In the recent past, the growing presence of renewable-energy research in academic journals and industrial companies has led to an increase in its contribution: 19% to global energy consumption and 22% to U.S. electricity generation in 2012 and 2013, respectively. National renewable-energy markets are expected to continue growing strongly in the coming decade and beyond for many reasons. First of all, clean energy comes from unlimited and natural resources, e.g., the movement of wind and water, and the heat and light of the Sun. Secondly, it reduces global warming and pollution, and improves environmental quality. Furthermore, it creates jobs and enhances economies.
Although ambient mechanical and thermal energy are classified as the largest forms of renewable energy among those available, they are also considered to provide desired power for low-power electronic devices by using piezoelectric and pyroelectric materials. Ambient mechanical and thermal energy are produced naturally and non-naturally; for example, ambient mechanical energy is produced naturally from different sources, such as hydroelectricity, ocean or river waves, and wind. It is also produced nonnaturally due to the forced motion of objects, such as human and machine motion. Conversely, thermal energy is generated naturally from sun rays or geothermal waves, and non-naturally from artificial light and microwaves.
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