Performance model and annual yield comparison of parabolic-trough solar thermal power plants with either nitrogen or synthetic oil as heat transfer fluid

• N2 is proposed as heat transfer fluid for parabolic-trough solar power plants.
• N2 is a clean fluid that can avoid some of the limitations of synthetic oils.
• 2 performance models of parabolic-trough plants with N2 or oil have been developed.
• 50 MWe parabolic-trough plants with 6 h of storage have been simulated.
• Similar annual electricity yields have been obtained for N2 and synthetic oil.

The majority of commercial parabolic-trough plants in the world operate with synthetic oil as heat transfer fluid in the solar field. However, the synthetic oils that are available at affordable cost present some challenges such as their flammability, environmental toxicity and a temperature limitation of around 400 °C. As alternative, this work proposes the use of pressurized nitrogen as heat transfer fluid. In order to analyze the feasibility of this technology, a comparison between a plant with nitrogen and a conventional plant with synthetic oil has been carried out. In both cases, 50 MWe parabolic-trough plants with 6 h of thermal storage are used as reference. A performance model including the solar field, the thermal storage system and the power block has been developed for each plant in the TRNSYS simulation software. This paper also describes the specifications, design and sizing of the solar field and explains the basic operation strategy applied in each model. Both annual simulations have been performed considering the same location, Almería (Spain), and meteorological data. In summary, the results show that similar net annual electricity productions can be attained for parabolic-trough plants with the same collection area using either nitrogen or synthetic oil as heat transfer fluid.