Author: Tran Charles
Institution: Biochemistry
Date: December 2007
Oil prices near $100US per barrel, and home heating this winter makes a significant dent in your wallet. The rising demand for fossil fuels has driven a surge in the cost of oil, and has forced consumers to look for ways to reduce their energy utility bill. Aside from energy conservation practices, alternative energy sources could mean lower energy bills in the long run.
This review will look at solar energy as an alternative energy source, and describe some of the recent developments in solar, or photovoltaic (PV) panel technology. Many of these improvements to PV panel production have focused on reducing their manufacturing costs because conventional PV panels are expensive to produce.
Solar panels, shown here, are used to generate electricity for a small town near Tempe, AZ. Image courtesy of Stock.XCHNG.
PV panels have no greenhouse gas emissions once manufactured, and have typical operating lifespans of 20-25 years (1). Besides providing electricity directly for a building or installation, PV panels power street and garden lights, and water pumps in developing countries (2).
A schematic of a PV cell. Sunlight passing through the glass (grey) and anti-reflective layer (yellow) strikes the p-type silicon (blue), releasing free electrons from the silicon atoms in that layer. The electrons are attracted to the positively-charged n-type silicon (red). Connecting the cell to an electrical circuit through the electrical contacts (green) will provide electrical energy to drive a load (e.g. light bulb).
The most common type of PV panel today is manufactured from silicon. There are several variants on silicon-based PV panels such as crystalline and non-crystalline or amorphous silicon. Crystalline silicon PV panels currently dominate the market, with monocrystalline silicon being the most common (1).
Current Limitations of Solar Panels
The various silicon PV panels differ in their efficiency. Simply put, efficiency refers to the process of converting light energy to electrical energy by the PV panel. Crystalline silicon is more efficient than amorphous silicon, but is also more expensive. The highest reported efficiency of silicon PV cells to date is 24.5% (3), and the highest PV cell efficiency reported is 42.8% (4). Although these numbers are promising, the newest developments in solar panel technology may not be incorporated for some time, and most monocrystalline silicon PV panels today (the most common type) have efficiencies of only 10-16% (1).
In addition to their low efficiency, silicon PV panels are also expensive both in terms of material and manufacturing costs. Developments in the semiconductor industry have reduced manufacturing costs for PV panels, but increased demand for high-grade silicon required for both semiconductor and PV panel production has also increased the price of refined silicon.
Nanoparticles can improve efficiency
To remedy the low efficiency, Munir Nayfeh and Matthew Stupca, both physicists at the University of Illinois, discovered that a coating of silica nanoparticles could increase the power output of PV cells (5). Their research was published in the August 2007 issue of the Journal of Applied Physics.
"Integrating a high-quality film of silicon nanoparticles 1 nanometer in size directly onto silicon solar cells improves power performance by 60 percent in the ultraviolet range of the spectrum," said Nayfeh.
Typically, ultraviolet light is absorbed by the silicon layer and lost as heat. The silicon nanoparticle coating improves power output by absorbing ultraviolet light and converting some of that energy into electrical energy. Nayfeh's process increases the efficiency of the solar cell by taking advantage of a broader range of the light spectrum for electrical energy production.
Nayfeh believes that his silicon nanoparticle coating can be readily incorporated into the manufacturing process without any significant increase to the manufacturing cost.
Cadmium telluride makes PV panels cheaper
Further improvements have focused on finding alternative materials to the expensive silicon for PV panel manufacturing. Walajabad Sampath, a mechanical engineer at Colorado State University in Fort Collins, developed a PV fabrication process that involves coating glass panels with cadmium telluride instead of crystalline silicon (6). Cadmium telluride is a crystalline compound that can deposited in a very thin layer on glass, and it can be substituted for silicon in the conventional PV cell. In terms of manufacturing costs, Sampath's cadmium telluride process delivers power for less than $2 US per watt, which is half of the price per watt for conventional solar panels.
PV technology based on cadmium telluride was first proposed by Joseph Loferski in the late 1950s (7), and the first functional cadmium telluride cells were developed in the 1970s by Dieter Bonnet (1). However, difficulties with scaling up the manufacturing process for cadmium telluride cells and finding a process that would give durable PV cells limited their use until recently.
The cadmium telluride process used by Sampath yields PV panels with an efficiency of 11-13%, which is comparable to current silicon PV panel efficiencies. However, the reduced cost of manufacture of this type of PV panel makes it more attractive than current silicon-based PV panels in both consumer and commercial applications.
