Ravi Anant Kishore

Ultra-Low Wind Speed Piezoelectric Windmill

We report an ultra-low start-up speed windmill that consists of a 72 mm diameter horizontal axis wind turbine rotor with 12 alternating polarity magnets around its periphery and a 60 mm × 20 mm × 0.7 mm piezoelectric bimorph element having a magnet at its tip. This wind turbine operates at very low Reynolds number of around 2×104, but still it has reasonably high power coefficient of about 11% at the optimal tip speed ratio of 0.7. It was found to produce peak electric power of 450 μW at the rated wind speed of 4.2 mph. An extremely low start-up wind speed of 2.4 mph was achieved by operating the bimorph in actuator mode.

A comprehensive optimization study on Bi2Te3-based thermoelectric generators using the Taguchi method

Low-grade waste heat recovery is a promising source of renewable energy; however, there are practical challenges in the recovery process. Thermoelectric generators (TEGs) are a viable solution, but their efficiency remains low, thereby limiting their implementation. The performance of TEGs can be enhanced by optimizing the module configuration; however, optimizing all the parameters experimentally using traditional experimental techniques will require several trials, and therefore, it is cumbersome and expensive. Here, we demonstrate the Taguchi method for optimizing TEG modules and demonstrate that full optimization can be achieved in just 25 experiments. The optimization has been achieved in four stages. In the first stage, a numerical model of thermoelectricity is developed and used in the second stage to optimize key geometric parameters of TEGs through the Taguchi method. In the third stage, the Taguchi method is used to optimize the geometric dimensions of the heat sink. Lastly, in the fourth stage, the effect of operating conditions on the performance of TEGs is investigated. The results reveal that the Taguchi method is capable of predicting the near-optimal configuration of TEGs.

Thermo-Magneto-Electric Generator Arrays for Active Heat Recovery System

Continued emphasis on development of thermal cooling systems is being placed that can cycle low grade heat. Examples include solar powered unmanned aerial vehicles (UAVs) and data storage servers. The power efficiency of solar module degrades at elevated temperature, thereby, necessitating the need for heat extraction system. Similarly, data centres in wireless computing system are facing increasing efficiency challenges due to high power consumption associated with managing the waste heat. We provide breakthrough in addressing these problems by developing thermo-magneto-electric generator (TMEG) arrays, composed of soft magnet and piezoelectric polyvinylidene difluoride (PVDF) cantilever. TMEG can serve dual role of extracting the waste heat and converting it into useable electricity. Near room temperature second-order magnetic phase transition in soft magnetic material, gadolinium, was employed to obtain mechanical vibrations on the PVDF cantilever under small thermal gradient. TMEGs were shown to achieve high vibration frequency at small temperature gradients, thereby, demonstrating effective heat transfer.

Taguchi optimization of bismuth-telluride based thermoelectric cooler

In the last few decades, considerable effort has been made to enhance the figure-of-merit (ZT) of thermoelectric (TE) materials. However, the performance of commercial TE devices still remains low due to the fact that the module figure-of-merit not only depends on the material ZT, but also on the operating conditions and configuration of TE modules. This study takes into account comprehensive set of parameters to conduct the numerical performance analysis of the thermoelectric cooler (TEC) using a Taguchi optimization method. The Taguchi method is a statistical tool that predicts the optimal performance with a far less number of experimental runs than the conventional experimental techniques. Taguchi results are also compared with the optimized parameters obtained by a full factorial optimization method, which reveals that the Taguchi method provides optimum or near-optimum TEC configuration using only 25 experiments against 3125 experiments needed by the conventional optimization method. This study also ...

Miniature Shape Memory Alloy Heat Engine for Powering Wireless Sensor Nodes

Abstract Shape Memory Alloys (SMAs) exhibit temperature-dependent cyclic deformation. SMAs undergo reversible phase transformation with heating that generates strain which can be used to develop heat engine. In this study, we build upon the concept where environmental heat is first converted into mechanical energy through SMA deformation and then into electrical energy using a microturbine. This SMA heat engine was tailored to function as a miniature energy harvesting device for wireless sensor nodes applications. The results showed that 0.12 g of SMA wire produced 2.6 mW of mechanical power which was then used to drive a miniature electromagnetic generator that produced 1.7 mW of electrical power. The generated electrical energy was sufficient to power a wireless sensor node. Potential design concepts are discussed for further improvements of the SMA heat engine for the wireless sensing platform.

Optimization of segmented thermoelectric generator using Taguchi and ANOVA techniques

Recent studies have demonstrated that segmented thermoelectric generators (TEGs) can operate over large thermal gradient and thus provide better performance (reported efficiency up to 11%) as compared to traditional TEGs, comprising of single thermoelectric (TE) material. However, segmented TEGs are still in early stages of development due to the inherent complexity in their design optimization and manufacturability. In this study, we demonstrate physics based numerical techniques along with Analysis of variance (ANOVA) and Taguchi optimization method for optimizing the performance of segmented TEGs. We have considered comprehensive set of design parameters, such as geometrical dimensions of p-n legs, height of segmentation, hot-side temperature, and load resistance, in order to optimize output power and efficiency of segmented TEGs. Using the state-of-the-art TE material properties and appropriate statistical tools, we provide near-optimum TEG configuration with only 25 experiments as compared to 3125 experiments needed by the conventional optimization methods. The effect of environmental factors on the optimization of segmented TEGs is also studied. Taguchi results are validated against the results obtained using traditional full factorial optimization technique and a TEG configuration for simultaneous optimization of power and efficiency is obtained.

