This study investigates the feasibility of improving the structural integrity of thermoelectric modules (TEMs) with varying geometry. For this purpose, six different TEM models with various thermoelectric leg geometries were designed and modeled in order to perform a thermal stress FEA using ANSYS Workbench. Temperature dependent material properties were used since some properties such as coefficients of thermal expansion change with temperature. Significant decrease in thermal stresses and leg deformations were observed with some models. Particularly, the cylindrical TE leg geometry model has approximately 54% lower Von Mises stresses (294MPa) and 13% lower TE leg deformations (3.9μm) than those of the typical TE leg geometry model (635MPa and 4.5μm). Power generation analyses of the models were performed to evaluate the effect of new TE leg geometries on the performance. TEM model with cylindrical TE leg geometry has the highest power generation (29.3mW) among all the models.
Piezoelectric composite (p-NC) made of a polymeric matrix and piezoelectric nanoparticles with conductive additives is an attractive material for many applications. As the matrix of p-NC is made of viscoelastic materials, both elastic and viscous characteristics of the matrix are expected to contribute to the piezoelectric response of p-NC. However, there is limited understanding of how viscoelasticity influences the piezoelectric performance of p-NC. Here we combined analytical and numerical analyses with experimental studies to investigate effects of viscoelasticity on piezoelectric performance of p-NC. The viscoelastic properties of synthesized p-NCs were controlled by changing the ratio between monomer and cross-linker of the polymer matrix. We found good agreement between our analytical models and experimental results for both quasi-static and dynamic loadings. It is found that, under quasi-static loading conditions, the piezoelectric coefficients (d) of the specimen with the lowest Young's modulus (∼0.45 MPa at 5% strain) were ∼120 pC N, while the one with the highest Young's modulus (∼1.3 MPa at 5% strain) were ∼62 pC N. The results suggest that softer matrices enhance the energy harvesting performance because they can result in larger deformation for a given load. Moreover, from our theoretical analysis and experiments under dynamic loading conditions, we found the viscous modulus of a matrix is also important for piezoelectric performance. For instance, at 40 Hz and 50 Hz the storage moduli of the softest specimen were ∼0.625 MPa and ∼0.485 MPa, while the loss moduli were ∼0.108 MPa and ∼0.151 MPa, respectively. As piezocomposites with less viscous loss can transfer mechanical energy to piezoelectric particles more efficiently, the dynamic piezoelectric coefficient (d') measured at 40 Hz (∼53 pC N) was larger than that at 50 Hz (∼47 pC N) though it has a larger storage modulus. As an application of our findings, we fabricated 3D piezo-shells with different viscoelastic properties and compared the charging time. The results showed a good agreement with the predicted trend that the composition with the smallest elastic and viscous moduli showed the fastest charging rate. Our findings can open new opportunities for optimizing the performance of polymer-based multifunctional materials by harnessing viscoelasticity.
Fluctuating heat radiating from the environment is an unused and unknown source. This study investigates pyroelectric energy harvesting as a way to tap a fluctuating radiation heat source. Appropriate circuitry coupling and the frequency of the radiation source play a key role in the ability to harvest this energy. Hence, a design of experiment approach that limits factors to temperature change frequency, electrical resistance, and capacitance is utilized to develop a full-factorial model at three levels for each factor. In order to quantify and maximize the harvested energy, a response surface model was developed. The optimum values for temperature change frequency, resistance, and capacitance were predicted to be 0.05 Hz, 7330 kΩ, and 100 µF, respectively, for a PZT-5A sample with a volume of 0.684 cm3. The maximum response of 62.89 µJ was predicted for the optimum values.
Methods of electrostatic conversion are available for harvesting energy where there are ambient vibrations. However, most of the previous work in the literature has addressed applications with high frequencies. In this study, we are not only implementing an electret-based energy harvester for low-frequency applications but also evaluating the effect of parameters, including vibration rates, accelerations, electret surface potential, e.g. on the efficiency of electrostatic energy harvesting (EH). A prototype system, with the size of 4 × 28 cm 3 , was built and constructed to accomplish experimental analysis, and the corona triode process was used to prepare electrets by charging Teflon FEP films. In the electret surface potential range of 300-1800 V, vibration frequency range of 2-45 Hz, and acceleration range of 0.1-1.0 g, the effect of parameters on the EH efficiency was experimentally tested. To predict and maximize the performance of the system, a mathematical response surface model (RSM), validated experimentally < 9.5% error. The maximum peak-peak voltage output of 318 V was predicted using this model for the electret surface potential of −1800 V, and vibration frequency of 16 Hz. Moreover, harvested energy was ∼ 900 µJ (∼0.8 µJ per mechanical cycle) in a minute though low frequencies (<20 Hz), which can be easily enhanced to more than 1 mJ with system optimization. We suggest our device can be used in numerous low-frequency applications, and our predictive model can also be used to optimize the efficiency of other electrostatic energy harvesters based on electrets.
This study demonstrates an energy harvester that combines a piezoelectric nanogenerator and an electret-based electrostatic generator. The device consists of an in-house fabricated nanocomposite (polydimethylsiloxane/barium titanate/carbon nanotube) as a piezoelectric layer and a monocharged Teflon fluorinated ethylene propylene as an electret electrostatic layer. The mechanical impedance of the structure can be altered easily by changing the nanocomposite monomer/cross-linker ratio and optimizing various mechanical energy sources. The energy harvester's performance was characterized by performing measurements with different frequencies (5–20 Hz) under applied dynamic loading. A total volumetric power density of ∼8.8 μW cm−3 and a total stored energy of ∼50.2 μJ min−1 were obtained. These findings indicate that this versatile, lightweight, and low-cost energy harvester can be employed as a power supply source for microelectronics in applications, such as wearables.
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