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Efficiency under the sun

Researchers from Rio Grande do Sul develop more productive solar cells

Calculations by the Ministry of Mines and Energy indicate that some 14 million Brazilians who live in rural areas have no access to electricity. One of the solutions to help to make good this deficiency may lie in the hands of researchers from Rio Grande do Sul, who have developed a more efficient technology for getting solar energy.

Coordinated by Adriano Moehlecke, from the Pontifical Catholic University of Rio Grande do Sul (PUCRS), they developed solar cells on the basis of silicon, with a level of utilization of 17%, the highest degree of efficiency attained with similar technology in the country, up to the end of last year. This means that from each 1,000 watts that the equipment captures at midday, on a sunny day, it manages to produce 170 watts of electricity. Accordingly, a system like this, installed on the roof of a house, for example, can produce an average of 130 kilowatt/hours a month, sufficient for a family of three persons who use an electric shower and domestic appliances.

Australia has reached 24.7% of utilization, but using silicon obtained by the technique known as FZ (for float zone) and a very complex manufacturing process. These ingots of silicon monocrystals are the most pure and free from defects, but they are also the most expensive. Coupled with the complexity and the cost of the process, the chances of making these cells on a large scale in Brazil are slight.

Ultrapure silicon
The choice of the raw material was a strategic one, according to the researcher. Brazil has the largest reserves of good quality quartz, fundamental for getting ultrapure silicon, which is intended for the production of computer chips, transistors and solar cells. For the time being, the company has not yet mastered the technology for ultrapurification, which is limited to just three or four industrial concerns in the world. With purity of between 98% and 99%, the silicon produced in Brazil is called metallurgical and serves to make aluminum alloys (for autoparts) and silicones (used in cosmetics and in the electrical-electronic industry).

The process for manufacture developed by the group of researchers from PUCRS uses low cost chemical products (acetone, propanol, hydrochloric acid, hydrogen fluoride, potassium hydroxide, amongst others) and gases (nitrogen and oxygen). “These two factors, national raw material and technology, are going to contribute towards a reduction in the costs of the industrial process”, Moehlecke explains.

The generation of photovoltaic energy takes place when sunlight falls on the silicon cell, producing negative and positive charges, and these are separated by an electric field that is in the device. An electric currently is thus set up, and when it passes through the region of the field, this generates a voltage and produces electricity. In countries like Germany, Spain, the United States and Japan, the generation of electricity by photovoltaic cells is at an advanced stage. In the 90’s, the market for photovoltaic modules grew 20% a year. Between 2000 and 2001, the increase was 40%, with world-wide production in the order of 300 megawatts (MW) a year.

A study by the EPIA – European Photovoltaic Industry Association in Europe estimates that in 2010 there will be an annual production of modules in the order of 630 MW. Of these, 33% will be intended for stand-alone systems, and 29% for systems connected to the electricity network. “The use of photovoltaic energy is now a process that is well advanced and in full growth in developed countries”, Moehlecke points out.

In Brazil, photovoltaic systems account for 12 MW of the electricity currently produced, according to the Electrotechnical and Energy Estimate of the University of São Paulo. They are pieces of equipment that are installed in fishermen’s villages, small settlements in the northeast, indigenous tribes in the Amazon and isolated regions, which other systems do not reach. The device developed at the university in Rio Grande do Sul may be used, without any great modifications in technology, both in small communities and in large urban centers. “The high levels of solar radiation, added to the low population density in some regions, make this option for electricity very competitive from the economic and environmental point of view all over the country”, the researcher points out.

Young scientist
The work, a result of three years of research, was awarded first place in the Graduates category of the Young Scientist Prize XVIII in 2002. Moehlecke, aged 37, is a doctor in engineering by the Solar Energy Institute of the Polytechnic University of Madrid, and a professor of the School of Physics and of the Postgraduate Studies Course in Materials Engineering and Technology at PUCRS.

The researcher recognizes that the cost of photovoltaic solar energy, compared with conventional electricity, is a barrier to its use on a large scale. The price for photovoltaic electricity is US$ 3.5 per watt, while conventional electricity works out at around US$ 1 to US$ 1.4 per watt. “But, in compensation, the costs of maintenance of the system and of the transmission of the electricity are more expensive when generated from water.” He also points out that, in the cost-benefit comparison, the hydroelectric power stations call for large distribution centers, have a given working life, and cause irreversible environmental damage, while photovoltaic energy is inexhaustible and clean from the environmental point of view.

“If we compare the photovoltaic systems with thermoelectric plant, fired by coal or gas, an option for which Brazil is giving incentives, there are even greater advantages, as far as pollution is concerned.”To reduce the price paid by the user of solar energy, Moehlecke believe that the State could absorb part of the spending on implementing the equipment. “In Germany and Spain, for example, the subsidy is in the order of 60%. Gas stations, parking meters and other equipment that serves the population are using photovoltaic energy more and more.” Large groups, such as British Petroleum and Isofoton in Europe; Shell Solar and Astropower, in the United States; Sharp, Kyocera and Sanyo, in Japan; these are the ones that set off ahead in the production and marketing of photovoltaic energy systems and equipment.

Moehlecke did not patent his system, because he regards having the know-how for making the equipment more important than this. Some businessmen have already shown interest in producing the cell in Brazil. Conversations began to be engaged in October last year, promoted by the Secretariat for Science and Technology of Rio Grande do Sul. “The world’s sources of energy are at a stage of transition and are moving towards exhaustion”, he says. In the researcher’s assessment, this is the best moment to take a decision about the large-scale adoption of solar energy.