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The measurement and calculation of radiative heat transfer between uncoated and doped tin oxide coated glass surfaces
Abstract
Accurate measurements are reported of the radiative heat transfer between parallel surfaces of uncoated and doped tin oxide coated glass. The infrared properties of the tin oxide coated glass are calculated by a novel approach by fitting a three parameter Drude model to reflectance measurements at a specified angle, and over a restricted range of wavelengths. Estimates of the radiative heat transfer between uncoated and coated glass surfaces are calculated from a consideration of the directional spectral emittances and the blackbody spectrum. This procedure gives results that are in more consistent agreement with experimental measurements than results obtained by considering the hemispherical total, or directional total emittances. Copyright © 1996 Elsevier Science Ltd.
... In the BIPV case, where the solar cell is encapsulated into a glazing, radiative transfer is reduced to a parallel plate problem, where directional and wavelength averages are needed. 8 Lastly, as thermal emittance varies with temperature, a knowledge of the total hemispherical thermal emittance as a function of temperature is necessary. 9,10 The knowledge of total hemispherical emittance can also be applied for calculating radiative heat transfers for the back of solar panel (facing the ground), [11][12][13] general BIPVs, 8,[14][15][16][17][18][19] for designing thermal-control coatings for spacecrafts, 20,21 and for optimizing rapid thermal processes for silicon solar cells. ...
... 8 Lastly, as thermal emittance varies with temperature, a knowledge of the total hemispherical thermal emittance as a function of temperature is necessary. 9,10 The knowledge of total hemispherical emittance can also be applied for calculating radiative heat transfers for the back of solar panel (facing the ground), [11][12][13] general BIPVs, 8,[14][15][16][17][18][19] for designing thermal-control coatings for spacecrafts, 20,21 and for optimizing rapid thermal processes for silicon solar cells. 22 It is not accurate to assume total hemispherical emittance to be equal to spectral emittance at normal incidence. ...
... Total hemispherical emittance may be approximated using the spectral emittance at normal incidence (calculated from measured spectral reflectance at normal incidence) using correlation values reported in the standard ISO 10292:1994 17 or by a mathematical correlation 14,18,19 reported in the standard BS EN 12898:2019. 19 Zhang et al. 8 compared different methods used to calculate total hemispherical emittance and the error introduced in the calculation of glass temperature of doped tin-oxide-coated vacuum glazing compared to experimental results. The authors found an error of 5% between the calculated temperature using total hemispherical emittance approximated from the standard ISO 10292:1994 and the measured temperature obtained by the guarded hot plate method. ...
The performance and lifetime of a solar cell is sensitive to its operating temperature. In applications such as building-integrated photovoltaics, an accurate knowledge of the total hemispherical emittance as a function of temperature is required to calculate total radiative heat transfer and hence solar cell operating temperature. Here, we report a direct and absolute calorimetric method based on Stefan-Boltzmann’s law for determining total hemispherical emittance of a surface. This method is applied to measure front and rear total hemispherical emittance of silicon and perovskite solar cells with and without encapsulation. Results show that the encapsulation material and the type of rear electrode strongly influence the emittance of the cell. Additionally, total hemispherical emittance values of solar cells determined in this work are considerably lower than spectral emittance values previously measured. These findings demonstrate the importance of front and rear materials selection in the optimization of radiative cooling for efficiency and lifetime.
... There is no such ideal surface existing. One method to solve this problem is to define a directional total emittance ε(θ) so the hemispherical emittance ε hemispherical can be written as [63]: ...
... The errors resulting from using either Eqs. (3.6) or (3.8) were typically about 4% for radiative heat transfer between two glass surfaces [63]. ...
... For spectral surfaces having reflectance which depend on both wavelength and angle, the net radiative heat transfer can be calculated by integrating over wavelength λ and angle θ [63]: ...
... No practical surface possesses such ideal properties. As noted by Zhang et al. (1997), an accurate calculation of radiative heat transfer between two surfaces therefore requires an integration over angle and wavelength of the black body spectra for the two surfaces, weighted by the effective angle and wavelength dependent emittance for these surfaces. As discussed below, such a procedure is essential in the calibration of the small area guarded hot plate apparatus. ...
... As discussed below, such a procedure is essential in the calibration of the small area guarded hot plate apparatus. Zhang et al. (1997) have shown, however, that the simplistic Equation 2 provides an accurate representation of radiative heat transfer, including its temperature dependence, provided that the value of effective emittance is only slightly different from that obtained conventionally by combining the hemispherical emittances as in Equation 3. They further show that the effective emittance of clear, and doped tin oxide coated glass is essentially independent of temperature over the range -20°C to +20°C. ...
... Heat transfer in this "standard sample" is therefore entirely by radiation between the two glass sheets. The radiative heat transfer coefficient is determined as discussed in Section 2.3 above, using the method described by Zhang et al. (1997). The validity of this calibration procedure has been demonstrated by comparison with measurements using a larger area (~50 mm diameter) guarded hot plate having an accurately known geometry. ...
The heat flow through a 1m x 1m sample of vacuum glazing has been measured in two independent ways. One measurement method uses a guarded hot box to make a direct determination of overall heat-transmission coefficient under standard test conditions. The other method uses a small-area guarded hot plate (specially designed for this purpose) to measure the local radiative and (negligible) gas conductance between the glass sheets. This instrument is also used to validate calculations of the heat flow through the support pillars. The overall heat flow is then obtained by combining the contributions from these processes with those due to heat flow through the conducting edge seal. The results obtained with the two methods are in very good agreement for the same external conditions. These results provide the first independent validation of estimates of heat flow through vacuum glazing based on the five separate processes that contribute to this heat transfer. RES
... Theoretically the emittance value depends on the surface temperature, the wavelength, the angle of incidence of the radiation to the normal [11]. Equation 3 was assumed to be independent of these parameters, which resulted in an error of 4% [11]. ...
... Theoretically the emittance value depends on the surface temperature, the wavelength, the angle of incidence of the radiation to the normal [11]. Equation 3 was assumed to be independent of these parameters, which resulted in an error of 4% [11]. ...
