Photovoltaic plant cloud shadowing and energy drops in Northern Europe

In this paper the effect of shadowing due to clouds on the energy production of a photovoltaic power plant installed in northern latitudes in Finland is analyzed. Numerical techniques, which can be also implemented on an embedded system because of their low computational and memory requirements, have been used for having an efficient processing of millions of experimental current vs. voltage curves acquired on the photovoltaic plant. The analysis allows to affirm that, at high latitudes, clouds can have a significant instantaneous effect on the power produced by the plant, but the time duration of the power drop is so short that the energy production is only marginally affected. This result is in full agreement with other studies appeared in the recent literature and suggests that, in such contexts, control electronics producers should avoid to implement related maximum power point tracking techniques for facing mismatched conditions, since their effect on energy production appears to be negligible.

I. Introduction

Mismatching effects are the main source of missed energy production from a photovoltaic (PV) field. They can be due to soiling, temperature differences among the cells due to the effect of the wind, bird droppings, cells damages, but the main source of mismatching is the shadowing [1]. The relationship between the shadowed PV array area and the output power drop is highly non-linear, which means that even a small fraction of shadowed PV array area causes a significant reduction of the power at the array terminals. Shadowing may be due to two main causes, which are clouds and obstacles. The latter ones create sharp shadows that repeat daily, with some change due to the position of the Sun along the year. Such shadows move slowly on the array and they can affect the area of the plant for a long time, depending on the layout of modules installation, day of the year etc. Shadows due to clouds, instead, have a gently sloping irradiance transition region compared to the one caused by obstacles, and they move on the modules with a speed that depends on the weather conditions [2].