For the output power estimation of photovoltaic devices in indoor applications, models are needed that perform accurately at the low illumination levels encountered. As a robust and scalable solution, we propose a data-driven modeling method, spanning an interpolated surface between two reference I-V curves. The proposed approach is evaluated based on experimental data of two exemplar PV panels at indoor illumination levels. The results are compared to two common parameter extraction methods for the one-diode circuit model. This investigation demonstrates that the proposed surface model has a high performance under all test conditions, whereas the reference models show a performance dependency on the PV panel type. It can be concluded that the surface model is a competitive alternative for output power estimations at indoor illumination levels, removing many of the uncertainties of traditionally used physical parameter extraction and scaling methods.

Pressure fluctuation energy harvesting devices are promising alternatives to power up wireless sensors in fluid power systems. In past studies, classical Helmholtz resonators have been used to enhance the energy harvesting capabilities of these harvesters. Nevertheless, for fluctuations with frequency components in the range of less than 1000 Hz, the design of compact resonators is difficult, mostly for their poor acoustic gain. This paper introduces a space-coiling resonator fabricated using 3D printing techniques. The proposed resonator can achieve a better acoustic gain bounded by a small bulk volume compared to a classic Helmholtz resonator, improving the energy harvesting capabilities of pressure fluctuation energy harvesters. The resonator is designed and evaluated using finite-element-method techniques and examined experimentally. Three space-coiling-resonators are designed, manufactured and compared to classic Helmholtz resonators for three frequencies: 280 Hz, 480 Hz and 920 Hz. This work displays the possibility of compact, high-performance pressure fluctuation energy harvesters and the advantages of the space-coiling printed resonators to enhance the harvesting performance.

The variable reluctance principle can be used to convert rotational kinetic energy into electrical energy, creating a Variable Reluctance Energy Harvester (VREH) based on electromagnetic induction. This can be used to implement self-sustaining wireless sensors in rotating applications. In this paper, we present and investigate a novel design of a VREH with high volumetric power density that targets low-speed rotating applications. The design uses an m-shaped pole-piece and two opposing magnets. We theoretically analyze key design parameters that influence the VREH’s output power, and relate these parameters to geometrical design factors of the proposed structure. Key design factors include the coil height, the permanent magnet height and the tooth height. A method based on numerical simulations is introduced, enabling to determine the optimal geometrical dimensions of the proposed structure under given size-constraints. The results demonstrate that the method leads to optimal structure configurations, which has been evaluated for different cases and is verified experimentally. Good agreement between numerical simulations and experiments are reported with deviations in output power estimation below 3%. The optimized m-shaped VREH, moreover, provides output power levels sufficient for wireless sensor operation, even in low-speed rotating applications.

Energy-autonomous wireless sensor systems have the potential to enable condition monitoring without the need for a wired electrical infrastructure or capacity-limited batteries. In this paper, a robust and low-cost energy-autonomous wireless rotational speed sensor is presented, which harvests energy from the rotary motion of its host using the variable reluctance principle. A microelectromechanical system (MEMS) gyroscope is utilized for angular velocity measurements, and a Bluetooth Low Energy System-on-Chip (SoC) transmits the acquired samples wirelessly. An analysis on the individual subsystems is performed, investigating the output of the energy transducer, the required energy by the load, and energy losses in the whole system. The results of simulations and experimental measurements on a prototype implementation show that the system achieves energy-autonomous operation with sample rates between 1 to 50 Hz already at 10 to 40 rotations per minute. Detailed investigations of the system modules identify the power management having the largest potential for further improvements.

Models of photovoltaic devices are used to compare the properties of photovoltaic cells and panels, and to predict their I-V characteristics. To a large extent, modeling methods are based on the one-diode equivalent circuit. Although much research exists on the implementation and evaluation of these methods for typical outdoor conditions, their performance at indoor illumination levels is largely unknown. Consequently, this work performs a systematic study of methods for the parameter extraction of one-diode models under indoor conditions. We selected, reviewed and implemented commonly used methods, and compared their performance at different illumination levels. We have shown that most methods can achieve good accuracies with extracted parameters regardless of the illumination condition, but their accuracies vary significantly when the parameters are scaled to other conditions. We conclude that the physical interpretation of extracted parameters at low illumination is to a large extent questionable, which explains errors based on standard scaling approaches. 

