History and Background of Energy Harvesting Systems
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, like those used in wearable electronics and wireless sensor networks. The demand for these small autonomous devices is driving research and development of energy harvesting systems. The concept of energy harvesting was proposed in the 1960s, which led to the development of the earliest harvesting technologies in the 1970s and 1980s. The earliest forms of energy harvesting converted heat, light, and vibration to electricity. Over the past few decades, advancements in materials and semiconductors have enabled the scaling down of energy harvesting and storage systems to power small sensor nodes and wireless devices.

Current Types of Energy Harvesting Systems

There are many types of energy harvesting systems that convert ambient sources of energy into electric power. Some of the most commonly used energy harvesting technologies include:

- Photovoltaic/solar energy harvesting: Systems that convert light energy from the sun into electricity. They are commonly used to power outdoor applications that have constant exposure to light.

- Thermal energy harvesting: These systems harness electricity from temperature differences and gradients. They are typically used where large and constant temperature changes are available, such as industrial processes.

- Vibration/kinetic energy harvesting: Systems that convert ambient vibrational energy from the motion of physical objects or human activity into electricity. They can power devices in industrial environments, vehicles, and wearable technologies.

- RF/WiFi Energy Harvesting System: Involves collecting ambient radio waves/electromagnetic radiation from nearby WiFi routers and cellular networks. It is most applicable for powering IoT devices located near strong wireless signals.

- Salinity gradient power: Converts the chemical energy from mixing fresh and saltwater into electricity. It has potential for coastal and offshore applications.

Applications and Opportunities for Energy Harvesting

There are numerous commercial applications emerging that utilize energy harvesting as a viable power solution. Some of the promising opportunities for energy harvesting technologies include:

- Wireless sensor networks: Energy harvesters allow sensor nodes to operate forever without battery replacement. This enables applications in industrial automation, infrastructure monitoring, precision agriculture and environmental sensing.

- Wearable devices and smart clothing: Integrating energy harvesters into fabrics, shoes, and accessories can power flexible displays, health/activity trackers and provide truly "wear-and-forget" convenience.

- Industrial IoT: Vibration-based harvesters combined with wireless communication allow remote condition monitoring of motors, pumps and other equipment at scale.

- Transportation sector: Kinetic energy harvesters from road/rail vibrations could power onboard telemetry systems and environmental sensors for asset tracking.

- Buildings and smart cities: Windows that double as solar panels, streetlights with thermoelectric harvesters, and "grid-less" infrastructure monitoring are emerging applications.

- Medical implants: Energy harvesting ensures year-plus operational lifetimes for ingested or implanted devices without battery replacement surgeries.

- Embedded/remote systems: Energy autonomy improves reliability/longevity of systems in tough industrial/outdoor environments like oil & gas installations.

Power Conversion and Energy Management

For energy harvesting to become the primary power source, efficient conversion and intelligent management of the captured energy is crucial. Advanced power conversion circuits maximize extraction from varied ambient sources into regulated DC power. Energy storage such as capacitors or rechargeable batteries buffers short-term variability. Power management ICs monitor available energy and judiciously allocate it based on application priority. Over-voltage and over-current protection safeguard sensitive electronics during harvesting/charging. The coupling of harvesting front-end, power conversion and energy storage/management is enabling self-powered operation of diverse systems without embedded batteries.

Challenges and the Future of Energy Harvesting

Despite progress, energy harvesting still faces challenges of non-continuous power availability, intermittency based on ambient conditions, and relatively low instantaneous power levels - especially for small-scale harvesters. Device-level energy efficiency gains through low-power circuit design are also required to minimize power consumption. Advancements in nanomaterials, microfabrication and integration are working to further scale down harvesters while improving conversion efficiency and power density. Standardization on optimized system architectures and components can help accelerate real-world adoption. Multimodal energy harvesting that combines complementary sources is also being explored to enable a round-the-clock powered future. With continued R&D, energy harvesting promises to revolutionize how we power portable/wearable devices and support the explosive growth of IoT systems worldwide. It heralds an exciting future of truly self-sustaining embedded electronics.

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