Performance evaluation and durability studies of W/WAlSiN/SiON/SiO₂ based spectrally selective solar absorber coating for high-temperature applications: A comprehensive study on thermal and solar accelerated ageing

Highlights

• The coating exhibits a high solar absorptance of 0.955 and a low thermal emissivity of 0.157 at 500 °C

• The solar absorber coating displays good performance and durability under various accelerated ageing tests.

• The coating depicts good thermal stability up to 550 °C for 200 cycles under thermal shock test.

• The solar accelerated ageing studies carried out at various concentrated solar flux depict good thermal stability.

Abstract

We report a solar selective coating of W/WAlSiN/SiON/SiO₂ fabricated using a Four-Cathode Reactive Unbalanced Direct Current (DC) magnetron sputtering system with high thermal stability, good durability and resistance to outdoor testing conditions. The coating also exhibits superior mechanical properties (Hardness ∼12 GPa). In addition, thermal shock tests at high temperatures and solar accelerated ageing measurements are investigated in-depth to analyze the performance of the solar absorber coating. The thermal shock tests of the samples at various temperatures in the range of 500–600 °C depict the excellent thermophysical resistance of the as-deposited samples. To test the thermal durability of the solar absorber coating, we applied 200 cycles of solar accelerated ageing on the samples using a solar accelerated ageing facility with concentrated flux density varying from 50 to 250 kW/m². These cycles have been defined to replicate real high solar flux and temperature on the front side of the samples along with high cooling and heating rates, reproducing the abrupt variations of solar irradiation due to cloudy weather and subsequent thermal shocks for a given receiver. Overall, the durability tests of the solar absorber carried out under various conditions indicate a minimal change in the optical properties (Δα = 0.002 and Δε = 0), thus, making it a potential candidate for high-temperature solar thermal applications.

Introduction

The increasing energy demands for various applications using fossil fuels and deterioration of environmental conditions have channelized the researcher to use alternative renewable energy sources such as solar, wind and hydro for energy generation. Among these renewable energy sources, solar energy is the most abundantly available energy source and concentrated solar power (CSP) technologies have drawn tremendous interest in power generation (Ahmadi et al., 2018, Thappa et al., 2020). The CSP uses reflective mirrors to concentrate the solar radiation onto the receiver, either point or line focus based on the technology. The solar radiation concentrated on the receiver tube absorbs the radiation and transfers the heat to a heat transfer fluid that generates steam. The steam is passed through a steam turbine to generate electricity (Hernández Moris et al., 2021). The CSP plant utilizes the spectrally selective solar absorber coatings to convert the solar irradiation to electricity. Ideally, the spectrally selective solar absorber coatings deposited on the receiver tubes must exhibit a high solar absorptance (α ≥ 0.95) and low thermal emissivity (ε ≤ 0.05) (Cao et al., 2018, Selvakumar and Barshilia, 2012, Kennedy and Price, 2005). Thus, the selective absorber coating absorbs the solar radiation in the solar spectral range of UV–Vis–NIR and in the infrared (IR) region it reflects the thermal radiation from the surface to reduce the thermal losses (Zhang et al., 2017). A multilayered solar absorber consists of an IR layer to reflect the thermal radiation in the IR region, an absorber layer for efficient absorption of solar irradiation and an anti-reflection layer to reduce the surface reflections and act as a diffusion barrier for oxidation at elevated temperatures (Xu et al., 2020). In the past decade, several groups have reported transition metal nitrides and/or oxynitrides based multilayered solar absorber coatings due to their excellent optical properties, high oxidation resistance and thermal stability (Ibrahim et al., 2018, Xu et al., 2020). Some of the solar absorber coatings fabricated by various groups are: W/AlSiTiN_x/SiAlTiO_yN_x/SiAlO_x (AL-Rjoub et al., 2020), W/WSiAlN_x /WSiAlO_yN_x/SiAlO_x (AL-Rjoub et al., 2018), Al/NbMoN/NbMoON/SiO₂ (Song et al., 2017), AlCrSiN/AlCrSiON/AlCrO (Zou et al., 2016), TiAlC/TiAlCN/TiAlSiCN/TiAlSiCO/TiAlSiO (Jyothi et al., 2017), W/WAlN/WAlON/Al₂O₃ (Dan et al., 2019). The above mentioned spectrally selective solar absorber coatings exhibit good optical properties (α = 0.95 and ε = 0.15 @ 500 °C) and thermal stability at high temperatures.

