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Introduction

Electromagnetism

Electromagnetic Radiation

Photoelectric Effect

Semiconductors

Photovoltaics

Electric Sunshine

Conclusion

References

Electric Sunshine

    Our sun is an average sized star lying at the center of our solar system at a distance of about 150 million kilometers from earth.¹⁶  Sunlight is the radiant energy left over from the process of fusing four Hydrogen atoms into two Helium atoms; a process that has been occurring in the sun’s core for roughly four and a half billion years at a variable rate of about four billion kilograms of hydrogen per second.¹⁶  Incident solar radiation, called isolation, is the radiant energy which travels at the speed of light from the suns photosphere to Earth’s upper atmosphere.

    But how much energy are we talking about?  The isolation received at the surface for any point on Earth will vary between 0 and 1050 watts per meter squared (Wm^-2) depending on the season, time of day, and cloudiness.⁷  From the figure below we can take the mean daily isolation of the United States to be about 4700 Watt-hours per meter squared per day (Whm^-2/day), and then multiply this by the surface area of the United States 9.862 × 10¹² m² to obtain the incident power as 4.64 × 10¹⁶ W h or 46,400 Tera Watt-hours (TWh).²³

A map of average daily insolation upon the United States.¹⁸                                      

    Comparing this to the total energy consumed by the US from all sources in 2005 given as 29,000 TWh, we can see that the energy from sunlight incident on the US in a single day could satisfy our nations energy requirements for an entire year!²³  Of course this would mean turning the entire country into a giant solar panel having a 100% conversion efficiency, and this is certainly not practical nor possible; the comparison though does serve to demonstrate the extraordinary amount of energy contained in ordinary sunlight.

    A typical silicon solar cell having a surface area of 100cm² will produce about 1.5 W at 3 amps and 0.5 V on a sunny day under insolation of 0.1 Wcm^-2.³ The output is direct current and we can assemble individual cells into an array and then wire arrays together to form a flat panel or module.⁸  Commercially available modules are generally about 1 m² and will yield an average of one kilo-watt hour per day (kWh); modules are wired in series to obtain a desired voltage level such as 12 VDC, and then series sets are wired in parallel to obtain a desired current output.⁸  Residential applications frequently have the modules placed on the roof, and a typical residence will consume anywhere form 5 - 15kWh per day.⁸  So installing photovoltaic modules will significantly off-set the amount of energy drawn from the grid to power the various devices within a given residence.

    One thing you might be wondering is why we only pull about 100 W out of our modules when we have upwards of 1000 W incident?  The answer is that typical solar cells have conversion efficiencies of about 12-14%.  This is due to a number of factors including cell reflectance, configuration of the conducting grid, internal quantum efficiency, and the spectral dependence of the silicon comprising the cell.  That last item is a big one because it determines how much of the incident spectrum can be harvested.  The following diagram shows the spectral conversion for silicon having a band gap of 1.1 eV.

              A graph of the energy of the solar spectrum at sea level as a function of wavelength.  The
              portion of the spectrum convertible by a 1.1eV silicon solar cell is shaded in green.²                              

    As you can see, a large portion of the incident spectrum is essentially invisible to our photovoltaic array.  The conversion efficiency can be increased by stacking layers of silicon with varying spectral dependencies, but this significantly increases the manufacturing costs.  However there are alternatives involving third generation photovoltaics which include more exotic semiconductor materials, tandem cells, and even nano-technology via quantum dots.   Whether or not these technologies become viable, integrated sources of energy for our homes and businesses remains to be seen, but the potential is certainly vast.