Grand Challenges for Engineering  -  Sep 30, 2014

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Make solar energy economical

Solar energy provides less than 1% of the world's total energy, but it has the potential to provide much, much more.

As a source of energy, nothing matches the sun. It out-powers anything that human technology could ever produce. Only a small fraction of the sun’s power output strikes the Earth, but even that provides 10,000 times as much as all the commercial energy that humans use on the planet.

Why is solar energy important?

Already, the sun’s contribution to human energy needs is substantial — worldwide, solar electricity generation is a growing, multibillion dollar industry. But solar’s share of the total energy market remains rather small, well below 1 percent of total energy consumption, compared with roughly 85 percent from oil, natural gas, and coal.

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.

Many of the technologies to address these issues are already in hand. Dishes can concentrate the sun’s rays to heat fluids that drive engines and produce power, a possible approach to solar electricity generation. Another popular avenue is direct production of electric current from captured sunlight, which has long been possible with solar photovoltaic cells.

How efficient is solar energy technology?

But today’s commercial solar cells, most often made from silicon, typically convert sunlight into electricity with an efficiency of only 10 percent to 20 percent, although some test cells do a little better. 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.

Prospects for improving solar efficiency are promising. Current standard cells have a theoretical maximum efficiency of 31 percent because of the electronic properties of the silicon material. But new materials, arranged in novel ways, can evade that limit, with some multilayer cells reaching 34 percent efficiency. Experimental cells have exceeded 40 percent efficiency.

Another idea for enhancing efficiency involves developments in nanotechnology, the engineering of structures on sizes comparable to those of atoms and molecules, measured in nanometers (one nanometer is a billionth of a meter).

Recent experiments have reported intriguing advances in the use of nanocrystals made from the elements lead and selenium. [Schaller et al.] In standard cells, the impact of a particle of light (a photon) releases an electron to carry electric charge, but it also produces some useless excess heat. Lead-selenium nanocrystals enhance the chance of releasing a second electron rather than the heat, boosting the electric current output. Other experiments suggest this phenomenon can occur in silicon as well. [Beard et al.]

Theoretically the nanocrystal approach could reach efficiencies of 60 percent or higher, though it may be smaller in practice. Engineering advances will be required to find ways of integrating such nanocrystal cells into a system that can transmit the energy into a circuit.

How do you make solar energy more economical?

Other new materials for solar cells may help reduce fabrication costs. “This area is where breakthroughs in the science and technology of solar cell materials can give the greatest impact on the cost and widespread implementation of solar electricity,” Caltech chemist Nathan Lewis writes in Science. [Lewis 799]

A key issue is material purity. Current solar cell designs require high-purity, and therefore expensive, materials, because impurities block the flow of electric charge. That problem would be diminished if charges had to travel only a short distance, through a thin layer of material. But thin layers would not absorb as much sunlight to begin with.

One way around that dilemma would be to use materials thick in one dimension, for absorbing sunlight, and thin in another direction, through which charges could travel. One such strategy envisions cells made with tiny cylinders, or nanorods. Light could be absorbed down the length of the rods, while charges could travel across the rods’ narrow width. Another approach involves a combination of dye molecules to absorb sunlight with titanium dioxide molecules to collect electric charges. But large improvements in efficiency will be needed to make such systems competitive.

How do you store solar energy?

However advanced solar cells become at generating electricity cheaply and efficiently, a major barrier to widespread use of the sun’s energy remains: the need for storage. Cloudy weather and nighttime darkness interrupt solar energy’s availability. At times and locations where sunlight is plentiful, its energy must be captured and stored for use at other times and places.

Many technologies offer mass-storage opportunities. Pumping water (for recovery as hydroelectric power) or large banks of batteries are proven methods of energy storage, but they face serious problems when scaled up to power-grid proportions. New materials could greatly enhance the effectiveness of capacitors, superconducting magnets, or flyweels, all of which could provide convenient power storage in many applications. [Ranjan et al., 2007]

Another possible solution to the storage problem would mimic the biological capture of sunshine by photosynthesis in plants, which stores the sun’s energy in the chemical bonds of molecules that can be used as food. The plant’s way of using sunlight to produce food could be duplicated by people to produce fuel.

For example, sunlight could power the electrolysis of water, generating hydrogen as a fuel. Hydrogen could then power fuel cells, electricity-generating devices that produce virtually no polluting byproducts, as the hydrogen combines with oxygen to produce water again. But splitting water efficiently will require advances in chemical reaction efficiencies, perhaps through engineering new catalysts. Nature’s catalysts, enzymes, can produce hydrogen from water with a much higher efficiency than current industrial catalysts. Developing catalysts that can match those found in living cells would dramatically enhance the attractiveness of a solar production-fuel cell storage system for a solar energy economy.

Fuel cells have other advantages. They could be distributed widely, avoiding the vulnerabilities of centralized power generation.

If the engineering challenges can be met for improving solar cells, reducing their costs, and providing efficient ways to use their electricity to create storable fuel, solar power will assert its superiority to fossil fuels as a sustainable motive force for civilization’s continued prosperity.

 

References

Beard, M.C., et al.  2007.  Multiple Exciton Generation in Colloidal Silicon Nanocrystals.  Nano Letters 7(8): 2506-2512.  DOI: 10.1021/nl071486l S1530-6984(07)01486-5

DOE (U.S. Department of Energy).  2007.  Solar America Initiative: A Plan for the Integrated Research, Development, and Market Transformation of Solar Energy Technologies.  Report Number SETP-2006-0010.  Office of Energy Efficiency and Renewable Energy Solar Energy Technologies Program.  Washington, D.C.: DOE.

DOE.  Solar Energy Technologies Program Multi-Year Program Plan 2007-2011.  Office of Energy Efficiency and Renewable Energy.  Washington, D.C.: DOE.

Lewis, N.S.  2007.  Toward Cost-Effective Solar Energy Use.  Science 315(5813): 798-801.  DOI: 10.1126/science.1137014

Ranjan, V., et al.  2007.  Phase Equilibria in High Energy Density PVDF-Based Polymers.  Physical Review Letters 99: 047801-1 - 047801-4.  DOI: 10.1103/PhysRevLett.99.047801

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

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U.S. Department of Energy Solar Energy Technologies Program
The Solar Energy Technologies Program leads the U.S. effort to research, develop, and deploy cost-effective technologies toward growing the use of solar energy.

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