High-temperature optical properties and stability of selective absorbers based on quasicrystalline AlCuFe

Solar selective absorbers based on AlCuFe thin films and a cermet of AlCuFe particles embedded in an Al₂O₃-matrix were produced by sputter deposition. Si-wafers coated with copper layers were used as substrates. According to X-ray diffraction and high-resolution transmission electron microscopy (HRTEM), the AlCuFe layers are quasicrystalline or consist of a mixture of quasicrystalline and crystalline phases. Reflectance of the selective absorber based on the AlCuFe cermet was measured in the wavelength region 400–1700 nm at temperatures up to 395°C with the absorber held in high vacuum. Changes of reflectance with temperature can mostly be assigned to changes of the optical properties of the copper layer. Stability at high temperatures in air was studied by heating the absorbers to 400°C for different time intervals up to 450 h. Only small changes in optical properties were found during this heat treatment, while rapid degradation within about 40 h occured at 500°C. According to elastic recoil detection analysis (ERDA), samples degraded by diffusion of copper and silicon to the surface and their subsequent oxidation.     

Computer modeling of the performance of some metal/dielectric multilayers for high-temperature solar selective absorbers

Solar thermal performance of metal/dielectric multilayer coatings of various material combinations was calculated using a computer program. The simulated material combinations were selected with respect to their use in high-temperature (T ≥ 500°C) solar thermal energy conversion. Maximum solar absorptances α = 0.94 with low emittances = 0.16 (T = 1100 K) were calculated for Al₂0₃/noble metal (Rh, Ir) coatings. Simulations on multilayers of noble metal oxides, like RuO₂ and IrO₂, were carried out to optimize solar absorption a using optical data in the region from 300 nm to 2.3 μm. Absorptances as high as α = 0.96 were obtained for both systems.     

Optical, thermal and structural characteristics of carbon nanoparticles embedded in ZnO and NiO as selective solar absorbers

Selective solar absorber coatings of carbon embedded in ZnO and NiO matrices on aluminium substrates have been fabricated by a sol–gel technique. Spectrophotometry was used to determine the solar absorptance and the thermal emittance of the composite coatings. The surface morphology of the samples was studied by scanning electron microscopy (SEM). Cross-sectional high-resolution transmission electron microscopy (X-HRTEM) was used to study the fine structure of the samples. Chemical composition analysis was done by energy dispersive spectroscopy (EDS) and electron energy loss spectroscopy (EELS). The crystal structure of ZnO and NiO samples was also investigated with an X-ray diffraction (XRD) technique. Samples were subjected to an accelerated ageing test for 95 h, with condensation at relative humidity of 95% and at a climate chamber temperature of 45 °C. The thermal emittances of the samples were 6% for the ZnO and 4% for the NiO matrix materials. The solar absorptances were 71% and 84% for ZnO and NiO samples, respectively. The SEM revealed a smooth featureless surface for both C–ZnO and C–NiO samples. Some C–NiO samples showed dentritic features. X-HRTEM, EDS and EELS studies revealed a nanometric grain size for both types of samples. The C–ZnO and C–NiO coatings contained amorphous carbon embedded in nanocrystalline ZnO and NiO matrices, respectively. Selected area electron diffraction (SAED) showed that a small amount of Ni grains of 30 nm diameter also existed in the NiO matrix. The accelerated ageing tests produced performance criterion (PC) values of 0.15 and 0.054 at 95 h for the C–ZnO and C–NiO samples, respectively. Based on these results, C–NiO samples proved to have better solar selectivity behaviour than the C–ZnO counterparts.     

Optical properties of dense zirconium and tantalum diborides for solar thermal absorbers

Ultra-high temperature ceramics (UHTCs) are interesting materials for a large variety of applications under extreme conditions. This paper reports on the production and extensive characterization of highly dense, pure zirconium and tantalum diborides, with particular interest to their potential utilization in the thermal solar energy field. Monolithic bulk samples are produced by Spark Plasma Sintering starting from elemental reactants or using metal diboride powders previously synthesized by Self-propagating High-temperature Synthesis (SHS). Microstructural and optical properties of products obtained by the two processing methods have been comparatively evaluated. We found that pure diborides show a good spectral selectivity, which is an appealing characteristic for solar absorber applications. No, or very small, differences in the optical properties have been evidenced when the two investigated processes adopted for the fabrication of dense TaB₂ and ZrB₂, respectively, are compared.     

