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Here comes the sun

Posted: 01 Jul 2006 ?? ?Print Version ?Bookmark and Share

Keywords:solar cells? hydrogen fuel cells? Photovoltaic solar cells? Gartner Dataquest?

Sticker shock at the gas pumps combined with concerns over the diminishing supply of non-renewable energy sources like coal, oil and natural gas have added new urgency to the search for alternatives. Riding to the rescue are technology-based resources that will never run out because they are renewable and environmentally friendly, do not add to the greenhouse effect and are increasingly cheap to harvest, thanks to continuing technical breakthroughs.

Solar is here today, but at about three times the cost of conventionally generated electricity (18 to 22 cents/kW, compared with 5 to 10 cents/kW for conventional). However, thanks to advances including the use of "plastic" solar cells to replace the more expensive silicon versions, the U.S. Department of Energy (DOE) believes the cost of solar will be at par with that of conventional electricity within 10 years. By that time, two other contenders!hydrogen fuel cells and nanoscale electric generators!will be at about the same stage of development as solar cells are today.

Photovoltaic solar cells work by absorbing units of light, or photons, in a semiconductor, thus energizing its electrons enough to drive circuitry. "There is of course a huge interest in solar cells worldwide," said Jim Tully, VP and chief of semiconductor research at Gartner Dataquest. "The Japanese government, for example, expects that 50 percent of power for homes there will be from solar sources by 2030." Japan holds just 20 percent of the $3 billion to $4 billion world solar cell market today, according to Solarbuzz LLC. Germany has the greatest number of solar installations, accounting for 57 percent of the total in 2005, with the United States at only 7 percent, the rest of Europe at 6 percent and the rest of the world at 10 percent.

Of the various solar cells available, efficiencies range from about 6 percent for the least expensive amorphous-silicon models on glass or plastic substrates to as high as 30 percent for multijunction GaAs cells on monocrystalline wafers, which cost up to 100 times more. Monocrystalline and polycrystalline silicon solar cells, the most popular types, have efficiencies ranging from about 10 percent to 18 percent. Ready-to-install modules sell for about $4 per watt.

Devices with lower conversion efficiency than mono- or polycrystalline silicon cells include amorphous silicon, cadmium telluride, copper indium diselenide and other similar alloys.

Monocrystalline and polycrystalline silicon solar cells hold 93 percent of the worldwide market today, according to Solarbuzz, and offer electricity for about 18 to 22 cents/kW. One sign of their promise: The world's largest vendor of chip-fabrication equipment, Applied Materials Inc., has expanded its product portfolio to include thin-film solar cells with its recent acquisition of Applied Films for a record $464 million in cash.

Thin-film solar cells on glass or stainless-steel substrates already have a 7 percent worldwide market share. They are less efficient than silicon cells, at 5 percent to 8 percent, but correspondingly cheaper. Shell Solar just sold its mono- and polycrystalline silicon solar cell unit to Germany's solar powerhouse, Solarworld AG, to concentrate exclusively on thin-film solar cells deposited on inexpensive glass substrates.

Since wafers account for up to 50 percent of the cost of silicon solar cells, these non-silicon cells slash costs by just switching to inexpensive glass, stainless steel or even flexible polymer (plastic) substrates. Formulations include thin films of amorphous silicon, nanocrystalline silicon, cadmium telluride, copper indium (gallium) diselenide and other inorganic alloys.

The DOE's National Renewable Energy Laboratory reported last year that HelioVolt Corp. had succeeded with its copper indium gallium selenide-based thin-film photovoltaic cells. HelioVolt says that its patented intra-absorber junction depends on depositing two films and forming a semiconducting junction between them by using a heated template to "print" the crystalline lattice structure into the thin film. Using a flash-heating technique similar to anodic wafer bonding, the technique forces selenium into the crystalline lattice with a reusable template capable of mass-producing the material.

Poised over the horizon with the promise of large-area solar cells are dye-stabilized and organic-polymer (plastic) cells. However, organic materials have lower carrier mobility and lower current-carrying capabilities than traditional inorganic materials. Moreover, they can't match today's silicon cell longevity of 25 years.

"Plastic devices using inkjet-style manufacturing hold great promise, but in general suffer from relatively low charge-carrier mobility today," said Tully of Gartner Dataquest. "That's certainly improving, but it represents a disadvantage in the short term." Tully believes that initially, plastic solar cells "will be restricted to applications with relatively low-frequency and low-power uses. But even so, there could still be significant opportunities for low-power personal electronic devices."

