C. Richards

Efficiency of energy conversion for devices containing a piezoelectric component

Recent developments in miniaturized sensors, digital processors and wireless communication systems have many desirable applications. The realization of these applications however, is limited by the lack of a similarly sized power source. Micro-scale concepts to generate electrical power include devices which use the stored energy in fuels to those which harvest energy from the environment. Many proposed power generation systems employ a piezoelectric component to convert the mechanical energy to electrical energy. Of primary importance is the efficiency of power conversion. In this paper, an exact formula is developed that predicts the power conversion efficiency for a device that contains a piezoelectric component. This formula transparently and quantitatively reveals a trade-off effect on efficiency caused by the quality factor and electromechanical coupling factor of the device. In particular, decreasing the structural stiffness leads to the largest gains in efficiency, followed by decreasing the mechanical damping and increasing the effective mass.

Optimization of electromechanical coupling for a thin-film PZT membrane: II. Experiment

In this two-part paper, the optimization of the electromechanical coupling coefficient for thin-film piezoelectric devices is investigated both analytically and experimentally. The electromechanical coupling coefficient is crucial to the performance of piezoelectric energy conversion devices. A membrane-type geometry is chosen for the study. In part I a one-dimensional model is developed for a membrane composed of two layers, a passive elastic material and a piezoelectric material. The lumped-parameter model is then used to explore the effect of design and process parameters, such as residual stress, substrate thickness, piezoelectric thickness and electrode coverage, on the electromechanical coupling coefficient. The model shows that the residual stress has the most substantial effect on the electromechanical coupling coefficient. For a given substrate material and thickness an optimum piezoelectric thickness can be found to achieve the maximum coupling coefficient. The substrate stiffness affects the magnitude of the maximum coupling coefficient that can be obtained. Electrode coverage was found to be important to electromechanical coupling. The model predicts an optimum electrode coverage of 42% of the membrane area. The model developed in part I formed the basis for the parameters studied experimentally in part II.

Optimization of the dynamic and thermal performance of a resonant micro heat engine

The dynamic behavior of a flexing membrane micro heat engine is presented. The micro heat engine consists of a cavity filled with a saturated, two-phase working fluid bounded on the top by a flexible expander membrane and on the bottom by a stiff evaporator membrane. A lumped parameter model is developed to simulate the dynamic behavior of the micro heat engine. First, the model is validated against experimental data. Then, the model is used to investigate the effect of the duration of the heat addition process, the mass of the expander membrane and the thermal storage or thermal inertia associated with the engine cavity on the dynamic behavior of the micro engine. The results show the optimal duration for the heat addition process to be less than 10% of the engine cycle period. Increasing the mass of the flexible expander membrane is shown to reduce the resonant frequency of the engine to 130 Hz. Operating the engine at resonance leads to increased power output. The thermal storage or thermal inertia associated with the engine cavity is shown to have a strong effect on engine performance.

Estimation of parasitic losses in a proposed mesoscale resonant engine: Experiment and model

A resonant engine in which the piston-cylinder assembly is replaced by a flexible cavity is realized at the mesoscale using flexible metal bellows to demonstrate the feasibility of the concept. A four stroke motoring technique is developed and measurements are performed to determine parasitic losses. A non-linear lumped parameter model is developed to evaluate the engine performance. Experimentally, the heat transfer and friction effects are separated by varying the engine speed and operating frequency. The engine energy flow diagram showing the energy distribution among various parasitic elements reveals that the friction loss in the bellows is smaller than the sliding friction loss in a typical piston-cylinder assembly.

Mathematical modeling of a four-stroke resonant engine for micro and mesoscale applications

In order to mitigate frictional and leakage losses in small scale engines, a compliant engine design is proposed in which the piston in cylinder arrangement is replaced by a flexible cavity. A physics-based nonlinear lumped-parameter model is derived to predict the performance of a prototype engine. The model showed that the engine performance depends on input parameters, such as heat input, heat loss, and load on the engine. A sample simulation for a reference engine with octane fuel/air ratio of 0.043 resulted in an indicated thermal efficiency of 41.2%. For a fixed fuel/air ratio, higher output power is obtained for smaller loads and vice-versa. The heat loss from the engine and the work done on the engine during the intake stroke are found to decrease the indicated thermal efficiency. The ratio of friction work to indicated work in the prototype engine is about 8%, which is smaller in comparison to the traditional reciprocating engines.

Characterization of a dielectric microdroplet thermal interface material with dispersed nanoparticles

This work presents the fabrication and characterization of a dielectric microdroplet thermal interface material (TIM). Glycerin droplets, 1 μL, were tested as TIMs in this study. Copper nanoparticles having a diameter of 25xa0nm were dispersed in glycerin at different volume fractions to enhance its thermal conductivity. An increase of 57.5xa0% in the thermal conductivity of glycerin was measured at a volume fraction of 15xa0%. A minimum thermal interface resistance of 30.37 mm2xa0K/W was measured for the glycerin microdroplets at a deformed droplet height of 10.2xa0μm. Good agreement between experimental measurements and the predictions of a model based on Maxwell’s equation of rules of mixtures was obtained. The effect of nanoparticles size on the effective thermal conductivity of glycerin was studied. Nanoparticles with diameters of 60–80 and 300xa0nm were dispersed in glycerin at a volume fraction of 5xa0%, and their results were compared to those of the 25xa0nm particles.

Experimental and Numerical Study of Evaporative Heat Transfer From Ten- Micro Microchannels

Experimental and numerical results are presented for evaporative heat transfer from ten-micron square open-top channels. The radial channels are fabricated in epoxy photoresist on a two micron thick silicon membrane. The working fluid is pumped by capillary forces from a reservoir at the edge of the silicon membrane into the channels where it evaporates. The electrical power dissipated in a thin-film heater in the center of the membrane, the conduction heat transfer rate radially out of the membrane, and the rate of evaporation of the working fluid are measured. A three-dimensional finite difference, time-domain integration is used to predict sensible and latent heat transfer rates. Only 5-10% of the energy dissipated as heat in the thin film heater is carried away as latent heat by the evaporating working fluid. Computed temperatures and heat transfer rates are shown to match the experimental results.Copyright

Characterization and Modeling of the Dynamic Behavior of a Liquid-Vapor Phase Change Actuator

In this work a study of the dynamic performance of a liquid-vapor phase change actuator is presented. The actuator consists of a cavity filled with a two phase fluid bounded by a thin membrane into which heat is added and a cover slip which is displaced by the expansion of the vapor. An experimental actuator was designed so that a parametric study of geometry and operation parameters could be conducted. A lumped parameter model of the system was developed to predict forces and displacements produced by the addition of heat. The input to the model is the heat and the output is the displacement of the actuator. FFT analysis of the actuator deflection and heat input are performed. This procedure allows the measurement of the transfer function between actuator displacement as output and heat as input over a frequency range of 10 to 500 Hz. These data are compared to the predictions of the lumped parameter model. Agreement is favorable.© 2007 ASME