Freeze-protection economics can reverse the heat-transfer-fluid preference for cold high-altitude parabolic-trough concentrating solar power: a four-dimensional (4E) and thermal-storage market-value assessment

Abstract

Heat-transfer-fluid (HTF) selection for parabolic-trough concentrating solar power is usually settled at sea level, yet a growing share of new capacity sits on high plateaus where reduced air density and low temperatures reshape both heat loss and freeze protection. We assess molten salt (Solar Salt) against thermal oil (Therminol VP-1) for a parabolic-trough plant at Golmud (2801 m, Qinghai) using a four-dimensional (energy, exergy, economic, environmental) framework coupled to a thermal-storage market-value model with time-of-use pricing. Altitude-dependent air properties are propagated through the receiver heat-loss model, molten-salt freeze parasitics, and component-level exergy. The direct altitude effect on collector heat loss is small (~0.2%), because reduced convective loss is largely offset by increased radiative loss; freeze protection, by contrast, is the dominant controllable penalty for molten salt, requiring roughly 20 times the maintenance power of thermal oil. Under a CNY cost basis anchored to recent Chinese large-scale CSP data, an idealized clear-sky comparison reproduces the conventional preference for molten salt (levelized cost 480 versus 523 CNY/MWh at each fluid's optimal storage). But when the freeze penalty is charged on realistic typical-meteorological-year weather, the ranking reverses: the salt's levelized cost rises to about 1040 CNY/MWh against 870 for the oil, because cold-weather freeze protection and lower net output erase the storage-cost advantage. The decisive driver is therefore not elevation-induced air-density change by itself but freeze-protection economics under cold plateau weather; mitigating that penalty — for example with lower-melting salts — is the key route to restoring molten salt's storage advantage.