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Selective properties of high-temperature stable spinel absorber coatings for concentrated solar thermal application
Solar Energy 199 (2020) 453–459
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Selective properties of high-temperature stable spinel absorber coatings for concentrated solar thermal application S.R. Atchutaa,b, S. Sakthivela, Harish C. Barshiliab, a b
T
⁎
Centre for Solar Energy Materials, International Advanced Research Centre for Powder Metallurgy and New Materials, Balapur PO, Hyderabad 500 005, India Academy of Scientific and Innovative Research (AcSIR), CSIR-National Aerospace Laboratories Campus, HAL Airport Road, Kodihalli, Bangalore 560017, India
ARTICLE INFO
ABSTRACT
Keywords: Solar selective absorber coating Wide angular selectiveness Thermal stability Photothermal conversion efficiency
In concentrated solar thermal (CST) system, receiver tube is one of the key important elements in the photothermal conversion process. The high photothermal efficiency of the receiver tube greatly depends upon the coating type, angular selectiveness of the coating, and radiative, conductive and convective losses. Apart from the efficiency, cost-effectiveness of the components is the major hurdle for the CST system to make it a viable technology. In this regard, we have implemented wet-chemical based spinel absorber coatings in a tandem layer approach to make the coating more selective in terms of high absorptance (95%), low emissivity (13%) and wide angular selectiveness (0 to 60°). The coatings are tested for thermal stability and corrosion resistance to check their stability in open-air atmosphere condition. Further, the photothermal conversion efficiencies of the developed coatings are calculated at different temperatures ranging from 300 to 500 °C by considering the actual thermal emissivity values of the absorber coating at that particular temperature.
1. Introduction Solar energy is one of the most reliable energy source in comparison with other renewable energy sources like biomass, wind and hydro, etc. (Li et al., 2018) The utilization of solar energy has a wide range of applications starting from the hot water generation, cooking, desalination, process heat application, electricity generation, water splitting and hydrogen generation, etc. Depending on the application, solar energy can be utilized in an efficient and economical way. At present scenario, for the production of electricity solar photovoltaic (PV) and for the process heat application (50 to 400 °C) solar thermal are the best choice of technologies available in the world (REN21, 2019). In solar thermal, different types of technologies are available to convert the solar radiation into thermal, like non-concentrated and concentrated collectors to achieve the desired temperature in the range from 50 to 800 °C. Technology like non-concentrated flat plate solar collectors can be used to get water temperatures of < 100 °C for domestic and commercial applications. Whereas, Concentrated Solar Thermal (CST) systems can attain temperatures of ≥100 °C. In CST, different types of technologies are categorized by their concentration ratios (the ratio of concentrator and receiver aperture areas) ranging from 20 to 1500 to attain temperatures in the range of 100–800 °C. Apart from this, the concentration ratio depends upon the focus type: either line or point focus. In line focus technologies, namely linear Fresnel reflectors (LFR) ⁎
and parabolic trough collectors (PTC), the concentration ratio is typically 20–100, leading to temperatures of 100–400 °C with single-axis tracking from east to west. On the other hand, for the point focus technologies like heliostat and parabolic dish, the concentration ratio varies from 1000 to 1500, leading to temperatures of 400 to 800 °C with dual-axis tracking both from east to west and north to south directions (Weinstein et al., 2015). Among all these above technologies, LFR and PTCs are the most viable CST technologies for the process heat application ranging from 100 to 400 °C. Whereas, for the other technologies like heliostat and parabolic dishes, the high temperature can be used for electricity production. However, the economic viability of the CST technologies is not yet attained as compared to solar PV electricity production (Boubault et al., 2016). Hence, from the past one decade, more research is focusing on LFR and PTC technologies for the process heat application (Manikandan et al., 2019). In both LFR and PTC, different components are involved in the system like reflective mirror, receiver tube and other optical and mechanical sub-components. From all of these components, receiver tube is one of the important components of the system where the solar radiation is converted into heat. Researchers are focusing more on this component to achieve higher photothermal conversion efficiencies and for the cost-effective process (Boubault et al., 2017). Studies have been carried out starting from black paints to vacuumdeposited coatings to achieve the higher efficiencies to the CST systems
Corresponding author. E-mail address: [email protected] (H.C. Barshilia).
https://doi.org/10.1016/j.solener.2020.02.048 Received 25 November 2019; Received in revised form 7 February 2020; Accepted 12 February 2020 0038-092X/ © 2020 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.
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(Atkinson et al., 2015). The photothermal conversion efficiency is mainly affected by the absorber coating in the receiver tube and the presence of medium between the glass envelope and absorber tube (like vacuum or non-vacuum). At the first, the coating should be selective (Chopra and Reddy, 1986) to convert the incoming solar radiation (ranging from 0.3 to 2.5 µm) into heat (2.5–25 µm). There are two factors defining the selectiveness of the coating, which are absorptance (α) and emittance (ε). Ideally to achieve the selectiveness of one, the coating should absorb all the incoming solar radiation and should not emit any thermal radiation (Olson and Talghader, 2012). But, no ideal material exists in the nature, hence the solar selective coating has selectivity of less than one. The solar absorptance can be calculated by the following Eq. (1): 2
=
1
(1
R ( )) Is d 2
1
prepared with good selectivity using wet-chemical method as discussed elsewhere (Atchuta et al., 2019a, 2019b). The present work is more focused on tandem layer approach of coatings on the spinel based absorber layers say binary (Ni and Co) and ternary (Cu, Ni and Co) metal spinels to get enhanced optical properties. Apart from the optical selectivity, the angular selectivity of each layer (single and tandem absorber layers with different spinel-based materials) has been studied to bring out the novelty of the coatings. We have also exposed the coatings for thermal stability studies in air up to 400 °C and also evaluated their corrosion behavior in accelerated ageing environment. Finally, the photothermal conversion efficiencies of all the absorber layers are measured by assuming the only loss is radiated from the absorbing surface and compared the same with different spinel absorber layer stacks at various temperatures.