The Internet of Things introduces Internet connectivity to things and objects in the physical world and thus enables them to communicate with other nodes via the Internet directly. This enables new applications that for example allow seamless process monitoring and control in industrial environments. One core requirement is that the nodes involved in the network have a long system lifetime, despite limited access to the power grid and potentially difficult propagation conditions. Energy harvesting can provide the required energy for this long lifetime if the node is able to send the data based on the available energy budget. In this paper, we therefore analyze and evaluate which common IoT communication technologies are suitable for nodes powered by energy harvesters. The comparison includes three different constraints from different energy sources and harvesting technologies besides several communication technologies. Besides identifying possible technologies in general, we evaluate the impact of duty-cycling and different data sizes. The results in this paper give a road map for combining energy harvesting technology with IoT communication technology to design industrial sensor nodes. 

Energy harvesting converts ambient energy to electrical energy that can be used to power, for example, sensors and sensor systems. Variable reluctance energy harvesting is a suitable candidate for the conversion of rotary kinetic motion, an energy form commonly found in industrial applications. The implementation of a variable reluctance energy harvester, however, has a significant effect on its performance and is not well studied. In this paper, we therefore conduct a survey on different structures of variable reluctance energy harvesters. Six existing structures, previously used in variable reluctance sensors, are presented and analyzed according to their approaches for magnetic flux change improvement. Together with a newly proposed structure, these structures are evaluated based on a finite element analysis, and their results are compared. It is demonstrated that the choice of structure considerably affects the power output of the harvester and is dependent on the improvement approaches the structure utilizes. The newly proposed structure outperforms all existing structures with respect to power output and power density, which comes at a cost of higher parasitic torque generation. A 53-fold power improvement over the reference and an 1.2-fold power improvement over the next best structure is observed. As a result, applications of variable reluctance energy harvesting become viable even at low angular velocities.

Hydraulic pressure fluctuation energy harvesters are promising alternatives to power up wireless sensor nodes in hydraulic systems. The characterization of these harvesters under dynamic and band-limited pressure signals is imperative for the research and development of novel concepts. To generate and control these signals in a hydraulic medium, a versatile apparatus capable of reproducing pressure signals is proposed. In this paper, a comprehensive discussion of the design considerations for this apparatus and its performance is given. The suggested setup enables the investigation of devices tailored for the harvesting of energy in conventional hydraulic systems. To mimic these systems, static pressures can be tuned up to 300 bar, and the pressure amplitudes with a maximum of 28 Bar at 40 Hz and 0.5 bar at 1000 Hz can be generated. In addition, pressure signals found in commercial hydraulic systems can be reproduced with good accuracy. This apparatus proves to be an accessible, robust, and versatile experimental setup to create environments for the complete performance estimation of pressure fluctuation energy harvesters. 

Wireless sensor nodes in state of the art fluid power systems used in monitoring and maintenance prediction demand long lasting power sources that do not rely on batteries. Energy harvesting is a promising technology that can provide the required energy to power wireless sensors. Pressure fluctuation energy harvesters can be employed in conventional hydraulic systems to convert the acoustic pressure fluctuation to electrical power. Present studies have explored the overall efficiency of these devices while experimentally describing losses in piezoelectric and circuit interfaces, nevertheless there is no study on the fluid to mechanical force transmission efficiency. In this paper we investigate the pressure to force transmission rate of two types of fluid to mechanical interfaces: a flat metal plate and a conventional hydraulic piston. The interfaces are investigated in conditions similar to those found in conventional hydraulic systems. The study shows that flat plate exhibit good force transmission for low pressure applications with a constant rate across frequencies, while exhibiting a decrease in force transmission at higher pressures. On the other hand the piston exhibit a more robust pressure design, with a constant force transmission rate at all pressures but with a dampening of force at higher frequencies. It is shown that small differences in force transmission ratios can have a considerable impact on the power generation.