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Chapter 3

Direct Solar Energy

future technology innovations (Section 3.7).The two final sections cover cost trends (Section 3.8) and the policies needed to achieve the goals for deployment (Section 3.9). Many of the sections, such as Section 3.3, are segmented into subsections, one for each of the five solar technologies.

The solar irradiance reaching the Earth’s surface (Figure 3.1) is divided into two primary components: beam solar irradiance on a horizontal surface, which comes directly from the Sun’s disk, and diffuse irradiance, which comes from the whole of the sky except the Sun’s disk. The term ‘global solar irradiance’ refers to the sum of the beam and the diffuse components.

3.2

Resource potential

The solar resource is virtually inexhaustible, and it is available and able to be used in all countries and regions of the world. But to plan and design appropriate energy conversion systems, solar energy technolo- gists must know how much irradiation will fall on their collectors.

Iqbal (1984), among others, has described the character of solar irradi- ance, which is the electromagnetic radiation emitted by the Sun. Outside the Earth’s atmosphere, the solar irradiance on a surface perpendicular to the Sun’s rays at the mean Earth-Sun distance is practically constant throughout the year. Its value is now accepted to be 1,367 W/m² (Bailey et al., 1997).With a clear sky on Earth, this figure becomes roughly 1,000 W/m2 at the Earth’s surface. These rays are actually electromagnetic waves—travelling fluctuations in electric and magnetic fields. With the Sun’s surface temperature being close to 5800 Kelvin, solar irradiance is spread over wavelengths ranging from 0.25 to 3 µm.About 40% of solar irradiance is visible light, while another 10% is ultraviolet radiation, and 50% is infrared radiation. However, at the Earth’s surface, evaluation of the solar irradiance is more difficult because of its interaction with the atmosphere, which contains clouds, aerosols, water vapour and trace gases that vary both geographically and temporally. Atmospheric condi- tions typically reduce the solar irradiance by roughly 35% on clear, dry days and by about 90% on days with thick clouds, leading to lower average solar irradiance. On average, solar irradiance on the ground is 198 W/m2 (Solomon et al., 2007), based on ground surface area (Le Treut et al., 2007).

There are several ways to assess the global resource potential of solar energy. The theoretical potential, which indicates the amount of irradi- ance at the Earth’s surface (land and ocean) that is theoretically available for energy purposes, has been estimated at 3.9×106 EJ/yr (Rogner et al., 2000; their Table 5.18). echnical potential is the amount of solar irradiance output obtainable by full deployment of demonstrated and likely-to-develop technologies or practices (see Annex I, Glossary).

3.2.1

Global technical potential

The amount of solar energy that could be put to human use depends significantly on local factors such as land availability and meteorologi- cal conditions and demands for energy services. The technical potential varies over the different regions of the Earth, as do the assessment meth- odologies. As described in a comparative literature study (Krewitt et al., 2009) for the German Environment Agency, the solar electricity technical potential of PV and CSP depends on the available solar irradiance, land use exclusion factors and the future development of technology improve- ments. Note that this study used different assumptions for the land use factors for PV and CSP. For PV, it assumed that 98% of the technical potential comes from centralized PV power plants and that the suitable land area in the world for PV deployment averages 1.67% of total land area. For CSP, all land areas with high direct-normal irradiance (DNI)—a minimum DNI of 2,000 kWh/m2/yr (7,200 MJ/m2/yr)—were defined as suitable, and just 20% of that land was excluded for other uses. The

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Figure 3.1 | The global solar irradiance (W/m2) at the Earth’s surface obtained from satellite imaging radiometers and averaged over the period 1983 to 2006. Left panel: December, January, February. Right panel: June, July, August (ISCCP Data Products, 2006).

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