The thermal performance of vacuum glazing was predicted using two dimensional (2-D) finite element and three dimensional (3-D) finite volume models. In the 2-D model, the vacuum space, including the pillar arrays, was represented by a material whose effective thermal conductivity was determined from the specified vacuum space width, the heat conduction through the pillar array and the calculated radiation heat transfer between the two interior glass surfaces within the vacuum gap. In the 3-D model, the support pillar array was incorporated and modeled within the glazing unit directly. The difference in predicted overall heat transfer coefficients between the two models for the vacuum window simulated was less than 3%. A guarded hot box calorimeter was used to determine the experimental thermal performance of vacuum glazing. The experimentally determined overall heat transfer coefficient and temperature profiles along the central line of the vacuum glazing are in very good agreement with the predictions made using the 2-D and 3-D models.
... The emittance value depends on the surface temperature, the wavelength, the angle of incidence of the radiation to the normal in a manner specific to a particular glazing material [16]. Eq. (3) was assumed to be independent of these parameters, which would result in a maximal error of 4% [16]. ...
... The emittance value depends on the surface temperature, the wavelength, the angle of incidence of the radiation to the normal in a manner specific to a particular glazing material [16]. Eq. (3) was assumed to be independent of these parameters, which would result in a maximal error of 4% [16]. ...
Thermal performanceofvacuumglazingpredictedbyusingtwo-dimensional(2-D)finiteelementand
three-dimensional(3-D)finitevolumemodelsarepresented.Inthe2-Dmodel,thevacuumspace,
includingthepillararrays,wasrepresentedbyamaterialwhoseeffectivethermalconductivitywas
determined fromthespecifiedvacuumspacewidth,theheatconductionthroughthepillararrayand
the calculatedradiationheattransferbetweenthetwointeriorglasssurfaceswithinthevacuumgap.In
the 3-Dmodel,thesupportpillararraywasincorporatedandmodelledwithintheglazingunitdirectly.
The predicteddifferenceinoverallheattransfercoefficientsbetweenthetwomodelsforthevacuum
window simulatedwaslessthan3%.Aguardedhotboxcalorimeterwasusedtodeterminethe
experimentalthermalperformanceofvacuumglazing.Theexperimentallydeterminedoverallheat
transfer coefficientandtemperatureprofilesalongthecentrallineofthevacuumglazingareinvery
good agreementwiththepredictionsmadeusingthe2-Dand3-Dmodels.
... Theoretically the emittance value depends on the surface temperature, the wavelength, the angle of incidence of the radiation to the normal [11]. Equation 3 was assumed to be independent of these parameters, which resulted in an error of 4% [11]. ...
... Theoretically the emittance value depends on the surface temperature, the wavelength, the angle of incidence of the radiation to the normal [11]. Equation 3 was assumed to be independent of these parameters, which resulted in an error of 4% [11]. ...
Thermal performance of vacuum glazing predicted by using two-dimensional (2-D) finite element and three-dimensional (3-D) finite volume models are presented. In the 2-D model, the vacuum space, including the pillar arrays, was represented by a material whose effective thermal conductivity was determined from the specified vacuum space width, the heat conduction through the pillar array and the calculated radiation heat transfer between the two interior glass surfaces within the vacuum gap. In the 3-D model, the support pillar array was incorporated and modelled within the glazing unit directly. The predicted difference in overall heat transfer coefficients between the two models for the vacuum window simulated was less than 3%. A guarded hot box calorimeter was used to determine the experimental thermal performance of vacuum glazing. The experimentally determined overall heat transfer coefficient and temperature profiles along the central line of the vacuum glazing are in very good agreement with the predictions made using the 2-D and 3-D models.
... For a VIG with glass sheets having mean temperature T m and individual hemispherical emittances ε hot and ε cold (at temperatures T hot and T cold ), the thermal conductance h r associated with radiative heat transport through the evacuated space is (Zhang et al. 1997): ...
This is the second of two papers concerning errors in the measurement of the thermal insulating properties of Vacuum Insulating Glass (VIG) due to non-uniformities in the heat flow due to the support pillars. Part 1 deals with the situation where the measurement area is in direct thermal contact with the glass sheets. This paper discusses how the non-uniformities and associated measurement errors can be reduced using thermally insulating buffer plates on each side of the specimen. A single parameter is developed that characterises the maximum error for measurement areas of all sizes. Values of this parameter are given for all practically relevant designs of the VIG and properties of the buffer plates. Methods are developed for selecting measurement conditions that lead to acceptable tradeoffs between reducing the errors associated with non-uniformities in the heat flow and errors due to heat flow through the edges of the specimen.
... To a good approximation, the radiative heat flow between plane parallel surfaces of area A and hemispherical emittances ε hot and ε cold at temperatures T hot and T cold is given by Zhang et al. (1997): ...
Non-uniformities in the heat flow through the support pillars in vacuum insulating glass (VIG) can lead to significant errors in the measurement of the thermal insulating properties of these devices. This paper discusses these errors in instruments for which the measurement area is in direct thermal contact with the glass sheets. The spatial non-uniformities of the heat flow in different VIG designs are modelled using the finite element method. For measuring areas with large dimension compared with the separation of the support pillars, the errors are unacceptably large for all practical designs of VIG when using guarded hot plate instruments. These errors are less for heat flow meter instruments due to the construction of the heat flux transducer.
... Low-e coats affect radiation in the thermal wavelengths of the infrared region of the spectrum [19,20]. They are typically metal or doped metal oxides [19,21]. ...
While energy-efficient window designs have been available for decades, their uptake in the Australian residential sector has been slow. This is partially due to inadequate policies and a scarcity of research about the optimal designs across different climate zones around Australia. This study addresses this gap by investigating the impact of different window designs on the annual energy loads of a house for all eight Australian capital cities covering diverse climate zones. The analysis focussed on two types of double glazing: uncoated; and coated with low-emittance films for improved thermal insulation (U-value) or lower solar heat gain. The results show that while low-emittance double glazing provides superior performance, the low solar gain option should only be installed in sunnier climate zones. The thermal performance of different frames (timber, aluminium, thermally broken aluminium and uPVC) was also analysed. The best performance is provided by timber and uPVC frames – especially for the sunnier climate zones, where uPVC frames provide the most energy savings for cost. This paper highlights the need for a more nuanced understanding of windows and their performance in delivering improved performance of housing. It also demonstrates limitations of current policies and modelling software that must be addressed to improve outcomes.
