Portfolio 3
Recently, I have read an article about solar energy, which mainly tell me the importance of solar energy and the current efficiency of using solar energy. Here is a summary of the article.
As we all know ,the sun is the most powerful source of energy. Even though only small fraction of the sun' s power reaches the earth, it produces "10,000 times as much as all the commercial energy that humans use on the planet", the article says.
The importance of the solar energy is because it is sustainable and clean. Those fossil fuels cannot remain the dominant sources of energy forever. Whatever the precise timetable for their depletion, oil and gas supplies will not keep up with growing energy demands. Coal is available in abundance, but its use exacerbates air and water pollution problems, and coal contributes even more substantially than the other fossil fuels to the buildup of carbon dioxide in the atmosphere.
For a long-term, sustainable energy source, solar power offers an attractive alternative. Its availability far exceeds any conceivable future energy demands. It is environmentally clean, and its energy is transmitted from the sun to the Earth free of charge. But exploiting the sun’s power is not without challenges. Overcoming the barriers to widespread solar power generation will require engineering innovations in several arenas — for capturing the sun’s energy, converting it to useful forms, and storing it for use when the sun itself is obscured.
The article take solar cell as an example to describe the current situation of solar energy usage. Solar photovoltaic cell is one of the popular production nowadays, which direct produce electric current from captured sunlight. However, today's commercial solar cells typically convert sunlight into electricity with "an efficiency of only 10 percent to 20 percent"[Schaller et al]. Given their manufacturing costs, modules of today’s cells incorporated in the power grid would produce electricity at a cost roughly "3 to 6 times higher than current prices, or 18-30 cents per kilowatt hour" [Solar Energy Technologies Program]. To make solar economically competitive, engineers must find ways to improve the efficiency of the cells and to lower their manufacturing costs.
The author points out the bright future of solar energy at the last line, solar energy provides less than 1% of the world's total energy, but it has the potential to provide much much more.

References
Make solar energy economical. Retrieved October 1st, 2008, from http://engineeringchallenges.org/cms/8996/9082.asp
DOE. Solar Energy Technologies Program Multi-Year Program Plan 2007-2011. Office of Energy Efficiency and Renewable Energy. Washington, D.C.: DOE.
Schaller, R.D., and V.I. Klimov. 2004. High Efficiency Carrier Multiplication in PbSe Nanocrystals: Implications for Solar Energy Conversion. Physical Review Letters 92(18): 186601-1 - 186601-4. DOI: 10.1103/PhysRevLett.92.186601