Solar Absorber Surfaces Research Articles

An Investigation on the performance Improvement of greenhouse-type agricultural dryers

Drying is simply the process of moisture removal from a product. It can be performed by various methods. In these methods, thermal drying is most commonly used for drying agricultural products. These drying systems are usually classified into low and high temperature dryers according to their operating temperature ranges. In low temperature drying systems, natural-circulation greenhouse-type dryers appear the most attractive option for use in rural locations. They are superior operationally and competitive economically to natural open sun drying. Therefore, two different types of natural-circulation greenhouse crop dryers were designed, constructed and tested for their performance in this investigation. Each dryer mainly consisted of a framework constructed from black coated metal bars, corrosion-resistant plastic mesh and a black coated solar radiation absorber surface. The frameworks of the dryers were clad with clear polyethlene sheet on the all sides. The cladding at rear side was arranged to allow put the moist products into the drying chamber or get dried product from there. The clear plastic cladding at the bottom edge of the front side and rear side was also arranged to allow air to flow into the chamber, while the rectangular stream at the top of the end served as the exit for the moist exhaust air. The dryers were tested in the summer conditions. They were aligned lengthwise in the north-south axis during experiments. All dryers were experimented without crops (no product loaded) and with crops (pepper loaded). The dryers were also tested with chimney constructed from a galvanised iron sheet and without chimney in order to determine the effect of the chimney on the air flows. In addition, pepper was dried in the open-sun drying in order to compare the greenhouse dryers with open air drying. During experiments, temperature distribution inside drying chamber, pepper surface temperature, moist exaust air velocity, wind velocity, solar radiation, temperature and relative humidity of the atmosphere air were measured. Pepper samples, collected from open sun dryer and different positions in greenhouse dryers were also weighted to note amount of water evaporated. The results of the study show that the use of natural-circulation greenhouse dryers for drying agricultural products, is 2…5 times more efficient than open air drying and using black coated solar radiation absorber surface and chimney improve the performance of these dryers. Besides, the output of the greenhouse type dryers has also high quality, compared with open air drying.

Towards corrosion testing of unglazed solar absorber surfaces in simulated acid rain

Abstract Electrochemical impedance spectroscopy and potentiodynamic polarization tests were utilized for determining corrosion probabilities of unglazed C/Al2O3/Al solar absorber surfaces in simulated acid rain. Previously, the main degradation mechanism found was exponentially temperature-related hydration of aluminium oxide. In acid rain tests the main corrosion determinant was the pH value of the rain. Results indicate that these methods measure corrosion characteristics of Al substrate instead of the C/Al2O3/Al surface, probably mainly due to the rough and non-uniform microstructure of the latter. Further analyses of the test methods are required in order to estimate their applicability on Al-based uniform sputtered absorber surfaces.

Degradation of unglazed rough graphite-aluminium solar absorber surfaces in simulated acid and neutral rain

Degradation mechanisms of unglazed solar absorber surfaces based on aluminium substrate were studied. Rough graphite-aluminium surfaces were total-immersion subjected to aerated and de-aerated simulated neutral and acid rain. Test conditions were based on calculated absorber stagnation temperature and global rain acidity measurements. Changes in optical properties, elemental composition and sample mass were examined by spectrometry, energy dispersive X-ray spectrometry and thermogravimetry, respectively. The absorbers exhibited almost no degradation at pH value of 3.5. At pH 5.5 alumina on the surface hydrated significantly degrading the optical properties of the surfaces severely in most cases. Therefore these absorber surfaces can not be recommended to be used in non-glazed applications if they are exposed to rain with pH exceeding ∼3.5–4.5. The total-immersion test needs to be developed further as the test results exhibited poor temperature and time dependency thus preventing accurate service lifetime estimates. Still, these tests were useful in determining favourable and non-favourable operating conditions for the absorber surfaces based on aluminium substrate.

Spectrally selective solar absorbers in different non-black colours

Abstract Silicon paints were prepared from yellow, ochre, dark ochre, green and blue pigments. To improve the solar absorptance, a s , of these coatings, an existing black paint was admixed in different ratios. The optical properties of the mixed paints thus formed are expressed in terms of the Kubelka–Munk absorption and scattering coefficients in the spectral region 400–17000 nm. The scattering coefficient obtained for all paints was essentially equal. In the visible region the absorption coefficient follows the spectral characteristics of each respective colour. In the infrared absorption at ∼9000 nm and above 12000 nm are seen in all cases which result in a thickness-dependent increase of the thermal emittance, e T , of the coating. The metric chroma ( C ab * ) and lightness ( L * ) in CIELAB colour space were calculated for wide-angle observer in average daylight conditions. A range of non-black spectrally selective solar absorber surfaces with a s >0.8 and e T L * C ab *

