How low must the freezing limit of a nitrate salt be? Freeze-protection cost sets the oil–salt parity boundary for cold-site parabolic-trough plants

Abstract

Molten nitrate salt lets parabolic-trough plants deliver heat at 540 °C and store it directly, but at cold sites the receiver inventory must be kept above its freezing limit whenever the field is not collecting. This study asks how low that limit must be for a direct Solar-Salt plant to match a thermal-oil plant with indirect salt storage. A one-loop equivalent plant at Golmud, China (2801 m; annual DNI 1898 kWh m⁻²) is simulated hourly with a segmented receiver enthalpy model, an explicit storage ledger, three receiver-maintenance policies and separate grid import and export accounts. With steady electric maintenance, salt needs 206 MWh of heating electricity per year against 6 MWh for oil; its net-energy levelized cost rises from 739 to 906 CNY MWh⁻¹, above the 846 CNY MWh⁻¹ of oil. A fixed-weather pressure experiment changes annual output by only 0.1%, so the penalty comes from the freeze-protection burden rather than from air density. With storage re-optimised over hourly candidates, salt reaches parity once its minimum operating temperature falls to 196.5 °C. Across 98 storage-cost and accounting combinations, 61 give a parity temperature between 90 and 240 °C, and the cost-optimal storage moves between 0 and 7 h. A ternary-salt-like window of 170–500 °C gives 804 CNY MWh⁻¹, 4.7% below oil, which would tolerate a salt price of about 10.8 CNY kg⁻¹ in the storage inventory. The results give salt developers a quantitative freezing-limit target and show which operating and accounting assumptions move it.