Friday, July 30, 2010

Breakthrough in Thin-Film Solar Cells

Scientists at Johannes Gutenberg University Mainz (JGU) in Mainz have made a major breakthrough in their search for more efficient thin-film solar cells. Computer simulations designed to investigate the so-called indium/gallium puzzle have highlighted a new way of increasing the efficiency of CIGS thin-film solar cells. Researchers to date have achieved only about 20% efficiency with CIGS cells although efficiency levels of 30% are theoretically possible.

Thin-film solar cells are gaining an ever increasing proportion of the solar cell market. As they are only a few micrometers thick, they offer savings on material and manufacturing costs. Currently, the highest level of efficiency of about 20% is achieved by CIGS thin-film solar cells, which absorb the sunlight through a thin layer made of copper, indium, gallium, selenium, and sulphur. However, the levels of efficiency achieved to date are nowhere near the levels theoretically possible.

The research team at Mainz University headed by Professor Dr Claudia Felser is using computer simulations to investigate the characteristics of CIGS, whose chemical formula is Cu(In,Ga)(Se,S)2. This research forms part of the comCIGS project funded by the Federal German Ministry for the Environment, Nature Conservation, and Nuclear Safety (BMU). IBM Mainz and Schott AG are collaborating with the Johannes Gutenberg University Mainz, the Helmholtz Center Berlin for Materials and Energy and Jena University in the project that is targeted at finding ways of optimizing CIGS solar cells. The researchers focused in particular on the indium/gallium puzzle that has been baffling scientists for years: Although it has been postulated on the basis of calculations that the optimal indium:gallium ratio should be 30:70, in practice, the maximum efficiency level has been achieved with the exactly inverse ratio of 70:30.

With the support of IBM Mainz, Christian Ludwig of Professor Felser's team undertook new calculations with the help of a hybrid method in which he used a combination of density functional calculations and Monte Carlo simulations. "Density functional calculations make it possible to assess the energies of local structures from the quantum mechanical point of view. The results can be used to determine temperature effects over wide length scale ranges with the help of Monte-Carlo simulations," Dr Thomas Gruhn, head of the theory group in Professor Felser's team, explains the methods used. Christian Ludwig is able to use a mainframe for his investigations that was recently donated to Mainz University by IBM as part of a Shared University Research (SUR) science award.

Production at high temperatures promotes homogeneity of the material

With the aid of the simulations, it was discovered that the indium and gallium atoms are not distributed evenly in the CIGS material. There is a phase that occurs at just below normal room temperature in which the indium and gallium are completely separate. If the material is heated to above this demixing temperature, differently sized clusters of indium and gallium atoms do form. The higher the temperature, the more homogeneous the material becomes. It has now become apparent that gallium-rich CIGS is always less homogeneous than indium-rich CIGS. Because of this lack of homogeneity, the optoelectronic properties of the gallium-rich material are poorer, resulting in the low efficiency levels of gallium-rich CIGS cells -- an effect that has now been explained for the first time. The calculations also provide a concrete indication of the best way to manufacture CIGS solar cells. If it is produced at higher temperatures, the material is significantly more homogeneous. To retain the desired homogeneity, the material then needs to be cooled down sufficiently rapidly.

In practice, it was the limited heat resistance of the glass used as a substrate for solar cells that has always restricted process temperatures, but a significant breakthrough has also recently been made here. Schott AG has developed a special glass with which the process temperature can be increased to well above 600°C. The cells that result from this process are considerably more homogeneous, meaning that the production of cells with a much greater efficiency level has become possible. But the comCIGS project researchers are already thinking ahead of this. "We are currently working on large-format solar cells which should outperform conventional cells in terms of efficiency," states Gruhn. "The prospects look promising."

The work of the scientists in Mainz, conducted as part of the federal government-funded comCIGS project, has been published in the latest edition of the journal Physical Review Letters.

Journal Reference:

1.Christian Ludwig, Thomas Gruhn, Claudia Felser, Tanja Schilling, Johannes Windeln, Peter Kratzer. Indium-Gallium Segregation in CuIn_{x}Ga_{1-x}Se_{2}: An Ab Initio%u2013Based Monte Carlo Study

Thursday, July 08, 2010

IRENA becomes a fully fledged International Organisation

The International Renewable Energy Agency (IRENA) becomes a fully fledged International Organization. Created in January 2009 with 75 Member States, IRENA has grown in just over a year to become one of the largest international organizations. 147 countries and the European Union already signed its statute.

The 25th instrument of ratification was deposited in Berlin on June 8th and according to Article XIX, the treaty enters into force 30 days after. 29 Member States have now ratified the Treaty.