... Radiation reflectance was assumed to be independent of the wavelength, angle of the incident radiation and surface temperature. The maximum error associated with ignoring these effects has been shown to be about 4% for two uncoated glass sheets (Zhang et al., 1997). Interactions between the levels of heat conduction through the pillars and radiation between the two internal glass surfaces within the evacuated gap for a well made vacuum glazing when compared to the total heat flow through the overall glazing system is small and can be ignored. ...
... Zhang et al. [7] studied experimentally and numerically the radiative heat transfer through the thin oxide-coated glass, considering the multiple reflections on the surfaces of the glass and the coated layer. That approach generated results that have shown good agreement with experimental data, although they did not cover the glass-surrounding interface reflections. ...
... Radiation reflectance was assumed to be independent of the wavelength, angle of the incident radiation and surface temperature. The error associated with calculating emittance using equation 2 in this research and ignoring dependence on surface temperature, the wavelength and angle of incidence of the radiation to the normal is about 4% [16]. ...
The electrochromic (EC) vacuum glazing (VG) comprised three 0.5 m by 0.5 m glass panes with a 0.12 mm wide evacuated space between two 4 mm thick panes sealed contiguously by a 6 mm wide metal edge seal with either one or two low-emittance (low-e) coatings supported by a 0.32 mm diameter square pillar grid spaced at 25 mm. The third glass pane on which the EC layer was deposited was sealed to the evacuated glass unit. Simulation demonstrated that with the EC glazing installed with the EC component facing the outdoor environment, for an incident solar radiation of 300 Wm -2 , when the EC layer is opaque for winter conditions, the inside glass pane of the unit due to solar radiation absorbed by the low-e coatings within the vacuum gap and EC layer is a heat source with heat transferred from the glazing to the interior environment.
... Radiation refl ectance was assumed to be independent of the wavelength, angle of the incident radiation and surface temperature. The error associated with calculating emittance using equation 2 in this research is about 4% (Zhang et al., 1997). This ignores dependence on surface temperature, the wavelength and angle of incidence of the radiation. ...
The thermal performance of triple vacuum glazing subjected to various solar insolation levels was simulated using a fi nite volume model. Simulation results show that with increasing insolation, the temperatures of the three glass panes increase; however the rate of temperature increase of the middle glass pane is the largest. This is due to the high thermal insulation provided by the vacuum gaps either side of the middle glass pane; consequently the heat absorbed from the sun by the middle glass pane cannot be easily transferred to the indoor and outdoor environments. For 0.5 m by 0.5 m and 1 m by 1 m triple vacuum glazing exposed, to isolation levels greater than 200 W.m -2 and 180 W.m -2 respectively with four 0.03 emittance coatings, the middle glass sheet temperature is larger than that of the indoor and outdoor glass sheets. Thus the heat absorbed from solar radiation by the middle glass sheet fl ows to both the indoor and outdoor glass sheets. When the insolation is less than 200 W.m -2 and 180 W.m -2 for 0.5 m by 0.5 m glazing and 1 m by 1 m glazing respectively, the heat fl ows from the indoor glass sheet to the middle glass sheet and then to the outdoor glass sheet. For a 0.5 m by 0.5 m triple vacuum glazing with four 0.18 emittance coatings, when insolation is greater than 400 W.m -2 , the middle glass sheet temperature is higher than that of indoor glass sheet.
... Theoretically, the emittance value depends on the surface temperature, the wavelength, and the angle of incidence of the radiation to the normal. The error that resulted from using equation 3 is approximately 4% (Zhang et al., 1997). ...
The thermal performance of a vacuum glazing employing coatings with emittance values between 0.02 and 0.16 were simulated using a three-dimensional finite volume model. The thermal performance of vacuum glazings with hard and soft coatings with emittance values of 0.04, 0.12 and 0.16 were fabricated and characterised using a guarded hot box calorimeter. Both simulated and experimentally characterised vacuum glazings consisted of two 400mm by 400mm glass panes which were separated by a 0.15mm wide evacuated space sealed contiguously by a 6mm wide metal edge seal. The vacuum space formed between the glass sheets was supported by a 0.4mm diameter array of pillars spaced 25mm apart on a square grid pattern. Each glazing had either one or two low-emittance (low-e) coatings. Details of the effect of different low-e coatings on the thermal performance of the simulated glazing are presented. Experimentally determined thermal performance for vacuum glazing with soft (emittance 0.04) and hard coatings (emittance 0.12 and 0.16) were in good agreement with theoretical predictions. The simulations show that for a low value of emittance (e.g. 0.02), the use of two low-e coatings gives limited improvement in thermal performance of the glazing system. The use of a single high performance low-e coating in the vacuum glazing has been shown to provide excellent performance.
... Radiation reflectance was assumed to be independent of the wavelength, angle of the incident radiation and surface temperature. The maximum error associated with ignoring these effects has been shown to be about 4% for two uncoated glass sheets (Zhang et al., 1997). ...
An electrochromic (EC) vacuum glazing (VG) is formed when a vacuum glazing is combined with an electrochromic glazing. Three glass panes are required, two of which may have a low-emittance coating separated by a pillar array, the space formed being evacuated and sealed contiguously by a metal edge seal, the third glass pane with an EC layer is sealed to the evacuated glass unit. With the EC glazing installed with the electrochromic component facing the outdoor environment, for an incident solar radiation of 300 W m−2, when the EC layer is opaque for winter conditions, the inside glass pane of the unit due to solar radiation absorbed by the low-emittance coatings within the vacuum gap and electrochromic layer is a heat source with heat transferred from the glazing to the interior environment.
... The error incurred by using Eq. (3) was approximately 4% (Zhang et al., 1997). ...
