Total Solar Energy Transmittance Research Articles

04/02052 Evaluation of window energy rating models for different houses and European climates: Karlsson, J. and Ross, A. Solar Energy, 2004, 76, (1–3), 71–77

Abstract In this paper different window energy rating systems (WERS) are evaluated and compared. The comparisons are made for different European climates, types of buildings and orientations. The purpose of the paper is to evaluate how complex a WERS needs to be in order to be able to provide a reliable energy rating which can be applied to different buildings in different climates and orientations. The results indicate that Europe needs to be separated into several different climate zones. A simple linear heating energy-rating model of the form Ag–BU (where g is the total solar energy transmittance, U is the thermal transmittance of the window, and A and B are empirical coefficients), may be sufficient within certain regulations. For warm locations a linear cooling energy rating, depending only on the g factor may also yield an acceptable rating. More advanced models that take the building type into account and that use hourly climate files showgood fit with building simulation programs.

Airflow patterns and thermal behavior of mechanically ventilated glass double façades

Glass double façades (GDFs) are finding increasing use in commercial buildings, but are difficult to design. Of key significance for the cooling load of the building and thermal comfort of its occupants during summertime are the total solar energy transmittance to the interior and the temperatures of the inner pane. A procedure for modeling such façades—comprising a spectral optical and a computational fluid dynamic model—is described and simulated results are compared with data derived from an experimental investigation of two mechanically ventilated GDFs built in an outdoor test facility. It is shown that simple models assuming piston-flows can lead to inaccurate results.

Total solar energy transmittance of glass double façades with free convection

Double façades made of glass layers with a ventilated mid-pane shading device are often used in commercial buildings. The solar-collector-like construction leads to high temperatures in the façade cavities during summertime and the possible overheating of the building. Of key significance for the cooling load of the building and thermal comfort of its occupants is the total solar energy transmittance to the interior. A procedure for modeling such façades—comprising a spectral optical and a computational fluid dynamic (CFD) model—is described and simulation results are compared with data derived from an experimental investigation of a single-story glass double façade (GDF) with free convection, incorporated in an outdoor test facility. The influence of the layer sequence and ventilation properties on the thermal behavior is discussed. It is shown that, for a given set of layers, total solar energy transmittance can easily vary by a factor greater than five. Hence, for reliable prediction of the total solar energy transmittance of a designed façade, models are needed that factor in all the relevant parameters. A spectral optical model combined with a CFD model that includes convection, conduction and radiation, is therefore recommended for analyzing and optimizing glass double façades.

Evaluation of window energy rating models for different houses and European climates

In this paper different window energy rating systems (WERS) are evaluated and compared. The comparisons are made for different European climates, types of buildings and orientations. The purpose of the paper is to evaluate how complex a WERS needs to be in order to be able to provide a reliable energy rating which can be applied to different buildings in different climates and orientations. The results indicate that Europe needs to be separated into several different climate zones. A simple linear heating energy-rating model of the form Ag– BU (where g is the total solar energy transmittance, U is the thermal transmittance of the window, and A and B are empirical coefficients), may be sufficient within certain regulations. For warm locations a linear cooling energy rating, depending only on the g factor may also yield an acceptable rating. More advanced models that take the building type into account and that use hourly climate files showgood fit with building simulation programs.

Facade systems with variable solar control using thermotropic polymer blends

Thermotropic layers integrated into glazing or transparent insulation systems reversibly reduce the total solar energy transmittance by becoming opaque when heated. Characteristics of both systems have been measured as a function of temperature and incidence angle, including the total solar energy transmittance for a thermotropic heat-mirror insulating glazing unit (IGU). Properties of complex thermotropic glazing systems have been simulated, starting from data characterising the thermotropic layer. The results show good agreement with measured data. A prototype system sample of a thermotropic heat-mirror IGU was integrated into an outdoor test facility and a thermotropic TEIF (transparent exterior insulation and finish) prototype system was installed into the facade of a demonstration house. Both systems have been monitored and showed the expected switching properties, resulting in the desired overheating protection. Computer simulations of the building integration of the thermotropic TEIF system have been performed, showing a strong dependence of the optimum switching temperature on the total solar and internal gains of the building.

