The process of acquiring the energy surrounding a system and converting it into usable electrical energy is termed power harvesting. In the last few years, the field of power harvesting has experienced significant growth due to the ever increasing desire to produce portable and wireless electronics with extended life. Current portable and wireless devices must be designed to include electrochemical batteries as the power source. The use of batteries can be troublesome due to their finite energy supply, which necessitates their periodic replacement. In the case of wireless sensors that are to be placed in remote locations, the sensor must be easily accessible or of disposable nature to allow the device to function over extended periods of time. Energy scavenging devices are designed to capture the ambient energy surrounding the electronics and covert it into usable electrical energy. The concept of power harvesting works towards developing self-powered devices that do not require replaceable power supplies. The development of energy harvesting systems is greatly facilitated by an accurate model to assist in the design of the system. This paper will describe a theoretical model of a piezoelectric based energy harvesting system that is simple to apply yet provides an accurate prediction of the power generated around a single mode of vibration. Furthermore, this model will allow optimization of system parameters to be studied such that maximal performance can be achieved. Using this model an expression for the optimal resistance and a parameter describing the energy harvesting efficiency will be presented and evaluated through numerical simulations.The second part of this paper will present an experimental validation of the model and optimal parameters.

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This paper performs an analysis of maximum power output of piezoelectric energy harvesters. It has been observed that there exists an overall power limit that can be obtained by tuning energy harvesting circuits, including both linear and nonlinear. The significance of the power limit is that it represents the maximum possible power output or capacity of an energy harvester. In other words, the harvested power is always capped by this limit regardless of the type and tuning of the energy harvesting circuit interface. The power limit and the optimal generalized electrical load or impedance to reach this power limit are first obtained directly by using the electromechanically coupled equations of the system, and then obtained by using the equivalent circuit analysis and impedance matching approach. Both are commonly used methods in energy harvesting research. This paper presents an effort to unify them but also offer insights on the power limit from two different perspectives. In the second part of this paper, the power limit and impedance matching results are applied to a linear energy harvesting circuit interface, i.e., resistive energy harvesting (REH) circuit, and a nonlinear circuit interface, i.e., standard AC–DC energy harvesting (SEH) circuit, to study their physical constraints on the impedance matching and clearly explain their power behaviors such as the maximum power and the effect of electromechanical coupling on the power. In addition, closed-form expressions, a relationship between the mechanical damping and the effective electromechanical coupling coefficient, to define the three types of coupling, i.e., weak, critical, strong, are obtained. It is found that the SEH harvesters require about 1.5 times of minimum electromechanical coupling of that of REH harvesters to reach the power limit, and the frequency bandwidth between the two power limit frequencies of a SEH harvester is narrower than that of a REH harvester given the same level of strong electromechanical coupling.

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Health monitoring of structures and people requires the integration of sensors and devices on various 3D curvilinear, hierarchically structured, and even dynamically changing surfaces. Therefore, it is highly desirable to explore conformal manufacturing techniques to fabricate and integrate soft deformable devices on complex 3D curvilinear surfaces. Although planar fabrication methods are not directly suitable to manufacture conformal devices on 3D curvilinear surfaces, they can be combined with stretchable structures and the use of transfer printing or assembly methods to enable the device integration on 3D surfaces. Combined with functional nanomaterials, various direct printing and writing methods have also been developed to fabricate conformal electronics on curved surfaces with intimate contact even over a large area. After a brief summary of the recent advancement of the recent conformal manufacturing techniques, we also discuss the challenges and potential opportunities for future development in this burgeoning field of conformal electronics on complex 3D surfaces.

Power harvesting describes the process of acquiring the ambient energy surrounding a system and converting it into usable electrical energy. Much of the work over the past two decades has focused on the conversion of ambient vibration energy sources using piezoelectric, electromagnetic and electrostatic transduction. Attempts were made to obtain a general model that could be applied to any transduction mechanism. Of the most interest is an electromagnetic generator model that was used by many researchers to model piezoelectric power harvesters. Two major results from the model are the power limit expression and the equal relationship between the electrically induced damping and the mechanical damping to reach the power limit. However, piezoelectric power harvesters cannot be accurately modeled by this electromagnetic model due to the essential difference in physics. There have also been attempts to obtain the power limit expression based on piezoelectric relationships, but they either neglect the piezoelectric backward coupling to the structure, or assume the power limit occurs at the resonance of the system. This paper obtains the power limit expression based on the piezoelectric coupled equations without those assumptions. In addition, the relationship between the electrically induced damping and mechanical damping at the power limit is studied. Furthermore, a closed-form criterion is derived and proposed to define strongly and weakly coupling power harvesters, whose differences in power characteristics are explained through analytical and numerical analysis. While most of the discussion is focused on linear power harvesters connected to a resistive circuit, the aim of this paper is to provide a comprehensive and deep understanding of this simple configuration, answers to important questions, and a starting point to develop a more general theory on power harvesters because similar system characteristics are observed in power harvesters with more complexities.

The concept of power harvesting works towards developing self-powered devices that do not require replaceable power supplies. One important parameter defining the performance of a piezoelectric power harvesting system is the efficiency of the system. However, an accepted definition of the energy harvesting efficiency does not currently exist. This article will develop a new definition for the efficiency of an energy harvesting system which rather than being defined through energy conservation as the ratio of the energy fed into the system to maintain the steady state to the output power, we consider the ratio of the strain energy over each cycle to the power output. This new definition is analogous to the material loss factor. Simulations will be performed to demonstrate the validity of the efficiency and will show that the maximum efficiency occurs at the matched impedance; however, for materials with high electromechanical coupling the maximum power is generated at the near open and closed-circuit resonances with a lower efficiency.

A piezoelectric based energy harvesting scheme is proposed here which places a capacitor before the load in the conditioning circuit. It is well known that the impedance between the load and source contributes heavily to the performance of the energy harvesting system. The additional capacitor provides flexibility in meeting the optimal impedance value and can be used to expand the bandwidth of the system. A theoretical model of the system is derived and the response of the system as a function of both resistance and capacitance is studied. The analysis shows that the energy harvesting performance is dominated by a bifurcation occurring as the electromechanical coupling increases above a certain value, below this point the addition of an additional capacitor does not increase the performance of the systems and above the maximum power can be achieved at all point between these two bifurcation frequencies.Additionally, it has been found that the optimal capacitance is independent of the optimal resistance. Therefore, the necessary capacitance can be chosen and then the resistance determined to provide optimal energy harvesting at the desired frequencies. For systems with low coupling the optimal added capacitance is negative (additional power to the circuit) indicating that a second capacitor should not be used for. For systems with high coupling the optimal capacitance becomes positive for a range of values inside the bifurcation frequencies and can be used to extend the bandwidth of the harvesting system. The analysis also demonstrates that the same maximum energy can be harvested at any frequency; however, outside the two bifurcation frequencies the capacitor must be negative.