Thermal advantages for solar heating systems with a glass cover with antireflection surfaces

Investigations elucidate how a glass cover with antireflection surfaces can improve the efficiency of a solar collector and the thermal performance of solar heating systems. The transmittances for two glass covers for a flat-plate solar collector were measured for different incidence angles. The two glasses are identical, except for the fact that one of them is equipped with antireflection surfaces by the company SunArc A/S. The transmittance was increased by 5–9%-points due to the antireflection surfaces. The increase depends on the incidence angle. The efficiency at incidence angles of 0° and the incidence angle modifier were measured for a flat-plate solar collector with the two cover plates. The collector efficiency was increased by 4–6%-points due to the antireflection surfaces, depending on the incidence angle. The thermal advantage with using a glass cover with antireflection surfaces was determined for different solar heating systems. Three systems were investigated: solar domestic hot water systems, solar heating systems for combined space heating demand and domestic hot water supply, and large solar heating plants. The yearly thermal performance of the systems was calculated by detailed simulation models with collectors with a normal glass cover and with a glass cover with antireflection surfaces. The calculations were carried out for different solar fractions and temperature levels of the solar heating systems. These parameters influence greatly the thermal performance associated with the antireflection surfaces.     

Angular dependent optical properties from outdoor measurements of solar glazings

A method for careful outdoor characterization of the angular dependent solar transmittance of solar glazing is developed and used for investigating the performance of structured and antireflection treated glass. Two identical collectors are series connected in order to get an identical flow. The reference collector is always covered with a thin Teflon foil and the test collector is furnished with the investigated covers. The method is carried out in six steps: (1) The collectors are long term tested in order to get the collector parameters, especially the U-values. (2) The performances of the collector at a temperature close to ambient are monitored during a clear day. (3) The effect of heat losses is eliminated from the knowledge of the U-values, and the optical efficiencies of the collectors are derived. (4) The ratio between optical efficiency of the test — and the reference collector is calculated. (5) Both collectors are tested with a Teflon film, with well-known optical properties, as a cover and the relative optical efficiency is derived. (6) From the relative transmittance between the investigated glazings and Teflon, the absolute value of the angular dependent transmittance is derived. The results show good agreement with conventional optical characterization. The commercial antireflection treatment increases the solar transmittance by 4% and the annual output by 9%. The structured glass shows similar angular performance as a flat glass if the structure is faced outwards, but shows a lower transmittance for incidence angles exceeding 40° if the structure is faced inwards. This means that the annual performance is decreased by 4% by facing the structure inwards instead of outwards.     

Effect of dirt on transparent covers in flat-plate solar energy collectors

The effect of dirt on the transmittance of solar radiation through various inclined glass plates and plastic films, which are used as a transparent cover for flat-plate collectors, has been studied. The dirt correction factor for glass plate inclined at an angle of 45 deg from the horizontal is 0·92, which is significantly different from the value of 0·99 given by Hottel and Woertz[1]. The correction factor is greater for plastic film than it is for glass plate for any inclination.     

Chemical vapour deposited SnO2:Sb heat mirror coatings for cylindrical solar collectors

A chemical vapour deposition technique for growth of SnO₂:Sb heat mirror coatings on the inner walls of long cover glass tubes for cylidrical solar collectors is reported. The best performance of the tin-oxide film is obtained for those films grown from a source of SnCl₂ + 1 mol% Sb on Corning glass tubes at 520°C. These films, supported on 2 mm glass substrates, have a solar transmittance of 0.85 and an infrared reflectance of 0.8. The heat mirror coatings are observed to increase the stagnation temperature of the absorber in anevacuated tubular collector from 142 to 161°C under incident optical flux of 1150 W/m²     

全玻璃真空太阳集热管光—热性能

从能量平衡议程导出全玻璃真空太阳集热管的3个光－热性能参数,即热损系数UTL、闷晒太阳曝辐量H和空晒性能参数Y与真空热管理的罩管玻璃的太阳透射比τ、选择性吸收表面的太阳吸收比a、发射比ε、和集热管内的气体压强p,太阳辐照度、环境温度和真空太阳集热管的几何尺寸等函数关系。理论计算的真空太阳集热管的3个热性能参数与实验的结果比较一致。上述全玻璃真空太阳集热管的光－热性能参数已经被国家标准《全玻璃真空太阳集热管》（GB／T17049－1997）采用。     

