and Haugwitz, 2010; Ruhl et al., 2010). The possible solar cell produc- tion will also depend on the material use per Wp. Material consumption could decrease from the current 8 g/Wp to 7 g/Wp or even 6 g/Wp (which could increase delivered PV capacity from 31 to 36 to 42 GW, respec- tively), but this may not be achieved by all manufacturers.
Forecasts of the future costs of vital materials have a high-profile history, and there is ongoing public debate about possible material shortages and competition regarding some (semi-)metals (e.g., In and Te) used in thin-film cell production. In a recent study, Wadia et al. (2009) explored material limits for PV expansion by examining the dual constraints of material supply and least cost per watt for the most promising semicon- ductors as active photo-generating materials. Contrary to the commonly assumed scarcity of indium and tellurium, the study concluded that the currently known economic reserves of these materials would allow about 10 TW of CdTe or CuInS2 solar cells to be installed.
In CSP electricity generation, the solar collector field is readily scalable, and the power block is based on adapted knowledge from the existing power industry such as steam and gas turbines.The collectors themselves benefit from a range of existing skill sets such as mechanical, structural and control engineers, and metallurgists. Often, the materials or compo- nents used in the collectors are already mass-produced, such as glass mirrors.
By the end of 2010, strong competition had emerged and an increas- ing number of companies had developed industry-level capability to supply materials such as high-reflectivity glass mirrors and manufac- tured components. Nonetheless, the large evacuated tubes designed specifically for use in trough/oil systems for power generation remain a specialized component, and only two companies (Schott and Solel) have been capable of supplying large orders of tubes, with a third company (Archimedes) now emerging. The trough concentrator itself comprises know-how in both structures and thermally sagged glass mir- rors. Although more companies are now offering new trough designs and considering alternatives to conventional rear-silvered glass (e.g., polymer-based reflective films), the essential technology of concentra- tion remains unchanged. Direct steam generation in troughs is under demonstration, as is direct heating of molten salt, but these designs are not yet commercially available. As a result of its successful operational history, the trough/oil technology comprised most of the CSP installed capacity in 2010.
Linear Fresnel and central-receiver systems comprise a high level of know-how, but the essential technology is such that there is the poten- tial for a greater variety of new industry participants. Although only a couple of companies have historically been involved with central receiv- ers, new players have entered the market over the last few years. There are also technology developers and projects at the demonstration level (China, USA, Israel, Australia, Spain). Central-receiver developers are aiming for higher temperatures, and, in some cases, alternative heat
Direct Solar Energy
transfer fluids such as molten salts. The accepted standard to date has been to use large heliostats, but many of the new entrants are pursuing much smaller heliostats to gain potential cost reductions through high- volume mass production. The companies now interested in heliostat development range from optics companies to the automotive industry looking to diversify. High-temperature steam receivers will benefit from existing knowledge in the boiler industry. Similarly, with linear Fresnel, a range of new developments are occurring, although not yet as devel- oped as the central-receiver technology.
Dish technology is much more specialized, and most effort presently has been towards developing the dish/Stirling concept as a commercial product. Again, the technology can be developed as specialized compo- nents through specific industry know-how such as the Stirling engine mass-produced through the automotive industry.
Within less than 10 years prior to 2010, the CSP industry has gone from negligible activity to over 2,400 MWe either commissioned or under construction. A list of new CSP plants and their characteristics can be found at the IEA SolarPACES web site.3 More than ten different com- panies are now active in building or preparing for commercial-scale plants, compared to perhaps only two or three who were in a position to build a commercial-scale plant three years ago. These companies range from large organizations with international construction and project management expertise who have acquired rights to specific technolo- gies, to start-ups based on their own technology developed in-house. In addition, major independent power producers and energy utilities are playing a role in the CSP market.
The supply chain does not tend to be limited by raw materials, because the majority of required materials are bulk commodities such as glass, steel/aluminium, and concrete. The sudden new demand for the specific solar salt mixture material for molten-salt storage is claimed to have impacted supply. At present, evacuated tubes for trough plants can be produced at a sufficient rate to service several hundred MW per year. However, expanded capacity can be introduced readily through new fac- tories with an 18-month lead time.
Solar fuel technology is still at an emerging stage—thus, there is no supply chain in place at present for commercial applications. However, solar fuels will comprise much of the same solar-field technology being deployed for other high-temperature CSP systems, with solar fuels requiring a different receiver/reactor at the focus and different down- stream processing and control. Much of the downstream technology, such as Fischer-Tropsch liquid fuel plants, would come from existing expertise in the petrochemical industry. The scale of solar fuel dem- onstration plants is being ramped up to build confidence for industry, which will eventually expand operations.