The sun is a sphere of hot gaseous matter made up of 74 percent hydrogen, 25 percent helium and trace quantities of heavier elements. According to Tom P. Hough, the author of "Solar Energy: New Research," the surface temperature of the sun is approximately 5,700 K. Energy is generated by nuclear fusion of hydrogen nuclei to helium. Solar radiation, or energy from the sun, supports life on earth via photosynthesis -- plants capture the energy and convert it into another form. Glucose is synthesized from sunlight, carbon dioxide and water, and oxygen is released as a byproduct.
Solar energy is clean and environment-friendly. Flat plate collector, solar distillation, microclimate greenhouse technology and photovoltaic systems are applications to harness solar energy. Solar thermal systems are more popular than solar electric systems because the energy payback time is shorter. Hybrid systems provide an economically viable alternative to solar electric systems by combining photovoltaic systems with thermal systems. Cooking, space heating and swimming pool heating are examples of domestic applications; pre-heating of boilers is an example of the commercial application of solar technology.
The availability of technology coupled with considerable wind energy density has fostered the emergence of wind energy systems. A wind turbine converts wind power into electricity. Wind turbines are connected to electrical networks such as battery charging circuits, isolated networks, residential power systems or large utility grids. Turbines ranging from 1.5 to 5 megawatts have been deployed in large utility grids in China, Europe, India and the United States.
The aerodynamic force of lift is used to produce a net positive torque on a rotating shaft. This results in the production of mechanical power, which is transformed to electricity in a generator. A limitation of wind energy is that wind cannot be stored for later use; hence, the output is fluctuating and non-dispatchable. Modern wind turbines such as the horizontal axis wind turbine (HAWT) serve to reduce total electrical load in larger networks and serve storage, backup and specialized systems in smaller networks.