Low emittance coatings and the thermal performance of vacuum glazing

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.     

高效平板太阳能集热器盖板的热工性能实验研究

针对目前平板太阳能集热器高温下集热效率低的缺点,研究通过改善透明盖板性能来提高集热效率的方法。在实验基础上,综合各种因素,通过对比实验方法比较了中空玻璃(DG)、充氩气中空玻璃(ADG)、Low-E中空玻璃(LDG)和Low-E充氩气中空玻璃(LADG)作为盖板对太阳能集热参数和效率的影响,并研究了上述4种盖板对太阳辐射的透射率和保温性能的不同影响。     

Thermal performance analysis of an electrochromic vacuum glazing with low emittance coatings

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 m⁻², 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 m⁻², the outdoor glass pane temperature exceeds the indoor glass pane temperature, consequentially the outdoor glass pane transfers heat to the indoor glass pane.     

Predicted thermal performance of triple vacuum glazing

The simulated triple vacuum glazing (TVG) consists of three 4 mm thick glass panes with two vacuum gaps, with each internal glass surface coated with a low-emittance coating with an emittance of 0.03. The two vacuum gaps are sealed by an indium based sealant and separated by a stainless steel pillar array with a height of 0.12 mm and a pillar diameter of 0.3 mm spaced at 25 mm. The thermal transmission at the centre-of-glazing area of the TVG was predicted to be 0.26 W m⁻² K⁻¹. The simulation results show that although the thermal conductivity of solder glass (1 W m⁻¹ K⁻¹) and indium (83.7 W m⁻¹ K⁻¹) are very different, the difference in thermal transmission of TVGs resulting from the use of an indium and a solder glass edge seal was 0.01 W m⁻² K⁻¹. This is because the edge seal is so thin (0.12 mm), consequently there is a negligible temperature drop across it irrespective of the material that the seal is made from relative to the total temperature difference across the glazing. The results also show that there is a relatively large increase in the overall thermal conductance of glazings without a frame when the width of the indium edge seal is increased. Increasing the rebate depth in a solid wood frame decreased the heat transmission of the TVG. The overall heat transmission of the simulated 0.5 m by 0.5 m TVG was 32.6% greater than that of the 1 m by 1 m TVG, since heat conduction through the edge seal of the small glazing has a larger contribution to the total glazing heat transfer than that of the larger glazing system.     

Effect of glass thickness on the thermal performance of evacuated glazing

Flat evacuated glazing consists of two plane glass panes separated by a narrow internal evacuated space. Separation in the space is maintained by an array of support pillars typically 0.32 mm in diameter and 0.12 mm high arranged on a regular square grid with an inter-pillar separation of up to 40 mm. A detailed three-dimensional finite volume model has been employed to determine the variation of thermal performance of an evacuated glazing as a function of glass pane thickness. It was predicted that for evacuated glazing of dimensions of 0.3 m by 0.3 m and 0.5 m by 0.5 m, reducing glass pane thickness gave improved thermal performance. For evacuated glazings with dimensions of 1 m by 1 m, the opposite was predicted.     

The effect of glazing shape upon the thermal performance of buildings

The great incidence that glazing has in a building energy conservation makes it one of the most important parameters to be taken into account especially in commercial buildings, where the surface occupied by glass areas is very important. So, different shapes of glass areas and their influence in the energy consumption of a commercial building are studied in this paper. Horizontal glazing (with different heights) and vertical glazing (with the same area as the horizontal ones), separated by opaque areas are considered in a base case building. A traditional wall and a curtain-wall are considered, and the different annual consumptions per conditions unit surface, both in winter and summer, are obtained.     

Smart switchable glazing for solar energy and daylight control

The exciting field of chromogenic materials for smart windows and other large-area applications is discussed. A selection of switchable glazing devices that change color electrically are detailed. The types of devices covered are the electrochromic which change color electrically, covering electrochromic, dispersed liquid crystal and dispersed particle glazing that switch under an applied electric field. Device structures and switching characteristics are compared. The status of prototype and commercial devices from commercial and university labs through out the world are covered. A discussion of the future of this technology is made including areas of necessary development for the realization of large area glazing in excess of 1m².     

Spectrally selective laminated glazing consisting of solar control and heat mirror coated glass: preparation, characterization and modelling of heat transfer

In this study, solar control coatings were prepared by sequential depositions of thin films of ZnS (40 nm)–CuS (150 nm) and ZnS (40 nm)–Bi₂S₃ (75 nm)–CuS (150 nm) from chemical baths on 3 mm thick commercial sheet glass. These were laminated to 3 mm thick clear glass or commercially available SnO₂ based heat mirror coating of sheet resistance 15 Ω on float glass of 3 mm thickness using a poly(ethylene vinyl acetate), EVA, sheet of 0.36 mm thickness in a vacuum process at 120 °C for 30 min. In total, the thickness of the glazing was 6.35 mm. The glazings possess visible transmittance, weighted for D65 solar spectra and sensitivity of the human eye for daylight vision, of 36% or 14% with solar absorptance of 71% or 78% depending on the coating type, i.e ZnS–CuS or ZnS–Bi₂S₃–CuS-heat mirror respectively. The solar heat gain coefficient (SHGC) was evaluated for these glazings at exterior temperatures of 15 and 32 °C for an exterior convective heat transfer coefficient (h_ex) of 6–100 Wm⁻² K⁻¹ using a mathematical model. The model predicts the extent of reduction in SHGC through the presence of the heat mirror coating as a function of h_ex and hence helps to decide on the relative benefit, which may be derived through their use in different locations. Though the deposition technique mentioned here involves longer duration compared with vacuum techniques, it may be developed into a low throughput, low-capital alternate technology for small-scale production.     

