This (also known as power harvesting or energy scavenging) is the process by which energy is derived from external sources (e.g., solar power, thermal energy, wind energy, salinity gradients, and kinetic energy), captured and stored. Frequently, this term is applied when speaking about small, wireless autonomous devices, like those used in wearable electronics and wireless sensor networks. Traditionally, electrical power has been generated in large, centralized plants powered by fossil fuels, nuclear fission, or flowing water. Large scale ambient energy, such as sun, wind and tides, is widely available but technologies do not exist to capture it with great efficiency. Energy harvesters currently do not produce sufficient energy to perform mechanical work but instead provide a very small amount of power for powering low-energy electronics. While the input fuel to large-scale generation costs money (oil, coal, etc.), the ‘fuel’ for energy harvesters is naturally present and is therefore considered free. For example, temperature gradients exist from the operation of a combustion engine and in urban areas, there is also a large amount of electromagnetic energy in the environment because of radio and television broadcasting.