The most advanced thermal energy storage for solar thermal power plants is a two-tank storage system where the heat transfer fluid (HTF) also serves as storage medium. This concept was successfully demonstrated in a commercial trough plant (13.8 MWe SEGS I plant; 120 MWh(t) storage capacity) and a demonstration tower plant (10 MWe Solar Two; 105 MWh(t) storage capacity). However, the HTF used in state-of-the-art parabolic trough power plants (30-80 MWe) is expensive, dramatically increasing the cost of larger HTF storage systems. An engineering study was carried out to evaluate a concept, where another (less expensive) liquid medium such as molten salt is utilized as storage medium rather than the HTF itself. Detailed performance and cost analyses were conducted to evaluate the economic value of this concept. The analyses are mainly based on the operation experience from the SEGS plants and the Solar Two project. The study concluded that the specific cost for a two-tank molten salt storage is in the range of US$ 30-40/kWh(th) depending on storage size. Since the salt storage was operated successfully in the Solar Two project, no major barriers were identified to realize this concept in the first commercial parabolic trough power plant. (C) 2003 Elsevier Ltd. All rights reserved.