Despite the sun's enormous size, and because of its distance from the earth, it is not quite a point source. It actually occupies 1/2° in the sky. When making a concentrator, the architecture of the system needs to take into account this subtended angle of the sun.
The maximum theoretical concentration of line focus is 212:1. Line focus solar thermal plants are reporting 80-100x concentration, with some claiming 112x — in other words, people are achieving about half of the maximum theoretical concentration. It's hard to get more than this because of errors in the parabolic shape, thermal expansion and shifting of parts over time, and optical alignment of all the moving parts. At these concentrations a steam turbine can be run at roughly 25% efficiency. Even with great technological advancements, the ceiling is set at a maximum of 212:1, so there is not much room for growth.
Point focus, however, has a much higher maximum concentration ratio at 44,000:1. Current technology is reaching 1,000x concentration. Despite being a small fraction of the maximum concentration ratio, point focus' concentration can run a steam turbine at anywhere from 35-50% efficiency. Also, with point focus concentration, there is a lot of room to further improve and run at higher temperatures, and thus even higher efficiencies.
Solar thermal technology faces some challenges. The most obvious is competition with abundant and inexpensive coal. Until more nations begin taxing carbon emissions, especially the United States and China, the cost of coal-fired plants will remain economical. Every ten days another coal power plant is built in the People's Republic.
Obviously, such growth will have a natural cap, but currently such expansion and consumption is severely impacting global climate change. With the development and advancement of solar thermal, the rapid growth of China and other industrializing nations will hopefully be diverted away from coal. Large scale conversion to solar around the world will not occur until solar is the cheaper alternative, and industry leaders hope to reach that point within a decade.
Such technological replacements allow the exploitation of the Leapfrog Effect, which will be an important factor in global development and emergent markets in an era facing serious climate change. The Leapfrog Effect is a principle that certain technological progressions are necessary, but only once. The end result, or product, is autonomous from all the preceding stages. For example, look at the slow transition that industrialized nations are making right now from coal to alternative energies. They all needed coal technology in order to develop new, cleaner methods of energy production. Now that these new technologies are developed, however, developing nations and emerging markets need not follow the same path, but instead could just leapfrog over coal-fired to the cleaner technologies.
> Land Requirements
This image from NASA shows the relative size of the Earth compared to the sun.
eSolar technicians configuring a point focus mirror array.