M. Perez-Garci’a1* , J. L. Bosch1 and A. Fernandez2

1 University of Almeria (UAL), Department of Applied Physics, E-04120 Almeria, Spain
2 CIEMAT-PSA E-04200 Tabernas (Almeria), Spain
Corresponding Author, mperez@ual. es

Abstract

This work describes the configuration and the performance of a small scale device aimed to be used as educational tool to support the contents related to the interconnection modes in a PV generator as well as to visualize the effect caused by the partial shading of one or several of its cells. The device consists of three main components: a set of m-Si cells, a variable light source and a load simulator connected to general purpose electrical meter. All the elements of the device are low cost and easily available in electricity and electronics laboratories. The performance of the device allows to draw I-V curves for each one of the cells and their parallel — series combinations under controllable illumination levels. This educational kit has been developed for its use in the Laboratory of Energy and Environment of the Department of Applied Physics of the University of Almeria.

Keywords: solar cells, interconnection, mismatch, hotspot

1. Introduction

The understanding of the solar cells characteristics as electricity generators is one of the key topics in any course on photovoltaic solar energy fundamentals or applications. In this sense, the students might get used to the solar radiation and temperature dependence of the solar cells parameters and, as consequence, their energy yield in function of local climate as well as they should be also able to extract complementary information from the shape of I-V curves as, for example, that requested for optimizing PV systems performance by the application of the maximum power tracking concept.

The step from individual cells characteristics to PV modules, arrays or plants characteristics is immediate because it consists essentially in the series-parallel combination of single elements until matching the voltage and/or current levels requested for the inverters or any other final PV system component. This combination, although in practice is not usually further analyzed by the students once PV system design is concluded, must however be still carefully studied because the lack of equilibrium between interconnected parts or strings can cause a serious problem in the whole PV generator under certain conditions. The above is provoked by two main reasons [1]. The first one, intrinsic, is the finite manufacturer’s tolerance in cell characteristics and, consequently, the eventual interconnection of non exactly identical elements. In an PV array, in these circumstances, the output power of the combination is less than the sum of maximum output power of its constituents. The second reason arises due to external causes as accidental impacts provoking the partial or full opening of a string due to cell

cracking and, especially, the shadowing of a certain fraction of the generator because predictable (near buildings, trees, mast,…) or unpredictable (bird droppings or fallen leaves) sun radiation blocking.

All the losses in the PV generator performance caused by the above are called mismatch losses As greater the difference between mismatched parts in regard to the rest of the generator elements, the higher losses because the output of the entire PV generator is determined by the solar cell with the lowest output. In addition to this undesired effect, when the difference exceeds certain level, the unbalanced elements in the generator become reverse biased, acting as loads instead of generators and, if not appropriate protection exists, overheating of these parts (hot-spot effect) can arise and, in severe cases, the system can be irreversibly damaged [2].

The methods for correcting fault tolerance for the electrical mismatch consist on limiting of component malfunctions by redundant circuit design together to on site extensive modules checking. This increase the installation time and costs, specially in multi-MWp plants, but, usually, the retrieved energy justify this extra effort. On the other hand, the hot spot effect in the partially shadowed solar cell arrays is minimized considerably by installing bypass diodes connected in parallel, but with opposite polarity, to the eventually affected solar cell. Many studies have been done to optimize the number and configuration of these diodes in PV modules and generators [3-4]. The performance of by-pass diodes is based on the following: under normal operation, each solar cell will be forward biased and therefore the bypass diode will be reverse biased and will effectively be an open circuit. However, if a solar cell is reverse biased due to the a mismatch in short-circuit current between several series connected cells, then the bypass diode conducts, thereby allowing the current from the good solar cells to flow in the external circuit rather than forward biasing each good cell. Although modules manufacturers and PV systems designers are used on hot spot prevention, the new needs as bigger modules for grid connected applications and the requirement of the absence of connection box for architecturally integrated PV applications in windows or roofs, make necessary case specific configurations to protect the systems against this risk.

The effect of unbalanced cells in PV generators curves can be easily visualized by a simple a procedure and its study can provide to the students a large amount of specific skills on cells electrical performance. The equipment proposed in this work consists of a) a set of cells prepared to be manually interconnected by leads in different modes c) a set of controllable light sources, each one linked to specific cell and d) a load simulator able to provide corresponding I-V pairs to draw characteristic curve of each cell.

2. Theoretical basis