THE BEST SOLAR POWER PLANT

Towers versus troughs
Rajessh Chhabara, 4 Decemeber 2009 (CSP Today)

"The moment for solar power tower technology has arrived…[I]s trough technology about to be sidelined? …Parabolic troughs, currently the most proven technology, concentrate sunlight onto thin tubes carrying thermal oil. The heat from the hot oil is transferred to the water in a heat exchanger to produce steam, which then is used to drive turbines to generate power…

"…[P]ower tower technology[uses] a large number of heliostats or mirrors [to] concentrate sunlight onto a boiler atop a tower. The heat is used to produce very hot steam from water, which rotates the turbines to generate power…Towers have the potential to be much more efficient than troughs, because they have far higher concentration ratios (300 to 800 suns vs. 80 or so for troughs)…While troughs produce heat at around 400 degree Celsius, towers can produce up to 550 degree Celsius…[allowing the use of more efficient turbines…And towers also have the potential for more efficient storage using molten salt as their fluid]…"

eSolar's breakthrough power tower technology. (click to enlarge)

"A further advantage of tower technology is its flexibility when it comes to siting. Whereas an even or leveled land area is needed for parabolic troughs, heliostats do not need to be sited on an even surface. Tower technology can even be deployed in a moderately hilly area…[T]echnological innovations have [also] advanced solar thermal power technology…[T]he software industry has made it possible to accurately track and control hundreds of thousands mirrors…

"…[T]he main barrier to [power tower] promotion comes from the fact that only a handful of tower plants are currently operational…More commercial tower plants need to come online before the technology can be considered ‘proven’…In addition to existing solar tower projects, including Abengoa Solar’s 11MW PS10, and 20MW PS20 solar towers in Sevilla, Spain, and the 10MW Solar One tower, (later expanded and renamed as Solar Two) in California…there are a number of ambitious projects in the pipeline…[from BrightSource Energy in California, Torresol Energy in Spain and eSolar in California]…[eSolar] unveiled the company’s first [5MW] tower plant…[in 2009 and has announced projects in California, New Mexico and China of more than 3,000MW]…"

Schematic of the parabolic trough technology. (click to enlarge)

"Some observers, however, say that banks are still cautious in financing tower plants…[E]ach tower technology provider will need to build a few plants with special financing, for example with government loan guarantees, before the technology will be routinely bankable…The upcoming tower projects by Bright Source, Torresol and [eSolar] will be watched closely and their success will likely trigger investment into tower plant project development around the world.

"Will towers replace parabolic troughs in future? Observers say both technologies will co-exist as both have their own unique advantages. While towers offer higher efficiencies leading to reduction in costs, parabolic troughs come with their own set of strengths [such as modularity that allows easier capacity expansion and]… the incremental development of very large-scale plants. When it comes to scale, however, tower technology has yet to prove itself."