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AZ Dept. of Commerce "Bright Ideas" Report on Photovoltaics - Introduction

                    

Photovoltaics (PV) - Introduction

Throughout the history of mankind the sun has inspired worship, awe and fear.  In ancient Egypt, it was the sun god Ra who held the supreme place among all deities as the giver of life.  In Greek and Roman legend, the sun was a fiery chariot driven across the face of the sky.

We can only imagine how those ancient people might respond to the sight of thin, shiny rectangles harnessing the sun’s awesome energy.  Even today with knowledge of scientific principles, observers are astonished by the technology of photovoltaics.

Photovoltaic cells convert sunlight directly into electricity.  They have successfully powered space satellites since 1958 and now furnish electricity for a wide variety of applications on earth.

Solar-generated electricity powers water pumps, weather monitoring stations, fire watchtowers, and billboard lights, irrigation system, streetlights, boat battery chargers, and numerous other devices in Arizona and throughout the world.  Glistening photovoltaic panels can easily be seen at Tucson bus stops, atop many roadside emergency telephones, and near artwork along Phoenix freeways.

Solar cells in space work in a vacuum at extremely high or extremely low temperatures while exposed to intense radiation. Other systems have been used on frozen tundra, in scorching deserts and on mountain peaks.  Tougher tests for a young technology would be hard to design.

Nevertheless, photovoltaic systems have established a record of reliability and have proven cost effective for many uses.  They produce no pollution in creating electricity and require no water to operate.  As environmental problems escalate and solar cells costs are reduced, these systems will almost certainly play an important role in our energy future.

The remainder of the "Photovoltaics" pages are here:

Electrical Generation - PV & Thermal Solar



Photovoltaics



How a PV Cell Works

 
Power Tower



Dish Sterling



Trough

Electrical Generation

The generation of electricity from solar energy can be achieved through two major technology alternatives. One uses the light from the sun to generate electricity directly, (photovoltaic technologies), and the other uses the heat from the sun to increase the temperature of a working fluid which-in turn can be used to generate electricity, (solar thermal technologies). Each of these major alternatives can, in turn, be subdivided into variants of the major technology. Photovoltaic technologies fall into crystalline, multi-crystalline, thin-film or concentrator variants while the solar thermal technologies fall into trough, power tower, dish engine and thermal electric variants.

Photovoltaics

Generally speaking, How a PV Cell Works describes the use of a semiconductor material that is exposed to sunlight. The energy of the incident light displaces electrons from their normal atomic orbits and an electrode grid structure on the surface of the semiconductor collects these electrons and makes them available for use in an external circuit. This is very similar to the way that the chemical reaction and the electrodes in a dry battery cell make electrons available for external use.

The terms crystalline, thin film and concentrator describe the manner in which the semi-conducting material is processed and optimized as a photovoltaic cell. Crystalline cells are fabricated from ingots of the semiconductor material, usually silicon, that are cut into relatively thin slices, processed to optimize the electron collection efficiency and laminated into a protective enclosure. Thin film cells are extremely thin layers of semi-conducting material that are evaporated onto a substrate, and concentrating cells use a plastic lens to concentrate sunlight from a large area onto a much smaller area of crystalline semi-conducting material. All types have their merits and problems and are described in detail in the referenced locations.

Download the Arizona Consumer's Guide - this booklet is designed to guide you through the process of buying a solar electric system.  This document is somewhat out of date (2000) and does not mention some newer information. NOTE: You will need Adobe's Acrobat Reader to open, view, and print this document.  Acrobat is freely available and can be downloaded from Adobe's Web site. Arizona Consumer's Guide (PDF Format)

Visit the National Geographic's web site and take the: PV Quiz

Solar Thermal

Both the trough and power tower solar thermal technologies use mirrors to concentrate the heat from the sun onto a vessel containing a heat transfer fluid. The fluid is then pumped into a steam generator where the heat is transferred to water turn it into steam. The steam can then be used to spin a conventional steam turbine connected to a generator to make electricity. As you can see, the size of the concentrating systems makes them impractical for homes.

In the case of the trough, the mirror is a long parabola with a steel tube containing the heat transfer fluid running along the focal axis of the mirror. The axis of the mirror is usually aligned in a North-South direction and the mirror is rotated from East to West as the day progresses so that the energy from the sun is continually focused onto the steel tube. Rows of mirror/tube assemblies are used to form large multi-acre solar fields from which the heated transfer fluid is collected and used in the generation of steam.

The power tower system is a little different in that all of the transfer fluid heating is achieved in a heat receiver on the top of a tower located in the center of a field of computer controlled mirrors, or heliostats. Cold fluid is pumped up to the top of the tower, the heliostats focus the sun's energy onto the receiver and heat the fluid which is subsequently returned to the ground and used in a steam generator in the same way as the heat transfer fluid in the trough system.

Dish/engine systems are somewhat different in that the heat from the sun is used to heat a working fluid within a heat engine. The rotating shaft of the engine is connected to a generator, which produces electricity without the need to go through a steam generation process. The engine is located at the focal point of a parabolic dish mirror, which is made to track the sun across the sky throughout the day.

Good Resources:

 

Environmental Benefits of Renewable Energy

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Renewable energy—wind, solar, geothermal, hydroelectric, and biomass—provides substantial benefits for our climate, our health, and our economy. Each source of renewable energy has unique benefits and costs; this page explores the many benefits associated with these energy technologies.

Solar, wind, and hydroelectric systems generate electricity with no associated air pollution emissions. In comparison, coal and natural gas plants have air and water pollution that is linked to breathing problems, neurological damage, heart attacks, and cancer.

Environmental damage affects everyone, but the costs and benefits of reducing environmental damage are not well matched or shared.  Air and water pollution can travel great distances and affect areas well removed from the sources of pollution.  There is a lot of evidence that the global environment is being degraded by the net effect of many activities that produce pollution either directly or indirectly.  The production of energy is a major contributor, especially the burning of fossil fuels.  Pollution of the environment can be reduced by adopting renewable energy, but there are costs involved. 

Some interesting links:

More Articles ...

  1. Solar Architecture in Ancient Greece
  2. Some Barriers to Implementation of PV Systems
  3. Concentrator photovoltaics (CPV)
  4. Barriers to PV Implementation