Adopting Nitrate/Nitrite Salt Mixtures as the Heat Transport Fluid in Parabolic Trough Power Plants

The present generation of commercial parabolic trough solar power plant uses a synthetic oil as the heat transport fluid in the collector field. The plants are currently operating at the upper temperature limit of the fluid, and further improvements in the solar-to-electric conversion efficiency are likely to be incremental. In contrast, adoption of a nitrate salt, or a nitrate/nitrite salt, mixture as the heat transport fluid would allow the collector field outlet temperature to increase by 50 to 100 °C, which translates into an increase in the gross Rankine cycle efficiency from the present 37.5 percent to new values in the range of 40 to 41 percent. Further, the low cost and the low vapor pressure of the candidate salt mixtures allow the heat transport fluid to also act as the storage medium in a thermal storage system. Using a salt mixture in the collector field should reduce the unit cost of thermal storage by approximately half compared to the current indirect designs. The principal, and far from minor, liability of the candidate salt mixtures are freezing points in the range of 120 °C to 220 °C. As a consequence, all salt components, including the collector field, will require some form of electric heating for freeze protection. Further, collector designs will need to be demonstrated, or developed, which are tolerant of a limited number of freeze/thaw cycles. The candidate salts are also corrosive to the current ball joint sealing materials. This paper outlines the problems which need to be solved before a commercial salt project could reasonably be considered by a project developer, the elements of a test and demonstration program to solve the problems, and the contributions which will be necessary from the salt component vendors, the project developers, and the financial community.