In June 2009, Abu Dhabi was elected as IRENA’s Headquarters. The first international organization of the 21st century is also the first to be headquartered in the Gulf Region. Hélène Pelosse, Interim Director-General comments, “With 98 billion barrels, Abu Dhabi is the 7th proven oil reserve in the world. Nevertheless Abu Dhabi is committed to achieve 7% renewable energy in 2020 and to invest 10% of its GDP in Masdar, a zero carbon city. The United Arab Emirates are IRENA’s home and this far sighted country has continuously proved a very strong support for the Agency”.

Hélène Pelosse was elected as Interim Director-General one year ago at the age of 39. Mother of three, she states, “We cannot rely on energy of the past to power our future. Now renewables account for 18% of world electricity production but potential scenarios show it can reach 50% or even higher. It is the only energy source which can serve the needs of the predicted nine billion earth population in 2050”.

Improving the renewable energy share in the energy mix is a direct way of tackling climate change and GHG emission reduction. Whilst also encouraging energy security and independence. Furthermore, it also offers strong support for both economic and social growth.

The International Renewable Energy Agency is aiming to become the global voice for renewable energy. The Agency’s mandate is to assist its Member States define their strategy across the fields of all renewable energies: bioenergy, geothermal, hydropower, ocean, solar and wind.

Saturday, July 03, 2010

Switching off your lights has a bigger impact, according to a new study

We supported the Earth Hour 2010 by switching off our lights on March 27. During the hour people across the world turned off their lights and joined a common movement to protect our climate and combat global warming.

Earth Hour was organized by WWF, one of the world’s largest and most respected independent conservation organizations on a mission to stop the degradation of the Earth’s natural environment and build a future where people live in harmony with nature.

Switching off lights, turning the television off at the mains and using cooler washing cycles could have a much bigger impact on reducing carbon dioxide emissions from power stations than previously thought, according to a new study published this month in the journal Energy Policy. The study shows that the figure used by government advisors to estimate the amount of carbon dioxide saved by reducing people's electricity consumption is up to 60 percent too low.

The power stations that supply electricity vary in their carbon dioxide emission rates, depending on the fuel they use: those that burn fossil fuels (coal, gas and oil) have higher emissions than those driven by nuclear power and wind. In general only the fossil fuel power stations are able to respond instantly to changes in electricity demand.

Dr Adam Hawkes, the author of the new study from the Grantham Institute for Climate Change at Imperial College London, says the government should keep track of changing carbon emission rates from power stations to ensure that policy decisions for reducing emissions are based on robust scientific evidence. The new study suggests that excluding power stations with low carbon emission rates, such as wind and nuclear power stations, and focussing on those that deal with fluctuating demand would give a more accurate emission figure.

Scientists advising government on for the best ways to reduce electricity demand currently use an estimated figure for emission rates. The new study shows that, at 0.43 kilograms of carbon dioxide per kilowatt hour of electricity consumed, this figure is 60 percent lower than the actual rates observed between 2002 and 2009 (0.69 kilograms of carbon dioxide per kilowatt hour), meaning that policy studies are underestimating the impact of people reducing their electricity use.

Dr Adam Hawkes, author of the paper, and a Visiting Fellow at the Grantham Institute for Climate Change at Imperial College London, said: "One way governments are trying to mitigate the effects of climate change is to encourage people to reduce their energy consumption and change the types of technologies they use in their homes. However, the UK government currently informs its policy decisions based on an estimate that, according to my research, is lower than it should be.

"This means any reduction we make in our electricity use -- for example, if everyone switched off lights that they weren't using, or turned off electric heating earlier in the year -- could have a bigger impact on the amount of carbon dioxide emitted by power stations than previously thought. However, this also acts in reverse: a small increase in the amount of electricity we use could mean a larger increase in emissions than we previously thought, so we need to make sure we do everything we can to reduce our electricity use," added Dr Hawkes.

Dr Hawkes drew upon 60 million data points showing the amount of electricity produced in each half-hour period by each power station in Great Britain from the start of 2002 to the end of 2009. He also calculated the emissions of each different type of generator by examining government data showing their average annual fuel use. Finally, he took these two sets of data to calculate the emissions rate that should be attributed to a small change in electricity demand.

The results show that, for 2002-09, the carbon dioxide emission rate for estimating the effect of a small change in electricity demand is 0.69 kilograms of carbon dioxide per kilowatt hour of electricity consumed. This is 30 percent higher than the average emissions rate across all power stations, which is 0.51 kilograms of carbon dioxide per kilowatt hour, and 60 percent higher than the figure currently used by government advisors, which is 0.43 kilograms of carbon dioxide per kilowatt hour.

Professor Sir Brian Hoskins, Director of Imperial's Grantham Institute for Climate Change, said: "This is a very important study that could help policy makers make more informed decisions to reduce our carbon emissions. The government needs a good understanding of the figures it uses to support policy analysis, because this has a big impact on which technologies we employ to reduce our energy use. With a more accurate picture of what is going on, we will be much better equipped to tackle our carbon dioxide emissions."
A.D. Hawkes: Estimating marginal CO2 emission rates for national electricity systems (Energy Policy 2020)

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