How We Get Power from the Sun
The Many Different Faces of Solar Energy
The Sun bathes the Earth in a constant flow of electromagnetic energy and there are many different ways that we can harvest that clean energy for our use.
Solar power, or energy derived from the electromagnetic radiation from the sun, has been touted as the new solution to a wide variety of energy related problems over the years; the latest being global warming. In reality mankind has been using solar power in its many varieties since the dawn of man. Understanding the many ways that we can use solar energy may be a key part in understanding how we can respond to many of our energy related problems.
Photosynthesis
Green plants use the energy of the sun to convert water and carbon dioxide to complex sugars and carbohydrates in a process called photosynthesis. Mankind has used those chemicals for food and fuel since the beginning of mankind. In fact, it has been said that, since coal and oil may be derived from pre-historic plants, they may actually be considered photosynthetic fuels.
Modern fuels like ethanol are derived from biomass provided by photosynthetic processes. Sugars are fermented to alcohol and carbohydrates like lignin and cellulose are broken down into their component sugars to be fermented into ethanol. The ethanol is then burned to form Carbon Dioxide and water, providing further fuel for photosynthesis.
Solar Heating
A simple form solar energy is used for direct heating of buildings or water (Nansen, Sun Power). While mankind has made houses and other buildings to use this energy to advantage, it was only in the 1970’s that serious study was made of ways to passively heat living spaces and water using solar energy.
Active solar heating systems use a working fluid to collect the sun’s energy and move it to where it can perform work. Flat plate solar collectors can heat a working fluid to up to 80° C (176° F) while compound parabolic concentrators can achieve working temperature of up to 315° C (600° F) (Welford, The Optics of Nonimaging Concentrators).
Electricity from the Sun
Since the energy from the Sun is delivered via electromagnetic radiation it is possible to directly convert that radiation to electricity. There are two different means of accomplishing this direct conversion; photovoltaic, and photogalvanic (Tien, in Solar Energy, ed Hautala). The photovoltaic cell, commonly known as a solar cell, produces an electromotive force (voltage) upon absorbing photons. A photogalvanic cell is essentially a battery; charged by a photo-redox reaction.
There are also two indirect means for converting solar radiation to electricity. The first is by photo-disassociation of water into oxygen and hydrogen. These chemicals can then be recombined in fuel cells to produce electricity. Another method is to use solar energy to heat a working fluid to drive an electric generator in much the same way as most conventional electric power is generated.
Finally solar power is the ultimate source of the energy produced by wind farms and hydroelectric dams. Unequal solar heating of the Earth’s surface causes wind, and that wind power can be used to turn turbines to generate electricity (Nansen). Solar heating is responsible for the evaporation of water from the ocean that ultimately falls as rain in the mountains. The flowing water, en route back to the ocean, turns turbines that generate electricity.
Conclusion
The Sun is the ultimate source of most of the energy that mankind has used through the ages. Direct conversion of solar radiation to heat or electricity is a modern usage of that power, but humanity has been using the energy of photosynthesis, of blowing wind and of flowing water for thousands of years. All of these types energy trace their origins back to the Sun.
