The freeze protection mode was intended to keep the HTF lines warm enough to prevent the fluid in the lines from solidifying. It was typically employed at night when the solar field was turned off and the TES system was not discharging heat for steam generation. Protections were also in place to keep the molten salt above its solidification point.
    HTF was circulated by the main HTF pump throughout the solar field and the lines leading to steam generation. A separate pump recirculated HTF through the TES heat exchanger. Hot salt was pumped through the exchanger to the cold salt tank to warm this HTF recycle stream. This hot HTF was bled into the HTF circulating through the solar field as necessary to offset the heat lost to the atmosphere. An equal amount of cold HTF was bled back into the HTF loop recirculating through the TES heat exchanger to keep the loops in balance. The amount of HTF in the heat exchangerloop was large in relation to the amount of the bleed so that the temperature in the exchanger will not drop below the freezing point of the salt.