The applicability of accelerated life testing for assessment of service life of solar thermal components

Abstract To achieve successful commercialisation of new advanced windows and solar facade components for buildings, the durability of these need to be demonstrated prior to installation by use of reliable and well-accepted test methods. In Task 27 Performance of Solar Facade Components of the IEA Solar Heating and Cooling Programme work has therefore been undertaken with the objective to develop a general methodology for durability test procedures and service lifetime prediction methods adaptable to the wide variety of advanced optical materials and components used in energy efficient solar thermal and buildings applications. As the result of this work a general methodology has been developed. The proposed methodology includes three steps: (a) initial risk analysis of potential failure modes, (b) screening testing/analysis for service life prediction and microclimate characterisation, and (c) service life prediction involving mathematical modelling and life testing. The applicability of the working scheme to be employed in the development of durability test procedures has been analysed for selective solar absorber surfaces and polymeric glazing materials in flat plate solar collectors. The examples show the great applicability of the general methodology for accelerated life testing. This will allow much shorter development cycle times for new products and will allow improvements to be identified and readily incorporated in new products prior to market introduction.

Anti-reflection coatings for solution-chemically derived nickel—alumina solar absorbers

In order to suppress reflections from a solar absorber surface an anti-reflection (AR) layer is usually deposited on top of the solar absorbing coating. For the AR coating to function well it should be made of a material with a lower refractive index than the underlying surface. Provided that the AR coating is sufficiently thin, it will not increase the thermal emittance value. Besides increasing the solar absorptance it is equally important that the AR layer is long term stable in order to create a successful solar selective coating. The objective was to create a highly efficient, flexible, inexpensive and durable AR coating for solar absorbers using simple techniques. Elastic properties are requested because it prohibits the formation of micro-cracks. Alumina, silica and hybrid silica AR coatings were synthesized using solution chemistry and sol–gel techniques. The AR coatings were deposited on the absorber surface using spin coating. The absorbers used without an AR layer had a normal solar absorptance of 0.79–0.81 and a normal thermal emittance of 0.03. The optimum anti-reflection coated sample acquired a normal solar absorptance of 0.93 while the thermal emittance remained unaltered. The study has shown that by adding a proper AR coating to a solar absorber the spectral selectivity can be considerably enhanced.

Comparison of AC voltage and periodic-reverse current nickel pigmented anodized aluminium as solar selective absorber

Abstract The optical properties of nickel pigmented anodized aluminium obtained by AC voltage and by periodic-reverse current impregnation have been compared. A solar selective absorbing surface is obtained from about the same threshold nickel content (γNi≅0.60 g/m2). Moreover in both cases, the impregnation faradic yield at the threshold nickel content is the same, whereas the structural parameters of the impregnated layers were different compared to the structural parameters of the initial DC anodic porous layer.

Thermal stability and moisture resistance of C/Al2O3/Al solar absorber surfaces

Mechanically manufactured C/Al2O3/Al solar absorber surfaces were exposed to thermal stability and moisture resistance tests following the IEA Solar Heating and Cooling Programme recommendations (draft ISO/DIS 12592). The main degradation mechanism is found to be hydration of aluminium oxide to pseudoboehmite and boehmite. We estimated the absorber service lifetime (with an optical performance more than 95% of its initial) based on two literature references, where time of wetness frequency distribution of a solar collector microclimate was measured. The estimated average service lifetime in normal use was 20 or 25 years, depending significantly on the time of wetness frequency distribution of the surface. r 2004 Elsevier B.V. All rights reserved.

Microstructural optimization and extended durability studies of low-cost rough graphite–aluminium solar absorber surfaces

Mechanically manufactured low-cost selective inhomogeneous rough graphite–aluminium (C/Al 2O 3/Al) solar absorber surfaces provide, at the current state of development, solar absorptance α=0.90 and thermal emittance ε=0.22. Based on a literature review, significantly lower emittance values could theoretically be achieved by utilization of surface plasmons in rough surfaces with an optimal graphite layer thickness, groove depth and sinusoidal surface profile periodicity. It is possible that an arbitrary form of roughening could produce equally good optical properties. In order to achieve lower emittance manufacturing parameters, the composition of silicon carbide grinding pad and the corresponding grinding pattern are required to be enhanced. The commercially available grinding pads used so far have not yielded optimal results. An antireflection coating could improve α to >0.90. Absorber samples were subjected to 383 days of temperature and irradiance cycling. In total, the samples were exposed to ultraviolet (UV) irradiation equivalent to 5–15 years of normal outdoor use. The results show that the samples are not sensitive to natural levels of UV irradiation or cycling-induced degradation of optical properties. A clear increment in absorptance was observed after the first 50 days of cycling. The elevated temperature of 130 °C is the probable cause for the increase. Reference samples indicated similar aging behaviour both after 4 years of natural exposure and after relatively short constant temperature tests at 120 and 180 °C.

