• Energy Research

Mismatching operating conditions affect the energetic performance of PhotoVoltaic (PV) systems because they decrease their efficiency and reliability. The two different approaches used to overcome this problem are Distributed Maximum Power Point Tracking (DMPPT) architecture and reconfigurable PV array architecture. These techniques can be considered not only as alternatives but can be combined to reach better performance. To this aim, the present paper presents a new algorithm, based on the joint action of the DMPPT and reconfiguration approaches, applied to a reconfigurable Series-Parallel-Series architecture, which is suitable for domestic PV application. The core of the algorithm is a deterministic cluster analysis based on the shape of the current vs. voltage characteristic of a single PV module combined with its DC/DC converter to perform the DMPPT function. Experimental results are provided to validate the effectiveness of the proposed algorithm and to demonstrate evidence of its major advantages: robustness, simplicity of implementation and time-saving.

The paper presents analytical formulas for computation in the time domain of electromagnetic (EM) fields generated by tortuous cloud-to-cloud (CC) lightning channels over a perfectly conducting ground. For the first time, the study was not limited to a horizontal lightning path but was extended to take into account the natural, tortuous geometry of the lightning channel. After the calculation of the step response, a convolution integration was applied for the computation of the fields generated by an arbitrary current source. The produced electric and magnetic fields were then compared with the fields generated by a horizontal channel. The method can be of primary importance to evaluating the hazards for electric and electronic systems of flying aircraft, estimating the voltages induced on overhead transmission lines by CC lightning, and, in general, evaluating the induced effects on sensitive electric and electronic components. Moreover, it may represent a simple, robust, and time-saving tool for estimating important physical parameters that characterize lightning phenomena.

In this paper, the theoretical analysis and the experimental validation of a modified version of the maximum power point tracking technique, that is known with the acronym TEODI, are presented and discussed. The modified version of TEODI (MTEODI) outperforms TEODI in photovoltaic (PV) applications operating under mismatching conditions. The working principle of MTEODI is based on the periodic measurement of the short-circuit currents of the PV units. The knowledge of such currents allows not only the determination of whether mismatching conditions occur but identification of the values of the suitable correction factors, on which the working of MTEODI is based, as well.

Abstract In order to overcome the drawbacks associated to mismatching operating conditions in PV systems it is possible to adopt one DC/DC converter (microconverter) for each PV module. The aim of microconverters is that of carrying out the Distributed Maximum Power Point Tracking (DMPPT), that is the MPPT of each PV module rather than of the whole PV field. Standard PV systems adopts instead central inverters which carry out the Central Maximum Power Point Tracking (CMPPT), that is the MPPT of the whole PV array. The DMPPT alone is not enough in order to get the actual maximization of the energetic efficiency of the PV system. Instead, it is necessary to couple the DMPPT technique with a suitable CMPPT technique. In this paper it will be shown how to properly optimize the CMPPT technique. In particular, the considered CMPPT technique is based on the periodic scan of the Power vs. Voltage characteristic seen at the input by the inverter. The aim of such a scan is to locate the optimal operating value of the bulk inverter voltage v b , that is the value of v b in correspondence of which the power extracted from the PV source is the maximum one. It will be shown that, despite of the apparent simplicity of such a CMPPT technique, much care is needed in order to avoid errors due to the peculiar shape assumed, under mismatching operating conditions, by the Power vs. Voltage characteristic seen at the input by the inverter. In fact, such a characteristic may exhibit the presence of multiple peaks and/or flat parts and/or nearly vertical portions which may easily lead the CMPPT technique to the error and, consequently, may cause a potentially significant waste of the available energy. The results of numerical simulations fully confirm the validity of the theoretical predictions.

The following two approaches can address the drawbacks associated with mismatching phenomena in photovoltaic (PV) plants: distributed maximum power point tracking (DMPPT) architecture and reconfigurable PV array architecture. Until now, these two approaches have represented alternative solutions. In this paper, for the first time, it is suggested that the two approaches can be used together. In particular, it will be shown how the joint adoption of the DMPPT and reconfiguration approaches can improve the performances of mismatched PV plants; here, performance is understood as the best compromise between the efficiency and reliability of the entire PV system. Numerical results confirm the above assumptions, providing the hints for the development of innovative reconfiguration techniques suitable for distributed applications.

Abstract In this paper a simple and fast re-configuration algorithm which is suitable for a PV array with Series–Parallel architecture is presented and discussed. The main advantage of such an algorithm is represented by its capability to find a nearly optimal configuration by testing only a very small subset of all the possible configurations. In particular it is shown that, in an actual PV array composed by 24 PV modules which operate under mismatching conditions (which are quite common in urban environments due to chimneys, streets lighting poles, antennas, neighboring buildings, etc.), the proposed algorithm is able to lead to energetic performances which are more or less comparable with those ones which can be obtained by adopting a Monte Carlo based algorithm employing a much higher number of trials. Moreover, by means of a specific example, it is shown that maximization of extracted energy and absence of dangerous operating conditions, possibly leading to the premature aging of the PV field and/or to hot spot phenomena, are contrasting requirements. It is nonetheless possible to find proper configurations able to lead to a suitable compromise between such two contrasting requirements. Further work is needed and is in progress in order to design suitable re-configuration algorithms able to identify such compromise configurations.

Abstract 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. Such a strategy is based on the evaluation of an estimate of the optimal operating range of the inverter input voltage and on the evaluation of an estimate of the optimal operating voltages of the PV modules. The main advantage of the proposed technique is represented by the possibility to evaluate in closed-form the above estimates, provided that the PV modules short circuit currents are known. The closed-form evaluation of the above estimates allows in turn the fast identification of a set of optimal operating points for the inverter and for the PV modules; such a fast identification allows to obtain a marked increase of the speed of tracking of the maximum power point of the whole PV system. Moreover, a further advantage of the proposed technique is represented by the capability to avoid that the operating value of the inverter input voltage remains trapped in the neighborhood of a suboptimal operating point thus lowering the energetic efficiency of the PV system, as it may happen when standard MPPT techniques (such as the Perturb and Observe technique) are adopted.

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.