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Direct Solar Energy

Chapter 3

resulting technical potentials for 2050 are 1,689 EJ/yr for PV and 8,043 EJ/yr for CSP.

Analyzing the PV studies (Hofman et al., 2002; Hoogwijk, 2004; de Vries et al., 2007) and the CSP studies (Hofman et al., 2002; Trieb, 2005; Trieb et al., 2009a) assessed by Krewitt et al. (2009), the technical potential varies significantly between these studies, ranging from 1,338 to 14,778 EJ/yr for PV and 248 and 10,791 EJ/yr for CSP. The main difference between the studies arises from the allocated land area availabilities and, to some extent, on differences in the power conversion efficiency used.

The technical potential of solar energy for heating purposes is vast and difficult to assess. The deployment potential is mainly limited by the demand for heat. Because of this, the technical potential is not assessed in the literature except for REN21 (Hoogwijk and Graus, 2008) to which Krewitt et al. (2009) refer. In order to provide a reference, REN21 has made a rough assessment of the technical potential of solar water heating by taking the assumed available rooftop area for solar PV appli- cations from Hoogwijk (2004) and the irradiation for each of the regions. Therefore, the range given by REN21 is a lower bound only.

3.2.2

Regional technical potential

clear-sky irradiance and sky clearance are adopted with an assumption of maximum available land used. As Table 3.1 also indicates, the world- wide solar energy technical potential is considerably larger than the current primary energy consumption.

3.2.3

Sources of solar irradiance data

The calculation and optimization of the energy output and economical feasibility of solar energy systems such as buildings and power plants requires detailed solar irradiance data measured at the site of the solar installation. Therefore, it is essential to know the overall global solar energy available, as well as the relative magnitude of its two primary components: direct-beam irradiation and diffuse irradiation from the sky including clouds. Additionally, sometimes it is necessary to account for irradiation received by reflection from the ground and other surfaces. The details on how solar irradiance is measured and calculated can be found in the Guide to Meteorological Instruments and Methods of Observation (WMO, 2008). Also important are the patterns of seasonal availability, variability of irradiation, and daytime temperature onsite. Due to significant interannual variability of regional climate conditions in different parts of the world, such measurements must be generated over several years for many applications to provide sufficient statistical validity.

Table 3.1 shows the minimum and maximum estimated range for total solar energy technical potential for different regions, not differentiat- ing the ways in which solar irradiance might be converted to secondary energy forms. For the minimum estimates, minimum annual clear-sky irradiance, sky clearance and available land used for installation of solar collectors are assumed. For the maximum estimates, maximum annual

In regions with a high density of well-maintained ground measurements of solar irradiance, sophisticated gridding of these measurements can be expected to provide accurate information about the local solar irradi- ance. However, many parts of the world have inadequate ground-based sites (e.g., central Asia, northern Africa, Mexico, Brazil, central South America). In these regions, satellite-based irradiance measurements are

Note: Basic assumptions used in assessing minimum and maximum technical potentials of solar energy are given in Rogner et al. (2000):

Annual minimum clear-sky irradiance relates to horizontal collector plane, and annual maximum clear-sky irradiance relates to two-axis-tracking collector plane; see Table 2.2 in

WEC (1994). Maximum and minimum annual sky clearance assumed for the relevant latitudes; see Table 2.2 in WEC (1994).

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Table 3.1 | Annual total technical potential of solar energy for various regions of the world, not differentiated by conversion technology (Rogner et al., 2000; their Table 5.19).

Range of Estimates

Minimum, EJ

Maximum, EJ

181

7,410

113

3,385

25

914

4

154

199

8,655

412

11,060

372

9,528

41

994

39

1,339

116

4,135

73

2,263

1,575

49,837

3.2

101

REGIONS

North America Latin America and Caribbean Western Europe Central and Eastern Europe Former Soviet Union Middle East and North Africa Sub-Saharan Africa Pacific Asia South Asia Centrally planned Asia Pacific OECD TOTAL Ratio of technical potential to primary energy supply in 2008 (492 EJ)

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