Introduction to Energy Harvesting
Energy harvesting, 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 for small, wireless autonomous devices, sometimes without batteries. Energy harvesting devices can operate permanently without maintenance or replacement by converting ambient sources into electric power. They provide alternatives to conventional power sources like batteries which have limited lifetimes and require replacement or recharging, which can be inconvenient or impossible for small, inaccessible devices.
Types of Energy Harvesting
Vibration Energy Harvesting
Vibration energy harvesting utilizes kinetic energy from vibration or minor motions to power electronic devices. Piezoelectric and electromagnetic technologies are commonly used to convert mechanical energy from vibrations into electricity. Applications include harvesting energy from operating machinery, traffic on bridges, footsteps, and other human-induced vibration sources. Microelectromechanical systems (MEMS) technologies enable the miniaturization of vibration energy harvesters for powering small devices.
Thermal Energy Harvesting
Thermal Energy Harvesting System captures heat generated by ambient temperature differences or thermal gradients and converts it into electricity. The Seebeck effect in thermoelectric generators produces voltage when there is a temperature difference between junctions of dissimilar materials. Thermoelectric energy harvesters can utilize body heat, engine waste heat, solar heat, and more to power wearable or implanted medical devices, sensors on engines or pipes, and other applications where thermal differentials exist.
Solar Energy Harvesting
Photovoltaics, or solar cells, are the most widely used form of energy harvesting from ambient light energy. Solar cells powered by sunlight generate electricity that can charge batteries or power devices directly. As photovoltaic technologies advance and module costs decline, solar energy harvesting becomes increasingly viable for powering a growing number of applications from electric vehicles and rooftop installations to small solar-powered toys and calculators. Many portable consumer electronics already integrate solar cells to supplement or recharge batteries.
Electromagnetic Energy Harvesting
Electromagnetic energy harvesting exploits changing or fluctuating electromagnetic fields for power. Some examples are inductive power transfer from fluctuating magnetic fields and energy captured from radio frequency (RF) electromagnetic waves, typically from ambient radio signals in the environment or dedicated low-power broadcast systems. RF energy harvesting enables powering of passive remote sensors and low-power devices without batteries by scavenging milliwatts of power from FM radio signals, digital TV signals, mobile phone base station signals and more.
Challenges and Advancements in Energy Harvesting Systems
While energy harvesting technologies provide valuable alternatives to batteries, some challenges remain to increase their effectiveness and commercial viability. One issue is the relatively low power densities of most energy harvesting sources compared to conventional batteries. Technical advancements aim to maximize power output from given sources through improved materials, designs, power management circuits, energy storage, and multi-modal harvesting that combines outputs from different sources. Power output must often be increased by one or two orders of magnitude to meet the demands of most electronic applications.
Energy storage is also crucial to smooth out variability in harvested power and provide sufficient voltage and current for device operation between recharge cycles. Improved micro batteries and electrochemical capacitors able to store bursts of harvested energy are undergoing research and development. Another focus is increasing energy harvesting system lifetime, efficiency and durability to low-maintenance operation over 10 or more years as expected from permanent installations.
Overall, energy harvesting provides a promising solution for powering a growing number of small, wireless smart devices and sensor networks, especially those working in inaccessible locations or needing truly autonomous operation without maintenance over long periods. As harvesting technologies continue to advance extraction of low-grade ambient energy sources along with co-design of efficient storage and power management, widespread adoption and commercialization of energy harvesting systems is anticipated across various industry sectors. The capability to harness ubiquitous low-power environmental energy opens up new possibilities for distributed sensor networks and the emerging Internet of Things.
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