Highly insulating aerogel glazing for solar energy usage

Granular silica aerogels have been integrated into highly-insulating translucent glazing. This work was performed within the large R&D project ISOTEG pursued by the ZAE Bayern. To avoid settlement of the granules, which often occurred in earlier glazing concepts and even caused destruction of the glazing, the granules were sandwiched between a double skin sheet made of PMMA. The sheet was mounted between two low-e coated glass panes. To optimize the thermal insulation, krypton was used as filling gas. This construction allows to achieve heat transfer coefficients of less than 0.4 W/(m 2 K). Optimized granular layers provide high solar transmittance of 65% for a thickness of 20 mm. Thus a total solar energy transmittance of 35% for the whole glazing unit is achieved. The glazing has a thickness of less than 50 mm. Such aerogel glazings can be integrated into solar wall systems or used as lightscattering daylighting elements with vanishing energy losses over the heating period even for north facade integration. Optical and thermal properties of the developed granular aerogels as well as the thermal properties of the whole glazing unit are reported.

Modelling the optical and thermal properties of complex glazing: overview of recent developments

The recently completed ALTSET project was part of the European Commission’s Standards, Measurement and Testing programme. Its objective was the development of European standard test procedures for the determination of angular-dependent light and total solar energy transmittance for complex glazings and integrated shading elements. In parallel to the development of test procedures, models to predict the properties of a variety of complex glazings were developed and validated. We review the progress made during this collaborative programme. Models have been developed for a wide range of complex glazings, including those incorporating solar control films, transparent insulation and both fixed and variable blinds. The models are based on the concept that a complex glazing can be represented by a stack of layers, each layer representing one of the component elements of the glazing. If the optical and thermal properties of each layer are known, methods are available to calculate the corresponding properties of the stack. Used with some caution, the models are able to predict the luminous and total solar energy transmittance to an accuracy comparable to that obtainable experimentally. The models are suitable for integration into building energy simulation tools. One area requiring further investigation is an improved model for scattering components.

Energy performance of glazings and windows

The purpose of the work is to make it possible to compare the energy performance of different glazings or windows in an easy way. The energy performance is expressed by the net energy gain, which is given as the solar gain minus the heat loss. The net energy gain from glazings and windows both depend on the thermal transmittance (the U-value) and the total solar energy transmittance (the g-value). This fact makes it difficult to choose the glazings or windows with respect to energy performance in a given case. To compare different glazings or windows in an easy way, diagrams have been produced which give the net energy gain based on the orientation, the tilt, the U-value and the g-value of the glazings or windows. In addition, a single diagram showing the net energy gain in a one-family house has been produced which takes into account the orientation of the windows in the building. This makes it possible to evaluate the energy performance of different glazings or windows for a one-family house using a single diagram. The diagrams give an easy way of comparing different glazings or windows so the best solution in a given case can be chosen. In addition, the diagram showing the net energy gain in a one-family house is used in the Danish Energy Labelling and Rating system for classification of glazings.

Evaluation of overheating protection with sun-shading systems

Sun-shading systems have to provide thermal and visual comfort both reliably and economically. At the same time, they should prevent unwanted solar gains in summer and permit high solar gains in winter. This paper describes a method to assess the overheating protection offered by different types of sun-shading systems together with the associated control strategy. The objective of the new method is to provide a simple but reliable and realistic approach to evaluate the effectiveness of internal and external shading devices in combination with glazing, which is independent of the building type. The new method consists of the angle-dependent determination of the total solar energy transmittance, g, based on ray-tracing methods, which has been validated using calorimetric measurements. Combination with annual irradiance distributions allows for the evaluation of different control strategies. This paper shows that it is essential for the reliability of calculated cooling and heating loads, that the calculation is based on a control strategy, which reflects the priorities of the users of the building.

