Dr. Alexander Los, Director of EKO INSTRUMENTS Europe B.V. talks us through the complex (yet ever-more understood) world of forecasting – crucial for an effective PV undertaking.
Forecasting wind and solar radiation is probably one of the most complex topics in the renewable energy generation process. The variable and intermittent character of renewable energy sources forms an increasing challenge for grid operators with a growing density of renewable energy production facilities. To maintain a reliable and stable electrical power grid and to operate electrical power plants most economically it is necessary to use renewable energy resource forecasts on different timescales.
When there is sunshine there is also shade – and the same is perfectly true for PV power generation systems. Even though the sun always illuminates the Earth, it remains challenging to harvest all of this energy with PV plants. The atmosphere around our planet contains suspended particles like dust, water droplets and trace gases that scatter and absorb the solar radiation before it reaches the Earth’s surface. Clouds, for example, reduce solar radiation efficiently and change radiative fluxes around the cloud area – think of the cloud shadow or the glaring, white cloud edges.
While some regions are well known for their sunny climate, making them obviously suitable for PV and in particular CPV applications, there are far more regions requiring individual investigations to clarify which PV technology should be used and how to install the PV systems[1]. Once the PV power plant has been installed and it starts producing energy, its electrical power production is directly proportional to the available solar radiation. The maximum PV power is produced under ideal weather conditions, such as clear-sky days. More often, however, the magnitude of solar radiation at the Earth’s surface changes – mainly due to clouds and aerosols. Hence, the power produced by PV installations is reduced proportionally to the cloud-induced extinction of solar radiation. Sometimes solar radiation can also exceed the clear-sky level, a phenomenon caused by multiple reflections between clouds and the Earth’s surface[2].
Figures 1a and 1b show the solar radiative flux on 9th and 12th September 2012, respectively. The 9th of September was a nice, nearly ideal clear-sky day (Fig. 1a). The radiative fluxes under such ideal conditions can – in most cases – be predicted accurately by weather forecasting services using numerical weather prediction models and local observations. Fig. 1b depicts a quite different case, where low-level stratocumulus clouds are distributed all around the sky producing shadows and enhanced solar radiation levels all day long. In such circumstances it is obviously much more difficult to forecast the cumulative daily solar energy and it is even more challenging to accurately predict the local solar radiative flux at the PV site.