Cathodic arc deposition of solar thermal selective surfaces

There are some advantages of cathodic arc deposition techniques over other thin film deposition methods. In this paper we discuss and demonstrate the possibility of applying the cathodic arc deposition technology to the production of solar energy conversion devices, particularly solar thermal selective surfaces. We show that cathodic arc deposition can be operated in a plasma assisted chemical vapour deposition mode as well as reactive deposition mode to produce metal-dielectric cermet materials. Solar thermal selective surfaces were obtained using this method. The morphology of the surfaces can be varied from rough to smooth by controlling the macroparticles content by means of filtering. Rough but dense films can be deposited, which may have potential for enhancing solar absorption for both solar thermal selective surfaces and solar cells.     

Computations of the optical properties of metal/insulator-multilayers for solar thermal applications

A computer program has been developed to calculate the optical properties of metal/ insulator-multilayer systems from complex refractive indices of corresponding bulk materials. In addition, interdiffusion in multilayer structures could be detected by fitting experimental reflectance spectra. Among M/Al₂O₃-systems, M = (Ta, Mo, W, orPt), optimum multilayer structures were evaluated as selective absorbers for solar thermal applications at various operating temperatures. The influence of the kind of metal, the modulation number, and the layer thicknesses on the solar thermal performance were studied. At temperatures of T_B ≈ 1100 K and 100-fold sunlight concentration Mo/Al₂0₃- and W/Al₂O₃-multilayers with a modulation number of 6.5 and metal layer thicknesses of a few nm exhibited an absorptance of merit up to A_m ≈ 0.8. Lower temperatures favoured Pt-systems (A_m ≈ 0.94 at 400 K) and higher temperatures Ta-systems (A_m ≈ 0.72 at 1400 K).     

Optical constants of sputtered Ni/NiO solar absorber film—depth-profiled characterization

An optical analysis of a solar absorber with a graded index coating of nickel–nickel oxide deposited in a continuous process for reactive DC magnetron sputtering is reported. The optical constants were determined by a set of 14 samples deposited under the same sputtering conditions as in the continuous process representing the composition at 14 different depths in the continuously graded index coating solar absorber. The optical constants of the 14 layers were derived from optical measurements of reflectance and transmittance in the wavelength range 300–2500 nm. The optical constants were used to construct a model for the graded refractive index in the solar-absorbing coating. It was found that the graded index was not linear in depth profile and hence could be modelled with four layers: two metallic layers in the base closest to the substrate, a dielectric layer at the top of the coating and one intermediate layer representing the change from metallic to dielectric properties in the middle part of the coating.     

Spectrally selective Al/AlN/Al/AlN tandem solar absorber by inline reactive ac magnetron sputtering

Abstract

Spectrally selective Al/AlN/Al/AlN tandem solar absorbers were deposited onto soda lime glass substrates using inline ac magnetron sputtering in a reactive atmosphere containing argon and nitrogen. To achieve a reproducible and homogenous deposition process, the deposition of multilayer Al/AlN/Al/AlN films under different process conditions was investigated. Two main variables, ac power and the speed of substrate movement in the chamber, were varied in the ranges of 1–10 kW and 10·47–31·4 mm s^?1 respectively to obtain films with high absorption and low emittance. The effects of film thickness of different layers and deposition conditions on the optical performance of selective coatings were also studied. Structural features and surface morphology of the films were investigated by X-ray diffraction and field emission scanning electron microscopy analyses respectively. In the present study, tandem solar absorber films deposited onto glass substrate with optimised sputtering parameters can consistently achieve solar absorptance α of 0·864 and thermal emittance ? of 0·03 at 80°C.     