The thermal performances of vacuum glazings employing coatings with emittance between 0.02 and 0.16 were simulated using a three-dimensional finite volume model. Physical samples of vacuum glazings with hard and soft coatings with emittance of 0.04, 0.12 and 0.16 were fabricated and their thermal performance characterised experimentally using a guarded hot box calorimeter. Good agreement was found between experimental and theoretical thermal performances for both a vacuum glazing with a soft coating (emittance 0.04) and those with hard coatings (emittance 0.12 and 0.16). Simulations showed that for a low value of emittance (e.g. 0.02), the use of two low-emittance coatings gives limited improvement in thermal performance of the glazing system. The use of a single high performance low-emittance coating in a vacuum glazing has been shown to provide excellent performance.
... The effective area of this apparatus was determined by measuring the rate of radiative heat flow between uncoated soda lime glass sheets in a dynamically pumped sample. This heat flow is known to very high accuracy by integration of the infrared reflectance of the glass over all relevant wavelengths and angles for radiative heat transfer between the sheets [12]. The measurements of vacuum conductance made with this apparatus have been demonstrated to be accurate to about ±1%. ...
A method is described for measuring the thermal conductance of vacuum glazing that is well-suited for integration into the manufacturing process of such devices. The sample of vacuum glazing to be measured, initially at elevated temperature, is placed in contact with a second sample of vacuum glazing with a known thermal conductance. The external surfaces of the glazings are then cooled by forced flow of air at room temperature, and a measurement is made of the rate of decrease of the temperature of the contacting glass sheets of the two samples. The method is simple to implement, and can be automated. The results obtained with the method are quite reproducible. The measurement can be made as the production samples of vacuum glazing cool at the completion of the manufacturing process, resulting in significant savings in time and labour compared with other methods.
... However, these curves must be seen together withFig. 8 0.2 K m 2 /W [2] e Hemispherical surface emissivity 0.855 [22] a Cell efficiency parameter 0.5546 [20] s Stephan–Boltzmann constant 5.67 Â 10 À8 W/m 2 K À4 [19] b shows the cell temperature rise with increasing concentration. It shows that the maximum power points correspond with very high cell temperatures. ...
Cooling of photovoltaic cells is one of the main concerns when designing concentrating photovoltaic systems. Cells may experience both short-term (efficiency loss) and long-term (irreversible damage) degradation due to excess temperatures. Design considerations for cooling systems include low and uniform cell temperatures, system reliability, sufficient capacity for dealing with ‘worst case scenarios’, and minimal power consumption by the system. This review presents an overview of various methods that can be employed for cooling of photovoltaic cells. It includes the application to photovoltaic cells of cooling alternatives found in other fields, namely nuclear reactors, gas turbines and the electronics industry. Different solar concentrators systems are examined, grouped according to geometry. The optimum cooling solutions differ between single-cell arrangements, linear concentrators and densely packed photovoltaic cells. Single cells typically only need passive cooling, even for very high solar concentrations. For densely packed cells under high concentrations (>150 suns), an active cooling system is necessary, with a thermal resistance of less than 10−4 K m2/W. Only impinging jets and microchannels have been reported to achieve such low values. Two-phase forced convection would also be a viable alternative.
... The error resulting from using Eq. (20) is about 4% (Zhang et al., 1997). Two-dimensional experimentally validated finite element models have been used to simulate the thermal performance of double glazing window systems (Elmahdy and Frank, 1993;Wright and Sullivan, 1995;Curcija and Goss, 1994). ...
Flat vacuum glazings consisting of a narrow evacuated space between two glass panes separated by an array of small support pillars have been fabricated. A guarded hot box calorimeter was designed and constructed to measure their heat transfer coefficients. Experimental measurements of temperatures and rates of heat transfer were found to be in very good agreement with those predicted using a developed finite element model. A method for determining the heat transfer coefficient of the evacuated gap has been established and comparisons are made between the measured and predicted glass surface temperature profiles of the exposed glass area and the heat transfer coefficients of the total glazing system in order to validated the model.
The fully tempered vacuum glazing, consisting of two fully tempered glass plates detached through a limited vacuum medium (below 0.01 Pa), is presented. The heat transfer through fully tempered vacuum glazing is complex, including heat conduction, thermal radiation, and convection. To analyze heat conduction through a stainless steel support ball, the thermal resistance of the support ball is established based on Hertzian contact model. And the total thermal resistance of the unit cell with one support ball is defined. The three-dimensional finite element model for a center unit cell of fully tempered vacuum glazing is simulated to validate the result of the total thermal resistance. The simulation transmission U-values are 0.26 W/m² K. Meanwhile, the simulation transmission U-values of entire fully tempered vacuum glazing are 0.84 W/m² K without frame insulation.
The thermal performance (U-value) of 0.4 × 0.4 m and 1 × 1 m double vacuum glazing (DVG) and triple vacuum glazing (TVG) using annealed and tempered glass panes with pillar separations of 25 and 50 mm respectively was simulated. It was found that (1) for both dimensions of DVG with 0.03 emittance low-emittance (low-e) coatings, the U-values at the centre of the glazing area of the DVG made of annealed and tempered glass panes were 0.57 and 0.30 Wm−2 K−1, a reduction of 47.4 %; (2) for both dimensions of TVG with 0.03 emittance low-e coating, the U-values at the centre of glazing area of the TVG with annealed and tempered glass panes were 0.28 and 0.11 Wm−2 K−1, a reduction of 60.7 %. The reduction in U-values for both DVG and TVG results from the significant reduction in pillar number, leading to the large reduction in heat conduction through the pillar arrays. The reduction in U-values from using tempered glass panes instead of annealed glass panes for TVG is larger than that for DVG; this is because the radiative heat transfer of TVG with three glass panes is much lower than that in DVG with two glass panes; therefore, the heat conduction through the pillar array in TVG plays a larger role compared with that in DVG. The reduction in pillar number in TVG results in a larger reduction in U-value compared to DVG; thus, using tempered glass panes in TVG confers a greater advantage compared to DVG, given that DVG can also achieve a large reduction in U-value when switching from using annealed glass panes to tempered glass panes.
