Yabin Liao

Model of a single mode energy harvester and properties for optimal power generation

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.

Structural Effects and Energy Conversion Efficiency of Power Harvesting

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 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.

Optimal parameters and power characteristics of piezoelectric energy harvesters with an RC circuit

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 greatly to the performance of the energy harvesting system. The additional capacitor provides flexibility in achieving 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; above it, the maximum power can be achieved at all points 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, for providing 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 systems with high coupling, the optimal capacitance becomes positive for a range of values between 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.

Modeling and Comparison of Bimorph Power Harvesters with Piezoelectric Elements Connected in Parallel and Series

Power harvesting devices are designed to convert the ambient energy surrounding a system to usable electric energy. The strong desire to create self-powered systems, which do not rely on traditional energy sources such as electrochemical batteries has led to the rapid growth of this field. One type of energy harvesting is the use of piezoelectric materials to directly transform ambient vibration to electrical energy. In the majority of applications the piezoelectric is configured as a bimorph bender, where the two patches of piezoceramic material can be electrically connected in either series or parallel. A reduced model will be used here to develop a set of closed form solutions to the power harvesting performance of the system based on the electrical connection of the piezoelectrics. It will be theoretically and experimentally shown that the maximum power output and efficiency is independent of the electrical connection. However, the voltage (and current) outputs between a series and parallel conditions are related by a factor of two with a symmetric system. Additionally, a critical impedance will be derived to serve as a criterion on selecting the appropriate electrical connection between the piezoelectrics to tune the systems performance based on the impedance of external circuitry. Key

Optimal placement of piezoelectric material on a cantilever beam for maximum piezoelectric damping and power harvesting efficiency

The loss factor of an electromechanical system characterizes the induced piezoelectric damping of the system for shunt damping applications, and it can also be used to represent the power harvesting efficiency of the system. This paper investigates the effect of piezoelectric patch placement on the loss factor, with the aim of finding an optimal location for the same amount of piezoelectric material. An analytical relationship between the loss factor and placement is presented and discussed for beam configurations. In addition, a structural model is developed based on wave propagation which accounts for the effects of the PZT patch on the system and is used to obtain the natural frequencies and mode shapes of the composite system. After that, numerical studies demonstrate the effects of placement on damping of various vibration modes, and the effects of patch size on optimal placement. For small piezoelectric patches, the optimal location is very close to the location of the overall maximum bending moment, where the mechanical strain energy density is maximized. Larger patches could be used to improve the damping or power harvesting efficiency of the system; however, the associated optimal placement might not be at the point of overall maximum bending moment. Moreover, as the patch size extends beyond a critical point, increasing the size can actually deteriorate the system?s performance.

Piezoelectric Damping of Resistively Shunted Beams and Optimal Parameters for Maximum Damping

This paper studies the piezoelectric damping of resistively shunted beams induced by the conversion of the vibration energy into electrical energy that is dissipated in the resistor through Joule heating. Significant contributions have been made in the modeling and development of the resistive shunt damping technique; however, many approaches involve complex models that require the use of numerical methods to determine system parameters and predict damping. This paper develops a closed-form solution for the optimal parameter of a resistive shunt damping system. The model is validated through experimental testing and provides a simple yet accurate method to predict the induced damping in a smart structure.

On structural effects and energy conversion efficiency of power harvesting

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.