Annual energy window performance vs. glazing thermal emittance — the relevance of very low emittance values

This paper investigates the heating and cooling energy impact of low thermal emittance values for architectural glazings. The importance of low emittance values and the sensitivity for small changes in the emittance is investigated for three very different climates and for two different types of buildings. Our results imply that minor changes in the parameter values lead to very small changes in the energy performance and are mostly insignificant for the end user. Furthermore, the relative importance of the solar transmitting properties is recognized. For instance, if the thermal emittance is reduced by 2–3% and at the same time the total solar energy transmittance is also reduced by 2–3%, the energy performance of the window is worse if it is used in a south facing position in a residential building in a heating dominated location (Stockholm).

Measurement and prediction of angle-dependent optical properties of coated glass products: results of an inter-laboratory comparison of spectral transmittance and reflectance

The European Union ADOPT project has been concerned with the accurate determination of the optical properties of coated glass products, describing reliable procedures for angle-dependent measurements and the development of predictive algorithms to enable accurate angle-dependent performance characterisation based upon near-normal values only. An essential element of the ADOPT project was the inter-laboratory comparison of transmittance and reflectance measurements for commercially available glazings. Thirteen European and three USA laboratories participated. The selected samples represent major categories of coated glass products used in buildings. Each participant received a set of samples and measured the angle-dependent spectral transmittance and reflectance under appropriate polarisation conditions for incident wavelengths within the solar spectral distribution. Calibrated mirrors were also provided for reference purposes. This paper presents the results of the inter-comparison which show that near-normal solar, visible and spectral transmittance and reflectance can be measured with an uncertainty of better than ±0.02. The inter-comparison illustrates the difficulties still experienced in making accurate oblique incidence measurements. The predictive algorithms, which require near-normal input data only, are capable of predicting the angle-dependent transmittance, reflectance and total solar energy transmittance for double and triple glazed windows to an accuracy which enables reliable calculation of the annual solar heat gain through windows.

Evaluation of predictive models for the angle-dependent total solar energy transmittance of glazing materials

The predictions of angle-dependent optical properties of glazings are discussed. A categorisation of windows depending on the type of coating on the glazing is discussed as a way of improving the accuracy in the predictive models. Four approximate ways to predict the angle dependence of the total solar energy transmittance are compared. The impact on the energy performance of windows with different angle dependence is assessed in a heating and a cooling dominated climate, respectively. Results imply that by simply using the clear glass angular profile for all types of windows gives quite low errors in the angle dependence prediction, lower than some other previously proposed models. By using a model with window category as input, the errors in angle dependence prediction can be further reduced. The impact on the energy performance from incorrect angle dependence is considerable in some cases but not necessarily critical.

Modelling the angular behaviour of the total solar energy transmittance of windows

The angle dependence of the total solar energy transmittance, the g-value, is addressed. Factors that influence the angular behaviour and possible ways to simulate it are discussed. A simple and accurate model to simulate the angle dependence is presented. Besides the data for normal incidence, the model employs two inputs: the number of panes in the window and the type of coating used. Detailed information about the coating composition is not required. The model gives good results and could easily be incorporated in building simulation programs or window selection tools where the angular dependence of the g-value for windows needs to be known.

Prediction of optical properties of commercial soda–lime-silicate glasses containing iron

We provide a new method to predict the optical absorption of soda–lime-silicate glasses containing low levels (≲1 wt%) of iron. We assumed that octahedral and tetrahedral ferric iron as well as octahedral ferrous iron contribute to the optical absorption of the glasses. The absorption coefficients were determined for the respective colorants, and the total transmission spectra were calculated in the wavelength range from 200 to 2100 nm. It has been shown that the calculated spectra agree well with the experimental ones in the entire range of wavelength. We have further calculated such optical properties as visible light transmittance, total solar energy transmittance, and total solar ultraviolet transmittance. It has been demonstrated that these optical properties are satisfactorily reproduced by the calculations.

