There are multiple advantages of replacing traditional HTFs such as mineral or synthetic oil, with molten salts. “Its thermal properties enable the solar field to be operated at temperatures of up to 550ºC, making it possible to use more efficient, standardised steam turbines”, explains Carlos Marquez-Salazar, the report’s author.
Another benefit of using molten salt as the HTF in the solar field is that it eliminates the need for expensive heat exchangers. The molten salt medium has already proven to be a preferred choice for thermal energy storage for concentrating solar plants.
Other alternative HTFs highlighted in the report include compressed air and pressurized gas. “A key advantage of this approach is that it enables plants to operate at the high temperatures (around 1000oC / 1832oF) of today’s most advanced power generation cycles, such as Brayton or supercritical Rankine”, explains Salazar.
However, there are tradeoffs, warns the report, which provides detailed analysis of the direct impact on cost and performance in Parabolic Trough technology. Much thicker walled piping must be used due to the need for increased pressure. In a parabolic trough plant where there are literally miles of HTF piping, that thicker piping presents a huge cost increase over the traditional piping used.
Direct steam generation (DSG) in parabolic trough plants is yet another novel concept singled out by the report. However, DSG’s main strengths have yet to be demonstrated. A Spanish consortium comprising CIEMAT, IDAE, and Iberdrola, aims to build a 3MW plant in the municipality of Puertollano, Spain, by 2012.