XPS characterization of Co and Cr pigmented copper solar absorbers

XPS is used to characterize the chemical state of Co and Cr on the surfaces of electrochemically pigmented copper plates. Using both the measured 2p binding energies and the magnitude of the 3s multiplet splitting chemical states of 2+ and 3+ are assigned to Co and Cr, respectively. FTIR reflectance analyses reveal that the surfaces of the pigmented samples contain hydrogen bonded OH groups. Optimum solar absorbance and thermal emittance values obtained are α=0.92 and =0.17, α=0.96 and =0.04 for the Co and Cr pigmented copper collectors, respectively.     

Life expectancy prediction and application properties of novel polyurethane based thickness sensitive and thickness insensitive spectrally selective paint coatings for solar absorbers

A novel Thickness Sensitive Spectrally Selective (TSSS PU B: a_s=0.90, e_T=0.20) paint coating on aluminium substrate was prepared from commercially available polyurethane binder (Binder B) (HELIOS TBLUS, SI) and black pigment (spinel (Mn–Fe)), in combination with trisilanol polyhedral oligomeric silsesquioxane (POSS), which served as pigment dispersant. Polyurethane resin binder B was selected because of its higher thermal stability (determined from thermogravimetric measurements (TG)) than polyurethane resin binder A, which has previously been used for making Thickness Insensitive Spectrally Selective (TISS PU A) coatings (a_s=0.90, e_T=0.38) deposited on copper absorbers (Kuni?, 2009 [36]).Thermal degradation of the TSSS PU B and TISS PU A coatings, both deposited on aluminium substrates, was studied by following, as close as possible, the methodology worked out within TASK 10 of the IEA's Solar and Heating Programme. Thermal load tests were performed in the temperature range from 170 to 200 °C at various time intervals (1, 6, 10, 15, 21 days). Degradation of the coatings was assessed using a variety of degradation indicators: changes of solar absorptance and thermal emittance determined from the hemispherical IR and VIS/NIR spectra, intensity changes of selected vibrational modes attributed to the polymeric backbone and ester and urethane linkages and combined with peel-off tests used as adhesion and cohesion indicators. The results revealed that degradation of the polyurethane resin binder was attributable to the breaking of the urethane linkages, also shown from the AFM and XPS spectra measurements. For the TISS PU A coating, the life expectancy was estimated to be 22.77 years (activation energy (E_a)=163.2 kJ/mol, T_eff=113.4 °C) while for the TSSS PU B coatings, it was at least 25.96 years (activation energy (E_a)=96 kJ/mol, T_eff=102 °C).     

ERDA of Ni-Al2O3/SiO2 solar thermal selective absorbers

Thin film materials for the use in solar thermal absorbers have been investigated using time-of-flight energy elastic recoil detection analysis (ERDA). The ERDA measurements proved to be very efficient in detecting the elemental depth composition of a selective solar absorber. The three-layer absorber is composed of an 80% nickel-20% alumina film at the base, a 40% nickel-60% alumina layer in the middle and finally an AR film of silica or hybrid-silica film at the top. The difference between solution volume percent and actual volume percent could be investigated when studying individual nickel-alumina films with varying ratios coated on glass substrates. The result showed that there was a maximum difference of 3% between the calculated solution volume percent and the actual volume percentages in the solid films. The ERDA measurements also indicate that about 15% of the nickel found in the nickel-alumina composite films is bound in the form of NiO.     

Computer simulation of the optical properties of high-temperature cermet solar selective coatings

A computer simulation is developed to calculate the solar absorptance and thermal emittance of various configurations of cermet solar selective coatings. Special attention has been paid to those material combinations, which are commonly used in high-temperature solar thermal applications. Moreover, other material combinations such as two-, three- and four-cermet-layer structures as solar selective coatings have been theoretically analyzed by computer simulation using three distinct physical models of Ping Sheng, Maxwell–Garnett and Bruggeman. The novel case of two-cermet-layer structure with different cermet components has also been investigated. The results were optimized by allowing the program to manipulate the metal volume fraction and thickness of each layer and the results compared to choose the best possible configuration. The calculated results are within the range of 0.91–0.97 for solar absorptance and 0.02–0.07 for thermal emittance at room temperature.     