I ( )d
2. Experimental details
(1)
where R(λ) denotes the total reflectance of the sample at that particular wavelength and I represents solar radiation intensity (AM 1.5, ASTM G173-03, ISO). Similarly, the spectral emittance (ε(λ, T)) can be characterized for an opaque substance in the infrared region (2.5–25 μm) by using Kirchhoff's law as following equation (2):
( , T) = 1
R ( , T)
Wet-chemical based cobaltite spinel absorber coatings, nickel cobaltite spinel (binary metal spinels) absorber coatings and nickel copper cobaltite spinel (ternary metal spinels) have been prepared as reported elsewhere (Atchuta et al., 2019a, 2019b) by dip-coating method. Here, we have achieved optical properties of α: 92% and ε: 14% for the binary metal spinel (NixCoyO4) absorber and α: 91% and ε: 14% for the ternary metal based spinel (CuxNiyCoz-x-yO4) absorber in a single layer deposition. Further, to enhance the optical properties of both the spinel absorber layers, an optical enhancement layer (OPEL) using 2 wt% SiO2 nanoparticles sol on top of the base absorber layer has been added with the help of dip coater and then films were cured at 300 °C for 1hr. UV–Vis-NIR spectrophotometer of Cary Varian Model 5000 with 110 mm diameter integrating sphere was used to measure the total absorptance of the absorber layers using the weight average calculation of Eq. (1). The spectral emittance measurements were carried out by a Bruker Vertex 70 Fourier Transform Infra-Red (FTIR) spectrometer in the infrared wavelength region (2.5–25 µm with a scanning velocity of 2.5 kHz) using standard integrating sphere. The thermal emissivity measurements were also carried out using the FTIR instrument (Bruker, VERTEX70) attached with blackbody source and high-temperature sample cell (Zhang et al., 2018) at different temperatures from 100 to 500 °C. For the measurements, sample in a temperature cell and blackbody were heated for a particular temperature by switching off the IR source. The emissions from sample and a blackbody were collected separately at a particular temperature and then calculated for the thermal emissivity, by making the ratio of sample emission with the blackbody emission. To determine the angular selectivity of the coatings, the reflectance measurements at different incident angles (from 10° to 80° with an interval of 10°) were measured by using Universal Measurement Accessory (UMA) attached with Carry Varian 5000 UV–Vis-NIR spectrophotometer. Thermal stability studies were carried out using Muffle furnace (Nabertherm). Each thermal cyclic test comprises of desired temperature of 400 °C with a heating rate of 10 °C/ min, dwell time of one hour and then sudden cooling to ambient temperature. To study the corrosion resistance, the coatings were exposed to salt spray test chamber of Ascott machine (S120ip) as per ASTM standard B117 using 5 wt% NaCl mist and 95% humidity conditions for 24 hrs. Further, the photothermal conversion efficiency of the absorber coatings was calculated by assuming only the radiative loss from the absorber surface at that particular temperature by using Eq. (3).
(2)
The photothermal conversion efficiency (ηT) of an absorber coating can be calculated by assuming that the only loss is radiation from the absorber surface (Weinstein et al., 2015) at that particular working temperature T by the following Eq. (3): T
=
4 (TH4 Tamb ) CI
(3)
where α is coating absorptance, ε is coating emissivity at that particular temperature, σ is the Stefan-Boltzmann constant (5.67 × 10−8 W m−2 K−4), TH is the absorber coating temperature, Tamb is the ambient temperature, C is the concentration ratio (for parabolic trough C = 20–80) and I denotes the solar Direct Normal Irradiance (DNI). Different types of coating technologies (viz, electrodeposition, vapour phase deposition and wet–chemical) are available for the preparation of absorber coatings (Bagheri et al., 2014; “Cathodic arc deposition of solar thermal selective surfaces,” 1996; Esposito et al., 2009; Lampert, 1987, 1979; Selvakumar and Barshilia, 2012; Youn et al., 2019). Solar selective coatings developed through vapour phase deposition techniques have been commercialized but have not attained the cost-effectiveness. Hence, the researchers have been trying from the past one decade to reduce the cost of solar selective receiver tube through different other deposition technologies. From the literature, the wet-chemical based sol-gel deposition technique is a promising method to make the CST receiver tube efficient and cost-effective (Ceratti et al., 2015; Naudin et al., 2017). In the sol-gel technique, researchers have tried different transition metal (Ti, Cr, Mn, Co, Ni and Cu) based solar selective absorber coatings (Boström et al., 2003; Li et al., 2012; Paquez et al., 2015) with the desired optical properties of α: > 95% and ε: < 20% but not the high thermal stable coatings in open-air atmosphere (> 400 °C), wide angular selectiveness and high efficient photothermal conversion (Bayón et al., 2010; Karas et al., 2018; Kim et al., 2016, 2015; Ma et al., 2016; Rubin et al., 2019). For the PTC, the optical concentration ratio is restricted by the finite solid angle of the sun in the sky on to the earth (Weinstein et al., 2015). Hence, we require an absorber coating with wide angular selectiveness (Chou et al., 2014; Hu et al., 2017; Yang et al., 2016; Zheng et al., 2015) for the receiver tube to get higher efficiencies in the CST system (Sai et al., 2003). The main aim of this study is to prepare a solar selective absorber coating with high photothermal conversion efficiency, thermal stability and corrosion resistance using a cost-effective method. In order to make this, different spinel based solar selective absorber coatings have been
3. Results and discussion 3.1. Optical properties and stability of coatings The optimized reflectance spectra of combined UV–Vis-NIR and FTIR of binary (NixCoyO4) and ternary metal based spinels (CuxNiyCoz-xyO4) with single and tandem layer approach of coatings as explained in the experimental section are compared with AM1.5 and ideal emittance 454
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Fig. 1. The combined UV–Vis-NIR and FTIR spectra of the optimized single and tandem absorber layer coatings of (a) binary metal base spinels and (b) ternary metal base spinels compared with standard solar AM1.5 spectrum and ideal emissivity.
3.2. Angular selectivity of the absorber coating
Table 1 The solar absorptance and emittance values of the optimized absorber layers with single and tandem layer approach and different type of base spinel absorber layers. Layer type
Absorptance (AM1.5) α (%)
Emittance ε (%)
Binary metal spinel absorber Binary metal spinel absorber + optical enhancement layer Ternary metal spinel absorber Ternary metal spinel absorber + optical enhancement layer
92 94
14 13
91 95
14 13
The selectivity of the absorber coating not only depends upon the optical properties of the coating at the normal incidence angle, but it also depends upon the wide angular selectivity of the coating for the effective capture of the sun light. Hence, here we have measured all four types of absorber layer configurations to see the range of angular selectivity to the absorber coatings. For this UMA attached with UV–Vis-NIR spectrophotometer was used and measured the reflectance spectra of the samples over a range of 10° to 80° with an interval of 10°. The measured reflectance graphs of the different spinel-based absorber coatings have been converted into absorptance spectra and made the contour plots (angle of incidence vs wavelength vs solar absorptance) as in Figs. 3 and 4. The calculated solar absorptance values of the coatings at different incident angles are reported in Table 4. Fig. 3(a) of the contour plot corresponds to the binary metal spinel absorber layer coating and Fig. 3(b) corresponds to tandem absorber layer of binary metal spinel based absorber and SiO2 optical enhancement layer. One can observe from the figures that, more absorption (the blue region of the contour plots) takes place in a wide range of incident angles from 10 to 60° in the active solar region of 300–1500 nm for the tandem absorber layer. This may be because of the optical interference spectra of the films and also the nanomorphology of the tandem absorber layer, which makes the light interaction high in higher angle of incidences (Shen et al., 2015; Wang et al., 2011). Form Fig. 3(c), we can infer that the angular selectivity has increased from 30° to 50° by the tandem layer approach of the absorber coating when compared with the single-layer spinel absorber coating. In the same way from Fig. 4(a)–(c), the angular selectivity of the tandem layer absorber coating has a wide range of selectivity from 10° to 60° and for the single layer of ternary metal-based spinel coating has the angular selectivity of 10° to 30°. Therefore, we conclude that the tandem layer approach of the spinel-based absorber coatings is good for a wide range of angular selectivity and is capable of achieving high concentration ratio, which leads to the high photothermal conversion efficiency to the receiver.
