ABSTRACT
Solar radiation reaches the Earth’s surface at a maximum flux density of about 10kWm−2 in a wavelength band between 0.3 and 25m. This is called short wave radiation and includes the visible spectrum. For inhabited areas, this flux varies from about 3 to 30MJm−2 day−1, depending on place, time and weather. The spectral distribution is determined by the 6000K surface temperature of the Sun. This is an energy flux of very high thermodynamic quality, from an accessible source of temperature very much greater than from conventional engineering sources. The flux can be used both thermally (e.g. for heat engines – see Chapters 5 and 6) or, more importantly, for photochemical and photophysical processes (e.g. photovoltaic power and photosynthesis – see Chapters 7 and 10). The temperatures of the Earth’s atmosphere, at about 230K, and the
Earth’s surfaces, at about 260-300K, remain in equilibrium at much less than the 6000K temperature of the Sun. Therefore the outward radiant energy fluxes emitted by the Earth’s atmosphere and surfaces are also of the order of 1kWm−2, but occur in an infrared wavelength band between about 5 and 25m, called long wave radiation, peaking at about 10m (see Wien’s law, Section 3.5.5). Consequently, the short and long wave radiation regions can be treated as quite distinct from each other, which is a powerful analytical method in environmental science. The main aim of this chapter is to calculate the solar radiation likely to be
available as input to a solar device or crop at a specific location, orientation and time. A secondary aspect is to explain the physical fundamentals associated with the atmospheric greenhouse effect and global climate change; the avoidance of which favours renewable energy. First, we discuss how much radiation is available outside the Earth’s atmosphere (Section 4.2). The proportion of this that reaches a device depends on geometric factors, such as latitude (Sections 4.4 and 4.5), and on atmospheric characteristics, such as infrared radiation absorption by water vapour, carbon dioxide and other such molecules (Section 4.6). Two final sections deal briefly with the measurement of solar radiation and with the more difficult problem of
how to use other meteorological data to estimate a solar measurement. The most basic information for solar energy devices is contained in Figures 4.7 and 4.15.