Thermal emittance of sputter deposited infrared reflectors in spectrally selective tandem solar absorbers

The thermal emittance of infrared reflectors, deposited by DC-magnetron sputtering on glass substrates of spectrally selective solar absorbers was studied. The deposition process was optimized in order to decrease the thermal emittance of the absorber. The sputter deposition process was optimized with regard to applied power and argon pressure for nickel–vanadium, copper–nickel and copper. The results show that the thermal emittance of the infrared reflector in a tandem solar absorber can be reduced from 0.12 to 0.06 by replacing nickel–chromium by copper–nickel. The copper–nickel alloy has a higher deposition rate and is less sensitive to the sputtering conditions, which is also favorable in large-scale industrial production.     

Optimisation of metal sputtered and electroplated substrates for solar selective coatings

This study describes the performance of selective coatings to have maximum solar absorptance and minimum thermal emittance, in relation to substrate preparation. Aluminium and copper substrates, covered with sputtered or electroplated metal base layer, have been used to see the influence of different types of substrates for solar selective coatings. The effect of the base layer material, thickness, deposition process and deposition condition, on the optical performance of selective coatings has been analysed. Nickel was electroplated and nickel and vanadium were sputtered as a base layer on the Al and Cu substrates. A comparison of plated and sputtered nickel substrates for Ni:SiO₂ and V:Al₂O₃ composite solar selective coatings is presented. Theoretical results using computer simulation for solar selective composites on various substrates, and the effect of the base layer thickness on these substrates are compared with experimental results. The effects of the base layer thickness for cobalt and tungsten are also included. The sputtered base layers selective coatings produced higher absorptance along with higher emittance and electroplated base layer coatings resulted in comparatively lower absorptance and lower emittance. Hundred nanometre metal sputtered base layer is optimised for solar selective coatings.     

A novel design in composities of various materials for solar selective coatings

This study describes the development of multilayer metal-dielectric graded index solar selective coatings in which the metallic volume fraction increases with depth, from top (air–film interface) to bottom (film–substrate interface). The work is based on computer simulation followed by validation through fabrication of the coatings and optical measurements. The influence of the choice of the number of layers present in a graded index composite selective absorber and results obtained for a new destructive interference bilayer (four-layer system) coating, designed using the computer model, were studied. The design and optimization of the composite coating was undertaken using a computer tool developed within this program of research employing Bruggeman and Maxwell–Garnett effective medium formalisms. The design tool enabled all key design parameters, with the exception of particle size and orientation, to be varied systematically to permit the sensitivity of the optical properties of the selective absorber coating to be studied.The model was validated with a supporting program of experimental research in which many different selective absorbers were prepared by co-sputtering of metal and dielectric materials.Although the best compositional gradation can be achieved by increasing the number of layers, the variation in optical performance beyond a certain number of layers is minimal. The destructive interference produced between adjacent layers contributes to the absorptance. The effect of the number of layers (single, four and 10) has been calculated for various materials such as nickel, vanadium, tungsten, cobalt and chromium based coatings. Solar absorptance of 0.98 and 0.96 was achieved by simulation and experimental findings with less than 0.07 thermal emittance at 300 K for 200 nm thick, 4-PGSAC (four-layer system) of V : Al₂O₃ composites. Other designs showed lower optical performance for all the material combinations regardless of their individual optical properties. Use of such thin film coating on the absorbers of solar thermal appliances can reduce thermal losses significantly, which could be of importance to the relevant industry.     

Selective paint coatings for coloured solar absorbers: Polyurethane thickness insensitive spectrally selective (TISS) paints (Part II)

Red, green and blue paints were prepared for use as thickness insensitive spectrally selective (TISS) paint coatings for solar façade absorbers. The paints were composed of a polyurethane resin binder in which various pigments were incorporated in such a way that they formed stable paint dispersions, satisfying stability criteria for façade coatings. A low emittance of the paints was achieved by using low-emittance aluminium flake pigments combined with iron oxide (red coloured paints). Black pigment was added to adjust solar absorptance. Blue and green paints were made by the addition of coloured aluminium flake pigment and the solar absorptance was also adjusted by the addition of black pigment. Efficiency for photo-thermal conversion of solar radiation was assessed by evaluation of the corresponding performance criteria, which enabled the selection of paints whose performance criteria values were higher than 0 (spectrally non-selective black coating). The results confirmed that blue and green paints and to minor extent red ones, combined selectivity with colour. The morphology of the paints was assessed, revealing that the colours originated from the deposition of finely dispersed colour and/or black pigment on the surface of the aluminium flakes during paint preparation.     

