A eutectic salt high temperature phase change material: Thermal stability and corrosion of SS316 with respect to thermal cycling

Abstract

Thermal energy storage (TES) is a critical component in a concentrated solar power (CSP) plant since it is able to provide dispatchability and increase the capacity factor of the plant. Recently the Brayton power cycle using supercritical carbon dioxide (s-CO2) has attracted considerable attention as it allows a higher thermal to electric power conversion efficiency compared to the conventional Rankine cycle using subcritical steam. However, no commercial TES has yet been developed for integration with a s-CO2 based plant. One reason is the lack of a suitable storage material. This work explores the use of a eutectic NaCl-Na2CO3 salt as a reliable high temperature phase change material (PCM). The PCM has been thermally cycled up to 1000 times. Its thermo-physical properties have been measured before and after it has been subjected to the thermal cycling and its corrosion behavior has been investigated. This eutectic salt shows good thermal stability without degradation after cycling 1000 times between 600 and 650 ᵒC. The corrosion rate on stainless steel 316 (SS316) increases linearly up to 350 cycles, and thereafter it stabilizes at 70 mg/cm 2 .

Medium- and high-temperature latent heat thermal energy storage: Material database, system review, and corrosivity assessment

“…In another study, 145 the eutectic salt was found to have better thermal stability in a CO 2 environment below 700°C than in a N 2 environment. 146 BaCO…”

Development of a new methodology for validating thermal storage media: Application to phase change materials

“…Those PCMs include pure substances, eutectic mixtures, and composites that undergo either solid‐liquid or solid‐solid phase transitions. In contrast, only few stability studies have been found for PCMs intended to temperature applications above 100°C.…”

“…In order to perform cycling tests, some authors used differential scanning calorimetry (DSC), whereas the majority used set‐ups adapted and/or designed on purpose. Those set‐ups included thermostatic chambers, thermal water baths, electric hot plates with temperature control, weathering chambers, ovens with controlled heating, and, in few cases, also with a cooling control . The advantage of using a DSC apparatus for thermal cycling is that phase change temperature ( T ph‐ch ) and calculated enthalpy (Δ H ph‐ch ) are recorded directly for each cycle, so that their evolution can be monitored without interrupting the test or removing the sample.…”

“…57,60,74,75,77,80 Almost all references shown in Table 1 report on tests based on consecutive heating/cooling cycles. Only in a few cases, 28,29,32,36,55,[72][73][74] a stand-by time at constant temperature between the ramps was introduced. The usual temperature interval for the cycles was 10°C to 30°C above/below T ph-ch , and only a handful of researchers carried out tests with larger temperature intervals.…”

“…Sometimes, the use of additives 26 or the so-called water principle 53 reduced that ΔH ph-ch decrease. PCMs for high temperature applications (greater than 300°C) showed no degradation upon cycling (inorganic salts 60,[70][71][72][73] ) or a slight decrease in ΔH ph-ch (metallic alloys 47,74 ).…”