Assessment of a Molten Salt Heat Transfer Fluid in a Parabolic Trough Solar Field

TLDR

Solar electric plants in the Mojave Desert, such as SEGS, currently use a high‑temperature synthetic oil blend of biphenyl and diphenyl oxide as the heat transfer fluid. This study evaluates the feasibility of molten salt as both heat transfer fluid and thermal storage medium in a parabolic trough solar field to enhance performance and lower levelized electricity costs. The investigation examined critical issues, proposed solutions, and quantified performance and cost using preliminary inputs, comparing two candidates—solar salt (60 % NaNO₃/40 % KNO₃) and HitecXL (48 % Ca(NO₃)₂/7 % NaNO₃/45 % KNO₃). The analysis shows that a two‑tank storage system operating at 450 °C can reduce levelized electricity costs by 14.2 %, potentially reaching 17.6 % at higher temperatures, with thermocline salt storage offering even greater benefits.

Abstract

An evaluation was carried out to investigate the feasibility of utilizing a molten salt as the heat transfer fluid (HTF) and for thermal storage in a parabolic trough solar field to improve system performance and to reduce the levelized electricity cost. The operating SEGS (Solar Electric Generating Systems located in Mojave Desert, California) plants currently use a high temperature synthetic oil consisting of a eutectic mixture of biphenyl/diphenyl oxide. The scope of this investigation included examination of known critical issues, postulating solutions or possible approaches where potential problems exist, and the quantification of performance and electricity cost using preliminary cost inputs. The two leading candidates were the so-called solar salt (a binary salt consisting of 60% NaNO3 and 40% KNO3) and a salt sold commercially as HitecXL (a ternary salt consisting of 48% CaNO32, 7% NaNO3, and 45% KNO3). Assuming a two-tank storage system and a maximum operation temperature of 450°C, the evaluation showed that the levelized electricity cost can be reduced by 14.2% compared to a state-of-the-art parabolic trough plant such as the SEGS plants. If higher temperatures are possible, the improvement may be as high as 17.6%. Thermocline salt storage systems offer even greater benefits.