Pei-Chun Wu

Analysis of an array of piezoelectric energy harvesters connected in series

This paper investigates the electrical response of a series connection of piezoelectric energy harvesters (PEHs) attached to various interface electronics, including standard and parallel-/series-SSHI (synchronized switch harvesting on inductor) circuits. In contrast to the case of parallel connection of multiple oscillators, the system response is determined by the matrix formulation of charging on a capacitance. In addition, the adoption of an equivalent impedance approach shows that the capacitance matrix can be explicitly expressed in terms of the relevant load impedance. A model problem is proposed for performance evaluation of harvested power under different choices of interface circuits. The result demonstrates that the parallel-SSHI array system exhibits higher power output with moderate bandwidth improvement, while the series-SSHI system delivers a pronounced wideband at the cost of peak harvested power. The standard array system shows a mild ability in power harvesting between these two SSHI systems. Finally, comparisons between the series and parallel connection of oscillators are made, showing the striking contrast of these two cases.

Multiphoton imaging to identify grana, stroma thylakoid, and starch inside an intact leaf

BackgroundGrana and starch are major functional structures for photosynthesis and energy storage of plant, respectively. Both exhibit highly ordered molecular structures and appear as micrometer-sized granules inside chloroplasts. In order to distinguish grana and starch, we used multiphoton microscopy, with simultaneous acquisition of two-photon fluorescence (2PF) and second harmonic generation (SHG) signals. SHG is sensitive to crystallized structures while 2PF selectively reveals the distribution of chlorophyll.ResultThree distinct microstructures with different contrasts were observed, i.e. “SHG dominates”, “2PF dominates”, and “SHG collocated with 2PF”. It is known that starch and grana both emit SHG due to their highly crystallized structures, and no autofluorescence is emitted from starch, so the “SHG dominates” contrast should correspond to starch. The contrast of “SHG collocated with 2PF” is assigned to be grana, which exhibit crystallized structure with autofluorescent chlorophyll. The “2PF dominates” contrast should correspond to stroma thylakoid, which is a non-packed membrane structure with chrolophyll. The contrast assignment is further supported by fluorescence lifetime measurement.ConclusionWe have demonstrated a straightforward and noninvasive method to identify the distribution of grana and starch within an intact leaf. By merging the 2PF and SHG images, grana, starch and stroma thylakoid can be visually distinguished. This approach can be extended to the observation of 3D grana distribution and their dynamics in living plants.

Observation of spontaneous polarization misalignments in periodically poled crystals using second-harmonic generation microscopy.

Periodically poled crystal (PPC) is a key component for nonlinear optical applications. Its poling quality relies largely on successful domain inversion and the alignment of spontaneous polarization (SP) vectors in each domain. Here we report the unexpected observation of bulk second harmonic generation (SHG) in PPC when excitation propagating along its optical axis. Based on its tensorial nature, SHG is highly sensitive to the orientation of SP, and therefore the misalignment of SP in each domain of PPC can be revealed noninvasively by SHG microscopy. This nonlinear imaging modality provides optical sectioning capability with 3D sub-micrometer resolution, so it will be useful for in situ investigation of poling quality in PPC.

Series Connection of Multiple Piezoelectric Oscillators

This article investigates the electrical response of piezoelectric energy harvesters (PEHs) connected in series. Analytic estimates of harvested power output are proposed for a series PEH array system attached to various energy harvesting circuits, including standard and parallel-/series-SSHI (synchronized switch harvesting on inductor) interfaces. In contrast to the case of parallel connection of multiple oscillators, the results are presented through the matrix formulation of charging on capacitance. Besides, they are validated numerically by standard circuit simulations.Copyright

Mixed Parallel-Series Connection of Piezoelectric Oscillators for Wideband Energy Harvesting

The article studies the behavior of a mixed parallel-series connection of piezoelectric oscillators attached to the standard interface for wideband energy harvesting. The estimate of power output is obtained analytically considering the formulations of balance of charge, energy and system dynamics. It can be presented based on the generalized matrix formulation of charging on capacitance in terms of equivalent load impedance. The proposed model is subsequently validated numerically through circuit simulations. Finally, a design with a careful choice of parallel-series mixed connection of oscillators is proposed for illustration. With a proper circuit layout triggering the switching of connection, the result shows that the peak power of each array configuration is roughly uniform within the frequency range of interest. Hence, the bandwidth is enlarged without the loss of peak power.Copyright