Solar Energy Materials

AbstractFor Abstract see ChemInform Abstract in Full Text.

Graded selective coatings based on chromium and titanium oxynitride

In order to improve the performance of thermal solar collectors, the development of good, durable and reproducible spectrally selective solar absorber surfaces for photothermal conversion is of greatest importance. Spectrally selective cermet coatings were produced by DC magnetron sputtering using metallic chromium and titanium targets at constant target current, substrate bias and substrate temperature. These coatings are graded cermets with a metal concentration decreasing from the substrate to the coating surface. Basically, these coatings are formed with a layer structure consisting of a cermet layer enriched in metal, one or more cermet layers with less metal than the first one, and on top a pure ceramic layer as an antireflection layer. The metal layer was deposited by non-reactive DC sputtering, while the ceramic layers were deposited by DC reactive sputtering in argon–nitrogen or argon–oxygen atmosphere. The metallic and ceramic layers were deposited as a sub-layer system, consisting of alternating metallic and nitride or oxide sub-layers. The effect of the optical properties in dependence on the metallic fraction in the layers and the number of sub-layers in the film is discussed. For coatings based on chromium, the optimum selectivity achieved was a solar absorptance of 94% and a thermal emittance of 6% at 82 °C. For titanium oxynitride based coatings, the best selectivity achieved was a solar absorptance of 91% and a thermal emittance of 4%. The microstructure and thickness were studied by scanning electron microscopy (SEM). The surface microtopography was analysed by atomic force microscopy (AFM) and the metal concentration profile by Rutherford backscattering spectrometry (RBS).

Microstructural analysis of selective C/Al 2O 3/Al solar absorber surfaces

The elemental composition and geometrical structure of mechanically manufactured spectrally selective C/Al 2O 3/Al solar absorber surfaces were characterized by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy, energy dispersive X-ray spectroscopy and optical microscopy. The XPS analysis revealed that the surface contains Al 2O 3 and C in graphite form. Optical microscopy confirmed that graphite forms inhomogeneous agglomerated clusters on the surface. The thickness of the clusters varies, the maximum measured thickness being in the range of 300 nm. A solar absorptance of 0.90 and a thermal emittance of 0.22 have been achieved so far. Increasing the graphite coverage and decreasing the graphite cluster thickness could increase the absorptance to ⩾0.94 and lower the emittance. This could be achieved by altering the composition and the structure of the grinding pad used and by finding the suitable manufacturing parameters for the advanced pad.

High-temperature optical properties and stability of AlxOy–AlNx–Al solar selective absorbing surface prepared by DC magnetron reactive sputtering

AlxOy–AlNx–Al selective absorbing surface was prepared by DC magnetron reactive sputtering with aluminum alloy (LY13)1 in air and argon. The studies were carried out to access the high-temperature (400°C–600°C) optical properties and stability of the coatings. The coatings were found to withstand heating at 600°C for 30 min in 4.5×10−3 Pa vacuum with absorptance 0.94 and emittance 0.07 after annealing. After heating at 450°C for 10 h, the specimen still had good performance whose absorptance and emittance was 0.93 and 0.07, respectively. Auger electron spectroscopy was used to analyse the structure of the solar selective surface before and after annealing.

Mechanically manufactured selective solar absorber surfaces

Abstract This study describes the development of manufacturing processes for a mechanically manufactured selective C/Al2O3/Al surface and its basic characterization. The composition and structure of the surfaces were characterized by scanning electron microscopy and electron microprobe analysis. Spectroradiometer and FTIR-spectrometer were used for optical characterization. The manufacturing process has been developed from manual abrasive grinding followed with an acid bath to mechanically operated grinding alone. The surface consists mainly of Al2O3, with some C adsorbed on the surface during the grinding process. The microstructure of the surface consists of small grooves, organized in a heterogeneous two-dimensional matrix. The width of the grooves varies typically between 1 and 2 μm. The absorptance of a surface from the latest stage of development is 0.90 and the emittance 0.25.