Konarka Technologies Inc. is treading that decade-long path to organic solar cells. Konarka recently received $6 million from DARPA (on top of $60 million in venture capital it already had) to develop organic solar cells on flexible plastic substrates. "We are printing organic chemical solar cells onto a flexible polymer substrate that can be either transparent or opaque, depending on the formulation," said chief marketing officer Daniel McGahn. The company uses a low-temperature roll-to-roll manufacturing process similar to processing photographic film, and envisions its first products as small, light-activated, flexible "rechargers" that can absorb any kind of light (even from a desk lamp) to recharge cellphones, laptops and similar small electronic devices. For the military, Konarka is also developing a portable electric-generating tent. A flexible polymer coating on the outside turns the tent into an electricity generator for the equipment housed within it.

"When the spectral response of these plastic solar cells is widened to include infrared, they could potentially be woven into clothing to generate electrical energy from body heat in the dark and from sunlight during the day," said analyst Tully.

Konarka has developed formulations that cover different parts of the spectrum, concentrating on two basic types of solar cells!dye-sensitized cells with chemical cores and those with nanoparticle "grains" embedded in a polymer-matrix-like film.

"We are printing organic-chemical solar cells onto a flexible polymer substrate that can be either transparent or opaque, depending on the formulation," said McGahn. "Our material is very similar to photographic-film negative material, but our stack of materials is even simpler than photographic film." The material can also be screen-printed for camouflage.

Konarka reports 11 percent efficiencies from its dye-based solar cells and as much as 8 percent for its organic cells in the lab. It plans to partner with plastics manufacturers to make their building materials solar!such as adding a tinted layer to windows or solar-power-generating capabilities to wallboard. "Many of our formulations will be called architectural because we are engineering them to be more like the material for an awning than a module attached to a building," McGahn said.

Hydrogen fuel cells
Although hydrogen fuel cells are still a decade away from widespread use, the technology behind them is advancing and new architectures are beginning to appear. The story is much the same for nanoscale electricity generators that could someday keep batteries constantly charged!or eliminate them altogether.

Hydrogen fuel cells traditionally work by keeping fuel and oxygen in separate, adjacent compartments. As fuel enters the first compartment, it reacts with the anode catalyst, which breaks the fuel's molecular bonds so that its electrons can flow out the anode to power equipment. In the second compartment, oxygen flows over the cathode catalyst, which uses the electricity's return line from the load to bond oxygen to the leftover fuel's free hydrogen, resulting in water as exhaust.

Platinum is generally used for both the anode and cathode because it's good at both oxidizing fuel and reducing oxygen to water. But the fuel and oxygen must be kept completely separate; otherwise both will react on the same platinum electrode, producing combustion instead of electricity.

Newer fuel cell designs replace the metal flow-field plate that traditionally separates adjacent cells with a thin, perforated polymer membrane. Then, by coating the anode and cathode with a selective catalyst instead of using platinum, the fuel and oxygen may be mixed. The catalyst produces only the desired reaction, either breaking down fuel into hydrogen molecules at the anode or bonding free hydrogen to oxygen at the cathode.

The DOE has a four-year, $119 million effort under way to identify and overcome the technical and manufacturing challenges associated with developing cheap, commercial hydrogen fuel cells by 2010. The first goal, claiming $100 million of those funds, will seek to improve fuel cell membranes and liquid transport within the stack, and to develop novel cathode catalysts for innovative new designs.

For polymer membrane research, 12 competitively-awarded, cost-shared projects have been funded with $19 million over five years. The goal is to advance the membrane's durability and extend shelf life enough to enable commercially-feasible hydrogen fuel cell systems powerful enough for an automobile.

But before a single vehicle hits the road, U.S. researchers are already proposing organic-chemistry solutions for fuel cells that are "green." For instance, Lars Angenent, an environmental engineer at Washington University in St. Louis, has found a way to use bacteria from wastewater as the selective catalyst on the biofuel cell's anode and cathode instead of platinum. If realized fully, the biofuel cell could neutralize the biological matter while simultaneously producing electricity.

Fuel cells will be used far beyond future electric automobiles, according to MTI MicroFuel Cells Inc. The company has patented a direct-methanol fuel cell using a simplified architecture that permits small handheld units to power cellphones, laptops and the like. MTI Micro promises three to 10 times the "on" time of lithium batteries.

Other researchers have harnessed nanoscale piezoelectric effects to generate electricity from environmental motion. Zhong Lin Wang, a professor at the Georgia Institute of Technology, has created a piezoelectric nanogenerator that converts vibration energy!from acoustic or ultrasonic waves, hydraulic energy, or body fluids or blood flow!into electric energy that can be used to power small devices without requiring a battery.

Organic substitutes for batteries have also been proposed by professors Angela Belcher, Paula Hammond and Yet-Ming Chiang at MIT. The trio has genetically engineered living viruses to assemble thin-film nanowires as the anodes and cathodes of a flexible battery-like film. A film 100nm thick can conform to any shape. A commercial spin-off company, Cambrios Technologies Corp., aims to commercialize MIT's battery-like film.

- R. Colin Johnson
EE Times

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