Smart materials and structures for energy harvesters

Tian Liu1, Sanwei Liu1, Xin Xie1, Chenye Yang2, Zhengyu Yang3, and Xianglin Zhai4*

1 Department of Mechanical and Industrial Engineering, Northeastern University, Boston, MA 02115 2 Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology,
Cambridge, MA 02194 3 Department of Electrical and Computer Engineering, Northeastern University, Boston, MA 02115 4 Department of Chemistry, Louisiana State University, Baton Rouge, LA 70803

• alex.zhai@poochonscientific.com

Keywords: Energy harvester, Smart materials and structures, Energy conversion

Abstract

Vibrational energy harvesters capture mechanical energy from ambient vibrations and convert the mechanical energy into electrical energy to power wireless electronic systems. Challenges exist in the process of capturing mechanical energy from ambient vibrations. For example, resonant harvesters may be used to improve power output near their resonance, but their narrow bandwidth makes them less suitable for applications with varying vibrational frequencies. Higher operating frequencies can increase harvesters’ power output, but many vibrational sources are characterized by lower frequencies, such as human motions. This paper provides a thorough review of state-of-the-art energy harvesters based on various energy sources such as solar, thermal, electromagnetic and mechanical energy, as well as smart materials including piezoelectric materials and carbon nanotubes. The paper will then focus on vibrational energy harvesters to review harvesters using typical transduction mechanisms and various techniques to address the challenges in capturing mechanical energy and delivering it to the transducers.

Ambient energy sources Energy harvesters can extract energy from ambient sources, convert it to electrical energy, and use it to power electronic devices wirelessly. Energy harvesters based on solar energy [Tan 2011, Abdin 2013], thermal energy [Harb 2011], electromagnetic energy [Pinuela 2013], and mechanical energy [Saadon 2011] have been reported recently. Among many review papers, including [Steingart 2009, Harb 2011, Watral 2013], Harb et al. describes many harvesters that use the energy sources mentioned above and concludes that vibrations are the most available energy source and provide the highest output powers [Harb 2011].
Solar energy harvesting The sun supplies energy to the planet, and solar harvesting captures some of that sunlight directly and converts it to electricity. Solar technologies including solar photovoltaics (PV) and concentrating solar power (CSP) are widely used to convert sunlight into electrical energy. A recent review of solar technologies and applications can be found in [Devabhaktuni 2013].
One example is found in [Roundy 2003], in which a self-contained 1.9 GHz radio frequency transmit beacon is reported. The device is powered by a cadmium telluride (CdTe) solar cell that is attached on the back side of a PCB and charges a capacitor on the front. The solar powered beacon achieves 100% duty cycle under high light conditions.
Solar cells based on the photoelectric effect are suitable for applications relevant to energy harvesters such as wireless sensor networks and portable electronic devices. However, the availability of solar energy is unpredictable due to its high dependence on weather conditions. Moreover, devices that are required to be installed under clothes or inside the human body can hardly be charged by solar energy.

Thermal energy harvesting Thermal gradients arise as a result of many processes, including the burning of fuel and geothermal processes. Thermoelectric energy harvesters extract electrical energy from temperature gradients. The energy extraction is enabled by the Seebeck effect, in which the temperature difference across a conductor or semiconductor results in a net flow of electrons from the high-temperature side to the low-temperature side so that an electrical current (and voltage) are generated.
One example of thermoelectric harvesting may be found in [Kishi 1999]. Kishi et al. developed a thermoelectric device that can power a wrist watch. With a temperature difference of 2C between the high-temperature substrate and the low-temperature substrate, the thermoelectric device can produce an power output of 5.6 mW with a load resistance of 1 k. A wrist watch can be driven by connecting 16 such thermoelectric devices in series at room temperature.
However, the performance of such thermoelectric energy harvesters is limited by the available temperature difference of the application and the Carnot efficiency. The requirement of high temperature difference for high power output conflicts with the low temperature difference available in many applications, such as harvesting energy from the human body.

RF energy harvesting In the presenc

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