Structure and mechanical properties of co-deposited TiAl thin films

Titanium–aluminium compounds are very attractive materials for aerospace and high performance engine applications owing to their low density, high temperature strength and high specific strength. Specific compositions may also be suitable as solar absorber surfaces. Consequently, a detailed investigation of the titanium-aluminium binary alloy coating system is being carried out. Coatings have been deposited from pure titanium and aluminium cathodes, via the closed field unbalanced magnetron sputtering technique. Two different magnetron co-sputtering techniques were used: co-deposition, giving a gradient in coating composition across the surface of the substrates; and multi-layer deposition, giving fine multi-layers of different thickness. A range of compositions from 0 to 100% titanium was deposited. The coatings were studied in the as-deposited condition and after successive annealing treatments in order to obtain stable intermetallic phases. Changes in structure and phase composition were studied by SEM and XRD analysis after each annealing process. Mechanical analysis was also conducted, consisting of hardness and friction and wear testing. This paper concentrates mainly on the multi-layer deposited films and the results obtained to date are presented here.

Durability of solar energy materials

A general test procedure for qualification of solar absorber surface durability has been developed based on the results of a comprehensive case study performed within the framework of the IEA Solar Heating and Cooling Programme Task X. It was assumed, in the development of the qualification procedure, that the intended use of the absorber surface to be qualified, was in single glazed flat plate solar collectors for domestic hot water production. The absorber surface should be considered qualified if it met the requirement of a design service life of 25 years with maximum loss in the optical performance of the absorber surface corresponding to a 5 % relative reduction in the performance of a solar domestic hot water system. The testing procedure, consisting of three kinds of constant load accelerated life-time tests, was limited to simulation of the following kinds of absorber surface degradation processes: a) high temperature degradation, e.g. oxidation, b) degradation by the action of moisture or condensed water on the absorber surface, e.g. hydration or hydrolysis. To quantify expected environmental stress on the absorber surface related to the environmental factors of interest, microclimate data, representing typical service conditions for absorbers in single-glazed flat plate collectors for domestic hot water production were used.

AC anodization of aluminum, electrodeposition of nickel and optical property examination

Commercial grade aluminum was anodized to form porous anodic films using alternating current at a voltage of 15 V and a temperature of 25°C. The AC anodization was implemented in phosphoric acid and in an electrolyte modified by dissolving 1.1 g/l iron powder. The presence of iron species in the modified electrolyte did not make iron to be incorporated in the film and appeared not to change the pore diameter, but doubled the film thickness. Electrodeposition of nickel was carried out to pigment the films. Pores of the films produced in the modified electrolyte were also found more accessible for nickel pigmentation. Reflectance spectrum of a nickel-pigmented film produced in the modified electrolyte was measured, and its solar absorptance and thermal emittance were calculated. It has been shown that AC anodization of aluminum in phosphoric acid modified by a small number of intentionally added species such as iron species, followed by electrodeposition of nickel, has potential in developing a method for cheaply preparing selective solar absorbing surface of nickel-pigmented anodic films of aluminum.

00/02730 Qualification test procedure for solar absorber surface durability

In order to research the high temperature durability of the absorber surface,a test procedure was designed based on the abroad method and our national practice in this paper.According to the procedure,we calculate the service life by the optical performance reduce of the absorber surface,if the optical performance reduced less than 5%,the system could be thought qualified.

00/02731 Round robin on accelerated life testing of solar absorber surface durability

00/02731 Round robin on accelerated life testing of solar absorber surface durability

Qualification test procedure for solar absorber surface durability

A general test procedure for the qualification of solar absorber surface durability has been developed based on the results of a comprehensive case study performed within the framework of the IEA Solar Heating and Cooling Programme Task X. It was assumed, in the development of the qualification procedure, that the intended use of the absorber surface to be qualified, was in single-glazed flat plate solar collectors for domestic hot water production. The absorber surface should be considered qualified if it met the requirement of a design service life of 25 years with maximum loss in the optical performance of the absorber surface corresponding to a 5% relative reduction in the performance of a solar domestic hot water system. The testing procedure, consisting of three kinds of constant load-accelerated life-time tests, was limited to simulation of the following three kinds of absorber surface degradation processes: (a) high-temperature degradation, e.g. oxidation, (b) degradation by the action of moisture or condensed water on the absorber surface, e.g. hydration or hydrolysis, and (c) degradation caused by high humidity air containing a small concentration of sulphur dioxide as an airborne pollutant, e.g. atmospheric corrosion. To quantify expected environmental stress on the absorber surface related to the environmental factors of interest, microclimate data, representing typical service conditions for absorbers in single-glazed flat plate collectors for domestic hot water production were used.