Powered only by natural sunlight, an array of nanotubes is able to convert a mixture of carbon dioxide and water vapour into natural gas at unprecedented rates.
Such devices offer a new way to take carbon dioxide from the atmosphere and convert it into fuel or other chemicals to cut the effect of fossil fuel emissions on global climate, says Craig Grimes, from Pennsylvania State University, whose team came up with the device.
Although other research groups have developed methods for converting carbon dioxide into organic compounds like methane, often using titanium-dioxide nanoparticles as catalysts, they have needed ultraviolet light to power the reactions.
The researchers' breakthrough has been to develop a method that works with the wider range of visible frequencies within sunlight.
Enhanced activity
The team found it could enhance the catalytic abilities of titanium dioxide by forming it into nanotubes each around 135 nanometres wide and 40 microns long to increase surface area. Coating the nanotubes with catalytic copper and platinum particles also boosted their activity.
The researchers housed a 2-centimetre-square section of material bristling with the tubes inside a metal chamber with a quartz window. They then pumped in a mixture of carbon dioxide and water vapour and placed it in sunlight for three hours.
The energy provided by the sunlight transformed the carbon dioxide and water vapour into methane and related organic compounds, such as ethane and propane, at rates as high as 160 microlitres an hour per gram of nanotubes. This is 20 times higher than published results achieved using any previous method, but still too low to be immediately practical.
If the reaction is halted early the device produces a mixture of carbon monoxide and hydrogen known as syngas, which can be converted into diesel.
Copper boost
"If you tried to build a commercial system using what we have accomplished to date, you'd go broke," admits Grimes. But he is confident that commercially viable results are possible.
"We are now working on uniformly sensitising the entire nanotube array surface with copper nanoparticles, which should dramatically increase conversion rates," says Grimes, by at least two orders of magnitude for a given area of tubes.
This work suggests a "potentially very exciting" application for titanium-dioxide nanotubes, says Milo Shaffer, a nanotube researcher at Imperial College, London. "The high surface area, small critical dimensions, and open structure [of these nanotubes] apparently provide a relatively high activity," he says.
by Jon Evans
Sun-powered Device Converts CO2 into Fuel
Cheaper Materials Could Be Key To Low-cost Solar Cells
Unconventional solar cell materials that are as abundant but 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 globally significant source of low-carbon energy, the study authors said.
The analysis, which appeared online Feb. 13 in Environmental Science & Technology, 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 that 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 today.
The work provides a roadmap for research into novel solar cell types precisely when the U. S. Department of Energy and other funders plan to expand their efforts to link new basic research to deployment efforts as part of a national effort to greatly expand the use of clean energy, according to 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 of whether 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 are still limited by 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 that 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 some recent advances in nanoscale science to argue that the modest efficiency losses of unconventional solar cell materials would be offset 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 percent of the world's population remains without reliable or sufficient electrical energy. Scientific forecasts predict that 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.
The study by is by Wadia, Kammen and Alivisatos and will appear in the March print issue of Environmental Science & Technology.
The work was supported by the U.S. Environmental Protection Agency, the Energy Foundation, the Karsten Family Foundation Endowment of the Renewable and Appropriate Energy Laboratory and the Class of 1935.
Dutch Keep on Buying Expensive e-Bikes!
BREDA, The Netherlands – The economic crisis is not hampering the sale of e-Bikes in The Netherlands. Despite an average retail price of € 1,900 per electric bike the Dutch keep on buying the pedal assisted bicycles in record-breaking numbers. During the first three quarters of 2008 sales were up by 52% in value and 35% in volume.
The Dutch Dealer association BOVAG announced these statistics at its yearly Congress that was held yesterday. With the 35% rise in quantity during the first nine months of 2008, the BOVAG confirmed its earlier expectations that for the whole of 2008 the sale of e-Bikes in the Netherlands will end at 120,000 units and a share of the total bicycle market of about 10%. For the whole of 2007 e-Bike sales stood at 89,000 units (45,000 was the total 2006 number).
The money maker in every bike shop
In terms of turnover e-Bikes scored an even bigger increase. With this 52% plus in value electric bicycles in Holland are growing into the dominant money maker in every bike shop. The Dutch Dealer association said yesterday that e-Bikes currently account for 29% of the turnover made in the whole Dutch two-wheeler sector that next to the sale of new bikes and P&A also includes the sale of mopeds and scooters.
The BOVAG statistics did not mention which brands were the best sellers last year. However, Sparta (part of the Accell Group) is widely recognized as the leading e-Bike brand in Holland which thanks to its big share of the Dutch market, is also leading on the European e-Bike market. The ION (photo) with the batteries mounted in the down tube is Sparta’s most popular model.
Because of the boom in e-Bikes the sale of regular bikes in the 700 to 900 euro (retail) price bracket suffered in 2008. However, cheaper prices bikes in the 500 to 700 euro segment managed to increase in market share.
The BOVAG also stated that dealers increased their share in bike sales to 81% (was 75%). In terms of value the Dutch specialist trade controlled an even bigger share of the market – 91% which was 88% during the first nine months of 2007.
