Solar Energy

The mother of all fuels, solar energy reaches us through light and heat, wind and waves, and through photosynthesizing plants ("biomass" in FutureCars jagron). No fuel is more fundamental, more abundant or more powerful. Perplexingly, neither is any fuel more difficult to harness.

solar panels

photo by Alla Leitus

Production

There are two systems used to produce usable transportation fuel from solar energy:

Concentrating Solar Power (CSP)
CSP systems use mirrors to concentrate sunlight onto receivers that convert the energy to heat. This energy can drive a generator to produce electricity. Mark Mehos of NREL provides the following descriptions of the 3 primary types of CSPs:
A dish/engine system
uses a mirrored parabolic-shaped dish to collect and concentrate sunlight onto a receiver, which absorbs the heat and transfers it to a working gas within the engine. The heat causes the gas to expand against a piston or turbine to produce mechanical power, and the mechanical power runs a generator or alternator to produce electricity.
A power tower system
uses a large field of mirrors to concentrate sunlight onto a receiver on top of a tower, heating a working fluid—either molten salt or water/steam—that flows through the receiver. A conventional steam generator uses the heat from the working fluid to generate electricity.
Parabolic trough systems
concentrate the sun’s energy onto a receiver pipe using parabolically curved, trough-shaped reflectors. The sun heats oil flowing through the pipe, which boils water in a conventional steam generator to produce electricity. Both trough and tower systems can store energy collected in the field in large tanks filled with molten salt, allowing them to operate through cloudy conditions and into the evening.
Photovoltaics (PV)
Photovoltaics employ semiconducting cells (think solar panels) to convert solar energy into electricity. As described by the National Renewable Energy Laboratory (NREL), "They [solar cells] are made of semiconducting materials similar to those used in computer chips. When sunlight is absorbed by these materials, the solar energy knocks electrons loose from their atoms, allowing the electrons to flow through the material to produce electricity. This process of converting light (photons) to electricity (voltage) is called the photovoltaic (PV) effect."

Infrastructure

CSP plants can feed power into existing electric distribution infrastructure. Current CSP plants require substantial acreage, so land resources are an important consideration.

Advantages

Solar power is extremely clean to generate and distribute, and the U.S. DOE has substantial investments in solar technologies, primarily through the Solar Energies Technology Program CSPs are compatible with existing generators - the heat generated by a CSP can be used to run a conventional power plant. Concentrating solar plants take full advantage of the sun's power and, in the right location, can generate tremendous amounts of power and store it geothermally. On a personal transportation scale, the scalability of PVs makes them a viable option for zero-emissions onboard power generation. In fact, you've probably seen pictures of exotic solar-powered cars that hit the highway every year as part of the American Solar Challenge.

Disadvantages

PV cells are very inefficient and at present play a supplementary role. Even the most advanced cells available for purchase manage to convert only about 30% of received energy into power. PV cells are also not very useful in cloudy climates. CSPs require a lot of land. Even in the sunny, arid, sparsely populated Southwestern United States. The acres of mirrors require a large footprint and power lines must be strung along the distribution routes; this means lawsuits by private property owners. On a positive note, Bright Source Energy claims, "In general, a typical 100 MW solar thermal plant will occupy 600 to 800 acres. Installing solar power plants on an area covering only 1% of the Mojave Desert would provide enough solar power to serve 75% of the homes in California."

The Future

In a general sense, the future of solar is now. Both the public and private sectors are increasing investment in solar energy, motivated by decreasing costs, increasing efficiency, peak oil and environmental concerns. The future of solar energy as an onboard source of transportation fuel depends on the improved efficiency of photovoltaic cells. Somewhere in the middle is the increasing use of PV arrays in homes and businesses. An electric car that is recharged with sun-generated power is, effectively, a solar-powered car. See this page to read about solar cars in development.

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