Page:Quantifying a realistic, worldwide wind and solar electricity supply.pdf/8

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concentrated solar power (CSP) and solar photovoltaics (PV) on land and on buildings). We ﬁnd that of the total 146 Â 106 km2 of (non-ice covered) land, less than 5 Â 106 km2, or 3.5%, are deemed available for energy harvesting after exclusions due to elevation, land cover/use, slope, ecosystem protection, resource intensity and the additional availability factor. This stands in contrast to current (2008) total urban land area of $0.18%, and total cropland of 11% (FAO, 2013). Offshore, less than 10 Â 106 km2, or $2.2% of the 361 Â 106 km2, remain due to limitations based on sea ice, distance to shore, depth, competitive uses, jurisdiction, protection, resource intensity and the additional availability factor. The area exclusions are visualised in Fig. 2.

For buildings we ﬁnd a total of 62 Â 103 km2 of total current (2010) suitable area for PV, including roofs and fac¸ades, rising to 169 Â 103 km2 by 2070. This represents around 20% (2010) to 30% (2070) of our estimated total roof and fac¸ade space. All results for suitable and available area on land and sea are shown in Table 8. As expected, on land the assumptions on availability have a larger impact on the ﬁnal available area than the assumptions on suitability, i.e. the availability factor carries the greatest uncertainty affecting the results. It depends crucially on future societies’ attitudes towards devoting land and sea use to renewable energy, and economic competition with other uses. We have tried to estimate realistic availability factors which capture restrictions on

Fig. 2. Suitable area by technology. The suitable area on land and sea is found by successive exclusions for technical and economic reasons, using the resource intensity as a marker for economic viability. The ﬁgure shows available area before (left) and after (right) the resource exclusion step. From top to bottom: PV on land, CSP on land, Wind on land, Wind on sea. Dark/coloured denotes suitable area, grey/white denotes excluded area.