Kinetic and Solar Energy

Kinetic and solar energy can be considered the most pervasive and commonly coexisting ambient energy sources, where harvesting of both simultaneously can be significantly beneficial. A self-powered lantern based on a triboelectric-PV hybrid structure has been built for harvesting wind and light energy [165]. In this structure, a transparent polylactic acid (PLA) tube was connected to a central axis rod via bearings and transparent, interdigitated indium tin oxide (ITO) electrodes were coated on the inner wall of the tube. A rod coated with a layer of fluorinated ethylene propylene (FEP) film was placed on top of the ITO electrodes. A dye-sensitized solar cell (DSSC) was also mounted on the tube. In addition, a soft lithium battery (SLB) and 10 light-emitting diodes (LEDs) were connected. The PLA tube rotated when driven by the wind, and the freely moving FEP rod generated friction with the ITO electrodes, thus harvesting the wind energy through the triboelectric effect. Meanwhile, solar energy was harvested by the DSSC. The harvested energy was stored in the SLB and used for lighting the LEDs. Being charged for 0.88 hour, the SLB could be discharged for 4.12hour with 10 pA current [165]. This performance was equivalent to 100% and 33% efficiency increases compared to those of the individual triboelectric generator and solar cell, respectively. Apart from structural hybridization as introduced above, multisource energy harvesters can also be realized with multifunctional materials. Details have been given in the reference [166]. Recently, a polyvinylidene fluoride (PVDF)-ZnO composite has been reported [167]. The composite consisting of 33 wt% ZnO nanowires and PVDF polymer helped to increase the output voltage by over 300% compared to that using PVDF only.

Lee et al. [168] fabricated a tandem device which integrates a PVDF nanogenerator and silicon (Si) nanopillar solar cell. The SiNP solar cell was fabricated using a plasma etching technique and doping process by rapid thermal annealing and furnace annealing. The PVDF nanogenerator was stacked on top of the Si nanopillar solar cell using a spinning method. The optical properties and the device performance of nanowire solar cells were characterized, and the dependence of device performance versus annealing time or method has been investigated. Furthermore, the PVDF nanogenerator was operated with a 100 dB sound wave and a 0.8 V peak- to-peak output voltage was generated. This tandem device can successfully harvest energy from both sound vibration and solar light, demonstrating its strong potential as a future ubiquitous energy harvester.

Wind and Thermal Energy

The pyroelectric effect is known for the energy harvesting from temperature fluctuations. A pyroelectric harvester has been reported to utilize a vortex to generate the temperature fluctuation and thus harvest wind energy via the pyroelectric effect [169]. The vortex generator was a right-angled substrate on which a PVDF film was deposited on the horizontal part. The wind flowed along the horizontal direction and when it collided with the vertical part of the substrate, energy was harvested. The main advantage of this harvester was that it was able to capture energy even with a very weak air flow (down to 1 m/s velocity) [169]. Although the piezoelectric effect was not mentioned, PVDF is also ferroelectric and well known for its coexhibition of piezoelectric and pyroelectric effects. Therefore, this structure could naturally become a multisource harvester by employing the piezoelectric effect at the same time. Stronger ferroelectric materials, e.g., [170,171] could also replace the PVDF in order to improve the piezoelectric and pyroelectric performance.

Solar, Kinetic, and Radio Frequency Energy

A recently published patent has released the design of an umbrella apparatus which managed to incorporate PV, piezoelectric, electromagnetic, and radio frequency (RF) energy harvesters into different parts of an umbrella [172]. The canopy of the umbrella was replaced by a multilayer lamination combining PV and piezoelectric energy harvesters. The lamination consisted of an inverted polymer solar cell layer [173], a PVDF piezoelectric layer, electrode layers, and other supportive layers. When the energy harvesting canopy was open, solar and kinetic (i.e., wind and raindrops) energy could be harvested. Meanwhile, the open canopy could spin when subjected to wind force, thus driving a miniature windmill (electromagnetic energy harvester) embedded in the shaft of the umbrella. In addition, the shaft of the umbrella could act as a monopole antenna and an RF energy harvesting circuit could be installed in the shaft in order to harvest RF energy. Furthermore, the shaft, in conjunction with a movable ferrule, contained a magnet and coils. When the canopy was closed, the umbrella could be used as a cane. By striking the ferrule on the ground while walking, the magnet would be forced to move through the coils, forming another electromagnetic harvester. Apart from these energy harvesting components, the necessary DC-DC converters, switches, conductive leads, and a rechargeable battery were also installed in the umbrella [172]. Personal electronic devices could be charged through a port of the umbrella so that the entire umbrella became a self-powered, portable charging station.

Since the amount of power available by energy harvesting is quite limited, there has been interest in utilizing multiple forms of external energy simultaneously—such as light and heat, or light and vibrations—in order to collect a sufficient amount for practical use. In the past, this has been achieved by combining different kinds of devices, which leads to higher costs. Fujitsu Laboratories has developed a new hybrid harvesting device that captures energy from either light or heat, which are the most typical forms of ambient energy available for wide-scope application. This makes it possible for a single device to capture energy from either heat or light without combining two harvesting devices. In addition, as it can be manufactured from inexpensive organic materials, device production costs can remain low. This technology includes (a) new structure for hybrid generating devices, (b) development of an organic material for hybrid generating device. Until now, PV cells—which generate electricity from light, and ТЕ devices—which generate electricity from temperature differentials, have only been available as separate devices. This new technology from Fujitsu Laboratories doubles the energy-capture potential through the use of both ambient heat and light in a single device. If either the ambient light or heat is not sufficient to power the sensor, this technology can supply power with both sources, by augmenting one source with the other. In addition, the technology can also be used for environmental sensing in remote areas for weather forecasting, where it would be problematic to replace batteries or run electric lines.

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