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DOI: 10.1615/AnnualRevHeatTransfer.2012004122
pages 93-129

Andrej Lenert
University of Michigan

Youngsuk Nam
Device Research Laboratory, Department of Mechanical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA ; Department of Mechanical Engineering, Kyung Hee University, Yongin 446–701, Korea

Evelyn N. Wang
Device Research Laboratory, Massachusetts Institute of Technology, Cambridge, MA 02139, USA

KEY WORDS: Solar thermal, Heat transfer fluid, Figure of merit, Direct steam, Molten salt, Ionic liquid, Nanofluid.


The choice of heat transfer fluids has significant effects on the performance, cost, and reliability of solar thermal systems. In this chapter, we evaluate existing heat transfer fluids such as oils and molten salts based on a new figure of merit capturing the combined effects of thermal storage capacity, convective heat transfer characteristics, and hydraulic performance of the fluids. Thermal stability, freezing point, and safety issues are also discussed. Through a comparative analysis, we examine alternative options for solar thermal heat transfer fluids including water−steam mixtures (direct steam), ionic liquids/melts, and suspensions of nanoparticles (nanofluids), focusing on the benefits and technical challenges.