Photovoltaics Present Some Problems, But Mirrors and Sunflowers Hold the Key
Thursday, January 19th 2012 1:37 PM
By GetSolar Staff.
One of the biggest concerns about solar power as more residential and commercial solar installations proliferate around the country, is the intermittent nature of the power source. Photovoltaic solar panels produce electricity passively, absorbing sunlight when it is available. This makes for less work, but also means that energy cannot be created when demand is high and it is generally difficult to store the energy until it is needed.
The New York Times reports that one solution that could clear the way for even greater adoption of residential solar installations is the solar thermal power plant. These systems, which use mirrors to heat water that is then used to turn a turbine like a traditional power system, have a great deal more control over their output.
By generating electricity through an intermediary, solar thermal can smooth over some of the bumps in productivity made by clouds and other issues. Some new versions even boast energy storage in the form of molten salt, which can be tapped later on to produce steam even without sunlight.
One particular benefit of these solar plants is that they allow for more responsive electricity generation without relying on traditional "peakers," the fast-acting power plants that respond to peak demand. Most of these power plants use fossil fuels such as natural gas, and some even use petroleum.
Already these plants have seen growing interest, with one major 392-megawatt solar installation being planned in the Mojave Desert in California. The Ivanpah solar plant has drawn interest from major backers as well, including tech companies like Google.
However, while the open spaces of the Mojave Desert present a tempting locale for such a power plant, they do face a major challenge simply in the amount of land required. The plants are designed with a central tower that holds water surrounded by arrays of mirrors known as heliostats, which can take up a massive amount of space for the amount of energy being generated. The Ivanpah solar plant, with 347,000 heliostats, covers 5.6 square miles of desert, ultimately raising some environmental concerns by displacing local tortoises.
"Concentrated solar thermal energy needs huge areas," Alexander Mitsos, an assistant professor of mechanical engineering at the Massachusetts Institute of Technology, told MIT News. "If we’re talking about going to 100 percent or even 10 percent renewables, we will need huge areas, so we better use them efficiently."
Mitsos and his partners Corey Noone, also from MIT, and Manual Torrilhon of RWTH Aachen University in Germany have taken an interesting approach to trying to address this issue.
One of the biggest issues with solar thermal plants is that the heliostats must be arrayed in such a fashion that they will not block each other from the sun and reduce the amount of energy hitting the tower. To avoid this, most provide for ample room between heliostats and stagger them, much like in a movie theater. However, given the cost of land in many regions, this approach can be impracticable.
The researchers created several mathematical models to help improve the overall access to sunlight while reducing the footprint of the plant. After several computer models, the group was able to reduce the size of a standard plant by around 10 percent by creating a vague spiral pattern. This would already prove a substantial improvement, but the spiral reminded the group of a familiar subject for mathematicians - the sunflower. Sunflowers array their florets, the smaller flower-like structure that eventually form into seeds, in what is known as a Fermat spiral. This spiral uses the "golden angle" often found in nature of around 137.
A new model designed around this Fermat spiral allowed the researchers to bring the heliostats even closer into the tower, eventually reducing the footprint of the plant by 20 percent while slightly improving efficiency. With luck and effort, developments like this will allow adoption of a broader array of solar technologies that could help spur the entire market.
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One of the biggest concerns about solar power as more residential and 