Wideband Energy Harvesting by Multiple Piezoelectric Oscillators With an SECE Interface

This paper presents the development of wideband energy harvesting by the parallel connection of multiple piezoelectric oscillators attached to a synchronized electric charge extraction (SECE) interface circuit. It is shown that the electric response is determined by the matrix formulation of generalized Ohm’s law. The impedance matrix is explicitly expressed in terms of the equivalent load impedance and system parameters. In addition, the load impedance is found to be independent of external load, so is harvested power. The framework is subsequently validated numerically by SPICE circuit simulations. Finally, the performance evaluation is carried out with comparisons to other interface circuits, including the standard and synchronized switch harvesting on inductor (SSHI) interfaces. The result demonstrates that the bandwidth of an SECE array system is improved and its harvested power is not less than that based on the use of other interface circuits.Copyright

Design of Rectified Piezoelectric Power Generators by Finite Element Methods

The article proposes designing rectified piezoelectric power generators based on the direct finite element method. It accounts for the consideration of common interface circuits such as the standard and parallel-/series-SSHI (synchronized switch harvesting on inductor) circuits, and complicated structural configurations such as arrays of piezoelectric oscillators. The proposed model suggests replacing the energy harvesting circuit by an equivalent load impedance in series with negative piezoelectric capacitance. As the expression of the equivalent load impedance can be explicitly derived, the proposed finite element approach is capable of handling common interface circuits without resorting to circuit solvers. Finally, the model is extended to the consideration of SECE (synchronized electric charge extraction) circuit which offers an advantage of independence of harvested power on external loads.Copyright

Finite element modeling of piezoelectric energy harvesters

This article reports a novel finite element model of piezoelectric energy harvesters accounting for the effect of nonlinear interface circuits. The idea is to replace the energy harvesting circuit in parallel with the parasitic piezoelectric capacitance by an equivalent load impedance. This approach offers many advantages. First, the model itself can be implemented conveniently in commercial finite element softwares. Second, it directly provides system-level designs on the whole without resorting to circuit solvers. Third, the extensions to complicated structures such as array configurations are straightforward. The proposed finite element model is validated by considering the case of an array system endowed with the standard, parallel-/series-SSHI (synchronized switch harvesting on inductor) interfaces. Good agreement is found between simulation results and analytic estimates.

Imaging leukocytes in vivo with third harmonic generation microscopy

Without a labeling, we demonstrated that lipid granules in leukocytes have distinctive third harmonic generation (THG) contrast. Excited by a 1230nm femtosecond laser, THG signals were generated at a significantly higher level in neutrophils than other mononuclear cells, whereas signals in agranular lymphocytes were one order smaller. These characteristic THG features can also be observed in vivo to trace the newly recruited leukocytes following lipopolysaccharide (LPS) challenge. Furthermore, using video-rate THG microscopy, we also captured images of blood cells in human capillaries. Quite different from red-blood-cells, every now and then, round and granule rich blood cells with strong THG contrast appeared in circulation. The corresponding volume densities in blood, evaluated from their frequencies of appearance and the velocity of circulation, fall within the physiological range of human white blood cell counts. These results suggested that labeling-free THG imaging may provide timely tracing of leukocyte movement and hematology inspection without disturbing the normal cellular or physiological status.

Multiphoton imaging to distinguish grana and starch inside an intact leaf

We have demonstrated a straightforward and noninvasive method to identify the distribution of grana and starch within an intact leaf. Grana and starch are the major functional structures for photosynthesis and energy storage of plant, respectively. Both exhibit highly ordered molecular structures and appear as micrometer-sized granules inside chloroplasts. In order to distinguish grana and starch, we used multiphoton microscopy, with simultaneous acquisition of two photon fluorescence (2PF) and second harmonic generation (SHG) signals. Consequently, SHG is found on both grana and starch while 2PF from chlorophyll indicates the identity of grana.