4.3.3.5 Solar thermal electric
The proportion of solar radiation that reaches the Earth’s surface is more than 10,000 times the current annual global energy consumption. Annual surface insolation varies with latitude, ranging between averages of 1000 W/m2 in temperate regions and 1200 W/m2 in low-latitude dry desert areas.
Concentrating solar power (CSP) plants are categorized according to whether the solar flux is concentrated by parabolic trough-shaped mirror reflectors (30–100 suns concentration), central tower receivers requiring numerous heliostats (500–1000 suns), or parabolic dish-shaped reflectors (1000–10,000 suns). The receivers transfer the solar heat to a working fluid, which, in turn, transfers it to a thermal power-conversion system based on Rankine, Brayton, combined or Stirling cycles. To give a secure and reliable supply with capacity factors at around 50% rising to 70% by 2020 (US DOE, 2005), solar intermittency problems can be overcome by using supplementary energy from associated natural gas, coal or bioenergy systems (IEA, 2006g) as well as by storing surplus heat.
Solar thermal power-generating plants are best sited at lower latitudes in areas receiving high levels of direct insolation. In these areas, 1 km2 of land is enough to generate around 125 GWh/yr from a 50 MW plant at 10% conversion of solar energy to electricity (Philibert, 2004). Thus about 1% of the world’s desert areas (240,000 km2), if linked to demand centres by high-voltage DC cables, could, in theory, be sufficient to meet total global electricity demand as forecast out to 2030 (Philibert, 2006; IEA, 2006b). CSP could also be linked with desalination in these regions or used to produce hydrogen fuel or metals.
The most mature CSP technology is solar troughs with a maximum peak efficiency of 21% in terms of conversion of direct solar radiation into grid electricity. Tower technology has been successfully demonstrated by two 10 MW systems in the USA with commercial development giving long-term levelized energy costs similar to trough technology. Advanced technologies include troughs with direct steam generation, Fresnel collectors, which can reduce costs by 20%, energy storage including molten salt, integrated combined-cycle systems and advanced Stirling dishes. The latter are arousing renewed interest and could provide opportunities for further cost reductions (WEC, 2004d; IEA 2004b).
Technical potential estimates for global CSP vary widely from 630 GWe installed by 2040 (Aringhoff et al., 2003) to 4700 GWe by 2030 (IEA, 2003h; Table 4.2). Installed capacity is 354 MWe from nine plants in California ranging from 14 to 80 MWe with over 2 million m2 of parabolic troughs. Connected to the grid during 1984–1991, these generate around 400 GWh/yr at 100–126 US$/MWh (WEC, 2004d). New projects totalling over 1400 MW are being constructed or planned in 11 countries including Spain (500 MW supported by a new feed-in tariff) (ESTIA, 2004; Martinot et al., 2005) and Israel for the first of several 100 MW plants (Sagie, 2005). The African Development Bank has financed a 50 MW combined-cycle plant in Morocco that will generate 55 GWh/yr, and two new Stirling dish projects totalling 800 MWe planned for the Mojave Desert, USA (ISES, 2005) are estimated to generate at below 90 US$/MWh (Stirling, 2005). Installed capacity of 21.5 GWe, if reached by 2020, would produce 54.6 TWh/yr with a further possible increase leading towards 5% coverage of world electricity demand by 2040.