Solar radiation affects all the Earth’s processes related to the environment and plays a fundamental role in the development of human activities. Among these processes, solar radiation influences water evaporation into the atmosphere and, consequently, also humidity of ground and air. Therefore, solar radiation strongly affects the agricultural and ecological processes. The knowledge of solar radiation is also important for solar energy conversion systems, such as photovoltaic, thermal and thermosyphon applications. Finally, solar radiation determines the Earth’s energy balance and, therefore even, it is a key parameter for the understanding of the climatic change.
Solar radiation has been measured for a long time, but even today there are many unknown characteristics of its behaviour for remote areas with no direct measurement.
Along the last century, and particularly in its second half, a notably theoretical and experimental research effort has been conducted to develop solar energy conversion devices. These studies have contributed to great technological know-how in the use of solar energy and, nowadays, thermoelectric and photovoltaic solar energy production facilities are found in many countries of the world. The need of use of renewable energies, particularly in recent years, has contributed to the use of solar energy, too. The latter represents a small amount in relation to other type of energies, but a significant increase of solar facilities both thermal and photovoltaic is foreseen for the next years. Data given by the Official Energy Statistics of the U. S. Government in February 2007 [Report DOE/EIA-0383(2007) table 16] foresee a great increase in the renewable energy sector during the next years. Particularly, photovoltaic solar energy will undergo the most important increases between 2005 and 2030.
University of Jaen, Spain, e-mail: jtovar@ujaen. es
Despite the long period in solar radiation research, the most important advances took place in the last two decades. These advances triggered important improvements in the efficiency of the conversion methods. The solar energy conversion systems have a fast and non-linear response to incident radiation (Suehrcke and McCormick 1989). Therefore, the knowledge of temporal variability of solar radiation is important for the study of these systems based on thermal or photovoltaic principles, and the fluctuating nature of solar radiation has to be taken into account (Gansler et al. 1995).