spectra in Fig. 1(a) and (b). The effect of tandem layer approach on the base absorber layers by using SiO2 layer and their optical properties enhancement are reported in Table 1. Thermal stability and corrosion resistance of the absorber coating are the important parameters to define the life time of the receiver tube (Chen et al., 2019; Zhang et al., 2017). The CST systems operated at temperature < 250 °C application can use non-vacuum receiver tubes with air-stable absorber coatings to reduce the system cost. Whereas, for temperature > 250 °C vacuum receiver tubes are mandatory to avoid the major radiative and conductive losses. In the latter case, even though the coating operates in vacuum environment, there is possibility of breaching the vacuum during operation and later the coating gets exposed to an air atmosphere. Hence, the absorber coating in the receiver tube should be stable in air and corrosion resistance to give maximum efficiency for the longer duration. In this regard, we have prepared two sets of samples (both single and tandem layer absorbers) with reproducibility on SS substrate and tested for thermal stability at 400 °C temperature (50 thermal cyclic tests) and for corrosion resistance in salt spray chamber as per the ASTM B117 standard (24 h). The digital images of the set of samples exposed to thermal and corrosion resistance tests are shown in the Fig. 2. And also, the optical properties for the thermal stability tested samples are measured in every 10 cycles of test and the salt spray tested samples are measured in every 6 hrs of test and the corresponding values are tabulated in the Tables 2 and 3. From the experimental observations, the coatings were well stable and no change in the optical properties was observed (Tables 2 and 3) even after performing the cross-hatch tests (ASTM standard F2452) for the thermal stability and corrosion tested samples. Hence, we can conclude that the spinel-based absorber layers are suitable for open-air atmosphere condition till 400 °C even if vacuum got breached.
3.3. Radiative thermal loss and stability of the coatings To calculate the photothermal conversion efficiency of the absorber coating, one needs to measure the radiative loss of the coating at that particular temperature (Echániz et al., 2015). For this, as mentioned in the experimental section by using the FTIR with blackbody and temperature cell, the sample and the blackbody were heated at a particular temperature and then measured the radiative emission spectra over a range of 2.5–25 μm wavelength region (Zhang et al., 2018). The collected data for both binary and ternary spinels based absorber layers 455
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Fig. 2. Optical images of the optimized solar selective absorber coatings (binary spinel, ternary spinel, tandem layer coating on binary spinel, and tandem layer coating on ternary spinel) on SS 304 substrates (a) as-deposited coatings; (b) after thermal stability test at 400 °C; (c) before salt spray; and (d) after slat spray test. Table 2 The binary metal spinel absorber coating thermal stability at 400 °C and corrosion resistance in salt spray chamber at different time intervals with their corresponding optical properties. Binary metal spinel absorber + optical enhancement layer
0 hr 10 hrs 20 hrs 30 hrs 40 hrs 50 hrs
Thermal stability at 400 °C
Corrosion resistance in salt spray
Absorptance (AM1.5) α (%)
Emittance ε (%)
Number of hrs
Absorptance (AM1.5) α (%)
Emittanceε (%)
94.2 94.1 94.2 94.2 94.1 93.9
13 13 13 13 13 12
0 1 6 12 18 24
94.2 94.3 93.9 94.1 94.0 93.8
13 13 13 13 13 14
Table 3 The ternary metal spinel absorber coating thermal stability at 400 °C and corrosion resistance in salt spray chamber at different time intervals with their corresponding optical properties. Ternary metal spinel absorber + optical enhancement layer
0 hr 10 hrs 20 hrs 30 hrs 40 hrs 50 hrs
Thermal stability at 400 °C
Corrosion resistance in salt spray
Absorptance (AM1.5) α (%)
Emittance ε (%)
Number of hrs
Absorptance (AM1.5) α (%)
Emittanceε (%)
94.9 95.1 94.7 94.5 94.6 94.3
13 13 13 13 12 12
0 1 6 12 18 24
94.9 94.9 94.8 94.6 94.2 94.1
13 13 13 13 14 14
Fig. 3. The contour plots of (a) single layer coating of binary metal based spinel; (b) tandem layer coating on binary metal based spinel absorber and (c) comparison graph for the angular selectivity of single and tandem layer coatings.
456
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Fig. 4. The contour plots of (a) single layer coating of ternary metal based spinel; (b) tandem layer coating on ternary metal based spinel absorber and (c) comparison graph for the angular selectivity of single and tandem layer coatings. Table 4 The solar absorptance values at different angles of incidence on the optimized absorber layers with single and tandem layer approach and different type of base spinel absorber layers. Layer type
Binary metal spinel absorber Binary metal spinel absorber + optical enhancement layer Ternary metal spinel absorber Ternary metal spinel absorber + optical enhancement layer
Solar absorptance at different angles (%) 10°
20°
30°
40°
50°
60°
70°
80°
90.1 94.2 91.0 94.8
90.0 94.1 90.7 94.3
89.7 93.6 90.5 94.2
87.5 93.1 87.8 94.0
82.1 91.8 83.2 92.2
74.1 82.3 76.1 89.9
61.4 72.8 63.6 79.3
42.2 49.8 45.7 56.5
Fig. 5. Radiative emission spectra of: (a) the blackbody; (b) binary metal spinel absorber layer; and (c) tandem absorber layer with binary metal spinel measured through FTIR at various temperatures.
Fig. 6. Radiative emission spectra of: (a) the blackbody; (b) ternary metal spinel absorber layers and (e) tandem absorber layer with ternary metal spinel measured through FTIR at various temperatures.
with tandem coatings (Figs. 5(a)–(c) and 6(a)–(c)) has been processed for the thermal emissivity calculation, which is the ratio of radiative emission of the sample and the blackbody at that particular temperature. The measured data for different types of absorber layers over a
wide range of temperature from 200 to 500° C with an interval of 100 °C have been presented in Table 5. From the Figs. 5 and 6 and Table 5, we can observe that the tandem layer coatings have low radiative losses as compared to the single-layer spinel-based absorber 457
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Table 5 Radiative thermal emissivity values of the optimized absorber layers with single and tandem layer approach and different types of base spinel absorber layers. Temperature (°C)
200 300 400 500
Thermal emissivity Binary metal spinel absorber
Binary metal spinel absorber + optical enhancement layer
Ternary metal spinel absorber
Ternary metal spinel absorber + optical enhancement layer
0.13 0.17 0.19 0.20
0.08 0.12 0.17 0.18
0.13 0.18 0.19 0.20
0.09 0.14 0.17 0.18
Table 6 Photothermal conversion efficiencies of the optimized absorber layers with single and tandem layer approach and different types of base spinel absorber layers. Layer type
Binary metal spinel absorber Binary metal spinel absorber + optical enhancement layer Ternary metal spinel absorber Ternary metal spinel absorber + optical enhancement layer
α
0.92 0.94 0.91 0.95
Thermal emissivity (εT)
Photothermal conversion efficiency (%)
300 °C
400 °C
500 °C
300 °C
400 °C
500 °C
0.17 0.12 0.18 0.14
0.19 0.17 0.19 0.17
0.20 0.18 0.20 0.18
90.4 92.9 89.3 93.7
88.6 90.9 87.5 92.0
85.7 88.3 84.7 89.3
increases the concentration ratio to the absorber. Here, in this case the photothermal conversion efficiencies decrease with increasing temperature from 300 to 500 °C say about 94 to 89%, considering only the radiative losses from the coating. It is to be noted that, apart from the radiative losses, there are other losses from the absorber surface like conductive and convective losses at that particular temperature during the operation of the receiver system in field condition. The receiver efficiencies are greatly increased by creating the vacuum between the absorber and glass envelope to make the conductive loss low. The other loss (convective loss) majorly depends upon the glass envelope temperature, wind speed and ambient temperature. Hence, with the good mechanical design and conditions, one can achieve the overall receiver thermal efficiency to about 70% by using the spinel based absorber materials and tandem layer approach of design to the absorber layer in a cost-effective wet-chemical method (Osorio and Rivera-Alvarez, 2019). Fig. 7. The photothermal conversion efficiencies of different absorber layers at different temperatures by considering the actual radiative losses for the corresponding temperatures.