Antireflection of glazings for solar energy applications

Several antireflection treatments such as Teflon dipping, dipping in silica saturated fluosilic acid, have previously been tested. The films made with the fluosilicic acid showed an increased solar transmittance of up to 5%. Outdoor testing for more than 7 years indicated an excellent long-term stability. The problem with this process is the involved chemicals, which are too harmful to permit a large-scale production in Sweden. Therefore, a dip-coating process has been investigated, which uses silica sols. Two sols were investigated, one polydisperse and one monodisperse. The highest attained solar transmittance was achieved with the monodisperse sol. It is believed that the increase in the solar transmittance is connected to the size of the silica particles, the larger the smallest particles the higher the solar transmittance. A decrease by up to 5.2% in the solar reflectance has been noted. A problem is the mechanical properties and especially the adhesion tends to be low. It is, however, possible to improve these properties by baking the film at approximately 550°C during 30 min. The solar transmittance decreases when baking the film since it collapses partially, but the mechanical properties seemed to improve. During baking the temperature is close to that of commercial tempering. It should therefore be possible to bake and temper at the same time. The deposited film does not introduce any absorption. It is concluded that this process should be suitable for large-scale production, since the involved chemicals are harmless and the process is fairly rapid. Films were made at withdrawal rates of up to 1 cm/s.     

Effect of cover plate treatment on efficiency of solar collectors

Theoretical data are presented for prediction of solar collector efficiencies as the cover plates are varied by coating, de-reflecting, changing iron content, or evacuating. Tin oxide as a radiation shield appears to offer some improvement for a nonselective black with two cover plates but no improvement with a selective black with one cover plate. Low-iron and dereflected soda lime glass offer considerable improvement over regular soda lime glass over the whole range of operating temperatures. An evacuated single cover plate with a selective black performs at 50 per cent efficiency at a collector plate temperature of 430°F. An analysis is presented that shows the effect of support studs in an evacuated unit. The data show that support studs having a cross-sectional area equal to 5 per cent of the collector area will greatly lower the efficiency of the unit.Experimental data are presented for no-load (no flowing water) conditions where collector plates fabricated with either tin-oxide coated glass and low iron glass are compared to regular glass for a non-selective surface. The collector plates with the tin-oxide coated glass and low iron glass attain higher equilibrium temperatures.     

Effect of absorption of solar radiation in glass-cover(s) on heat transfer coefficients in upward heat flow in single and double glazed flat-plate collectors

Absorption of solar radiation in the glass cover(s) of a flat plate solar collector increases the temperature of cover(s) and hence changes the values of convective and radiative heat transfer coefficients. The governing equations for the case of single as well as double glazed collector have been solved for inner and outer surface temperatures of glass cover(s) with/without including the effect of absorption of solar radiation in the glass cover(s), with appropriate boundary conditions. The effects of absorption of solar radiation on inner and outer surface temperatures and consequently on convective and radiative heat transfer coefficients have been studied over a wide range of the independent variables. The values of glass cover temperatures obtained from numerical solutions of heat balance equations with and without including the effect of absorption of solar radiation in the glass cover(s) are compared. For a single glazed collector the increase in glass cover temperature due to absorption of solar radiation could be as high as 6°. The increase in temperatures of first and second glass covers of a double glazed collector could be as high as 14° and 11°, respectively. The effect on the convective heat transfer coefficient between the absorber plate and the first glass cover is substantial. The difference in the values of the convective heat transfer coefficients between the absorber plate and the first glass cover (h_cp1) of a double glazed collector for the two cases: (i) including the effect of absorption and (ii) neglecting the effect of absorption in glass cover, could be as high as 49%. Correlations for computing the temperatures of inner and outer surfaces of the glass cover(s) of single and double glazed flat plate collectors are developed. The relations developed enable incorporation of the effect of absorption of solar radiation in glass cover(s) in the relations for inner and outer surface temperatures in a simple manner. By making use of the relations developed for inner and outer surface temperatures of glass cover(s) the convective and radiative heat transfer coefficients can be calculated so close to those obtained by making use of surface temperatures of glass cover(s) obtained by numerical solutions of heat balance equations that numerical solutions of heat balance equations are not required.     

Probabilistic modeling of a parabolic trough collector power plant – An uncertainty and sensitivity analysis

In this work, an uncertainty and sensitivity analysis for the annual performance of a parabolic trough collector plant based on a probabilistic modeling approach of the solar-to-thermal energy conversion process has been accomplished. Realistic probability functions have been assigned to the most relevant solar field performance parameters. The Latin Hypercube sampling method has been used to create equal probable parameter combinations. The so obtained sample matrix has been used to run multiple annual electricity yield simulations in SimulCET, a validated parabolic trough collector plant simulation software, developed by the National Renewable Energy Center (CENER) in Spain García-Barberena et al., 2012. This procedure has led to a representative distribution for the annual plant performance, given the uncertainty in the input data. For this study the parabolic trough power plant model has been run in solar driven operation mode, without the use of thermal storage or fossil fuel back up. While being aware of the great influence of the solar irradiation on the power plant performance, only one single reference meteorological year has been used as solar input data. This has been done in order to emphasize the influence of technical design- as well as solar field maintenance parameters, factors that can be controlled or affected by mankind. In order to assess and rank the impact of each varied model parameter a multiple linear regression has been performed. The standardized regression coefficients, the Pearson correlation coefficients as well as the coefficient of multiple determination R² are discussed. Varied parameters are the collector mirror reflectance, the collector mirror cleanliness factor, the collector glass tube transmittance, the collector receiver tube absorptance, and the collector receiver tube heat loss characteristic. Based on existing and published bibliography, a set of parameter distributions and ranges have been chosen for this work and the simulation results show that the cleanliness factor has the strongest influence on the model output. The cleanliness is followed (in this sequence) by the mirror reflectance, the glass tube transmittance, the receiver tube absorptance and, finally, by the receiver tube heat loss characteristic.     

