Energy Harvesting Systems: Capturing Energy from the Environment

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 sensor networks and the Internet of Things (IoT). With the increasing number of devices being used in daily lives, energy harvesting systems provide a sustainable solution to power these devices without the need for frequent battery replacements.

History of Energy Harvesting
The concept of energy harvesting has existed for centuries, but the development of microsystems enabled the miniaturization of energy harvesting technologies which paved the way for wireless autonomous systems that can operate indefinitely without batteries. One of the earliest documented examples of energy harvesting was the windmill, which converts wind energy to mechanical energy. Sailing ships also harnessed wind energy to transport people and goods across oceans. More recently, solar photovoltaics have enabled the conversion of sunlight to electric power, first for large-scale electricity generation and later for smaller portable devices. The energy crisis in the 1970s provided further impetus for research and development of alternative renewable energy sources. Advances in micromachining and microelectronics allowed the miniaturization of energy harvesters to power low-power electronics.

Types of Energy Harvesters
There are several types of Energy Harvesting Systems that capture different forms of ambient energy:

- Solar/Photovoltaic Energy Harvesting: The most commonly used and well-established form of energy harvesting utilizes solar cells to convert light energy, usually from sunlight, into electrical energy. Solar energy harvesting is effective for outdoor applications or applications near windows.

- Thermal Energy Harvesting: Thermoelectric and pyroelectric materials enable the direct conversion of temperature differences to electric voltage. Common applications include harvesting waste heat from machines, human body heat, or the temperature difference between day and night.

- Vibration Energy Harvesting: Piezoelectric, electromagnetic and electrostatic transduction mechanisms can extract energy from ambient vibrations in the environment. Sources of vibration include machines, structures, and human motion. This form of harvesting works well for wearables, industrial equipment, and infrastructure monitoring sensors.

- RF Energy Harvesting: Ambient radio frequencies from sources like WiFi networks, TV and cell phone signals can be harvested using antennae and rectifier circuits. RF harvesting works best near transmitters and in indoor environments.

- Kinetic Energy Harvesting: Moving objects like cars, elevators and escalators possess kinetic energy that can be captured and transformed to electricity via electromagnetic induction or piezoelectric transduction. Common applications are in traffic monitoring sensors and building automation systems.

Research Trends and Applications
Research into energy harvesting systems has increased in recent times due to growing demand for self-powered wireless sensors and the internet of things. Some key trends and applications include:

- Multi-modal and Hybrid Harvesters: Combining multiple types of energy harvesters in a single device enables extracting energy from diverse ambient sources to maximize power output. For example, a hybrid harvester may include solar, thermal and vibration components.

- Micro Energy Storage: Efficient energy storage such as micro-supercapacitors and micro-batteries are critical to regulate and supply energy from harvesters to electronic systems. Research aims to increase energy and power densities of storage.

- Efficient Power Management Circuits: Power management ICs (PMICs) ensure optimal energy extraction from harvesters and regulate voltage/current levels for loads. Advanced PMICs minimize operating voltages and improve conversion efficiencies.

- Wireless Sensor Network Applications: Common applications are in environmental monitoring, industrial automation, infrastructure monitoring, healthcare monitoring, and smart homes/cities. Large-scale deployments are powered solely using energy harvesting.

- Consumer Electronics: Recent examples include energy harvesting smartwatches, fitness trackers and hearables that eliminate battery replacements and enable longer product lifetimes.

Challenges and Future
While energy harvesting provides an appealing alternative to batteries, some challenges still need to be addressed:

- Intermittency of energy sources: Harvested power varies with changing environmental conditions and may not reliably meet peak load requirements. Energy storage helps but has limitations.

- Low energy densities: Most harvesters currently produce small amounts of power, in microwatts or milliwatts range, limiting applications. Advanced materials research aims to increase power densities.

- Harvesting circuitry and power management overheads: Non-ideal conversions & leakage currents in transistors/capacitors consume part of harvested energy. Efficiency improvements are an active area of R&D.

- High initial costs: Production costs of energy harvesting systems, particularly multi-modal designs with storage, remain higher than conventional batteries currently. Economies of scale can lower costs.

Going forward, continued progress in microfabrication, power electronics, energy storage, and renewable materials will help advance energy harvesting technology and drive greater commercialization across various applications envisioned for smart and connected systems of the future. Widespread adoption of energy harvesting promises to reduce dependency on battery replacements leading to economic and environmental benefits on a large scale.