The Future of Electricity
Here comes the sun
by: Nancy S. Grant
Energy from the sun is the most plentiful and widely distributed renewable resource on our planet. Scientists figure that enough sunlight reaches the Earth's surface in just one hour to provide a year's worth of the energy needed around the globe.
But how soon will we be able to capture more of that solar energy to use where we need it?
That's tough to answer right now because we need a lot of technological breakthroughs to make solar power easier and more affordable than it is today.
There are several ways to capture the energy in sunlight.
Today, the most common and widespread method of using the sun's energy is with solar panels. You've probably seen these flat black panels mounted above highway signs. These solar devices are based on photovoltaic technology. In this system, tiny packets of energy within sunlight known as photons strike the panel and cause a physical reaction within it.
In a photovoltaic system, sunlight is converted directly into electricity. A single photovoltaic cell, or a group of them linked together, generates electricity because of the way the materials are put together inside the panel. The most common material used today is silicon. Other elements such as boron are also needed inside to allow the panel to function correctly.
That leads to the first problem with large photovoltaic systems--they are very expensive to build today.
A photovoltaic solar panel is really a multi-layered sandwich made of these different materials, each with special physical and chemical properties. Each layer is very thin and must be manufactured to extremely precise standards. There's no room for mistakes.
There's another big problem with today's most common style of photovoltaic cells: they're not very efficient. Only about 15 to 20 percent of the sunlight that falls on them is actually used to generate electricity.
Research to improve photovoltaic technology is going on now.
In Newark, Delaware, the University of Delaware heads up the Consortium for Very High Efficiency Solar Cells, a project supported by 15 colleges, companies, and laboratories. This diverse task force is working to reach goals set by the Defense Advanced Research Projects Agency.
The consortium is working to develop and produce 1,000 Very High Efficiency Solar Cell prototypes that are both affordable to manufacture and capable of operating at 50 percent or higher rates of efficiency. That's more than double the efficiency that solar cells coming off today's production lines can achieve. This long-term project will take many years.
The third issue limiting the use of solar power involves the electricity itself.
Photovoltaic cells generate electricity in the form of direct current, which is the kind of electricity produced by batteries. That's fine for use right at the site (perhaps to power a single piece of machinery or a traffic sign) but if the electricity is going to be used in your house, or go into the nation's grid of electric transmission lines, it must be converted to alternating current. That adds a step to the process, and to the cost of PV systems.
Direct current versus alternating current is not a new issue. More than 100 years ago, when American inventors were first experimenting with electric energy, direct current was often the first choice for electric power systems. But along the path of development, our nation chose alternating current as the system for our transmission grid, our appliances, and just about everything involving electricity for the general public.
Engineers have known for a long time that direct current transmission lines are more efficient than alternating current transmission lines. But the materials within such a system cost more and present some serious safety problems.
We already have a vast infrastructure worth billions of dollars based on alternating current operating now. Changing it to accommodate direct current from solar power systems may not be a wise use of financial resources. So, in the future, it's more likely that the direct current electricity from PV solar systems will either be used on-site for industrial machinery and kept separate from the national electric transmission grid, or converted to alternating current.
Photovoltaic systems can operate successfully in many climate zones, even places with cool air temperatures.
But another method of generating electricity from sunlight works best where the air is hot, such as the deserts in the American Southwest.
The heat of the light
In a solar thermal system, the heat in sunlight is captured and used to generate electricity. Unlike photovoltaic systems, in solar thermal systems the sunlight does not do the job directly--it's used as an intermediate step.
Concentrating Solar Power technology offers many improvements in the efficient use of solar energy. Different kinds of solar thermal systems have their own sets of complications--but new solar thermal technology may help us get around some of the problems with photovoltaic systems.
There are three types of CSP technology being developed today.
In a parabolic trough system, the sun's heat can be concentrated by taking advantage of several innovative design features. Pipes containing special fluids are part of the design.
A trough system looks a bit like a typical fixed photovoltaic array, but instead of flat panels, a parabolic trough system's devices are curved and can be tilted side to side or up and down throughout the day so that the sunlight falls onto the system's piping at a good angle. This continuous tracking allows the special fluid in the system's pipes to reach 400 degrees Fahrenheit. A system of heat exchangers transfers the heat to ordinary water to make steam as the driving force for turbines.
A parabolic system can concentrate the sun's heat from 30 to 100 times its usual intensity.
A solar dish system, another type of Concentrating Solar Power installation, looks a bit like a television satellite dish you might have mounted on your roof. But a TV dish is fixed in one position, and the satellite sending signals is also stationary above the earth. A solar dish system can rotate to precisely follow the sun as it moves through the sky. This exact tracking ability, plus the shape of the dish, allows this system to concentrate the sun's heat up to 2,000 times, resulting in a fluid temperature of up to 750 degrees Fahrenheit in the system's pipes.
Instead of the system of heat exchangers used in a parabolic system, a solar dish system uses the high heat in the pipes in a mechanical system to move other parts to produce electricity.
A third kind of CSP technology may be even more efficient.
A solar power tower is a two-part system that combines features of other thermal systems. Rotating mirrors at ground level reflect sunlight up to a central device mounted at the top of a tower.
A tower system should be able to concentrate the sun�s heat about 1,500 times--with no loss of energy because there's no need to have an elaborate system of heat exchangers. This is a very new technology with great potential to supply power to our nation's electric grid. An 11-megawatt solar power tower is already providing zero-emissions electricity to the Spanish city of Seville.
There are 11 utility-scale CSP generating facilities of various designs in the United States. One's installed in Nevada, one in Arizona, and nine operate in California, with many more planned for the near future both in the United States and worldwide. A Spanish company, Abengoa Solar, hopes to begin construction next year on a 280-megawatt CSP facility in Gila Bend, Arizona.
Another attractive feature of Concentrating Solar Power systems is their capacity to store heat from the sun for gradual use after sunset or during cloudy daytime intervals. The Solana project in Gila Bend will make use of storage areas filled with molten salt to accomplish this.
For the near term, CSP technology will continue to make big headlines. But Thomas Key, technical leader for renewables with the Electric Power Research Institute, a not-for-profit utility group, predicts that as the century progresses, photovoltaics will emerge as the more dominant technology.
Key says, "PV is a solid-state technology with many opportunities for cost reductions and increased efficiency. The direct conversion of sunlight into electricity offers more headroom for advancement."
Key also points out that PV technology works in more locations than CSP, and can be used closer to where people live and work.
KEYWORD EXCLUSIVE: SUN POWER SPOTS
Discover more links on solar energy, including a photo of the world's largest photovoltaic solar plant and a video news report on a solar tower.
Energy journalist NANCY GRANT is a member of the Cooperative Communicators Association and the American Society of Journalists and Authors.
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