Optical transmittance measurements on a solar collector cover of cylindrical glass tubes

The transmittance of unpolarized light through a solar collector cover made of cylindrical glass tubes in a coplanar parallel close-packed array is dependent on the orientation of the cylindrical axes of the tubes in the plane of the cover. A maximum transmittance occurs when the axes are perpendicular to the plane of the angle of incidence, and a minimum transmittance occurs when the axes are parallel to the plane.At all orientations and at all angles of incidence the tubular cover has a markedly greater transmittance than does a cover formed from two parallel sheets of window glass.     

Angular dependence of optical efficiency of evacuated tubular collectors with antireflection coatings and stationary specular reflectors

The efficiencies of η₀ of arrays of evacuated tubular collectors with non-imaging specular reflectors have been determined experimentally using a calorimetric technique and theoretically using a Monte-Carlo ray tracing technique. Results have been obtained for collectors incorporating reflectors of two concentrations, and efficiencies are compared with and without antireflection coatings on the glass envelopes for sunlight incident at angles 0–70°. The reflective properties of all optical components have been modelled in detail for the ray tracing calculations. Experimental and theoretical efficiencies agree within 0.02 for a wide range of angles of incidence. Antireflection coatings which increase the normal transmittance through a glass envelope by 5% result in an increase of 0.025 (about 4%) in collector efficiency. A theoretical study of the dependence of collector efficiency on absorptance of the absorber tube and specular reflectance of the reflectors is also discussed. Experimental and theoretical results have also been obtained for a collector incorporating a specular reflector with an accumulated dust cover. In this system, the reflector exhibits both specular and diffuse components of reflectance.     

Spherical antireflection coatings by large-area convective assembly of monolayer silica microspheres

Solution-processed surface textures are highly desirable for antireflection in high-performance cost-effective solar cells. Inorganic spherical surface textures can be formed with monolayers of microscale silica spheres partially immersed into spin-on-glass films. We report here a convective assembly process for the formation of large-area self-assembled monolayers of silica microspheres on glass, quartz, and silicon substrates. The structure of the self-assembled monolayers and their spatial extent are significantly influenced by sphere concentration in the suspension, dispersed suspension volume, solvent, coating plate speed, and wedge angle. Glass substrates up to 150×150 mm² are uniformly coated with monolayers of 2-μm silica spheres. It is found that the spherical coating improves the transmittance of quartz wafer from 89.2% to 92.7% around 400 nm and from 90.8% to 92.5% around 1100 nm, demonstrating its broad-spectrum nature. The spherical structure offers an attractive solution to antireflection in crystalline silicon solar cells, as well as thin-film, quantum dot, organic, and flexible solar cells.     

Stability of antireflection coatings for large area glazing

We draw attention to the excellent stability of the antireflection (AR) coatings on glass prepared by liquid-phase etching in a silica supersaturated flousilicic acid in comparison to dipping in a teflon sol. Both methods resulted in both sides of the glass being coated with antireflection films. For the best samples the transmission of the original glass was increased by as much as 5.5%. The samples have then been exposed to weathering for about seven years to test their durability, after which their optical properties were measured again. Even after this long period of outdoor exposure, the acid-etched samples show remarkable stability in their antireflection properties. After cleaning, their transmittance is restored to almost pre-exposure levels.     

Properties, performance and current status of the laminated electrochromic glass of Gesimat

In the last 10 years Gesimat has developed a large-area electrochromic glazing with an advanced polymer electrolyte and tungsten oxide and Prussian Blue as complementary electrochromic layers. This electrochromic glass has a switching range between 75% and 8% visible transmittance and between 56% and 6% solar transmittance. The polymer electrolyte is based on polyvinyl butyral (PVB), a polymer in use as an interlayer for laminated safety glass since more than 60 years. The electrochromic films are deposited by a new large-area electrodeposition process. For lamination the standard methods of laminated safety glass production can be used.     