4. Conclusion The photothermal conversion efficiency of the receiver tube mainly depends on the absorber coating type, angular selectiveness, and radiative, conductive and convective losses. Herein, we have prepared different types of spinel-based absorber layers to achieve high absorptance, low emissivity and high thermal stability. Further, to improve the selectiveness of the coating, the tandem layer approach has been chosen and an optical enhancement layer has been coated on base spinel absorber layer to achieve 95% absorption and 13% emission. The absorber coatings have been exposed to long-term thermal stability test at 400 °C in open air and corrosion resistance, and observed no change in optical properties. The stable tandem absorber layers exhibited wide angular selectiveness from 10 to 50° for binary based spinel tandem absorber and 10 to 60° for ternary based spinel tandem absorber, thus making them to attain higher concentration ratios. Apart from this, the real photothermal conversion efficiencies have been calculated by considering the radiative loss from the absorber surface and achieved about 90% of conversion efficiency for the tandem layer stacks of both binary and ternary metal spinel absorbers at 300 and 400 °C. From the observed results, the ternary spinel based tandem absorber exhibits high spectral selectivity, wide angular selectivity, low thermal emissivity and high photothermal conversion efficiency. Therefore, the tandem layer approach of ternary spinel-based absorber coating by the wet-chemical method is a good candidate for high photothermal conversion efficiency for the receiver tube in concentrated solar thermal
coatings. This may be because the optical enhancement layer of SiO2 nanoparticle acts as a barrier layer for low thermal emission and the uniform nanomorphology of the coating avoids trapping of oxygen in the pores (Heras et al., 2016). 3.4. Photothermal conversion efficiency The photothermal conversion efficiency (ηT) of an absorber coating is calculated by assuming that only the radiative loss from the absorber surface at that particular working temperature by the Eq. (3). Here, we have considered the parabolic trough concentration ratio of 80 in order to attain the temperatures of 500 °C and DNI of 800 W/m2. The conversion efficiencies of different absorber layer configuration and hightemperature stable spinel absorber coatings are calculated and tabulated in Table 6. From the Fig. 7 and Table 6, we can clearly observe that the photothermal conversion efficiencies are high for the tandem absorber layer stack (> 90% till 400 °C) compared to the single absorber layer (< 90%). Out of these two tandem layer coatings, ternary metal spinel based absorber shows higher conversion efficiency (about 1% higher) compared to binary metal spinel absorber. This is because of the improved optical properties of the tandem absorber and high angular selectivity nature of ternary tandem absorber, which indirectly 458
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Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was supported by a project of Technology Research Centre (AI/1/65/ARCI/2014) sponsored by Department of Science and Technology (DST), Govt. of India. Harish C. Barshilia also acknowledges the financial support of CEFIPRA through the project U-1-154. The authors are grateful to the Shri J.J. Jadhav, Director, CSIR-NAL, Dr. G. Padmanabham, Director, ARCI and Dr. T.N. Rao, Associate Director of ARCI for their support and encouragement. References Atchuta, S.R., Sakthivel, S., Barshilia, H.C., 2019a. Nickel doped cobaltite spinel as a solar selective absorber coating for efficient photothermal conversion with a low thermal radiative loss at high operating temperatures. Sol. Energy Mater. Sol. Cells 200, 109917. Atchuta, S.R., Sakthivel, S., Barshilia, H.C., 2019b. Transition metal based CuxNiyCoz-xyO4spinel composite solar selective absorber coatings for concentrated solar thermal applications. Sol. Energy Mater. Sol. Cells 189, 226–232. Atkinson, C., Sansom, C.L., Almond, H.J., Shaw, C.P., 2015. Coatings for concentrating solar systems – a review. Renew. Sustain. Energy Rev. 45, 113–122. Bagheri, M., Ashrafizadeh, F., Najafabadi, M.H., 2014. Black nickel coating and color anodized layers for solar absorber. Trans. Indian Inst. Met. 67, 927–934. Bayón, R., San Vicente, G., Morales, Á., 2010. Durability tests and up-scaling of selective absorbers based on copper-manganese oxide deposited by dip-coating. Sol. Energy Mater. Sol. Cells 94, 998–1004. Boström, T., Wäckelgård, E., Westin, G., 2003. Solution-chemical derived nickel-alumina coatings for thermal solar absorbers. Sol. Energy 74, 497–503. Boubault, A., Ho, C.K., Hall, A., Lambert, T.N., Ambrosini, A., 2017. Durability of solar absorber coatings and their cost-effectiveness. Sol. Energy Mater. Sol. Cells 166, 176–184. Boubault, A., Ho, C.K., Hall, A., Lambert, T.N., Ambrosini, A., 2016. Levelized cost of energy (LCOE) metric to characterize solar absorber coatings for the CSP industry. Renew. Energy 85, 472–483. Cathodic arc deposition of solar thermal selective surfaces, 1996. Sol. Energy Mater. Sol. Cells 44, 69–78. Ceratti, D.R., Louis, B., Paquez, X., Faustini, M., Grosso, D., 2015. A new dip coating method to obtain large-surface coatings with a minimum of solution. Adv. Mater. 27, 4958–4962. Chen, H., Gao, W., Liu, T., Lin, W., Li, M., 2019. An experimental study on the effect of salt spray testing on the optical properties of solar selective absorber coatings produced with different manufacturing technologies. Int. J. Energy Environ. Eng. 10, 231–242. Chopra, K.L., Reddy, G.B., 1986. Optically selective coatings. Pramana 27, 193–217. Chou, J.B., Yeng, Y.X., Lenert, A., Rinnerbauer, V., Celanovic, I., Soljačić, M., Wang, E.N., Kim, S.-G., 2014. Design of wide-angle selective absorbers/emitters with dielectric filled metallic photonic crystals for energy applications. Opt. Express 22, A144. Echániz, T., Setién-Fernández, I., Pérez-Sáez, R.B., Prieto, C., Galindo, R.E., Tello, M.J., 2015. Importance of the spectral emissivity measurements at working temperature to determine the efficiency of a solar selective coating. Sol. Energy Mater. Sol. Cells 140, 249–252. Esposito, S., Antonaia, A., Addonizio, M.L., Aprea, S., 2009. Fabrication and optimisation of highly efficient cermet-based spectrally selective coatings for high operating temperature. Thin Solid Films 517, 6000–6006. Heras, I., Krause, M., Abrasonis, G., Pardo, A., Endrino, J.L., Guillén, E., Escobar-Galindo,
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Selective properties of high-temperature stable spinel absorber coatings for concentrated solar thermal application S.R. Atchutaa,b, S. Sakthivela, Harish C. Barshiliab, a b
T
⁎
Centre for Solar Energy Materials, International Advanced Research Centre for Powder Metallurgy and New Materials, Balapur PO, Hyderabad 500 005, India Academy of Scientific and Innovative Research (AcSIR), CSIR-National Aerospace Laboratories Campus, HAL Airport Road, Kodihalli, Bangalore 560017, India
ARTICLE INFO
ABSTRACT
Keywords: Solar selective absorber coating Wide angular selectiveness Thermal stability Photothermal conversion efficiency
In concentrated solar thermal (CST) system, receiver tube is one of the key important elements in the photothermal conversion process. The high photothermal efficiency of the receiver tube greatly depends upon the coating type, angular selectiveness of the coating, and radiative, conductive and convective losses. Apart from the efficiency, cost-effectiveness of the components is the major hurdle for the CST system to make it a viable technology. In this regard, we have implemented wet-chemical based spinel absorber coatings in a tandem layer approach to make the coating more selective in terms of high absorptance (95%), low emissivity (13%) and wide angular selectiveness (0 to 60°). The coatings are tested for thermal stability and corrosion resistance to check their stability in open-air atmosphere condition. Further, the photothermal conversion efficiencies of the developed coatings are calculated at different temperatures ranging from 300 to 500 °C by considering the actual thermal emissivity values of the absorber coating at that particular temperature.