A glazing unit for solar control, daylighting and energy conservation

The glazing unit for solar control, daylighting and energy conservation is a system consisting of two prismatic panes. The prismatic ribs of the panes are inclined by a certain angle to the horizontal within the window plane, exhibit identical cross-sections in the shape of a rightangle-triangle with a certain basic prism angle, are facing each other and are positioned such that just a small gap remains between the two panes. The lower rib faces of the outer prismatic pane are coated with a specularly reflecting layer and the upper rib faces of the inner prismatic pane are coated with a diffusely reflecting layer. The prismatic glazing unit can be used for common window tilt angles and for window directions with significant solar irradiation at sites with a temperate climate. It does not reduce the view to the outside appreciably and achieves — in comparison to other window panes — relatively uniform illumination of a room with daylight. During the summer and the transitional seasons it provides improved protection against solar irradiation and distinctly reduced irradiated heat fluxes. The reflecting surfaces of the prismatic ribs do not create glare.     

Spectral effects on the transmittance, solar heat gain, and performance rating of glazing systems

This study investigates the potential changes in Solar Heat Gain Coefficient (SHGC) and Visible Transmittance (VT) ratings of vertical or tilted glazing systems that would result from a deliberate change in the reference spectrum used as Spectral Weighting Function (SWF). This SWF is necessary to evaluate broadband-average optical properties from their spectral values, and obtain the desired rating of such bulk properties. The SWFs currently specified by rating institutions in Europe and North America for SHGC and VT are now outdated, and their inadequacies are discussed. Six potential replacements, which have been recently adopted by ASTM are described, including three direct irradiance spectra and three global irradiance spectrum incident on tilted surfaces of various tilts (20°, 37° and 90°). Some of these spectra have been tailored for use in building energy applications, including Building-Integrated Photovoltaics (BIPV). The effect of tilt on the U-factor and hence SHGC of glazing systems used for skylights on roofs is discussed, using a representative dataset of 37 glazing system specimens. The spectral effects on SHGC induced by a change in the current North American SWF are also obtained for this dataset, and show small to moderate deviations from current ratings (−2% to +7% for windows, and −3% to +11% for skylights). The variations in VT are within ±2% for most glazing systems. To remove the current inconsistency in the SWFs used for SHGC and VT, it is recommended that a single SWF be used for both properties. For improved accuracy and reliance on active standards, it is also recommended that the SWF for SHGC and VT be either one of the two recent ASTM G197-08 global irradiance spectra, depending on application (incidence on a vertical surface for window applications, and incidence on a 20°-tilted surface for skylight applications). No change in colorimetric calculations (based on the D65 illuminant) is recommended, however.     

提高太阳能集热器热性能的实验研究

在能量平衡方程的基础上,以《家用太阳热水器热性能试验方法》（GB／T12915—1991）为依据,对两种有渐开线反光板且吸热体形状不同的热管真空管和一种无反光板的热管真空管进行了对比实验。结果表明,渐开线反光板可大幅度提高热管真空管热水器的日平均效率,加反光板且吸热体为圆柱形的集热器能够达到更好的效果。     

An evaluation on thermal performance of CPC solar collector

The main objective of this work is the investigation and improvement of thermal performance of evacuated CPC (Compound Parabolic Concentrator) solar collector with a cylindrical absorber. Modified types of this solar collector are always combined with the evacuated glass envelop or tracking system. The conventional stationary CPC solar collector has been compared with the single axis tracking CPC solar collector in outlet temperature, net heat flux onto the absorber and thermal efficiency. Numerical model has been analyzed based on the irradiation determined actually and the results have been calculated to predict the thermal efficiency. Based on the comparison of the measured and calculated results, it is concluded that the numerical model can accurately estimate the performance of solar collectors. The result shows the thermal efficiency of the tracking CPC solar collector is more stable and about 14.9% higher than that of the stationary CPC solar collector.     

Investigation on thermal performance of glazed solar chimney walls

This paper reports investigation on the thermal performance of glazed solar chimney walls (GSCW) under the tropical climatic conditions of Thailand. The GSCW consisted of double glass panes with an air layer and openings located at the bottom (room side glass pane) and at the top (ambient side glass pane). A prototype of GSCW was integrated into the southern wall of a small room of 2.8 m³ volume. Its dimensions were as follows: 0.74 m height, 0.50 m width and 0.10 m air gap. The size of openings was 0.05 × 0.5 m². With a clear glass of 6 mm thickness, velocity field measurement indicated that the induced airflow rate was about 0.13–0.28 m³/s. The temperature difference between room and ambient was less than that with a single layer clear glass window. The reduction of daylight due to the double glass layer is negligible. Comparison between simulated and experimental results showed a reasonable agreement, therefore, the developed numerical model is valid and could be used as a tool for the design of GSCW.     

