PV technologies performance comparison in temperate climates

Abstract

During the last decade, PV technology has been subject to a quick and remarkable improvement, affecting both the efficiency of modules and the manufacturing costs; during its period of greatest growth, the EU market was characterized by different major types of PV modules, but in many cases, due to the extremely rapid development of the sector, there is a lack of knowledge about the energy performance and behavior of such technologies under actual operating conditions. The aim of this work is thus to carry out a performance comparison of three different and representative PV technologies in temperate climatic conditions; in particular, the influence of climatic parameters on energy production was investigated, with a focus on the thermal and spectral analysis. The experimental results, referred to a monitoring period of one year, show a significant difference between the reference performances, rated under standard test conditions, and the actual daily/seasonal productivity, providing useful information about the different interaction with climate of the technologies tested.

Introduction

The actual performance of PV systems is strongly related to the environmental conditions; in detail, the parameters that most affect energy production are global irradiance, ambient temperature and solar radiation spectrum (Monokroussos et al., 2011, Huld et al., 2010, Strobel et al., 2009, Zinßer et al., 2008). These parameters directly affect the operating conditions of PV modules, with particular reference to cells temperature, which can be identified as one of the major factors affecting electricity production. However, each PV technology reacts differently to the variations of cells operating temperature, according to the temperature coefficient (Makrides et al., 2009): the performance of crystalline silicon modules (c-Si), as is well known, decreases when temperature increases (Radziemska, 2003); on the contrary, modules realized with single or multi-junction amorphous silicon cells (a-Si) are able to improve the electrical performance in high-temperature conditions (Makrides et al., 2012). This effect, which is called thermal annealing, allows to recover part of the nominal power initially lost due to Light Induced Degradation (LID), as a consequence of a prolonged exposure to high temperature (McEvoy et al., 2012). These modules have a strongly site-dependent performance, which is typically difficult to predict in terms of actual energy production.

In such a scenario, it is worth to note that the actual performance of PV modules in outdoor conditions is thus determined by the overlapping of several concurrent effects, which are difficult to differentiate and analyze individually (Fanni et al., 2011, Gottschalg et al., 2003).

The aim of this study is the definition and the performance comparison of three main categories of PV modules, made, respectively, of crystalline silicon cells (c-Si), micromorphous silicon cells (a-Si/μc-Si) and heterojunction with intrinsic thin layer (HIT). The first category is the most common all over the world (European Photovoltaic Industry Association, 2010), with a stable and predictable performance over time; the second is an emerging technology at commercial level, with very competitive manufacturing costs (Stannowski et al., 2013, Chopra et al., 2004, Zweibel, 1999), and the third achieves a very high energy conversion efficiency among the PV products for civil uses actually available on the market (Taguchi et al., 2014, Tsunomura et al., 2009).

The paper provides the experimental monitoring and a critical analysis of different representative commercial PV modules, applied at the experimental test facility of the Politecnico di Milano University from January 1 to December 31, 2013. The experimental analysis was carried out in order to evaluate in detail the actual performance of the chosen technologies in a temperate climate, under real operating conditions.