50 GW of Solar by 2020?

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Currently there are only 430 MW of concentrating solar thermal worldwide.

California and Spain are the biggest markets for these concentrating solar power systems. If renewable portfolio standards get passed in more states, we could see a much greater diversity of technologies beyond the solar trough and solar tower.

The Prometheus Institute forecasts that concentrating photovoltaic technologies will be used in midsize to large power plants that range from about 1 megawatt of production to about 100 megawatts.

Concentrating solar thermal systems, meanwhile, will dominate very large centralized power generation. __Cnet_via_Kurzweilai.net

The Bright Source solar tower heliostat design above provides efficient steam-turbine power production combined with air-cooling of steam--which is important in desert areas. Parabolic trough designs are more expensive to build and will likely require more upkeep.

Photovoltaics will continue to lag behind solar thermal in the utility scale market until better utility energy storage is perfected. For smaller, specialised plants, photovoltaics can be closer to ideal, being simpler systems with fewer mechanical moving parts. Just one breakthrough in storage--a perfected redox flow cell technology for example--would change the equation of cost/benefit very quickly.