Marco Balato

A Survey on Mismatching and Aging of PV Modules: The Closed Loop

In this paper, the different aging mechanisms taking place in photovoltaic modules are discussed, and the cause-effect links, which exist among such mechanisms, are evidenced. It is also shown that a closed-loop link exists between aging and mismatching since aging (which is nonuniform by its nature) causes mismatching among cells, whereas mismatching, in turn, mainly due to its thermal effects, leads to nonuniform aging.

Fast Hybrid MPPT Technique for Photovoltaic Applications: Numerical and Experimental Validation

In PV applications, under mismatching conditions, it is necessary to adopt a maximum power point tracking (MPPT) technique which is able to regulate not only the voltages of the PV modules of the array but also the DC input voltage of the inverter. Such a technique can be considered a hybrid MPPT (HMPPT) technique since it is neither only distributed on the PV modules of the PV array or only centralized at the input of the inverter. In this paper a new HMPPT technique is presented and discussed. Its main advantages are the high MPPT efficiency and the high speed of tracking which are obtained by means of a fast estimate of the optimal values of PV modules voltages and of the input inverter voltage. The new HMPPT technique is compared with simple HMPPT techniques based on the scan of the power versus voltage inverter input characteristic. The theoretical analysis and the results of numerical simulations are widely discussed. Moreover, a laboratory test system, equipped with PV emulators, has been realized and used in order to experimentally validate the proposed technique.

A new strategy for the identification of the optimal operating points in PV applications with Distributed MPPT

In this paper, a new control strategy allowing to optimize the performances of PV systems adopting Distributed Maximum Power Point Tracking (DMPPT), is presented and discussed. It is based on the estimate of the optimal operating range of the inverter input voltage and of the optimal operating voltages of the PV modules. The main advantage of the proposed technique is represented by the fast identification of a set of operating points for the inverter and for the PV modules, which allows to obtain a marked increase of the speed of tracking both of the inverter and of the DC/DC converters performing the DMPPT function. Moreover, a further advantage is represented by the possibility to avoid that, due to mistakes of the inverter MPPT technique, the operating value of the inverter input voltage remains trapped in the neighborhood of a suboptimal operating point.

Closed-form analysis of Switchless Electrostatic Vibration Energy Harvesters

Batteries drawbacks are represented by the high cost, the limited reliability and the frequent maintenance requirements (recharge or replacement). Energy Harvesting (EH) offers an alternative to batteries replacement or, at least it allows the increase of their lifetime. EH is the conversion of ambient energy, which is present in the environment, into electrical energy. Different energy sources for EH applications have been reported in the literature: light, radio frequency (RF), temperature gradients, solar energy, vibrations. In this paper we will focus on vibration EH. In a Vibration Energy Harvester (VEH) electrical energy is generated by allowing an inertial mass to vibrate with respect to a frame. In this paper a closed-form analysis of the model of a single-degree of freedom Out-of-Plane Switchless Electrostatic Vibration Energy Harvester (SEVEH) will be presented and discussed. We will demonstrate that, under suitable reasonable assumptions, it is possible to study SEVEHs by using models that admit closed form solutions and, at the same time, are enough accurate if the simplifying hypothesis are met. In particular, a circuital model will be developed which describes both the mechanical and the electrical subsystems composing a SEVEH.

Identification of the parameters of the equivalent electric circuit of electromagnetic harvesters

This paper concerns electromagnetic vibration energy harvesters. The knowledge of the equivalent electric circuit of electromagnetic harvesters is a fundamental need in order to be able to carry out numerical simulations of electromagnetic vibration harvesters. The aim of this paper is the description of a suitable procedure leading to the determination of the parameters of such an equivalent electric circuit. In fact, while in the case of the laboratory harvesters which are presented and discussed in the scientific literature such parameters are usually known, in the case of commercial devices instead such parameters must be extracted from the typical information which can be usually found in the manufacturers data sheet.

