Various energy harvesting systems have been proposed in the last years. The use of ambient energy, such as vibration energy, has evoked great interests. Most vibration-based converters generate the maximum power only when the generator is excited at its resonance frequency. To overcome this limitation, researchers focus on strategies for increasing the working bandwidth of an energy harvester. Due to the amount of different approaches for broadband energy harvesting, we suggest a categorization. This review presents a classification of the current techniques. Every technique has its own benefits and drawbacks and is suitable for different applications. This review presents the categorization and describes each conversion technique. Typical research approaches are shown to get a better understanding of the energy harvesting techniques. The advantages and drawbacks are presented, and the suitability is shown for each category.

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Summary The investigation of oilwell-drillstring dynamics is essential for understanding the complex behavior of downhole-vibration phenomena. Experimental test rigs allow a reproduction of critical vibrations in a laboratory environment with defined boundary conditions. Furthermore, measurements of mechanical quantities become possible, allowing a closer insight into the complex mechanisms of drillstring dynamics. This article presents a new scaled rig for analyzing drillstring vibrations. A review of existing test rigs is given, and the new laboratory rig is presented. The experimental system consists of a rotating shaft representing a section of the bottomhole assembly between two stabilizers. It is capable of reproducing lateral drillstring vibrations with and without contact. Particularly, one can investigate the vibration phenomena of stick/slip, forward whirl, backward whirl, and snaking. The shaft is positioned horizontally and supported with a fixed and a floating bearing. An electric drive provides the required rotational speed. To induce an axial force into the shaft and to simulate the weight on bit, the test rig is composed of an axial-force module. The borehole/drillstring interaction is realized with a contact module that is positioned at the shaft's center. This contact module is equipped with two eddy-current sensors to measure the lateral deflection of the rotating shaft and three force sensors to determine the normal and tangential contact forces. The paper provides a comparison of the new scaled rig to existing test setups and points out differences. Unique features of the new test rig are the application of mechanical similarity laws for the test-rig design as well as the possibility to measure the contact forces. Measurements of vibration phenomena with and without contact are shown to outline the potential of the new scaled rig.

This article illustrates the modeling of a piezomagnetoelastic energy harvesting system. The generator consists of a piezoelectric cantilever with a magnetic tip mass. A second oppositely poled magnet is attached near the free end of the beam. Due to the nonlinear magnetic restoring force the system exhibits two symmetric stable equilibrium positions and one instable equilibrium position. The equation of motion is derived and it is shown that the system can be modeled as Duffing oscillator. An analytical approach is given to derive the Duffing parameters from the system parameters. The Duffing equation is solved for an oscillation around both equilibrium positions by using the harmonic balance method. For small orbit oscillations the equation of motion is solved by applying the fourth-order multiple scales method.

Zusammenfassung Piezoelektrische Energy Harvesting Systeme zur Nutzung von Umgebungsenergie haben in den letzten Jahren an Bedeutung gewonnen. Viele vibrationserregte Wandler erzeugen die nötige Energiemenge nur, wenn sie in ihrer Eigenfrequenz angeregt werden. Magnetische Kräfte sind eine Möglichkeit die Energieausbeute zu verbessern. Dieser Beitrag stellt verschiedene Methoden dar durch nichtlineare Magnetkräfte die Bandbreite des Energy Harvestings zu verbessern.