The crossed compound parabolic concentrator (CCPC) is one of the most efficient non-imaging solar concentrators used as a stationary solar concentrator or as a second stage solar concentrator. In this study, the CCPC is modified to demonstrate for the first time a new generation of solar concentrators working simultaneously as an electricity generator and thermal collector. The CCPC is designed to have two complementary surfaces, one reflective and one absorptive, and is named as an absorptive/reflective CCPC (AR-CCPC). Usually, the height of the CCPC is truncated with a minor sacrifice of the geometric concentration. These truncated surfaces rather than being eliminated are instead replaced with absorbent surfaces to collect heat from solar radiation. The optical efficiency including absorptive/reflective part of the AR-CCPC was simulated and compared for different geometric concentration ratios varying from 3.6× to 4×. It was found that the combined optical efficiency of the AR-CCPC 3.6×/4× remained constant and high all day long and that it had the highest total optical efficiency compared to other concentrators. In addition, the temperature distributions of AR-CCPC surfaces and the assembled solar cell were simulated based on those heat flux boundary conditions. It was shown that the addition of a thermal absorbent surface can increase the wall temperature. The maximum value reached 321.5K at the front wall under 50° incidence. The experimental verification was also adopted to show the benefits of using absorbent surfaces. The initial results are very promising and significant for the enhancement of solar concentrator systems with lower concentrations.
Vacuum glazing is a new product of saving-energy glass, has an important role in urban construction which promote low carbon, environment protection, energy conservation, reducing building energy consumption. In this paper, as a new method of preparation of the vacuum glazing, use a "wired" approach instead of metal pillar approach. In a vacuum chamber, at 420°C, the use of automatic control equipment to complete exhausting, sealing and other operations to avoid exhaust gas leakage caused by the presence of exhaust port. Through the use of rectangular plate deflection with large ratio of length to width and fixed boundary calculation method and the finite element analysis to determine the number and arrangement of metal filament, identifying with a diameter of 0.1 mm stainless steel filaments arranged in intervals of 60 mm. The heat transfer coefficient (K value) of vacuum glazing is 2.58 W/m2 K, achieve a good thermal insulation effect, consistent with the idea of latest urban construction.
In greenhouse structures, given the major role played by glazing systems with regard to losses and gains of energy in the system, an accurate prediction of the radiative heat transfer through the glazing material and in the interface between cover and ground is of great importance in energy simulations involving solar radiation. Note that the solar radiation is not only a function of the extinction coefficient (consequently the transmission coefficient), but also depends on the multiple reflections inside the glazing and between the glazing and its surroundings
. Therefore, the objective of this study was to incorporate multiple reflections in the glazing and its surroundings, analyzing its influence on radiative heat transfer. In this paper, results show that the methods available in the literature that use only the multiple reflections on the glass cover can return an overestimation of energy transmitted of more than 58 %, while for methods that consider only the transmissivity of the glass this deviation could be even higher, with values of up to 85 %, using a reflectivity of the surroundings of 0.4. This paper also includes a sensitivity analysis, in which the transmitted energy are evaluated changing the quality of the glass and the reflectivity of the surroundings. Results show that the quality of the glass do not affect significantly the energy transmitted, but the reflectivity of the surroundings can considerably affect the prediction of energy gains of the system.
This paper describes the working principles and operation of a new instrument for making absolute measurements of heat flow. This instrument, called the guarded cold plate apparatus, differs from standard guarded hot plate designs in that the measured heat flow through the sample is towards the metering side, rather than away from it. A guarded cold plate apparatus is described for measurement of the radiative heat flow from high emittance surfaces at temperatures up to 200°C. Values of emittance obtained from such measurements agree with those derived from infrared reflectance data to within ±5%
Methods are described for characterizing the thermal insulating properties of vacuum glazing—two flat sheets of glass, hermetically sealed together around the edges containing a highly evacuated space, and separated by small pillars. The small-area guarded hot plate apparatus gives absolute measurements of the different heat flows through the glazing due to radiation, gaseous conduction and thermal conduction through the pillars. In the transient technique, a step temperature increase is applied to one side of the glazing, and the resultant slow temperature rise of the other glass sheet is measured. This method can be used in ageing studies to characterize glazings at elevated temperatures. In the cool-down method, one glass sheet of a glazing that is initially at high temperature is insulated, the opposite glass sheet is rapidly cooled, and the rate of cooling of the thermally insulated sheet is then measured.
Window is a critical component in the design of energy-efficient buildings. To minimize the heat loss, insulation performance of the glazing has to be improved. Manufacturing of vacuum glazing has been motivated by the possibility of making windows of very good thermal insulation properties for such applications. It is made by maintaining vacuum in the gap between two glass panes. Pillars are placed between them to withstand the atmospheric pressure. Edge covers are applied to reduce conduction through the edge. Accurate measurements have been made of the radiative heat transfer, the pillar conduction and the gas conduction using a guarded hot plate apparatus. Vacuum glazing is found to have low thermal conductance roughly below . Among the heat transfer modes of residual gas conduction, conduction through support pillar and the radiative heat transfer between the glass panes, the last one is the most dominant to the overall thermal conductance. Vacuum glazing using very low emittance AI-coated glass has an overall thermal conductance of about .
Vacuum glazing consists of two glass sheets with a narrow internal evacuated space. The separation of the sheets under the influence of atmospheric pressure is maintained by an array of small support pillars. The thermal resistances associated with the heat flow through individual pillars, and through the pillar array, are calculated using a simple analytic method, and by more complex finite element models. The results of both approaches are in very good agreement, and are validated by comparison with experimental data. It is shown that, for many purposes, the amount of heat which flows through the pillars can be determined without incurring significant errors by assuming that the heat flow is uniformly distributed over the area of the glass. Finite element modelling, and a superposition method, are used to determine the temperature distribution on the external surfaces of the glass sheets due to pillar conduction. Again the results obtained with both approaches are in very good agreement. An approximate method is described for calculating the magnitude of these temperature non- uniformities for all practical glazing parameters.
A temperature difference across a sample of vacuum glazing causes differential expansion of one glass sheet relative to the other. In vacuum glazing with a fused edge seal, this results in tensile and compressive stresses in the glass sheets, and bending of the structure. The physical origins of these stresses and deflections are discussed, and a finite element model is used to determine their magnitude. The model has been validated by comparison with experimental data for a well-characterised sample of vacuum glazing under accurately defined external conditions. Modelling data are presented for two glazing designs which have properties that are characteristic of the extremes of performance of this type of glazing. It is shown that mechanical edge constraints can profoundly alter the spatial distribution of stresses in the glazing.
