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The generation of electrical energy involves an energy-to-energy conversion process such as mechanical-to-electrical (ME), chemical-to-electrical (CE), solar-to-electrical (SE), radio frequency-to-electrical (RFE), and thermal-toelectrical (TE). ME conversion is used in hydroelectric and wind turbines for large-scale generation to meet the demands of cities. CE conversion is used in batteries to provide portable electrical energy. TE conversion is a technology under development. SE conversion uses solar energy to generate electrical energy. Solar farms provide electrical generation on a large industrial scale as well as for individual home owners. RFE converts electromagnetic energy in the millimeter (mm)-to-micron-wavelength range of the electromagnetic spectrum to electrical energy.
Currently, energy harvesting refers to the nonchemical generation of small amounts of electrical energy on a local scale using one of the above energy conversion principles. Among the above energy conversion processes for electrical energy harvesting, even under very low light levels, SE conversion has generally been found to be the best choice and is widely used in consumer goods and many other products. For example, SE conversion is used to power wrist watches, calculators, road signs, and in practically any other application where solar illumination is available and its space and power requirements can be met. The energy output of SE-based energy harvesters is limited by the size of the solar cell. However, in many energy-harvesting applications, a few microwatts of power may suffice and can be obtained. For 24/7 operation, storage devices such as rechargeable batteries and super-capacitors have been used.
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