SOLAR (Also see Solar Energy Flash Lesson)

There are three processes for converting solar energy

  1. heliochemical: the photosynthesis process
  2. heliothermal: heating of a secondary fluid (solar thermal)
  3. helioelectrical: photovoltaics (solar cells)

The advantages of solar energy include its nonpolluting nature; it is nondepletable, reliable, and free fuel. The disadvantages of solar energy are that the solar energy concentration is very dilute, so collectors with large surface area are needed. In addition, solar radiation is neither constant nor continuous for terrestrial applications (i.e., low capacity factor). The solar energy received depends on latitude, season, time-of-day, and atmospheric conditions.

There are three general categories of solar-energy collection systems:

  1. direct conversion of sun rays to electricity with solar cells (photovoltaics),
  2. flat-plate systems producing low-temperature (<150°F) thermal energy for heating and cooling of buildings; the thermal energy generated in the collector is usually removed by either air or an ethylene glycol-water solution, and
  3. concentrating solar collection systems that produce high-temperature thermal energy for the generation of electricity.

Photovoltaics

Solar photovoltaic (solar cell) is a direct conversion of the sun's electromagnetic radiation to electricity, and is not limited by Carnot cycle efficiency considerations. Photovoltaic (PV) cells employ a solid-state diode structure with a large area on a silicon wafer. The surface layer is very thin and transparent so that light can reach the junction region of the silicon sandwich. In that region the photons are absorbed, releasing charges from their atomic bonds. These charges migrate to the terminals, raising the potential. A single cell has an open circuit the voltage of approximately 0.6-1.0 volts and a short circuit current of a few mA. In order to increase both current and voltage, the individual cells are placed into (solar) arrays where cells may be connected in series to raise the voltage and current output can be raised by parallel connection of cells.

The solar cell structure consists of two layers of material: one layer is doped with an impurity such as boron to make it negative (n-type), and the other is similarly doped to make it positive. The area where the two layers touch is called the p-n junction. When sunlight penetrates to the p-n junction, positive and negative charges from the two layers cross the junction, creating a flow of electric current. The layers must be extremely thin to ensure light penetration. For silicon cells, these thin layers have been obtained at high cost by slicing an expensive silicon ingot; much of the silicon is lost in saw cuts.

Other substances, such as cadmium sulfide and gallium arsenide, are also used to make solar cells. Cadmium sulfide, although relatively cheap, has a low efficiency. Gallium arsenide is very efficient but also very costly. Additional methods of producing silicon cells--such as using amorphous, rather than crystalline, silicon--offer considerable promise.

Amorphous silicon cells, generated on a layer of film 1.5 µ thick and sandwiched between plate glass, have been installed in huge arrays. A California complex, built in 1992, covers five acres and generates 500 kW at a cost of about $0.25 per kilowatt hour. Thin-film cells are half as efficient as crystalline cells, but they cost considerably less to produce. New production methods may increase conversion efficiency to crystal-cell levels.

Typical efficiencies for solar cells currently run from 10 to 15%; efficiencies of 30% have been achieved, however, and researchers hope eventually to reach as high as 40%. Applications of photovoltaic cells include

Presently, photovoltaic is economical at remote, end-of-transmission line locations.

photovolt

Solar Cell Power Plant Diagram [Source: Texas Utilities (TU)].

solar

Solar Photovoltaic Power Plant Diagram [Source: Tennessee Valley Authority (TVA)].

Solar Thermal

Solar thermal is the use of a vapor power cycle that requires the concentration of solar energy to reach high temperatures and reasonable thermal efficiency. Solar thermal, energy concentration devices include parabolic mirrors and arrays of focused mirrors (heliostats). [Note that concentrating lens such as Fresnel lens have been used for solar cells.] The solar concentrator is most expensive component in a solar thermal installation representing 40-50% of total system cost. A solar thermal power station is highly capital-cost intensive.

The working fluid temperature is limited by the solar collector design

flat-plate collector 40 to 120°C (100 - 250°F)
parabolic concentrator 150 to 800°C (300 - 1500°F)
heliostats 250 to 1500°C (500 - 2700°F)

From the table one can see that either parabolic concentrating collectors or heliostats should be used. The parabolic concentrating collectors include (1) the parabolic trough collector, which is a half-cylinder mirror with a pipe parallel in the center to absorb the heat into the working fluid, and (2) the parabolic dish collector, which is a hemispherical collector. The heliostats are highly polished mirror-like devices which focus light to a single collector on top of a tower.

Solar thermal systems can be broken into three categories:

  1. large point focus: power tower systems with heliostats. Molten salts and liquid metals are used as the working fluid that then boils water for use in a Rankine cycle. Sizes of 100 kWe to 100 MWe.
  2. small point focus: use parabolic hemispherical dishes to reflect light to a focal point on each individual dish. These are for remote stand-alone systems (5-25 kWe), and
  3. line focus systems: use parabolic shaped troughs, and have lower efficiency.

Demonstration plants include

SolarONE

Solar One Power Plant [Source: The Geo-Images Project].

Another solar thermal concept is that of the solar salt pond, which is similar to the ocean thermal system. In this scheme a shallow (1 to 2 m deep) pond saturated with salt is the solar collector. The hot water sinks to the bottom of the pond since as the water temperature increases, it becomes more soluble, thus absorbing more salt which makes it heavier. The hot water is used to heat another fluid for use in a Rankine cycle.


Last updated: June 13, 2006

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