Multi-objective optimization of PV arrays performances by means of the dynamical reconfiguration of PV modules connections

Mismatching operating conditions (due to clouds, shading, non-uniform aging, manufacturing tolerances, etc.) strongly limit the energetic efficiency and the reliability of PV plants. The dynamical re-configuration of PV modules represents a promising solution in order to mitigate the drawbacks associated to mismatching phenomena. The goal of this paper is just to prove that, by using the dynamical reconfiguration of PV modules, it is possible to find configurations which allow to get a suitable compromise between energetic efficiency and reliability. In particular, in this paper, the expression of a suitable Objective Function whose maximization allows to get the required compromise between energetic efficiency and amplitude of thermal stresses (due to bypass diode conduction and/or reverse biasing of shaded cells) is introduced and discussed.

Simulation and laboratory characterization of a hybrid MPPT technique based on the fast estimate of the maximum power voltages in PV applications

In photovoltaic (PV) applications, under mismatching conditions, the adoption of a Maximum Power Point Tracking (MPPT) technique which is able to regulate not only the voltages of the PV modules of the array, but also the DC input voltage of the inverter is necessary. Such a technique can be considered an Hybrid MPPT (HMPPT) technique which is neither only distributed on the PV modules of the PV array or only centralized at the input of the inverter. In this paper a new HMPPT technique is presented and discussed. Its main advantages are the high MPPT efficiency and the high speed of tracking obtained by means of a fast estimate of the maximum power voltages of the PV modules and of the inverter. The new HMPPT technique is compared with traditional HMPPT techniques. Mathematical formulation, simulation and a test system for laboratory characterization of the presented technique are discussed.

Fast dynamical reconfiguration algorithm of PV arrays

In case of mismatch (due to clouds, shadows, dirtiness, manufacturing tolerances, aging, different orientation of parts of the PV field, thermal gradients, etc.), the Power versus Voltage (P-V) characteristic of a PV field may exhibit more than one peak, because of the adoption of bypass diodes, therefore Centralized Maximum Power Point Tracking (CMPPT) algorithms can fail. The consequent power drop can be avoided by using power optimizers, which are module dedicated dc/dc converters, or micro-inverters. The main drawbacks of such devices are the high cost and the reduced plant efficiency during the day hours in which the PV array works under a uniform irradiation level. This usually happens in the middle of the day, just when the PV power reaches the peak. Moreover, both the dc/dc and the dc/ac module dedicated converters introduce further components that may reduce the system lifetime, e.g. large electrolytic capacitances. A recent alternative is represented by the dynamical re-configuration of the PV array by means of active switches: in this way, the efficiency under uniform irradiation conditions is preserved and, in presence of mismatching, the best series/parallel modules configuration is achieved. This solution is cheaper than the one involving power optimizers and, besides the possibility of plant monitoring, offers additional advantages in terms of safety in case of fire. In this paper a novel approach to the PV string dynamical reconfiguration is presented and discussed.

Resonant electromagnetic vibration harvesters feeding sensor nodes for real-time diagnostics and monitoring in railway vehicles for goods transportation: A numerical-experimental analysis

In this paper, the results of a combined numerical-experimental analysis on a system, part of a wireless sensor network, composed by a resonant electromagnetic energy harvester, a suitable power electronic interface and a sensor node are presented and discussed. Such a system is to be used onboard for real-time diagnostics and monitoring in railway vehicles for goods transportation.

Resonant electromagnetic vibration energy harvesters: The harvester ideal utilization factor

In this paper an important figure of merit is introduced and discussed with reference to electromagnetic vibration energy harvesters which are loaded by diode bridge rectifiers. It is called Harvester Ideal Utilization Factor. It precisely quantifies how close to the optimum the harvester is loaded. Such a figure of merit can be defined not only for electromagnetic but also for piezoelectric harvesters as long as the assumptions which allow the modeling of such vibration harvesters by means of an equivalent linear electric circuit are valid.