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Y.Y. Deng et al. / Global Environmental Change 31 (2015) 239–252

� miss a part of the potential (e.g. assessing roofs, but not fac¸ades for solar photovoltaic (PV) energy systems) (Defaix, 2009; Hoogwijk, 2004; de Vries et al., 2007; Zhou et al., 2009; Denholm and Margolis, 2008), or � pertain to a single country or region, not the whole world with differences in approach or assumptions between regions (EEA, 2009; Siegfriedsen et al., 2003; DLR, 2006; Defaix, 2009; Zhou et al., 2009; Denholm and Margolis, 2008; Schwartz et al., 2010), or � cover the whole world, but do not provide sub-global detail (Jacobson and Delucci, 2011), or � focus on theoretical or technical potentials, taking only a limited number of, or no, additional barriers to implementation into account (EEA, 2009; Hoogwijk, 2004; de Vries et al., 2007), or � use low resolution datasets, potentially over- or under-estimating potential signiﬁcantly locally (Hoogwijk, 2004; de Vries et al., 2007). They commonly ﬁnd (very) large potentials: for land-based solar and wind potentials combined, results on the order of a few thousand to tens of thousands of exajoules of ﬁnal electric energy per annum are reported. In contrast to these existing studies this paper presents, for the ﬁrst time, a comprehensive, bottom-up assessment of renewable electricity potentials across the entire globe for all solar and wind electricity technologies: wind, solar photovoltaic (PV) and concentrated solar power (CSP) including land- and sea-based resources as well as building-based PV potential. The focus in this study is not on full technical potential, as is the case for many existing studies, but on an estimate of the realistic, or constrained, technical potential, which accounts for technical as well as nontechnical limitations, such as acceptance, cost, competition with other uses or remoteness. For land- and sea-based technologies, we achieve this through a detailed analysis of land use and cover in a geographic information system (GIS) with up to $1 km2 resolution, the most detailed, global analysis to date. We ﬁrst ﬁnd the technically suitable area, by successive exclusion of geographic and technical factors. In addition, we include a ﬁnal step which aims to assess a realistic future maximum availability share of all land which is technically suited for use. For building-based PV potentials we use the same technology and resource assumptions as on land, but calculate the available area on both roofs and fac¸ades at country level, starting from a building stock analysis of a representative set of reference countries. In addition, and for completeness, we present a meta-analysis of existing studies for hydro- and geothermal electricity, with extrapolations to assess realistic long-term potential. 2. Methods The overall framework to assess any solar or wind energy potential followed a three step approach by determining: a. the available area (on land, on sea, on building roofs and building fac¸ades) b. the amount of resource incident upon this area (wind speed, solar irradiation) c. the amount of energy a technology could capture of this total resource (i.e. conversion) The three steps above are then combined according to Eq. (1) to yield the overall potential estimate for three study periods: 2010, 2030 and 2070. Pð p; tÞ ¼ Að p; tÞ Á Ið p; tÞ

(1)

where P = potential in EJ per technology, for each study period; A = area in km2 per technology; I = resource intensity, i.e. potential per area, in EJ/km2, which is calculated differently for solar and wind technologies and can vary per study year; p = the study period (2010, 2030, 2070); t = the technology per category, i.e. CSP, wind on land or sea, PV on land or building roofs, or fac¸ades. The terms and steps above differ by technology (PV, CSP, wind) and category (sea, land, buildings) and are discussed in detail below. Of the two factors in Eq. (1), the ﬁrst factor, the available area, has a much larger uncertainty than the second factor, the effective resource per area. Note that the available area can also vary over time, due to land use changes, e.g. from urbanisation and deforestation. However, these changes are expected to have a much smaller bearing than other elements, most notably the ‘availability factor’ and have therefore not been taken into account here. For hydro- and geothermal electricity, the potential is much more discretely distributed across the world, as it is associated with speciﬁc localised features. These two resources were not assessed in depth, but via a meta-analysis of existing studies. 2.1. Available area Land- and sea-based resources were calculated using a geographic information system with datasets with up to 1 km Â 1 km resolution. The available area for these resources was calculated by starting with the total world surface area (146 Â 106 km2 on land and 361 Â 106 km2 on sea) and successively excluding areas which would not be suitable for use for a given technology (wind, PV, CSP). In a ﬁnal step we attempted to estimate the percentage of suitable area which would be realistically available for renewable electricity production. Note that this availability of a given type of area is the factor with the largest uncertainty. All these steps are described in detail below, ﬁrst for land-based resources, then for sea-based resources, and formalised in Eq. (2). Að p; tÞ ¼

X l

" al ðtÞ Á

X

Ai ð p; tÞ

(2)

i

where A = total available area for a given technology and period; al = availability factor per land type; Ai = suitable area in km2 in grid cell i for a given technology; p = the study period (2010, 2030, 2070); t = the technology, i.e. CSP, wind on land or sea, PV on land. 2.1.1. Land-based solar and wind resource The area available for land-based wind and solar electricity installations is restricted by the following factors. These factors have been used to estimate suitable area onshore based on the data sources in Table 1 in the following steps: � Exclusion of Antarctica: The land area of Antarctica was excluded. � Elevation: � Wind: Areas above 2000 m were excluded due to the signiﬁcantly lower power density at such elevation. � Land cover–Urban area: This was excluded for all technologies based on a combination of data on urban land cover and on population density. Note that for PV potential on buildings, a different approach, not based on GIS, was used (see below). � Land cover–Forests: � Solar: All forest areas were excluded for solar electricity production. Note that we also excluded mixed land covers (cropland or grassland with some forest) in this exclusion step.