively narrow band gap, one that responds to intermediate energies. And finally, the original wide band gap is still there to take care of high energies.

"The major issue in creating a full-spectrum solar cell is finding the right material," says Kin Man Yu."The challenge is to balance the proper composition with the proper doping."

In solar cells made of some highly mismatched alloys, a third band of electronic states can be created inside the band gap of the host material by replacing atoms of one component with a small amount of oxygen or nitrogen. In so -- called II-VI semiconductors (which combine elements from these two groups of Mendeleev's original periodic table), replacing some group VI atoms with oxygen produces an intermediate band whose width and location can be controlled by varying the amount of oxygen. Walukiewicz and Yu's original multiband solar cell was a II-VI compound that replaced group VI tellurium atoms with oxygen atoms. Their current solar cell material is a III-V alloy. The intermediate third band is made by replacing some of the group V component's atoms -- arsenic, in this case -- with nitrogen atoms.

Finding the right combination of alloys, and determining the right doping levels to put an intermediate band right where it's needed, is mostly based on theory, using the band anticrossing model developed at Berkeley Lab over the past 10 years.

"We knew that two-percent nitrogen ought to do the job," says Yu."We knew where the intermediate band ought to be and what to expect. The challenge was designing the actual device."

Passing the test

Using their new multiband material as the core of a test cell, the researchers illuminated it with the full spectrum of sunlight to measure how much current was produced by different colors of light. The key to making a multiband cell work is to make sure the intermediate band is isolated from the contacts where current is collected.

"The intermediate band must absorb light, but it acts only as a stepping stone and must not be allowed to conduct charge, or else it basically shorts out the device," Walukiewicz explains.

The test device had negatively doped semiconductor contacts on the substrate to collect electrons from the conduction band, and positively doped semiconductor contacts on the surface to collect holes from the valence band. Current from the intermediate band was blocked by additional layers on top and bottom.

For comparison purposes, the researchers built a cell that was almost identical but not blocked at the bottom, allowing current to flow directly from the intermediate band to the substrate.

The results of the test showed that light penetrating the blocked device efficiently yielded current from all three energy bands -- valence to intermediate, intermediate to conduction, and valence to conduction -- and responded strongly to all parts of the spectrum, from infrared with an energy of about 1.1 electron volts (1.1 eV), to over 3.2 eV, well into the ultraviolet.

By comparison, theunblocked device responded well only in the near infrared, declining sharply in the visible part of the spectrum and missing the highest-energy sunlight. Because it was unblocked, the intermediate band had essentially usurped the conduction band, intercepting low-energy electrons from the valence band and shuttling them directly to the contact layer.

Further support for the success of the multiband device and its method of operation came from tests"in reverse" -- operating the device as a light emitting diode (LED). At low voltage, the device emitted four peaks in the infrared and visible light regions of the spectrum. Primarily intended as a solar cell material, this performance as an LED may suggest additional possibilities for gallium arsenide nitride, since it is a dilute nitride very similar to the dilute nitride, indium gallium arsenide nitride, used in commercial"vertical cavity surface-emitting lasers" (VCSELs), which have found wide use because of their many advantages over other semiconductor lasers.

With the new, multiband photovoltaic device based on gallium arsenide nitride, the research team has demonstrated a simple solar cell that responds to virtually the entire solar spectrum -- and can readily be made using one of the semiconductor industry's most common manufacturing techniques. The results promise highly efficient solar cells that are practical to produce.

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