Studying the thermal process of concentrating system could help us better understand how photovoltaic system works and seek ways to increase electricity production so as to reduce the cost of power generation. Energy transfer of concentrating photovoltaic system includes the process of light to electricity and the process of direct current to alternating current. This paper presents the factors that affect the energy transfer efficiency of the former one. And at last author points out that the key factor to increase the power production of photovoltaic system is controlling the temperatu- re of solar cell.
Vacuum glazing consists of two sheets of glass separated by a narrow evacuated space. In this article we describe investigations of the causes of thermally induced degradation of vacuum glazing. It is shown that the rate of increase of pressure within the glazing during high temperature aging is mainly determined by the diffusion of water out of near-surface regions in the glass. In addition, adsorption of water molecules onto the internal surfaces influences the temperature dependence of the pressure in a degraded glazing. A prediction is made of the likely upper limit of the thermally induced degradation of vacuum glazing, and hence its properties as a thermal insulator, for a given production schedule and under conditions representative of practical applications. © 1999 American Vacuum Society.
Thermal performance of an electrochromic (EC) vacuum glazing (VG) was modelled under ASTM standard winter conditions. The EC VG comprised three 0.5 m by 0.5 m glass panes with a 0.12 mm wide evacuated space between two 4 mm thick panes sealed contiguously by a 6 mm wide indium based edge seal with either one or two low-emittance (low-e) coatings supported by a 0.32 mm diameter square pillar grid spaced at 25 mm. The third glass pane on which the 0.1 mm thick EC layer was deposited was sealed to the evacuated glass unit. The whole unit was rebated by 10 mm within a solid wood frame. The low-e coating absorbed 10% of solar energy incident on it. With the EC VG installed with the EC component facing the outdoor environment, for an incident solar radiation of 300 W m², simulations demonstrated that when the EC layer is opaque for winter conditions, the temperature of the inside glass pane is higher than the indoor air temperature, due to solar radiation absorbed by the low-e coatings and the EC layer, the EC VG is a heat source with heat transferred from the glazing to the interior environment. When the emittance was lower to 0.02, the outdoor and indoor glass pane temperatures of the glazing with single and two low-e coatings are very close to each other. For an insolation of 1000 W m², the outdoor glass pane temperature exceeds the indoor glass pane temperature, consequentially the outdoor glass pane transfers heat to the indoor glass pane. (author)
This paper reviews the current state-of-the-art of the science and technology of vacuum glazing. The construction of vacuum glazing, and its method of manufacture in the laboratory, is described. Experimental data are presented on the magnitude of heat flows through vacuum glazing. Gaseous heat transfer is negligible, and the internal vacuum is shown to be stable over many years, in well-manufactured glazing. Values of air-to-air, centre-of-glazing thermal conductance have been achieved ranging from 3Wm−2K−1 (for vacuum glazing with no internal low emittance coating) to 0.8Wm−2K−1 (for samples with two internal low emittance coatings). The overall heat transport rate through 1m×1m samples of vacuum glazing has been measured in accurately calibrated guarded hot box instruments. The results obtained agree to within experimental error (±6%) with those estimated on the basis of local measurements of heat transfer due to radiation, pillar conduction and lateral heat flow through the edge seal. Sources of mechanical tensile stress in vacuum glazing are identified. Stresses due to atmospheric pressure occur in the vicinity of the pillars, and (in poorly designed glazing) near the edge seal. Stresses due to temperature differences are influenced by many factors including the external heat transfer coefficients, level of insulation of the glazing, edge insulation, and edge constraints. Methods of estimating these stresses are discussed. It is shown that vacuum glazing can be designed with adequately low stresses, and low thermal conductance.
This paper describes the first long-term field tests done on vacuum glazing. In this preliminary study, glazing samples were mounted in an outdoor environment and observed for more than one year. The effects of large temperature differences and thermal cycling on the thermal performance and the mechanical stability of the glazings have been investigated. The results provide support for the viability of vacuum glazings in their intended application as thermally insulating windows.
This article discusses the design and construction of guarded hot plate instruments for measuring the heat flow through an evacuated space between plane-parallel glass surfaces. In this structure, the insulating region is surrounded by two pieces of relatively highly conducting material. High resolution measurements of heat flow using these instruments therefore requires the detection of quite small temperature differences (10−4 K) between the metering piece and the guard. The instruments are calibrated, and the linearity evaluated, by measuring radiative heat transfer through the evacuated space between uncoated soda lime glass sheets; this is because this heat flow can be calculated to high accuracy from the infrared optical properties of the glass. The level of parasitic heat flow in the instruments is estimated by measuring radiative heat flow between glass surfaces coated with very low emittance layers, such as evaporated gold. These instruments operate over a range of temperatures from 0 to about 70 °C. It is shown that the heat flow between evacuated glass surfaces can be measured with these instruments to high resolution (∼10 μW) and high accuracy (∼1%) over an area of ∼ 1 cm2. The departures from linearity, and the level of parasitic heat flow, are within the measurement resolution. For a temperature difference across the sample of 20 K, the measurement resolution corresponds to an uncertainty in the thermal conductance of the sample of ∼ 0.005 W m−2 K−1. © 1998 American Institute of Physics.
The thermal performance of an electrochromic vacuum glazing and a vacuum glazing with a range of low-emittance coatings and frame rebate depths were simulated for insolations between 0 and 1000 W m−2 using a three-dimensional finite volume model. The vacuum glazing simulated comprised two 0.4 m×0.4 m glass panes separated by a 0.12 mm wide evacuated space supported by a 0.32 mm diameter pillar array spaced at 25 mm. The two glass sheets were sealed contiguously by a 6 mm wide metal edge seal and had either one or two low-emittance coatings. For the electrochromic vacuum glazing, a third glass pane on which an electrochromic layer was deposited was assumed to be sealed to an evacuated glass unit, to enable control of visible light transmittance and solar gain and thus improve occupant thermal comfort. It is shown that for both vacuum glazing and electrochromic vacuum glazings, when the coating emittance value is very low (close to 0.02), the use of two low-emittance coatings only gives limited improvement in glazing performance. The use of a single currently expensive low-emittance coating in both systems provided acceptable performance. Deeper frame rebate depths gave significant improvements in thermal performance for both glazing systems.