Switchable glazing with a large dynamic range in total solar energy transmittance (TSET)

Modern, energy-saving buildings incorporate large areas of highly insulating glazing. The resulting solar gains lead to major savings in heating energy during winter, but protection against overheating in summer is also needed. Usually this problem is solved by using mechanical shading devices, with the disadvantages of high cost and low durability. The work on switchable glazing at Fraunhofer Institute for Solar Energy Systems, in cooperation with industrial partners, aims to present new and viable alternatives. Two types of switching layers, which are quite different in their structure and function, but are similar in having a large dynamic range in TSET, are being investigated—gasochromic and thermotropic. Gasochromic windows are actively switched between a clear and a coloured (but image-preserving) state by alternately introducing strongly diluted O 2 and H 2 gases. In contrast to classic electrochromic configurations, only one tungsten oxide film with a very thin catalyst coating is needed. At present, prototype windows with an area of 1.1×0.6 m 2 are being produced by sputtering. Careful adjustment of the layer structure, the gas concentration and its flow velocity is needed to obtain the desired switching rate. Homogeneous colouring of the whole area within seconds has been achieved. In addition to information on the colouring kinetics, the paper also discusses system aspects of these windows. Thermotropic layers switch reversibly and automatically, from a clear state with high transmittance to a milky white state with high diffuse reflectance, when their temperature rises. Depending on the composition of the material, the switching temperature can be chosen in the range needed. The measured optical properties of glass laminates with a thermotropic layer are presented. These are combined with the measured values for further panes to calculate the characteristic data for thermotropic insulated glazing units. The results are compared with those measured on a 1.1×1.65 m 2 prototype window. Stability results are also included. The effect of the two different types of switchable glazing on building energy savings is explored for a residential building model, using the TRNSYS building energy simulation program.

Influence of transmission models for special glazing on the predicted performance of commercial buildings

A new improved model has been developed to calculate the radiation transmittance, absorptance and reflectance values for glazing with optical thin films and/or tinted glass layers. The model is based on the optical behaviour of radiation in thin films and substrates. Simulation studies examined the thermal consequences of using glazing properties calculated using the improved method rather than properties calculated by applying the conventional Fresnel-based calculations for clear glass to special glazing. Two glazings (one heat-absorbing and one coated) which had the same basic window properties ( U-value and total solar energy transmittance) were studied using a commercial building, located in either a temperate, or an extreme continental, climate. The heating and cooling loads predicted by using the improved model differed from those using the conventional approximations by up to 19%. The results also demonstrated that conventional total solar energy transmittance values do not accurately reflect the thermal differences between glazing systems. Therefore, effective total solar energy transmittance values have been introduced which correctly represent the imposed heating and cooling effects of glazings. They permit more accurate predictions of heating or cooling loads from simplified (steady-state) calculation methods.

Correlation between absorption changes and electron transport in photosynthesis : Rumberg, B., P. Schmidt-Mende, J. Weikard and H. T. Witt, reprinted from Photosynthetic Mechanisms of Green Plants, Pub. 1145, National Academy of Sciences, National Research Council, received August 19, 1964, 34 pgs, illus.

In response to the increased occurrence of overheating problems in glazed buildings in recent years, EMPA has focussed on the determination and modelling of the total solar energy transmittance (TSET) of multiple glazing combined with different shading systems. Measurements were performed in a calorimetric outdoor test facility near Zurich, Switzerland. In the first phase, average TSET-values related to the incident global solar radiation were identified for various Venetian blind configurations with a measurement period of a few days each. As expected the results strongly depend on the slat tilt angle as well as on the solar reflectance of the slat surface. The influence of the directional distribution of the incident radiation is demonstrated by comparison of results from different measuring periods with the identical shading–glazing configuration. An extended identification model accounting for the angle-selective transmittance is presented. The experimental results are also compared with weighted g-values from numerical modelling, which are based on a view factor method extended for curved slats in a louver type shading device. In general, the agreement between the measured and calculated results is satisfactory. Some deviation is explained by a significant non-diffuse albedo component in the hemispherical radiation field.