1. Introduction Solar energy is one of the most reliable energy source in comparison with other renewable energy sources like biomass, wind and hydro, etc. (Li et al., 2018) The utilization of solar energy has a wide range of applications starting from the hot water generation, cooking, desalination, process heat application, electricity generation, water splitting and hydrogen generation, etc. Depending on the application, solar energy can be utilized in an efficient and economical way. At present scenario, for the production of electricity solar photovoltaic (PV) and for the process heat application (50 to 400 °C) solar thermal are the best choice of technologies available in the world (REN21, 2019). In solar thermal, different types of technologies are available to convert the solar radiation into thermal, like non-concentrated and concentrated collectors to achieve the desired temperature in the range from 50 to 800 °C. Technology like non-concentrated flat plate solar collectors can be used to get water temperatures of < 100 °C for domestic and commercial applications. Whereas, Concentrated Solar Thermal (CST) systems can attain temperatures of ≥100 °C. In CST, different types of technologies are categorized by their concentration ratios (the ratio of concentrator and receiver aperture areas) ranging from 20 to 1500 to attain temperatures in the range of 100–800 °C. Apart from this, the concentration ratio depends upon the focus type: either line or point focus. In line focus technologies, namely linear Fresnel reflectors (LFR) ⁎
and parabolic trough collectors (PTC), the concentration ratio is typically 20–100, leading to temperatures of 100–400 °C with single-axis tracking from east to west. On the other hand, for the point focus technologies like heliostat and parabolic dish, the concentration ratio varies from 1000 to 1500, leading to temperatures of 400 to 800 °C with dual-axis tracking both from east to west and north to south directions (Weinstein et al., 2015). Among all these above technologies, LFR and PTCs are the most viable CST technologies for the process heat application ranging from 100 to 400 °C. Whereas, for the other technologies like heliostat and parabolic dishes, the high temperature can be used for electricity production. However, the economic viability of the CST technologies is not yet attained as compared to solar PV electricity production (Boubault et al., 2016). Hence, from the past one decade, more research is focusing on LFR and PTC technologies for the process heat application (Manikandan et al., 2019). In both LFR and PTC, different components are involved in the system like reflective mirror, receiver tube and other optical and mechanical sub-components. From all of these components, receiver tube is one of the important components of the system where the solar radiation is converted into heat. Researchers are focusing more on this component to achieve higher photothermal conversion efficiencies and for the cost-effective process (Boubault et al., 2017). Studies have been carried out starting from black paints to vacuumdeposited coatings to achieve the higher efficiencies to the CST systems
Corresponding author. E-mail address: [email protected] (H.C. Barshilia).
https://doi.org/10.1016/j.solener.2020.02.048 Received 25 November 2019; Received in revised form 7 February 2020; Accepted 12 February 2020 0038-092X/ © 2020 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.
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(Atkinson et al., 2015). The photothermal conversion efficiency is mainly affected by the absorber coating in the receiver tube and the presence of medium between the glass envelope and absorber tube (like vacuum or non-vacuum). At the first, the coating should be selective (Chopra and Reddy, 1986) to convert the incoming solar radiation (ranging from 0.3 to 2.5 µm) into heat (2.5–25 µm). There are two factors defining the selectiveness of the coating, which are absorptance (α) and emittance (ε). Ideally to achieve the selectiveness of one, the coating should absorb all the incoming solar radiation and should not emit any thermal radiation (Olson and Talghader, 2012). But, no ideal material exists in the nature, hence the solar selective coating has selectivity of less than one. The solar absorptance can be calculated by the following Eq. (1): 2
=
1
(1
R ( )) Is d 2
1
prepared with good selectivity using wet-chemical method as discussed elsewhere (Atchuta et al., 2019a, 2019b). The present work is more focused on tandem layer approach of coatings on the spinel based absorber layers say binary (Ni and Co) and ternary (Cu, Ni and Co) metal spinels to get enhanced optical properties. Apart from the optical selectivity, the angular selectivity of each layer (single and tandem absorber layers with different spinel-based materials) has been studied to bring out the novelty of the coatings. We have also exposed the coatings for thermal stability studies in air up to 400 °C and also evaluated their corrosion behavior in accelerated ageing environment. Finally, the photothermal conversion efficiencies of all the absorber layers are measured by assuming the only loss is radiated from the absorbing surface and compared the same with different spinel absorber layer stacks at various temperatures.