A method is described for measuring the radiative and gas conductance of vacuum glazing while at room or elevated temperatures. Such measurements are used to determine the residual gas pressure, permitting studies to be made of the accelerated degradation of vacuum glazing. The heat flows associated with this method have been analysed using a detailed finite element model and the technique has been calibrated against both high accuracy measurements on a guarded hot plate apparatus, and calculations of the thermal conductance due to radiative heat transfer. The technique is reasonably accurate—total uncertainties are less than about ± 5 % and measurements are reproducible to about ± 2%, for temperatures over the range 20–200°C.
A physical model of the optical behaviour of aluminium nitride cermet solar coatings has been used to optimize the metal volume fraction and layer thickness of the coatings. A modified photo-thermal conversion efficiency for solar collector tubes is presented and used. The cermet layers are generally deposited by reactive sputtering in a gas mixture of argon and nitrogen. Sputtered aluminium Alsp is used as a metallic component in the cermet and its refractive index, evaluated in this study, is employed. Due to oxygen contamination, aluminium oxynitride (AlON) is used as a ceramic component in the cermet. Bruggeman approximations are used to calculate the dielectric function for composite materials. An initial ten-layer grade film optimized to one nearly identical to a double cermet film structure when maximizing photo-thermal conversion efficiency at 80 °C under a concentration of 1. The optimized films, F10Lm (for initial ten-layer graded film) and F3Lm (for three-layer film) have an identical solar absorptance of 0.957 and an identical hemispherical emittance of 0.048 at 80 °C. The optimized film consists of one anti-reflection layer and two cermet layers with metal volume fractions of 0.093 and 0.255, and thicknesses of 30 nm and 93 nm, respectively, going from the anti-reflection coating to the infrared reflector layer. The solar performance can be further improved using a lower refractive index anti-reflection layer and a lower emittance infrared reflector. For example, using an Al2O3 anti-reflection layer, the solar absorptance increases to 0.974, and using a Cu infrared reflector, hemispherical emittance decreases to 0.033 at 80 °C. For these different anti-reflection and infrared reflector materials, optimized calculations have also predicted that the double cermet layer film structures have the highest photo-thermal conversion efficiency.
The layer thickness and tungsten metal volume fraction of W-AlN cermet solar selective absorbing coatings on a W, Cu or Al infrared reflector with a surface aluminium oxynitride (AlON) or Al2O3 ceramic anti-reflector layer were optimized using physical modelling calculations. Due to limited published data for the refractive index of AlN, and likely oxygen contamination during reactive sputtering of AlN ceramic materials, AlON was used as the ceramic component and the published value of its refractive index was employed. The dielectric function and then the complex refractive index of W-AlON cermet materials were calculated using the Ping Sheng approximation. The downhill simplex method in multi-dimensions was used in the numerical calculation to achieve maximum photo-thermal conversion efficiency at 350 °C under a concentration factor of 30 for a solar collector tube. Optimization calculation results show that the initial graded (ten-step layers) cermet films all converge to something close to a three-layer film structure, which consists of a low metal volume fraction cermet layer on a high metal volume fraction cermet layer on a metallic infrared reflector with a surface ceramic anti-reflection layer. The optimized three-layer solar coatings have a high solar absorptance of 0.95 for AlON and 0.96 for the Al2O3 anti-reflection layer, and a low hemispherical emittance of 0.073 at 350 °C. For the optimized three-layer films the solar radiation is efficiently absorbed internally and by phase interference. Thermal loss is very low for optimized three-layer films due to high reflectance values in the thermal infrared wavelength range and a very sharp edge between low solar reflectance and high thermal infrared reflectance. The high metal volume fraction cermet layer has a metal-like optical behaviour in the thermal infrared wavelength range and makes the largest contribution to the increase of emittance compared with that of the metal infrared reflector.
This paper reports an experimental and theoretical study of the stresses in and durability of vacuum glazing fabricated at low temperature using an indium based edge seal. For the first time a finite-element model with support pillars incorporated directly, enabled the stresses in the whole structure to be explicitly calculated. Experimental validations of the finite element model predictions were undertaken. Modelling results are presented for a case with American Society of Testing and Materials standard winter boundary conditions. It was found that, for the particular system studied, the predicted stress level in the structure is essentially the same for indium sealed and solder glass sealed vacuum glazing, and the magnitude of stress values in the indium seal is comparable with that dictated by the indium strength characteristics.
Results for the thermal emittance of copper films deposited on glass are reported for the 40–460 °C range. The values found are substantially below those previously reported for pure copper samples and are smaller than is predicted on the basis of diffuse scattering of electrons by the metal surface.
The relationship between the electric-susceptibility mass m
s
of free carriers in a semiconductor and the optical properties of the material in the infrared region of the spectrum was first pointed out and applied by Spitzer and Fan [1]. As part of their general treatment of this subject, they showed that reliable values of m
s
could often be obtained from simple measurements of the normal reflectivity as a function of wavelength. Since that time, this method has been used widely as an experimental tool for studying the nature of charge carriers in semiconductors [2–23].
Directional emittance plays an important role in the calculation of radiative heat exchange. It partly determines the thermal insulation of single and multiple glazing and the efficiency of solar collectors. An emissiometer has been designed and built, capable for measurements of the directional emittance at angles up to 85 degree(s). The emissiometer can be used for absolute measurements, with a black body radiator as reference, and for relative measurements using a known reference sample. In the case of low-emissivity materials the directional emittance usually has a maximum at an angle between 80 degree(s) and 90 degree(s) (pseudo Brewster angle). The emissiometer is, therefore, especially useful for the accurate characterization of low-emissivity materials like spectral selective coated glazing and solar absorbers. The paper gives a description of the instrument and results are discussed for three coated glass samples with low, medium, and high emissivity, respectively, which were measured for a temperature of 283 K. Results are given of the calculation of the thermal transmittance (or U-value) and of three types of double glazing in which the low, medium, and high emissivity coating was applied.