Characterisation of high-temperature-resistant spectrally selective paints for solar absorbers

The spectrally selective paint coatings were prepared from organically modified siloxane resin and inorganic pigment (FeMnCuO_x-P320). To optimise the low-emittance properties, different thicknesses of paints were applied on high-reflecting Al foil by a draw bar coater. For all paints, optical and thermal properties were determined as well as their adhesion resistance. Pigment to volume concentration ratio was 20% and for thicknesses of about 1.7–2.0 g/m², the solar absorptance for these samples were a_s=0.90–0.92 with corresponding thermal emittance of e_T=0.20–0.25. Temperature stability of these samples was followed by FT-IR spectroscopy at 300°C. The obtained results indicated good temperature stability of prepared paint coatings.     

Sol–gel derived CuCoMnOx spinel coatings for solar absorbers: Structural and optical properties

A novel black coloured coating with the composition CuCoMnO_x was prepared using sol–gel synthesis. The coatings were deposited using the dip-coating technique from alcoholic sols based on Mn-acetate and Co- and Cu-chloride precursors. Thermogravimetric analysis showed that xerogels become crystalline at 316°C while X-ray diffraction analysis revealed that the coatings and powders correspond predominantly to CuCoMnO_x spinels. Rutherford back scattering (RBS) and transmission electron microscopic (TEM) studies combined with energy dispersive X-ray spectroscopy (EDXS) measurements confirmed that Cu, Mn and Co are present in the films in stoichiometric ratios close to that in the initial sols. IR spectroscopy has been employed to study the formation of sols by following the changes in the vibrational bands of the acetate groups during both thermal hydrolysis and the ageing of sols to xerogels. It was found that ageing of xerogels was accompanied by the formation of −COO⁻ bridging units, which at 250°C are no longer visible in the IR spectra but substituted by the vibrational modes characteristic for CuCoMnO_x. The solar absorptance (a_s) and thermal emittance (e_T) of the coatings when deposited on an Al-substrate are a_s=0.9 and e_T=0.05, which rank deposited black sol–gel CuCoMnO_x spinels among the promising candidates for spectrally selective absorber coatings for solar collectors and solar facades.     

Suitability analysis of selective solar absorber surfaces based on a total cost accounting approach

By adopting a total cost counting approach a sputtered solar absorber coating is compared with an electrochemically produced solar absorber coating for the application of domestic hot water production. The comparison is made assuming a 25 years service time and takes into account production cost, cost associated with initial non-ideal optical performance, possible cost due to reduction in long-term performance, end-of-life cost, and possible cost associated with ecological damage. The result from the analysis gives an understanding of the relative importance of the various factors contributing to the total cost and shows that the sputtered coating seems more favourable to the electrochemical coating in all respects.     

Synthesis and electro-optic properties of nanosized-boron doped cadmium oxide thin films for solar cell applications

Boron doped CdO thin films were prepared by sol–gel dip coating technique. Atomic force microscopy results indicate that the boron doped CdO films have the nanostructure. The influence of the boron doping on the film growth is resulted in a change of grain size. The optical band gap of the CdO films was significantly changed by boron dopant. The refractive index dispersion of the films obeys the single oscillator model. The dispersion parameters, oscillator and dispersion energy were changed by boron dopant. The optical absorption results show that the optical band gap of the CdO film can be engineered over a wide range of 2.27–2.45 eV by introducing B dopant. For solar cell applications of the CdO film, a p-Si/1% B doped n-CdO heterojunction solar cell was fabricated and the solar cell shows the best values of open circuit voltage, V_oc = 0.37 and short circuit current density, J_sc = 0.81 mA/cm² under AM1.5 illumination, despite the fact that V_oc and J_sc are lower than those reported in the literature without using frontal grid contacts and or post-deposition annealing. It is evaluated that this work is useful as a basis search for synthesis of the nanosized-boron doped cadmium oxide thin films for solar cell applications and more competitive p-Si/n-CdO based solar cells.