I ( )d
2. Experimental details
(1)
where R(λ) denotes the total reflectance of the sample at that particular wavelength and I represents solar radiation intensity (AM 1.5, ASTM G173-03, ISO). Similarly, the spectral emittance (ε(λ, T)) can be characterized for an opaque substance in the infrared region (2.5–25 μm) by using Kirchhoff's law as following equation (2):
( , T) = 1
R ( , T)
Wet-chemical based cobaltite spinel absorber coatings, nickel cobaltite spinel (binary metal spinels) absorber coatings and nickel copper cobaltite spinel (ternary metal spinels) have been prepared as reported elsewhere (Atchuta et al., 2019a, 2019b) by dip-coating method. Here, we have achieved optical properties of α: 92% and ε: 14% for the binary metal spinel (NixCoyO4) absorber and α: 91% and ε: 14% for the ternary metal based spinel (CuxNiyCoz-x-yO4) absorber in a single layer deposition. Further, to enhance the optical properties of both the spinel absorber layers, an optical enhancement layer (OPEL) using 2 wt% SiO2 nanoparticles sol on top of the base absorber layer has been added with the help of dip coater and then films were cured at 300 °C for 1hr. UV–Vis-NIR spectrophotometer of Cary Varian Model 5000 with 110 mm diameter integrating sphere was used to measure the total absorptance of the absorber layers using the weight average calculation of Eq. (1). The spectral emittance measurements were carried out by a Bruker Vertex 70 Fourier Transform Infra-Red (FTIR) spectrometer in the infrared wavelength region (2.5–25 µm with a scanning velocity of 2.5 kHz) using standard integrating sphere. The thermal emissivity measurements were also carried out using the FTIR instrument (Bruker, VERTEX70) attached with blackbody source and high-temperature sample cell (Zhang et al., 2018) at different temperatures from 100 to 500 °C. For the measurements, sample in a temperature cell and blackbody were heated for a particular temperature by switching off the IR source. The emissions from sample and a blackbody were collected separately at a particular temperature and then calculated for the thermal emissivity, by making the ratio of sample emission with the blackbody emission. To determine the angular selectivity of the coatings, the reflectance measurements at different incident angles (from 10° to 80° with an interval of 10°) were measured by using Universal Measurement Accessory (UMA) attached with Carry Varian 5000 UV–Vis-NIR spectrophotometer. Thermal stability studies were carried out using Muffle furnace (Nabertherm). Each thermal cyclic test comprises of desired temperature of 400 °C with a heating rate of 10 °C/ min, dwell time of one hour and then sudden cooling to ambient temperature. To study the corrosion resistance, the coatings were exposed to salt spray test chamber of Ascott machine (S120ip) as per ASTM standard B117 using 5 wt% NaCl mist and 95% humidity conditions for 24 hrs. Further, the photothermal conversion efficiency of the absorber coatings was calculated by assuming only the radiative loss from the absorber surface at that particular temperature by using Eq. (3).
(2)
The photothermal conversion efficiency (ηT) of an absorber coating can be calculated by assuming that the only loss is radiation from the absorber surface (Weinstein et al., 2015) at that particular working temperature T by the following Eq. (3): T
=
4 (TH4 Tamb ) CI
(3)
where α is coating absorptance, ε is coating emissivity at that particular temperature, σ is the Stefan-Boltzmann constant (5.67 × 10−8 W m−2 K−4), TH is the absorber coating temperature, Tamb is the ambient temperature, C is the concentration ratio (for parabolic trough C = 20–80) and I denotes the solar Direct Normal Irradiance (DNI). Different types of coating technologies (viz, electrodeposition, vapour phase deposition and wet–chemical) are available for the preparation of absorber coatings (Bagheri et al., 2014; “Cathodic arc deposition of solar thermal selective surfaces,” 1996; Esposito et al., 2009; Lampert, 1987, 1979; Selvakumar and Barshilia, 2012; Youn et al., 2019). Solar selective coatings developed through vapour phase deposition techniques have been commercialized but have not attained the cost-effectiveness. Hence, the researchers have been trying from the past one decade to reduce the cost of solar selective receiver tube through different other deposition technologies. From the literature, the wet-chemical based sol-gel deposition technique is a promising method to make the CST receiver tube efficient and cost-effective (Ceratti et al., 2015; Naudin et al., 2017). In the sol-gel technique, researchers have tried different transition metal (Ti, Cr, Mn, Co, Ni and Cu) based solar selective absorber coatings (Boström et al., 2003; Li et al., 2012; Paquez et al., 2015) with the desired optical properties of α: > 95% and ε: < 20% but not the high thermal stable coatings in open-air atmosphere (> 400 °C), wide angular selectiveness and high efficient photothermal conversion (Bayón et al., 2010; Karas et al., 2018; Kim et al., 2016, 2015; Ma et al., 2016; Rubin et al., 2019). For the PTC, the optical concentration ratio is restricted by the finite solid angle of the sun in the sky on to the earth (Weinstein et al., 2015). Hence, we require an absorber coating with wide angular selectiveness (Chou et al., 2014; Hu et al., 2017; Yang et al., 2016; Zheng et al., 2015) for the receiver tube to get higher efficiencies in the CST system (Sai et al., 2003). The main aim of this study is to prepare a solar selective absorber coating with high photothermal conversion efficiency, thermal stability and corrosion resistance using a cost-effective method. In order to make this, different spinel based solar selective absorber coatings have been
3. Results and discussion 3.1. Optical properties and stability of coatings The optimized reflectance spectra of combined UV–Vis-NIR and FTIR of binary (NixCoyO4) and ternary metal based spinels (CuxNiyCoz-xyO4) with single and tandem layer approach of coatings as explained in the experimental section are compared with AM1.5 and ideal emittance 454
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Fig. 1. The combined UV–Vis-NIR and FTIR spectra of the optimized single and tandem absorber layer coatings of (a) binary metal base spinels and (b) ternary metal base spinels compared with standard solar AM1.5 spectrum and ideal emissivity.
3.2. Angular selectivity of the absorber coating
Table 1 The solar absorptance and emittance values of the optimized absorber layers with single and tandem layer approach and different type of base spinel absorber layers. Layer type
Absorptance (AM1.5) α (%)
Emittance ε (%)
Binary metal spinel absorber Binary metal spinel absorber + optical enhancement layer Ternary metal spinel absorber Ternary metal spinel absorber + optical enhancement layer
92 94
14 13
91 95
14 13
The selectivity of the absorber coating not only depends upon the optical properties of the coating at the normal incidence angle, but it also depends upon the wide angular selectivity of the coating for the effective capture of the sun light. Hence, here we have measured all four types of absorber layer configurations to see the range of angular selectivity to the absorber coatings. For this UMA attached with UV–Vis-NIR spectrophotometer was used and measured the reflectance spectra of the samples over a range of 10° to 80° with an interval of 10°. The measured reflectance graphs of the different spinel-based absorber coatings have been converted into absorptance spectra and made the contour plots (angle of incidence vs wavelength vs solar absorptance) as in Figs. 3 and 4. The calculated solar absorptance values of the coatings at different incident angles are reported in Table 4. Fig. 3(a) of the contour plot corresponds to the binary metal spinel absorber layer coating and Fig. 3(b) corresponds to tandem absorber layer of binary metal spinel based absorber and SiO2 optical enhancement layer. One can observe from the figures that, more absorption (the blue region of the contour plots) takes place in a wide range of incident angles from 10 to 60° in the active solar region of 300–1500 nm for the tandem absorber layer. This may be because of the optical interference spectra of the films and also the nanomorphology of the tandem absorber layer, which makes the light interaction high in higher angle of incidences (Shen et al., 2015; Wang et al., 2011). Form Fig. 3(c), we can infer that the angular selectivity has increased from 30° to 50° by the tandem layer approach of the absorber coating when compared with the single-layer spinel absorber coating. In the same way from Fig. 4(a)–(c), the angular selectivity of the tandem layer absorber coating has a wide range of selectivity from 10° to 60° and for the single layer of ternary metal-based spinel coating has the angular selectivity of 10° to 30°. Therefore, we conclude that the tandem layer approach of the spinel-based absorber coatings is good for a wide range of angular selectivity and is capable of achieving high concentration ratio, which leads to the high photothermal conversion efficiency to the receiver.