One of the current goals for CSP receivers is to increase the durability, thermal stability, photothermal efficiency and operating temperatures of the solar absorber coating (Ahmadi et al., 2018). In this regard, a detailed study towards thermal stability, thermal shock, durability, performance validation and other outdoor tests of the coating needs to be carried out to determine the overall performance of the solar absorber. Along with optical properties of the solar absorber the studies should be extended towards their mechanical/adhesion stability, scratch resistance, environments test (under temperature and humidity). Dan et al. reported the W/WAlN/WAlON/Al₂O₃ based solar absorber coating with good stability towards humid and corrosive environments, with a nano-hardness of ∼9.6 GPa and the coating exhibits a high solar selectivity (α/ε) of 0.958/0.08 (Dan et al., 2018). Similarly, Ti/AlTiN/AlTiON/AlTiO based solar absorber coating was reported with improved corrosion resistance, adhesion, UV stability and thermal stability in air at 350 °C for 1000 h of cyclic heating conditions (Barshilia, 2014). Sallaberry et al. reported the various test conditions of ageing in different environments of solar absorbers to withstand temperatures higher than 1000 °C (Sallaberry et al., 2015). Furthermore, it is crucial to evaluate the long-term thermal stability and thermal shock test of the solar absorber at various temperatures as it will simulate the thermal shock resistance experienced by the absorber coating in real climatic conditions. The presence of different materials in the multilayer stack would experience localized thermal stresses during cyclic annealing/thermal shock, as different materials will have different expansion coefficients (Zhang et al., 2017, Abadias et al., 2018, Meindlhumer et al., 2019, Bilokur et al., 2020). Moreover, based on the thermal ageing data the activation energy of the solar absorber coating can be calculated from the relative changes in absorptance and emissivity with temperature (i.e., ln(Δα) versus 1/T slope) (Menzinger and Wolfgang, 1969, Köhl et al., 1989). According to the Arrhenius theory, the failure time at high temperatures is a function of temperature and of the activation energy of the dominant degradation process (Zhang et al., 2017). This method is a simplified way for calculating the activation energy. The higher activation energy results in higher thermal stability of the coating. Several groups have studied the influence of activation energy on the thermal degradation of solar absorber coatings and the impact on the service life predictions (Noč et al., 2019, Dinesh Kumar et al., 2015, Kaltenbach et al., 2017). In this context, Antonaia et al. designed a new graded WN-AlN cermet coating to operate at high temperatures which are stable up to 550 °C, with an activation energy of 325 kJ/mol. Theoretical calculations predicted the service life of 25 years with a 1.65% reduction in solar absorptance operating at 550 °C (Antonaia et al., 2016).

In addition to the above performance tests, solar accelerated ageing studies play a significant role in determining the degradation of solar absorbers in realistic situation. The studies provide more insight on the efficient performance of the solar absorber coatings under concentrated solar flux. An experimentally designed solar accelerated ageing facility has been reported for the first time for a parabola dish, wherein 15,000X solar radiation was concentrated on the sample surface (Boubault et al., 2012). They have also reported a numerical model for accelerated ageing and the preliminary experiments confirmed that the simulated and experimental results are in good agreement. Subsequently, these authors have used the ageing facility to investigate the influence of solar irradiance, prolonged exposure and cycle time on the performance of the solar absorber coating (A. Boubault et al., 2014). Recently, Reoyo-Prats et al. demonstrated that the concentrated solar flux (250–750 kW/m²) applied on four different sample surfaces (T91, T22, VM12 and Inconel 614 with different coatings) with high cooling and heating rates, resulted in insignificant changes in the optical properties of the sample. Moreover, replicating the drastic changes in weather conditions in a day was made possible by inducing maximum thermal stresses over a short duration of time and studying the subsequent thermal shock performance of the coating over 200 cycles (Reoyo-Prats et al., 2019).

In the present work, we report a spectrally selective solar absorber coating of W/WAlSiN/SiON/SiO₂ with excellent durability under harsh and elevated temperature environments. In addition, we report the performance evaluation studies related to adhesion, hardness, scratch resistance and stability under humid environment. The thermal shock studies of the solar absorber coating at various temperatures are discussed in detail and provide an insight of the degradation mechanisms. A comprehensive study of solar accelerated ageing tests at different concentrated solar flux densities (50–250 kW/m²) for a total of 200 cycles applied is also explored to determine the performance of the coating under realistic service conditions.