Environment

Solar power - Which renewable technology will prevail?

Energy thousands of times the world’s electricity use shines constantly on the Earth. The future of solar power technology is bright, but it is not yet clear. ...
Electricity generation is just one of the many uses of the sun\'s energy - are photovoltaics the .../ Credits: Reuters

Solar Photovoltaics

Despite its colossal potential, solar power provides less than one percent of global electricity generation. It is expensive, heavily subsidized, and relatively inefficient. 

Solar technology must mature. That means squeezing more kilowatt hours out of each solar plant, at lower cost, in order to reach so-called grid parity. 

“By around 2030 solar power will be produced at 5 to 10 cents per kilowatt hour and will be competitive with fossil fuels. We won’t need any financial support,” claims Eicke Weber, director of the Fraunhofer Institute for Solar Energy Systems. 

But although the Holy Grail is visible, nobody is quite sure how to get there. 

Weber reckons that rather than Big Bang breakthroughs incremental improvements in silicon-based solar photovoltaic (PV) technology—85 to 90 percent of the global PV market today—will win the day. 

“The big driver will remain crystalline silicon getting ever more efficient, with thinner wafers and cheaper silicon materials. It is a kind of Moore’s Law for solar PV,” he says. 

One example, says Weber, is using lasers in solar PV production. “This will revolutionize the way solar cells are made…you can improve efficiency and go to thinner wafers to save on materials.” 

Weber is also excited by the potential of ‘concentrated photovoltaics’, which the Fraunhofer Institute used in 2009 to scoop the world record for conversion of sunlight into electricity—41.1 percent efficiency. Current commercial solar PV cells range between 10 and 20 percent efficiency. 

Efficiency is lost because conventional PV cells can only absorb a small bandwidth of light. Concentrated PV broadens this spectrum using ‘triple junction’ solar cells made of three different materials— gallium indium phosphide, gallium indium arsenide, and germanium. Each material absorbs different colors in the sunlight’s spectrum. Special lenses amplify the energy intensity further by concentrating the beams up to 500 times. 

Markets, however, are reacting differently to new technologies, says Weber, pointing to a geographical split in solar PV research and development. 

While market-leading European manufacturers are “focusing on low-cost, high-volume production” of silicon cells, Americans are trying to get ahead by developing next generation photovoltaics. 

In early 2010, a team at the California Institute of Technology lab tested a silicon wire design which they said boasts a sensational 85 percent efficiency in plain sunlight and 95 percent at certain wavelengths. 

The researchers used tiny silicon wires one-millionth of a meter long instead of conventional wafers and encased them in a flexible polymer that can be rolled or bent. Aside from the flexibility, the added bonus is that they use just one percent of the silicon per cell area as a normal, brittle wafer.

Concentrated Solar Power (CSP)

Going from micro- to nano-scale is University of Texas scientist Xiaoyang Zhu, who in June 2010 reported that semiconductor nanocrystals, or ‘quantum dots’, could capture the sun’s energy currently lost as heat in conventional solar cells, theoretically boosting performance to more than 60 percent efficiency. 

Despite such promising research, concentrated solar power (CSP) still beats photovoltaics when it comes to efficiency. Most CSP system also have back-up power installed, usually a natural gas turbine, which helps guarantee electricity production during cloudy weather. 

But concentrating solar power with mirrors demands a large land and water footprint. CSP plants also have to be built in areas with very good solar resources that often tend to be far away from large population centers. 

But as with photovoltaic, the CSP industry is also locked in an “exciting technology race”, Weber says. 

The leading design is still the parabolic trough that uses mirrors to concentrate sunlight and heat synthetic fluids that then heat water to run steam turbines. But these ‘transfer fluids’ are costly and cannot be heated beyond 380 degrees Celsius, which limits efficiency and energy output. 

“The challenge is to enable the next generation of trough plants to produce steam at temperatures close to 500 degrees Celsius, thereby feeding state-of-the-art turbines without continuous backup from fuel,” says the International Energy Agency’s Concentrated Solar Power Roadmap. The Agency recommends replacing the transfer fluids with water and thereby using ‘direct steam generation’ in trough plants. 

Another way of turning up the heat is using solar power towers. In these systems, hundreds or even thousands of mirrors focus sunlight on the tip of a tower where it heats a liquid in a central tank to create superheated steam. 

Researchers are currently trying to increase temperatures—and therefore efficiency—to over 530 degrees Celsius in the planned Ivanpah CSP plant in California.

Storage is key

The biggest advantage of CSP over solar PV, however, is the ability to store solar energy at night or on sunless days: the Andasol solar power plant in Spain heats molten salt. In the evening, as the salt cools, it emits heat to make steam, giving the plant 7.5 hours of extra generating time even without sunlight. 

“Storage would open up new markets very quickly,” says Weber. “If you had cost effective storage for solar PV as you have for solar thermal it would be a real breakthrough.” 

Other storage methods under investigation include using solar power generated during the day to drive pumps to compress air underground. At night, the air could be released and spin a turbine.

For small solar PV installations, solar-powered electrolyzers could in future split water into hydrogen and oxygen and later recombine the two gases in a fuel cell to generate electrical currents. This is an expensive process but a new catalyst discovered by MIT researcher Daniel Nocera that makes splitting water cheaper could slash costs. 

Use electrolyzers on an industrial scale, and CSP could also provide hydrogen fuel, according to the IEA, helping to decarbonize global transport

If future generations are to avoid climate catastrophe solar power will have to become a greater part of the energy mix. The sun provides limitless energy for free. It’s just a matter of capturing it.


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