spectra in Fig. 1(a) and (b). The effect of tandem layer approach on the base absorber layers by using SiO2 layer and their optical properties enhancement are reported in Table 1. Thermal stability and corrosion resistance of the absorber coating are the important parameters to define the life time of the receiver tube (Chen et al., 2019; Zhang et al., 2017). The CST systems operated at temperature < 250 °C application can use non-vacuum receiver tubes with air-stable absorber coatings to reduce the system cost. Whereas, for temperature > 250 °C vacuum receiver tubes are mandatory to avoid the major radiative and conductive losses. In the latter case, even though the coating operates in vacuum environment, there is possibility of breaching the vacuum during operation and later the coating gets exposed to an air atmosphere. Hence, the absorber coating in the receiver tube should be stable in air and corrosion resistance to give maximum efficiency for the longer duration. In this regard, we have prepared two sets of samples (both single and tandem layer absorbers) with reproducibility on SS substrate and tested for thermal stability at 400 °C temperature (50 thermal cyclic tests) and for corrosion resistance in salt spray chamber as per the ASTM B117 standard (24 h). The digital images of the set of samples exposed to thermal and corrosion resistance tests are shown in the Fig. 2. And also, the optical properties for the thermal stability tested samples are measured in every 10 cycles of test and the salt spray tested samples are measured in every 6 hrs of test and the corresponding values are tabulated in the Tables 2 and 3. From the experimental observations, the coatings were well stable and no change in the optical properties was observed (Tables 2 and 3) even after performing the cross-hatch tests (ASTM standard F2452) for the thermal stability and corrosion tested samples. Hence, we can conclude that the spinel-based absorber layers are suitable for open-air atmosphere condition till 400 °C even if vacuum got breached.
3.3. Radiative thermal loss and stability of the coatings To calculate the photothermal conversion efficiency of the absorber coating, one needs to measure the radiative loss of the coating at that particular temperature (Echániz et al., 2015). For this, as mentioned in the experimental section by using the FTIR with blackbody and temperature cell, the sample and the blackbody were heated at a particular temperature and then measured the radiative emission spectra over a range of 2.5–25 μm wavelength region (Zhang et al., 2018). The collected data for both binary and ternary spinels based absorber layers 455
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Fig. 2. Optical images of the optimized solar selective absorber coatings (binary spinel, ternary spinel, tandem layer coating on binary spinel, and tandem layer coating on ternary spinel) on SS 304 substrates (a) as-deposited coatings; (b) after thermal stability test at 400 °C; (c) before salt spray; and (d) after slat spray test. Table 2 The binary metal spinel absorber coating thermal stability at 400 °C and corrosion resistance in salt spray chamber at different time intervals with their corresponding optical properties. Binary metal spinel absorber + optical enhancement layer
0 hr 10 hrs 20 hrs 30 hrs 40 hrs 50 hrs
Thermal stability at 400 °C
Corrosion resistance in salt spray
Absorptance (AM1.5) α (%)
Emittance ε (%)
Number of hrs
Absorptance (AM1.5) α (%)
Emittanceε (%)
94.2 94.1 94.2 94.2 94.1 93.9
13 13 13 13 13 12
0 1 6 12 18 24
94.2 94.3 93.9 94.1 94.0 93.8
13 13 13 13 13 14
Table 3 The ternary metal spinel absorber coating thermal stability at 400 °C and corrosion resistance in salt spray chamber at different time intervals with their corresponding optical properties. Ternary metal spinel absorber + optical enhancement layer
0 hr 10 hrs 20 hrs 30 hrs 40 hrs 50 hrs
Thermal stability at 400 °C
Corrosion resistance in salt spray
Absorptance (AM1.5) α (%)
Emittance ε (%)
Number of hrs
Absorptance (AM1.5) α (%)
Emittanceε (%)
94.9 95.1 94.7 94.5 94.6 94.3
13 13 13 13 12 12
0 1 6 12 18 24
94.9 94.9 94.8 94.6 94.2 94.1
13 13 13 13 14 14
Fig. 3. The contour plots of (a) single layer coating of binary metal based spinel; (b) tandem layer coating on binary metal based spinel absorber and (c) comparison graph for the angular selectivity of single and tandem layer coatings.
456
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Fig. 4. The contour plots of (a) single layer coating of ternary metal based spinel; (b) tandem layer coating on ternary metal based spinel absorber and (c) comparison graph for the angular selectivity of single and tandem layer coatings. Table 4 The solar absorptance values at different angles of incidence on the optimized absorber layers with single and tandem layer approach and different type of base spinel absorber layers. Layer type
Binary metal spinel absorber Binary metal spinel absorber + optical enhancement layer Ternary metal spinel absorber Ternary metal spinel absorber + optical enhancement layer
Solar absorptance at different angles (%) 10°
20°
30°
40°
50°
60°
70°
80°
90.1 94.2 91.0 94.8
90.0 94.1 90.7 94.3
89.7 93.6 90.5 94.2
87.5 93.1 87.8 94.0
82.1 91.8 83.2 92.2
74.1 82.3 76.1 89.9
61.4 72.8 63.6 79.3
42.2 49.8 45.7 56.5
Fig. 5. Radiative emission spectra of: (a) the blackbody; (b) binary metal spinel absorber layer; and (c) tandem absorber layer with binary metal spinel measured through FTIR at various temperatures.
Fig. 6. Radiative emission spectra of: (a) the blackbody; (b) ternary metal spinel absorber layers and (e) tandem absorber layer with ternary metal spinel measured through FTIR at various temperatures.
with tandem coatings (Figs. 5(a)–(c) and 6(a)–(c)) has been processed for the thermal emissivity calculation, which is the ratio of radiative emission of the sample and the blackbody at that particular temperature. The measured data for different types of absorber layers over a
wide range of temperature from 200 to 500° C with an interval of 100 °C have been presented in Table 5. From the Figs. 5 and 6 and Table 5, we can observe that the tandem layer coatings have low radiative losses as compared to the single-layer spinel-based absorber 457
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Table 5 Radiative thermal emissivity values of the optimized absorber layers with single and tandem layer approach and different types of base spinel absorber layers. Temperature (°C)
200 300 400 500
Thermal emissivity Binary metal spinel absorber
Binary metal spinel absorber + optical enhancement layer
Ternary metal spinel absorber
Ternary metal spinel absorber + optical enhancement layer
0.13 0.17 0.19 0.20
0.08 0.12 0.17 0.18
0.13 0.18 0.19 0.20
0.09 0.14 0.17 0.18
Table 6 Photothermal conversion efficiencies of the optimized absorber layers with single and tandem layer approach and different types of base spinel absorber layers. Layer type
Binary metal spinel absorber Binary metal spinel absorber + optical enhancement layer Ternary metal spinel absorber Ternary metal spinel absorber + optical enhancement layer
α
0.92 0.94 0.91 0.95
Thermal emissivity (εT)
Photothermal conversion efficiency (%)
300 °C
400 °C
500 °C
300 °C
400 °C
500 °C
0.17 0.12 0.18 0.14
0.19 0.17 0.19 0.17
0.20 0.18 0.20 0.18
90.4 92.9 89.3 93.7
88.6 90.9 87.5 92.0
85.7 88.3 84.7 89.3
increases the concentration ratio to the absorber. Here, in this case the photothermal conversion efficiencies decrease with increasing temperature from 300 to 500 °C say about 94 to 89%, considering only the radiative losses from the coating. It is to be noted that, apart from the radiative losses, there are other losses from the absorber surface like conductive and convective losses at that particular temperature during the operation of the receiver system in field condition. The receiver efficiencies are greatly increased by creating the vacuum between the absorber and glass envelope to make the conductive loss low. The other loss (convective loss) majorly depends upon the glass envelope temperature, wind speed and ambient temperature. Hence, with the good mechanical design and conditions, one can achieve the overall receiver thermal efficiency to about 70% by using the spinel based absorber materials and tandem layer approach of design to the absorber layer in a cost-effective wet-chemical method (Osorio and Rivera-Alvarez, 2019). Fig. 7. The photothermal conversion efficiencies of different absorber layers at different temperatures by considering the actual radiative losses for the corresponding temperatures.
