524 IEEE JOURNAL OF PHOTOVOLTAICS, VOL. 2, NO. 4, OCTOBER 2012 Performance Analysis of a Single-Axis Tracking PV System Alberto Dolara, Student Member, IEEE, Francesco Grimaccia, Member, IEEE, Sonia Leva, Member, IEEE, Marco Mussetta, Member, IEEE, Roberto Faranda, Member, IEEE, and Moris Gualdoni Abstract—In this paper a performance analysis of a photovoltaic (PV) tracking system is conducted, to study its efficiency based on experimental results of a specific power plant. A single-axis system is analyzed, taking into account different indexes to better characterize the overall system performance. Experimental data have been collected by an on-site monitoring system over a period of one year, bringing some final considerations and comparative results on PV-system efficiency. Index Terms—Energy efficiency, performance ratio (PR), photovoltaic (PV) plant monitoring, PV tracking system, trans- position factor (TR). I. INTRODUCTION S INCE renewable energy technologies and, in particular, photovoltaic (PV) power generation are playing an increas- ing role in electricity production, it is becoming more important to monitor PV plant performance and loss factors to evaluate and potentially enhance the overall system efficiency. In order to maintain high output power of the PV power gen- eration plant, several methods to increase the efficiency of solar cells exist (e.g. converting systems [1], control systems, maxi- mum power-point tracking schemes [2], etc.), and, in particular, different tracking methods are commonly used to optimize the incidence of the sun over cell modules throughout the entire day. Hence, the PV system performance analysis is becoming increasingly important with the aim to evaluate the quality of a PV system during its total lifecycle operations in terms of both annual performance and loss analysis. In this context, there is an increasing interest in developing and using sun tracking mounts for normal flat plate PV systems. A solar tracking system (TS) is a device to orient various PV panels toward the sun. Such systems are able to deliver a larger amount of energy at same nominal power, but their performances have to be carefully evaluated, considering the energy increase versus the inherent higher cost of the TS itself. Several studies in literature compare the performance of fixed PV systems based on measured data [3]–[7], while tracking Manuscript received January 27, 2012; revised April 13, 2012 and May 16, 2012; accepted May 20, 2012. Date of publication July 19, 2012; date of current version September 18, 2012. The authors are with the Department of Energy, Politecnico di Milano, 20156 Milano, Italy (e-mail: alberto.dolara@mail.polimi. it; francesco.grimaccia@polimi.it; sonia.leva@polimi.it; marco.mussetta@ polimi.it; roberto.faranda@polimi.it; moris.gualdoni@mail.polimi.it). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JPHOTOV.2012.2202876 systems’ performance estimation is still essentially based on theoretical analysis or simulated results, taking into account daily irradiation profiles and temperatures [8]–[11]. With respect to fixed PV plants, it is extremely important the use of performance ratio (PR) index which takes into account the net amount of energy produced by the PV system. However, when TS is used, the PR index is not enough to properly evaluate the efficiency of the TS itself nor to estimate its energy behavior [12], [13]. In this case, the transposition factor (TF) can be used to provide important information about the amount of captured irradiation by the PV panels with respect to fixed panel (see Section II). In [14], some measurements and results based on these differ- ent indexes with respect to the monitoring activity of a PV plant located in Italy are presented. The study also aimed to char- acterize the PV TS influence. All the indexes were evaluated using measured data compared with Photovoltaic Geographical Information System (PVGIS) historical data, which were used as a reference to estimate the energy increase due to TSs with respect to a fixed one. In this paper, a complete and more detailed analysis is pre- sented, with the aim to fully characterize all the advantages and drawbacks of a TS, taking into account all the other factors that affect PV-system performances (e.g. balance of the system (BOS) losses and TS energy consumption). In particular, the PV system under investigation is described here, giving some details about plant configuration and main features of its TS. Results obtained from data measured at the plant location are reported in terms of daily average irradiation trends (on hori- zontal plane and PV plane, respectively), PR and TF, and they are analyzed here, considering the uncertainty level of the data acquisition system. This paper is organized as follows. In Section II, TF and PR are defined following current standards; moreover, addi- tional monitoring analytics are introduced to better evaluate the performances of PV-TS. Section III describes the PV sys- tem considered in this study and gives more details about the adopted TS. Section IV describes the monitoring activity and its results, also considering instruments overall uncertainty. Con- clusion and general comments are reported in Section V. II. PERFORMANCE ANALYSIS AND INDEXES As mentioned in the previous section, if we consider a PV system with solar tracking, TF should be used to better evaluate the system performance. TF is a dimensionless quantity defined by the ratio between the incident solar energy over a differently oriented and inclined plane (the reference yield Y r ) and the solar 2156-3381/$31.00 © 2012 IEEE