"This technology offers a significant improvement in capital and labor productivity and overall manufacturing efficiency," explains Sampath. "The current market is over $5 billion annually and additional markets are developing."
AVA Solar, a company started by Sampath and his colleagues Kurt Barth and Al Enzenroth, was formed to commercialize the process. They were awarded a $3 million grant from the US Department of Energy's Solar America Initiative to up-scale and develop their process.
Organic and thin-layer PV cells
Organic solar cells (OSCs) and thin-layer PV cells are other alternatives that are less expensive in material cost and are potentially easier to manufacture. Both of these technologies aim to replace or reduce the silicon used for PV panel production. Organic solar cells use carbon-based polymers based upon molecules such as fullerenes (8, 9) (also known as "buckyballs" because some fullerenes have a spherical shape), while thin-layer silicon PV employ additional light-trapping techniques with a thinner silicon layer (1). Organic cells, however, have efficiencies of less than 10% (8), and need to be improved before they are viable for commercial use.
PV energy production
Despite these advances in PV technology, there are relatively few examples of PV installations as a major form of energy production. This is likely a consequence of the high cost of PV technology compared to other energy sources. In the UK for example, solar power currently provides less than one hundredth of one percent of home energy needs. Overall, PV energy production represents an extremely small fraction of energy production worldwide. The total annual energy produced by PV installations in the world, 2,204 megawatts (10), represents approximately one-tenth of a percent of the total energy use of the United States, which had an annual energy consumption of 3.3 trillion watts in 2006.
U.S. energy production in 2006 broken down by source. Data from the U.S. Energy Information Administration (11).
Future PV installations may use electricity produced by solar energy to split water (12) and produce hydrogen (e.g. for hydrogen fuel cells in cars), or to recharge electrically-powered vehicles. Many more possibilities for PV use exist, and they may even power manufacturing facilities for PV panels, which would reduce carbon dioxide emissions from PV manufacturing itself.
The numerous potential applications of PV installations and their use as a non-polluting, renewable source of energy are important advantages of PV technology. Improvements in their design and manufacturing will increase their attractiveness for use in energy production for all consumers.
References and further reading:
(1) Miles, R. W., et al. Photovoltaic solar cells: An overview of state-of-the-art development and environmental issues. 2005. Progress in Crystal Growth and Characterization of Materials 51:1-42.
(2) Partain, L. D., ed. Solar Cells and Their Applications. John Wiley & Sons, 1995.
(3) Green, M.A., et al. Solar Cell Efficiency Tables (Version 30). 2007. Prog. Photovolt: Res. Appl. 15:425-430.
(4) UD-led team sets solar cell record, joins DuPont on $100 million project. July 2007. http://www.udel.edu/PR/UDaily/2008/jul/solar072307.html
(5) Stupca, M., et al. Enhancement of polycrystalline silicon solar cells using ultrathin films of silicon nanoparticle. 2007. Appl. Phys. Lett. 91: 063107.
(6) Nelson, Taylour. Professor improves solar panels' efficiency. 2007. http://coloradoan.com/apps/pbcs.dll/article?AID=/20070905/CSUZONE01/709050333/1002/NEWS01
(7) Loferski, J. J., Theoretical Considerations Governing the Choice of the Optimum Semiconductor for Photovoltaic Solar Energy Conversion. J. Appl. Phys. 27:777
(8) Service, R. Materials Research Society Meeting: Organic Solar Cells Playing Catch-Up. 2004. Science 306: 2034.
(9) Somani, P.R., Somani, S. P., and Umeno, M. Toward organic thick film solar cells: Three dimensional bulk heterojunction organic thick film solar cell using fullerene single crystal nanorods. 2007. Appl. Phys. Lett. 91, 173503.
(10) Solarbuzz Reports World Solar Photovoltaic Market Growth of 19% in 2006. http://www.solarbuzz.com/Marketbuzz2007-intro.htm
(11) U.S. Energy Information Administration. Annual Energy Review: Energy Production by Source, 1946-2006. http://www.eia.doe.gov/emeu/aer/overview.html
(12) Mor, G. K., et al. Enhanced Photocleavage of Water Using Titania Nanotube-Arrays. 2005. Nano Letters 5:191-195.
Written by Charles Tran
Reviewed by Nira Datta, Pooja Ghatalia.
Published by Pooja Ghatalia.