Miniature Contactless Piezoelectric Wind Turbine

This study presents the development of a miniature wind turbine that can be used for powering wireless sensors deployed at remote locations. The wind turbine utilizes piezoelectric transducers in order to reduce the start-up speed. Six piezoelectric bimorphs with magnetic tip mass were actuated through attractive and repulsive force between permanent magnets. The design and characterization of the prototype is discussed along with a Finite Element Model (FEM) of the piezoelectric elements. The wind turbine was 23 cm high and 16.5 cm wide. It was able to achieve low cut-in speeds of 1m/s providing power outputs ranging between 0.5 mW and 3 mW for wind speeds in the range of 1 m/s to 4 m/s.

Self-Powered Temperature-Mapping Sensors Based on Thermo-Magneto-Electric Generator

We demonstrate a thermo-magneto-electric generator (TMEG) based on second-order phase transition of soft magnetic materials that provides a promising pathway for scavenging low-grade heat. It takes advantage of the cyclic magnetic forces of attraction and repulsion arising through ferromagnetic-to-paramagnetic phase transition to create mechanical vibrations that are converted into electricity through piezoelectric benders. To enhance the mechanical vibration frequency and thereby the output power of the TMEG, we utilize the nonlinear behavior of piezoelectric cantilevers and enhanced thermal transport through silver (Ag) nanoparticles (NPs) applied on the surface of a soft magnet. This results in large enhancement of the oscillation frequency reaching up to 9 Hz (300% higher compared with that of the prior literature). Optimization of the piezoelectric beam and Ag NP distribution resulted in the realization of nonlinear TMEGs that can generate a high output power of 80 μW across the load resistance of 0.91 MΩ, which is 2200% higher compared with that of the linear TMEG. Using a nonlinear TMEG, we fabricated and evaluated self-powered temperature-mapping sensors for monitoring the thermal variations across the surface. Combined, our results demonstrate that nonlinear TMEGs can provide additional functionality including temperature monitoring, thermal mapping, and powering sensor nodes.

Low-grade waste heat recovery using the reverse magnetocaloric effect

According to a recent study by Lawrence Livermore National Laboratory, about 59.1 quadrillion BTU of energy produced in the United States is rejected to the atmosphere, mostly in the form of waste heat. A major portion of the total rejected thermal energy has a low temperature (less than 230 °C), classified as low-grade waste heat. This energy loss is the result of the fact that current thermal energy harvesting technologies, primarily thermoelectric generators, have poor efficiency at low temperature gradients and therefore are not cost-effective. This study investigates the possibility of low-grade waste heat recovery using magnetocaloric materials, which were developed mainly for magnetic refrigeration. The working principle of energy harvesters using the reverse magnetocaloric cycle is described using thermodynamic analysis and the performance of more than 60 magnetocaloric materials is compared under different operating temperature conditions. Considering the ambient atmosphere as the heat sink (temperature ∼ 25 °C), it was found that oxide-based magnetocaloric materials, such as La2/3Ba1/3MnO2.98 (Curie temperature ∼ 38 °C), have a working potential as high as 53.5 J per kg per cycle at a heat source temperature of 50 °C. The working potential increases to 77.4 J per kg per cycle, when the heat source temperature is increased to 75 °C, and it further increases to 87.8 J per kg per cycle at a heat source temperature of 100 °C. The working potential up to 100 J per kg per cycle at a heat source temperature of 100 °C was estimated for a few other materials with higher Curie temperature, such as Gd5Si4 (Curie temperature ∼ 65 °C) and La2/3Ba1/3MnO3 (Curie temperature ∼ 63 °C).

Micro Wind Turbine for Powering Wireless Sensor Nodes

Abstract The goal of this study was to design a micro wind turbine having dimensions on the order of 1–10 cm3 for powering wireless sensor nodes. Using the parametric study based upon a computational model, the coupling factor between the blade and generator section was significantly enhanced. Building upon the formulation for the coupling factor, we derived efficiency metric for the rotational generators. A prototype was designed based upon the analytical study conducted through the combination of blade element momentum theory and ANSYS magnetics to match the performance of the generator to that of the blades. The blade diameter and depth was 72 mm and 9 mm, respectively. The generator diameter was 26 mm with total volume of 18.1 cm3. It was found that the micro wind turbine generated DC output power of 12.39 mW, 49.03 mW and 102.61 mW at 3.7 m/s, 6 m/s and 8 m/s wind speeds, respectively. The power density was computed to be 0.304, 1.204, and 2.52 mW/cm2, respectively, which are higher than all the other results reported in the literature.