Sung-Eun Jo

A resonant frequency switching scheme of a cantilever based on polyvinylidene fluoride for vibration energy harvesting

A mismatch between the ambient frequency and the resonant frequency of the vibrational energy harvester causes decrease of the energy transduction efficiency. Therefore, there is a great demand for the resonant frequency tuning of the vibrational energy harvester. In this paper, a flexible PVDF (polyvinylidene fluoride) cantilever, which can switch its resonant frequency automatically and maintain the switched resonant frequency without energy consumption, is proposed. The proposed energy harvester is composed of cantilever couples which are similar with a seesaw structure. When the proposed energy harvester is excited by an external vibration and the excited frequency fluctuates, the cantilever couples can be horizontally moved by using the large deflection of a flexible cantilever. So the beam length of each cantilever which corresponds to each arm of the seesaw structure can be changed and the resonant frequency of the proposed energy harvester can be switched in real time. The proposed energy harvester was realized by application of a piezoelectric polymer, PVDF. Also, it was confirmed that the proposed energy harvester can switch its resonant frequency in several seconds without an additional energy source.

Passive-self-tunable vibrational energy harvester

In this paper, a passive self frequency tunable energy harvester, which can transduce vibration energy into electrical energy, is proposed. The proposed harvester is composed of cantilever couples which can be horizontally moved by the large deflection characteristics of a flexible cantilever. As the external vibration frequency fluctuate, the resonant frequency of the harvester can be tuned in real time by the change of the cantilever beam length through the movement of the cantilever couples. It was confirmed that the harvester can tune its resonant frequency ranged from 24Hz to 40Hz in real-time without an additional energy source.

Triboelectric–thermoelectric hybrid nanogenerator for harvesting frictional energy

The triboelectric nanogenerator, an energy harvesting device that converts external kinetic energy into electrical energy through using a nano-structured triboelectric material, is well known as an energy harvester with a simple structure and high output voltage. However, triboelectric nanogenerators also inevitably generate heat resulting from the friction that arises from their inherent sliding motions. In this paper, we present a hybrid nanogenerator, which integrates a triboelectric generator and a thermoelectric generator (TEG) for harvesting both the kinetic friction energy and the heat energy that would otherwise be wasted. The triboelectric part consists of a polytetrafluoroethylene (PTFE) film with nano-structures and a movable aluminum panel. The thermoelectric part is attached to the bottom of the PTFE film by an adhesive phase change material layer. We confirmed that the hybrid nanogenerator can generate an output power that is higher than that generated by a single triboelectric nanogenerator or a TEG. The hybrid nanogenerator was capable of producing a power density of 14.98 mW cm−2. The output power, produced from a sliding motion of 12 cm s−1, was capable of instantaneously lighting up 100 commercial LED bulbs. The hybrid nanogenerator can charge a 47 μF capacitor at a charging rate of 7.0 mV s−1, which is 13.3% faster than a single triboelectric generator. Furthermore, the efficiency of the device was significantly improved by the addition of a heat source. This hybrid energy harvester does not require any difficult fabrication steps, relative to existing triboelectric nanogenerators. The present study addresses a method for increasing the efficiency while solving other problems associated with triboelectric nanogenerators.

Power generation of a thermoelectric generator with phase change materials

In this paper, a thermoelectric generator that embeds phase change materials for wasted heat energy harvesting is proposed. The proposed thermoelectric generator embeds phase change materials in its device structure. The phase change materials store large amounts of heat energy using the latent heat of fusion. When the heat source contacts the thermoelectric generator, dissipated heat from the heat source is stored in the phase change materials. When the heat source is removed from the thermoelectric generator, the output power of the thermoelectric generator slowly decreases, while the output power of conventional thermoelectric generators decreases rapidly without the heat source. The additional air layer in the proposed thermoelectric generator disturbs the heat dissipation from the phase change materials, so the thermoelectric generator can maintain the power generation for longer without a heat source. The experimental results for the thermoelectric generator fabricated clearly show the latent heat effect of the phase change materials and the embedded air layer.

Improvement of wireless power transmission efficiency of implantable subcutaneous devices by closed magnetic circuit mechanism

Induction coils were fabricated based on flexible printed circuit board for inductive transcutaneous power transmission. The coil had closed magnetic circuit (CMC) structure consisting of inner and outer magnetic core. The power transmission efficiency of the fabricated device was measured in the air and in vivo condition. It was confirmed that the CMC coil had higher transmission efficiency than typical air-core coil. The power transmission efficiency during a misalignment between primary coil and implanted secondary coil was also evaluated. The decrease of mutual inductance between the two coils caused by the misalignment led to a low efficiency of the inductive link. Therefore, it is important to properly align the primary coil and implanted secondary coil for effective power transmission. To align the coils, a feedback coil was proposed. This was integrated on the backside of the primary coil and enabled the detection of a misalignment of the primary and secondary coils. As a result of using the feedback coil, the primary and secondary coils could be aligned without knowledge of the position of the implanted secondary coil.

Heat energy management scheme of thermoelectric generator using phase change material

This paper reports a thermoelectric generator (TEG) with a phase change material (PCM) that provides a function of heat energy storage. The energy conversion efficiency of conventional TEGs decreases rapidly when the heat source is removed from the hot junction. The proposed TEG is composed of thermocouples and a polymer substrate embedding a PCM. The embedded PCM can store heat energy dissipated from a heat source. The TEG, thus, can maintain energy conversion efficiency continuously after removal of the heat source.