Diffuser design influence on the performance of solar thermal storage tanks

The effect of the inlet and outlet diffuser design on the performance of thermal stratification in a vertical water tank is investigated experimentally. Two sets of diffusers are used in the experiments, which are conducted with a moving thermocline (both up and down) for different flow rates. The results indicate that the preservation of the initial thermocline is excellent when using a settling mesh. It is also shown that the extraction efficiency of the tank is higher at low flow rates during charging, whereas it is lower at low flow rates during discharging.     

新型平板型太阳集热器热性能浅析

平板型太阳集热器有相对低廉的价格、成熟的制造技术及固有的热性能特点，在我国气候温暖的南方有很大市场，在寒冷的北方只有部分应用。这主要是由于平板型集热器保温性能差，与外界有热交换，高温效率特性不好的原因造成的。所以提高平板型集热器热性能指标，生产出新一代平板集热器是满足市场需求的唯一出路。     

Uncertainty in estimating the performance of solar thermal systems

The scope of this article is to present a methodology for the estimation of the uncertainty characterizing the energy performance of solar domestic hot water systems. The work concentrates on the uncertainty characterizing the expected annual energy output, as calculated through tests implemented according to the valid international standards. In order to cope with difficulties related to the algorithmic character of the measurement model, which cannot be explicitly formulated, Monte-Carlo simulation techniques are implemented. The component of uncertainty associated with measurement errors is estimated, on the basis of the metrological quality anticipated by the relevant testing standards. Errors due to imperfections of the energy model used through the test are also counted in, as well as uncertainties attributed to the variability of meteorological conditions. The proposed uncertainty analysis allows the realistic assessment of the actual energy provided to the user by a solar domestic hot water system. Implementation of the proposed methodology for a typical system leads to an expanded uncertainty in the order of 9% for the expected annual energy output.     

Influence of thermal radiation during aluminum thermal evaporation on organic solar cells

Organic solar cells (OSCs) based on the blends of poly(3-hexylthiophene) and fullerene derivative [6, 6]-phenyl-C₆₁ butyric acid methyl ester have been fabricated with p-type chromium oxide film as hole-transporting layer. The distribution of temperature and heat flow in vacuum chamber was investigated by temperature probes and finite element method. It is found that the two factors have significant effect on the quality of the thermally evaporated Al cathode. The bigger momentum impinging Al particles facilitates the formation of C-Al or C-O-Al bonds at the interface between photoactive layer and Al electrode, which is propitious to the electron transfer, and the corresponding devices have higher electron mobility. The higher substrate temperature that induced by thermal radiation during Al evaporation can act as a function of thermal annealing treatment, which can eliminate some electron traps and lead to better ohmic contacts for electrons to transport at the interface. The combination of the bigger momentum impinging Al particles and the higher substrate temperature can induce the improvement of electron mobility and OSC performance.     

平面真空玻璃作盖板的箱式太阳灶实验研究

用窄缝高真空平面玻璃作普通箱式太阳灶的透明盖板，对箱式太阳灶的集热性能进行了实验研究。将实验结果与用普通双层玻璃作盖板的箱式太阳灶进行了对比，发现用平面真空玻璃作盖板能够显著改善箱式太阳灶的性能。     

Study on thermal performance of solar methanol reforming MCFC-GT-ST-CHP system

In recent years, fuel cells have begun to attract the attention of many countries in the world. Based on Aspen Plus, this paper constructs a distributed energy system about solar assisted Methanol reforming-Molten Carbonate Fuel Cell (MR-MCFC), which is combined with GT-ST power generation system and heating system. The results show that the MCFC efficiency, total electrical efficiency and total thermal efficiency of the system are 44.12%, 58.62% and 90.36%, respectively. The research results will provide a theoretical basis for exploring a new method of highly complementary utilization of solar energy and chemical energy. By analyzing the variable working conditions of the system, the complementary utilization affinity characteristics of the solar MR-MCFC and the integrated system are revealed. We will continue to innovate and optimize the system in order to provide reference for the further development of the CHP (Combined Heat and Power) system.     

Influence of solar heating on the performance of integrated solar cell microstrip patch antennas

The integration of microstrip patch antennas with photovoltaics has been proposed for applications in autonomous wireless communication systems located on building façades. Full integration was achieved using polycrystalline silicon solar cells as both antenna ground plane and direct current power generation in the same device. An overview of the proposed photovoltaic antenna designs is provided and the variation characterised of the electromagnetic properties of the device with temperature and solar radiation. Measurements for both copper and solar antennas are reported on three different commercial laminates with contrasting values for thermal coefficient of the dielectric constant.