There has been increased interest in energy derived from nuclear and solar sources recently, which has led to renewed research efforts concerning the application of high temperature molten salts in nuclear and solar power stations. At first glance, nuclear energy and solar power seem to have little in common, but certain kinds of solar power plants convert sunlight into heat which is then used to generate electricity, just like the heat generated by nuclear power plants. Both energy sources also have one main ingredient in common - salt. As a nuclear fuel, salt is useful because it is immune to radiation and can operate at near-normal pressure and relatively low temperatures. Salt is also fairly stable within the nuclear fuel cycle. Now, engineers from the U.S. Department of Energy’s (DOE) Argonne and Oak Ridge national laboratories have employed decades of nuclear research on salts to advance a solar technology called concentrated solar-thermal power (CSP).
The scientists carried out an experiment (2019) in the course of which they used chloride salts, which can be heated up to 750°c, and let it flow through the hot and cold legs of a looped pipe. They installed sensors to measure the composition of the salts at different temperatures. They also removed salt impurities, such as hydroxychlorides and oxides that promote corrosion.
Argonne’s sensors reacted to electrochemical responses in the salt, analysing its health and spotting possible problems. When salt is exposed to water, for example, hydrochloric acid forms in it, which can corrode the metal pipes in the CSP system. This corrosion can cause chromium, iron, and other structural metal ions to form in the salt, which can shorten the system’s longevity. Once the sensors detected impurities, the system automatically turned on a separate electrolysis system, which added magnesium to the salt to remove the corrosive elements.
The successful implementation of chloride salts in this experiment is the result of previous meticulous research. In 2013, scientists researched a wide range of materials and alloys to enable efficient, long-term high temperature heat transfer. The experiment was carried out in several steps: first, they identified molten salt/material combinations that had a long service life and met performance needs. Secondly, corrosion rates, corrosion mechanisms, and methods to reduce corrosion were characterized. Then, they quantified corrosion rates and determined corrosion mechanisms under isothermal and non-isothermal operation. They also researched corrosion products and predicted corrosion potentials through thermodynamic modelling. Lastly, they looked at corrosion reaction kinetics and modelled corrosion reactions in realistic systems using CFD.
In 2016, corrosion was evaluated in several alloys in eutectic 34.42 wt.% NaCl – 65.58 wt.% LiCl at 650°c and 700°c in nitrogen atmosphere. Also, electrochemical evaluations were conducted using open-circuit potential followed by a potentiodynamic polarization sweep. Corrosion rates were determined with the help of Tafel slopes and Faraday’s law. A temperature increase of as little as 50°c more than doubled the corrosion rate of AISI stainless steel 310 and Incoloy 800H compared to the initial 650°c test. These alloys showed localized corrosion. Inconel 625 was the most corrosion-resistant alloy with a corrosion rate of 2.80 ± 0.38 mm/year. For TES applications, corrosion rates with magnitudes of a few millimetres per year are counterproductive because of economic considerations. Additionally, localized corrosion (intergranular or pitting) can be catastrophic.
Chloride salts are promising HTF/TES materials due to their low prices and wide operating temperature. Increasing the operation temperatures allows for higher conversion efficiencies of the power block. Furthermore, a larger temperature difference in the TES system reduces its capital costs. These benefits can result in reduced levelized costs of electricity (LCOE). Highly abundant chloride salts (e.g. NaCl, KCl) usually have a melting point above 750°c and boiling point above 1400°c.
However, molten chloride salts are corrosive; therefore, proper materials selection for plant hardware is vital. Current CSP plants use stainless steels and nickel-base alloys as materials of construction because of the desirable combination of mechanical properties and corrosion resistance.
The next steps for scientists at Argonne and Oak Ridge are to install the prototype’s major components, operate it successfully, and complete initial testing. If this model is successful, the scientists will build their CSP system at an even larger scale, demonstrating how proven technology from one industry can be adapted to another.