Solar energy availability in greenhouses as affected by condensation on cladding materials

To investigate the solar radiation transmission through cladding materials covered with condensation on their inner surface a new experimental set-up was constructed. Transmittance experiments were carried out on vertical glass plates and polyethylene films covered with condensation without and with runoff for several incidence angles in the horizontal plane using perpendicularly polarized radiation. It was observed that condensation on a glass plate slightly decreased the solar radiation transmittance when compared with the dry state. The reduction in transmittance increased from 0.8% at normal incidence to 8.5% at an incidence angle of 60°. Results for polyethylene pointed out that condensation affected the solar radiation transmission and so the available solar energy in the greenhouse much more than in a glasshouse. At normal incidence, a transmittance reduction of 20.8% with respect to the dry state was obtained. For higher incidence angles, the transmittance reduction decreased gradually to 7.2% at an incidence angle of 60°. For both glass and polyethylene, it was observed that the start of runoff gave rise to a transmittance increase of up to 2% when compared with the phase of condensation without runoff.     

Transmittance Study of a Tubular Cover Collector

Transmittance of a tubular cover collector has been calculated for a particular orientation. Tilt of the collector is optimised for the four typical days (two equinoxes and two solstices) at Delhi and Madras, a high and low latitude station respectively. Only the two nearest neighbouring tube elements are considered, in order to take into account the shadowing effect. In the present system, where the thickness of the tube and the flat glass sheets, has been taken to be the same, the transmittance of the tubular cover has been found to be comparitively less than that of a cover formed by two parallel sheets of window glass. However, because of its cylindrical geometry, very thin glass tubes can be used for all practical purposes, thereby, increasing the transmittance to a great extent. It has also been concluded that for all tubular covers including this system, hour angle instead of angle of incidence is an exact parameter to study the variation of effective transmittance. The difference in the mode of variation of transmittance with hour angle in the present system, with that in case of tubular collector, is attributed to the variation of average angle of incidence (local) with hour angle.     

Large-area electrochromic glazing with ion-conducting PVB interlayer and two complementary electrodeposited electrochromic layers

A new laminated large-area electrochromic glass consisting of two FTO-coated glass panes coated with complementary electrochromic thin films by electrodeposition and laminated together by the use of an ion-conducting PVB sheet is presented. The visible light transmittance can be changed between 77% and 8% and the solar transmittance between 56% and 6%.     

Effect of collector components on the collection efficiency of tubular evacuated collectors with diffuse reflectors

The efficiencies η_o of arrays of evacuated tubular collectors with diffuse reflectors have been determined experimentally using a calorimetric technique and theoretically using a Monte-Carlo ray tracing technique. Results have been obtained on collector arrays with various collector tube separations and collector-reflector distance, using two types of reflector, and efficiencies are compared for collector tubes with and without antireflection layers on the glass envelopes. The variation of collector efficiency with angle of incidence for sunlight has also been studied for two collector tube separations. The reflecting properties of the reflectors, glass envelope and selective absorber have been modelled in some detail for the ray tracing calculations. Experimental and theoretical efficiencies agree within the experimental and theoretical uncertainties, and all the trends observed experimentally are predicted by ray tracing. The efficiency of the collectors is not strongly dependent on the reflectance of the diffuse reflector, but depends strongly on the collector tube separation. Antireflection layers which increase the transmittance through the glass envelope by 5% result in an increase of 0.02 (about 3 per cent) in collector efficiency.     

Comparison of long-term flat-plate solar collector performance calculations based on averaged meteorological data

The use of averaged meterological data for collector performance calculations is studied. To this end, a steady state, two-dimensional, nodal, heat transfer analysis is developed for a flat-plate solar collector. The analysis accounts for the temperature gradients in the fluid flow and vertical directions in the collector, the physical and thermodynamic properties of the materials in the collector, the collector location, the orientation and dimensions of the collector, the number of cover plates and any thin film selective coatings on the cover plates or absorber. Also accounted for are the time dependent variations in the meteorological conditions, insolation, and collimated and diffuse solar irradiation. The spectral nature of radiation heat transfer in the collector is modeled by two spectral bands, solar and thermal, with 3.0 μm as the cutoff frequency between the solar and thermal bands of radiation. The results indicate that long term collector performance calculations based on averaged meteorological data will not correlate with calculations based on hourly data if the weather is highly variable. When the weather variations are mild, averaged data can give results very close to those based on hourly data.     

Analysis of a v-groove solar collector with a selective glass cover

The present paper describes a theoretical study of an air heating solar collector. Solar energy is collected in an absorbing triangularly corrugated plate with a selective surface. The collector also has a selective glass cover which reflects the long wavelength radiation emitted by the absorbing plate and transmits most of the short wavelength incoming solar radiation. The collector overall absorptance is further increased by the cavity effect.

Solar energy passing through the glass is incident on the selectively absorbing surface of the v-groove plate. The absorbed energy raises the temperature of the plate and heats air that is pumped through the triangular passages of the collector. The efficiency that can be obtained with the collector is determined as a function of a number of parameters of the problem.