A comprehensive set of optical constants and a partial set of bulk optical properties are listed for clear, absorbing, and low-iron glasses used for windows. The measurements extend from the near ultraviolet to the far infrared, covering the range of interest for calculating solar and thermal radiative transfer through windows. This information is also needed to calculate the properties of thin-film coatings on glass substrates. Large variations in solar absorption are observed among these glasses, whereas the far-infrared properties are almost constant for all glass types. An important quantity, the hemispherical total emissivity, is 0.837 at 20°C, as determined from reflectance data measured with an IBM Fourier-transform spectrometer. Solar properties are determined from conventional transmission and reflection measurements.
A model has been developed to explain the greatly reduced reflectivities of Ge samples implanted to doses 1.25–1.5×1018 O/cm2 with 45‐keV O+ ions, which have reflectivity values close to zero at 0.7 μm. The model divides the inhomogeneous implanted layer into a series of homogeneous sublayers with different volume fraction and thickness for each sublayer. The complex refractive indexes for every sublayer are calculated using the Maxwell Garnett (MG) and Persson–Liebsch (PL) theories. Using the expressions for the reflectivity of an absorbing multilayer layer systems, the reflectivities have been calculated. The reflectivity curve calculated in the PL approximation is closer to the experimental observations than is the MG approximation over the wavelength range 0.2–3.0 μm.
We review work on In2O3:Sn films prepared by reactive e-beam evaporation of In2O3 with up to 9 mol % SnO2 onto heated glass. These films have excellent spectrally selective properties when the deposition rate is ∼0.2 nm/s, the substrate temperature is ≳150 °C, and the oxygen pressure is ∼5×10−4 Torr. Optimized coatings have crystallite dimensions ≳50 nm and a C-type rare-earth oxide structure. We cover electromagnetic properties as recorded by spectrophotometry in the 0.2–50-μm range, by X-band microwave reflectance, and by dc electrical measurements. Hall-effect data are included. An increase of the Sn content is shown to have several important effects: the semiconductor band gap is shifted towards the ultraviolet, the luminous transmittance remains high, the infrared reflectance increases to a high value beyond a certain wavelength which shifts towards the visible, phonon-induced infrared absorption bands vanish, the microwave reflectance goes up, and the dc resisitivity drops to ∼2×10−4 Ω cm. The corresponding mobility is ∼30 cm2/V s. The complex dielectric function ε is reported.
These data were obtained from carefully selected combinations of spectrophotometric transmittance and reflectance data. It is found that ε can be reconciled with the Drude theory only by assuming a strongly frequency-dependent relaxation energy between the plasma energy and the band gap. We review a recently formulated quantitative theoretical model for the optical properties which explicitly includes the additive contributions to ε from valence electrons, free electrons, and phonons. The theory embodies an effective-mass model for n-doped semiconductors well above the Mott critical density. Because of the high doping, the Sn impurities are singly ionized and the associated electrons occupy the bottom of the conduction band in the form of an electron gas. The Sn ions behave approximately as point scatterers, which is consistent with pseudopotential arguments. Screening of the ions is described by the random phase approximation. This latter theory works well as a consequence of the small effective electron radii. Exchange and correlation in the electron gas are represented by the Hubbard and Singwi–Sjölander schemes. Phonon effects are included by three empirically determined damped Lorentz oscillators. Free-electron properties are found to govern the optical performance in the main spectral range. An analysis of the complex dynamic resistivity (directly related to ε) shows unambiguously that Sn ions are the most important scatterers, although grain-boundary scattering can play some role in the midvisible range.
As a result of this analysis one concludes that the optical properties of the best films approach the theoretical limit. Band-gap shifts can be understood as the net result of two competing mechanisms: a widening due to the Burstein–Moss effect, and a narrowing due to electron-electron and electron-ion scattering. The transition width—including an Urbach tail—seems to be consistent with these notions. Window applications are treated theoretically from detailed computations of integrated luminous, solar, and thermal properties. It is found that In2O3:Sn films on glass can yield∼78% normal solar transmittance and ∼20% hemispherical thermal emittance. Substrate emission is found to be insignificant. Antireflection with evaporated MgF2 or high-rate sputtered aluminum oxyfluoride can give ∼95% normal luminous transmittance, ∼5% normal luminous reflectance, little perceived color and little increase in emittance. A color purity <1% in normal transmission and <10% in normal reflection is achievable for a daylight illuminant within extended ranges of film thickness.
A guarded hot-plate apparatus has been developed for measuring the local thermal conductance of flat evacuated glazing. Parasitic heat flows in the apparatus have been reduced to below an equivalent thermal conductance of 0.01 W m2 K1. Techniques are described for determining the separate contributions to heat flow through the sample from pillar conduction, conduction through residual gas, and radiation. The accuracy of the measurement system is estimated to be better than ±2% and the reproducibility for sequential measurements is better than ±0.004 W m2 K1 for a measurement area of approximately 1.7 cm2.
An ellipsometer attachment for a commercially available Fourier transform spectrometer (Bruker IFS 66) was developed. Almost no modification of the basic spectrometer was necessary. The optical set-up, providing a parallel light beam with variable diameter, is placed in the sample compartment. The beam is deflected to an external goniometer attachment which allows measurements to be taken with incident angles of up to 85° depending on the sample size. The possible combination of our optical components (light sources, beam-splitters, polarizers, and detectors) allow measurements in the wavenumber range from 30 000 to 400 cm−1.The polarizers are aligned automatically. Determination of the refractive index of thin films is based on measurements at multiple angles of incidence and evaluation by the least squares fit method.
Measurement of angular emissivity of coated flat glass: extension of the angular range up to 85°
- P A Van Nijnatten
- F Simonis
Measured angular distribution of the emissivity and calculated radiation heat transfer of architectural coated flat glass, parts 1 and 2
- Geotti-Bianchini
Heat Transfer (SI Metric Edn
- J P Holman