4. Conclusion The photothermal conversion efficiency of the receiver tube mainly depends on the absorber coating type, angular selectiveness, and radiative, conductive and convective losses. Herein, we have prepared different types of spinel-based absorber layers to achieve high absorptance, low emissivity and high thermal stability. Further, to improve the selectiveness of the coating, the tandem layer approach has been chosen and an optical enhancement layer has been coated on base spinel absorber layer to achieve 95% absorption and 13% emission. The absorber coatings have been exposed to long-term thermal stability test at 400 °C in open air and corrosion resistance, and observed no change in optical properties. The stable tandem absorber layers exhibited wide angular selectiveness from 10 to 50° for binary based spinel tandem absorber and 10 to 60° for ternary based spinel tandem absorber, thus making them to attain higher concentration ratios. Apart from this, the real photothermal conversion efficiencies have been calculated by considering the radiative loss from the absorber surface and achieved about 90% of conversion efficiency for the tandem layer stacks of both binary and ternary metal spinel absorbers at 300 and 400 °C. From the observed results, the ternary spinel based tandem absorber exhibits high spectral selectivity, wide angular selectivity, low thermal emissivity and high photothermal conversion efficiency. Therefore, the tandem layer approach of ternary spinel-based absorber coating by the wet-chemical method is a good candidate for high photothermal conversion efficiency for the receiver tube in concentrated solar thermal
coatings. This may be because the optical enhancement layer of SiO2 nanoparticle acts as a barrier layer for low thermal emission and the uniform nanomorphology of the coating avoids trapping of oxygen in the pores (Heras et al., 2016). 3.4. Photothermal conversion efficiency The photothermal conversion efficiency (ηT) of an absorber coating is calculated by assuming that only the radiative loss from the absorber surface at that particular working temperature by the Eq. (3). Here, we have considered the parabolic trough concentration ratio of 80 in order to attain the temperatures of 500 °C and DNI of 800 W/m2. The conversion efficiencies of different absorber layer configuration and hightemperature stable spinel absorber coatings are calculated and tabulated in Table 6. From the Fig. 7 and Table 6, we can clearly observe that the photothermal conversion efficiencies are high for the tandem absorber layer stack (> 90% till 400 °C) compared to the single absorber layer (< 90%). Out of these two tandem layer coatings, ternary metal spinel based absorber shows higher conversion efficiency (about 1% higher) compared to binary metal spinel absorber. This is because of the improved optical properties of the tandem absorber and high angular selectivity nature of ternary tandem absorber, which indirectly 458
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Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was supported by a project of Technology Research Centre (AI/1/65/ARCI/2014) sponsored by Department of Science and Technology (DST), Govt. of India. Harish C. Barshilia also acknowledges the financial support of CEFIPRA through the project U-1-154. The authors are grateful to the Shri J.J. Jadhav, Director, CSIR-NAL, Dr. G. Padmanabham, Director, ARCI and Dr. T.N. Rao, Associate Director of ARCI for their support and encouragement. References Atchuta, S.R., Sakthivel, S., Barshilia, H.C., 2019a. Nickel doped cobaltite spinel as a solar selective absorber coating for efficient photothermal conversion with a low thermal radiative loss at high operating temperatures. Sol. Energy Mater. Sol. Cells 200, 109917. Atchuta, S.R., Sakthivel, S., Barshilia, H.C., 2019b. Transition metal based CuxNiyCoz-xyO4spinel composite solar selective absorber coatings for concentrated solar thermal applications. Sol. Energy Mater. Sol. Cells 189, 226–232. Atkinson, C., Sansom, C.L., Almond, H.J., Shaw, C.P., 2015. Coatings for concentrating solar systems – a review. Renew. Sustain. Energy Rev. 45, 113–122. Bagheri, M., Ashrafizadeh, F., Najafabadi, M.H., 2014. Black nickel coating and color anodized layers for solar absorber. Trans. Indian Inst. Met. 67, 927–934. Bayón, R., San Vicente, G., Morales, Á., 2010. Durability tests and up-scaling of selective absorbers based on copper-manganese oxide deposited by dip-coating. Sol. Energy Mater. Sol. Cells 94, 998–1004. Boström, T., Wäckelgård, E., Westin, G., 2003. Solution-chemical derived nickel-alumina coatings for thermal solar absorbers. Sol. Energy 74, 497–503. Boubault, A., Ho, C.K., Hall, A., Lambert, T.N., Ambrosini, A., 2017. Durability of solar absorber coatings and their cost-effectiveness. Sol. Energy Mater. Sol. Cells 166, 176–184. Boubault, A., Ho, C.K., Hall, A., Lambert, T.N., Ambrosini, A., 2016. Levelized cost of energy (LCOE) metric to characterize solar absorber coatings for the CSP industry. Renew. Energy 85, 472–483. Cathodic arc deposition of solar thermal selective surfaces, 1996. Sol. Energy Mater. Sol. Cells 44, 69–78. Ceratti, D.R., Louis, B., Paquez, X., Faustini, M., Grosso, D., 2015. A new dip coating method to obtain large-surface coatings with a minimum of solution. Adv. Mater. 27, 4958–4962. Chen, H., Gao, W., Liu, T., Lin, W., Li, M., 2019. An experimental study on the effect of salt spray testing on the optical properties of solar selective absorber coatings produced with different manufacturing technologies. Int. J. Energy Environ. Eng. 10, 231–242. Chopra, K.L., Reddy, G.B., 1986. Optically selective coatings. Pramana 27, 193–217. Chou, J.B., Yeng, Y.X., Lenert, A., Rinnerbauer, V., Celanovic, I., Soljačić, M., Wang, E.N., Kim, S.-G., 2014. Design of wide-angle selective absorbers/emitters with dielectric filled metallic photonic crystals for energy applications. Opt. Express 22, A144. Echániz, T., Setién-Fernández, I., Pérez-Sáez, R.B., Prieto, C., Galindo, R.E., Tello, M.J., 2015. Importance of the spectral emissivity measurements at working temperature to determine the efficiency of a solar selective coating. Sol. Energy Mater. Sol. Cells 140, 249–252. Esposito, S., Antonaia, A., Addonizio, M.L., Aprea, S., 2009. Fabrication and optimisation of highly efficient cermet-based spectrally selective coatings for high operating temperature. Thin Solid Films 517, 6000–6006. Heras, I., Krause, M., Abrasonis, G., Pardo, A., Endrino, J.L., Guillén, E., Escobar-Galindo,
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