Wearable sensor devices have transformed the fields of disease diagnosis and health management in recent years, providing exceptional portability and the ability to continuously monitor physiological signals in real time. This revolution has been largely facilitated by advancements in materials science and micro/nano-processing technologies. However, as the demand for multifunctionality and flexibility in wearable devices increases, the need for stable and sustainable miniaturized power supplies has become increasingly urgent.
Ambient energy harvesters are emerging as a promising solution to power these wearable sensors by collecting energy from the user’s surrounding environment. These harvesters can convert various forms of energy, such as thermal, mechanical, electromagnetic, and bioenergy, into usable electrical power. By integrating ambient energy harvesting capabilities, wearable devices can achieve self-powered and sustainable operation, overcoming the limitations of traditional battery power.
This article provides a comprehensive review of the current mainstream ambient energy harvesting technologies for powering wearable sensors, including supercapacitors, solar cells, biofuel cells, thermoelectric generators, radio frequency (RF) energy harvesters, and kinetic energy harvesters. We explore their fundamental energy conversion principles, characteristics, and typical applications across diverse fields. Notably, the crucial roles of nanomaterials and nano-processing techniques in enhancing the performance of these ambient energy harvesters are highlighted.
Furthermore, we discuss the emerging trend of hybrid power supply systems that integrate multiple energy conversion modes, leveraging the complementary strengths of various ambient energy sources. These hybrid systems can provide more continuous and reliable power to wearable devices, expanding their applications in personalized healthcare, environmental monitoring, and beyond.
Fundamentals of Ambient Energy Harvesting
Energy Sources for Wearable Devices
Wearable sensors can be embedded in a variety of products, such as wristbands, eyeglasses, masks, and clothing, and used for real-time monitoring and data collection of physiological indicators, including temperature, blood glucose, blood pressure, as well as movement status data, such as acceleration, gait, and fall detection. To operate these wearable sensing devices in a stable and efficient manner, an adequate, continuous, and stable power supply is essential.
Traditional battery-powered wearable devices face limitations in terms of size, weight, and limited operating lifetime. To address these constraints, researchers have explored alternative power supply technologies that can harvest energy from the user’s surrounding environment, known as ambient energy harvesters.
Principles of Energy Conversion
Ambient energy harvesters work by converting various forms of energy, such as thermal, mechanical, electromagnetic, and bioenergy, into usable electrical power. This energy conversion process is typically achieved through mechanisms like the Seebeck effect (thermoelectric), piezoelectric effect, Faraday’s law of induction (electromagnetic), and electrochemical reactions (biofuel cells).
The performance of ambient energy harvesters can be significantly enhanced by leveraging the unique properties of nanomaterials. For example, the high specific surface area and electrochemical activity of nanomaterials can improve energy conversion efficiency at the material interface, leading to better power output and increased energy density.
Limitations and Challenges
While ambient energy harvesters offer promising solutions to power wearable sensors, they also face several limitations and challenges. Achieving a balance between device performance, comfort, and user-friendly design remains a critical consideration. Additionally, the reliability and stability of ambient energy harvesters under complex real-world conditions, such as varying environmental factors and user movements, must be addressed to ensure consistent and uninterrupted power supply.
Applications of Ambient Energy Harvesters
Powering Wearable Sensors
Ambient energy harvesters can provide a sustainable power solution for a wide range of wearable sensor applications, including healthcare monitoring, athletic performance tracking, and environmental sensing. By integrating these harvesters into wearable devices, users can benefit from continuous data collection and personalized insights, without the need for frequent battery replacements or recharging.
Integration with Wearable Textiles
The development of flexible and stretchable ambient energy harvesters has enabled their seamless integration with wearable textiles, such as clothing and fabrics. This integration allows for the harvesting of energy from various human movements and activities, further expanding the capabilities of smart textiles and enhancing the overall user experience.
Enabling Self-Powered Devices
By harnessing ambient energy, wearable devices can achieve self-powered operation, eliminating the need for external power sources or batteries. This self-powered functionality enables the development of autonomous, maintenance-free wearable systems that can continuously monitor and respond to user needs without interruption.
Types of Ambient Energy Harvesters
Thermoelectric Generators
Thermoelectric generators (TEGs) operate on the Seebeck effect, where a temperature difference between two materials generates an electric voltage. TEGs can effectively harness the thermal energy generated by the human body and the surrounding environment, converting it into usable electrical power. Advancements in nanomaterials, such as bismuth telluride (Bi2Te3) and antimony telluride (Sb2Te3) alloys, have significantly improved the thermoelectric conversion efficiency of these devices.
Piezoelectric Generators
Piezoelectric generators rely on the piezoelectric effect, where the application of mechanical stress or strain on certain materials, such as lead zirconate titanate (PZT) and barium titanate (BaTiO3), generates an electric charge. These generators can effectively harvest energy from various human movements and activities, including walking, running, and joint movements.
Electromagnetic Generators
Electromagnetic generators operate based on Faraday’s law of induction, where a changing magnetic field induces an electric current in a conductive coil. These generators can capture energy from human motion, such as limb movements or gait, and convert it into electrical power.
Performance Optimization of Ambient Energy Harvesters
Efficient Energy Conversion Techniques
To maximize the power output of ambient energy harvesters, researchers have explored various techniques to optimize the energy conversion process. This includes impedance matching between the harvester and the load, as well as the integration of power management circuitry to efficiently regulate and condition the generated electrical energy.
Harvester Miniaturization
The development of microfabrication techniques and the incorporation of nanomaterials have enabled the miniaturization of ambient energy harvesters, making them more suitable for integration into compact wearable devices. These advancements have also improved the flexibility and conformability of the harvesters, allowing for seamless integration with the human body.
Emerging Trends in Ambient Energy Harvesting
Hybrid Energy Harvesting Systems
To provide more continuous and reliable power to wearable devices, researchers are exploring the integration of hybrid energy harvesting systems that combine multiple ambient energy sources, such as solar, thermal, and kinetic energy. By leveraging the complementary strengths of different energy conversion mechanisms, these hybrid systems can overcome the limitations of individual harvesters and deliver a more robust and sustainable power supply.
Flexible and Stretchable Harvesters
The growing demand for comfortable and user-friendly wearable devices has driven the development of flexible and stretchable ambient energy harvesters. Novel transducer materials, such as piezoelectric polymers and conductive composites, have enabled the fabrication of conformable harvesters that can seamlessly integrate with the human body and clothing, further enhancing the overall user experience.
The European Future Energy Forum (EFEF) is a leading platform that showcases the latest advancements and innovative solutions in the field of renewable energy. By exploring the potential of ambient energy harvesters for powering wearable electronics, the EFEF aims to contribute to the development of sustainable and user-centric energy technologies that can improve people’s lives and support Europe’s transition towards a greener future.