The Solar Energy Dilemma

Solar electricity generation is a growing, multi-billion dollar industry. Yet, the solar share of the total energy market is less than one percent of total energy consumption. Coal, natural gas, and oil cannot remain the dominant sources forever. For a long-term sustainable energy source, solar power is an attractive alternative. Its availability exceeds any conceivable future energy demands. It is environmentally clean, and its energy is transmitted from the sun to Earth free of charge.

But overcoming the barriers to widespread solar power generation requires engineering innovations for capturing the sun’s energy, converting it to useful forms, and storing it for use when the sun is obscured. Some technologies that address these issues are already in use. For example, reflective dishes concentrate the sun’s rays to heat fluids that drive engines and produce power.

Another involves direct production of electric current from captured sunlight, which is possible using solar photovoltaic cells.

Polycrystalline solar cells make up a blue colored panel. Individual crystals are visible from many angles. The cells are cut to a square shape which gives the best power production for a given solar panel size. These cells work slightly more efficiently in low light and perform well in high  temperatures and harsh environments.

However, today’s commercial solar cells typically made from silicon convert sunlight into electricity at an efficiency rate of only 10 to 20%. To make solar economically competitive, we must find ways to improve cell efficiency and lower manufacturing costs. Current standard cells have a theoretical maximum efficiency of 31% because of the electronic properties of the silicon material. But, new materials, arranged in novel ways can evade the limit, with some multi-layer cells reaching 34% efficiency. Experimental cells have exceeded 40% efficiency.

Another approach to improve efficiency is using 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. Experiments using nanocrystals made of lead and selenium show that in standard cells, the impact of a particle of light (photon) releases an electron to carry an electric charge. Lead-selenium nanocrystals increase the chance of releasing a second electron, boosting the electric current output. The nanocrystal approach could reach efficiencies of 60% or higher. Engineering advances will be required to find ways of integrating nanocrystal cells into a system that can transmit energy into a circuit.

Other new materials for solar cells may help reduce fabrication costs. Material purity is crucial: impurities block the flow of an electric charge. So, current solar cell designs require high purity and are, therefore, expensive. That problem could be reduced if electrical charges had to travel only a short distance through a thin layer of material. But thin layers would not absorb as much sunlight. One way around that issue is to use materials thick in one dimension for absorbing sunlight and thin in another dimension through which charges could travel. One approach involves cells made of tiny cylinders or nanorods. Light could be absorbed down the length of the rods, while charges could travel across the rods’ narrow width.

A major barrier to the widespread use of the sun’s energy is the need for storage. New materials could greatly improve the effectiveness of capacitors, superconducting magnets, or flywheels, all of which could provide convenient power storage in many applications. Another possible storage solution could 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. For example, sunlight could power the electrolysis of water, and generate hydrogen as fuel. Hydrogen could then power fuel cells, electrical-generating generating devices that produce no polluting byproducts.

For more information:
www.nae.org