FB720 project: development of new solar control glazing with passive seasonal discrimination of solar radiation incomes

Modern office buildings often have large glazed areas. Depending on the climate zone, incident solar radiation on fenestration can lead to high cooling demands during hot periods, although solar radiation can help reduce heating demands during cool periods. Previous studies have shown that a large proportion of the net energy demand of an office building is related to window heat loss and cooling demands induced by solar irradiance. Some experts [1] found that, even in what traditionally has been considered to be a heating-dominated climate, cooling demands dominate the net energy demand of an office building. Solar shading systems are vital to reduce the cooling demand of an office building and might also be necessary to reduce glare issues. In the tropics, glazed façade is also the cause of large solar heat gains inside buildings, and researchers [2] are focusing on reducing solar heat (infrared) transmittance and daylight (visible light) transmittance. It was also observed [3] that, in the case of double glazing units (DGU), the use of a solar control film is highly recommended to increase the separation between the sheet glasses. The energy gain was reduced by 55 % compared to the traditional DGU without solar control film. Fixed shading devices (e.g. vertical fins, diagonal fins and egg crate) are used to protect inner spaces from direct solar gain through openings, windows and large glazed surfaces to control air temperature and improve the illuminance level. The results [4] showed that the temperature in offices with shading devices was reduced to an acceptable level compared to offices without shading devices. The visual environment was improved by controlling the illuminance level, improving uniformity and eliminating glare. Other researchers [5] recommend accurately estimating the optical properties of the different types of dynamic shading devices (such as roller blinds and Venetian blinds) and including their effects in the glazing system analysis. The Venetian blind is an interesting solution because it blocks direct solar radiation, whilst enabling the transmission of diffuse radiation indoors. Further progress is inextricable linked to the dynamic control of sunlight and optimal control of solar heat gains, taking into account the variability in the solar radiation spectra incident on the building’s envelope and the variability in outdoor and indoor air temperature differences. Tuning control [6] of glazing’s transmittance dependence on the solar radiation wavelength is required to optimise daylighting with reference to people’s needs (their health and comfort) and energy (thermal and electrical load minimization). However, the energy-saving potential of regulating solar spectrum response properties is greater than that of regulating the long-wave thermal emission property in summer; the trend is reversed in winter [7]. By properly regulating the NIR spectrum transmission and reflection properties and the long-wave thermal emission property, the performance of ordinary glazing can be improved to approach the perfect window for summer or winter. An advanced fenestration system must combine the functions of daylighting regulation, glare protection and seasonal thermal control.

Currently, float glass products for façade glazing are available with a variable solar protection index value, obtained by the interposition of different kinds of solar radiation filters (microlouvers, metal mesh, etc.) or by alternating with reflective microprism surface treatments. However, these conventional protection solutions have three main disadvantages: they do not take into account the varying conditions of solar incidence due to latitude or the different orientations of each façade, and it is difficult to incorporate free glazing areas without any protective solar treatment in the same unit of glass. There is a search for innovative coatings to redirect incident solar radiation, thus simultaneously reducing glare and projecting daylight deep into the room. The solar gains are reduced for chosen angles corresponding to aestival elevations of the sun, thereby minimising heating loads in winter and cooling loads in summer. Ray-tracing programs [8] have been developed to optimise structures with the aforementioned goals. The chosen solution is based on embedded reflective surfaces that can be combined with a standard double glazed window.

The current challenge is to find a technical solution that is:

The aim of the research project FB720 (IDI-20090761) was to design and develop a lightweight façade for office buildings with low environmental impact and high energy efficiency, mainly for use in temperate climates, as in Spain [9]. One of the technical strategies to achieve this is the integration of new glass with passive seasonal discrimination of solar radiation incomes. The ultimate purpose of this strategy is to obtain clearly differentiated solar protection index values throughout the year, according to the angle of incidence of solar radiation. This strategy facilitates the desired thermal gains in the winter and reduces unwanted thermal contributions in the summer, without a loss of external views.

Unlike other solar control fenestration, FB720 solar control glass is based on the possibilities offered by the superimposition of various layers of glass in the case of laminated glazing. Solar control is achieved by means of a reflective coating treatment applied on every layer of glass [10] with a singular geometric pattern that is tailored to the specific characteristics of each architectural project; mainly the building façade’s orientation and the latitude position of the building (Fig. 1). The geometric pattern applied to each of the glass layers is slightly shifted on every layer to achieve a volumetric filtering effect similar to Venetian blinds, but without the inconvenience of handling and maintaining these types of blinds. The result is a customized laminated glass containing what could be described as “virtual slats”, which are designed to provide the highest protection from the incidence of direct solar radiation in summer and the greatest collection of solar energy in winter.

The procedure for designing the ideal geometric pattern of the “virtual slats” is based on different strips of metallic reflective coatings applied to both sides of the glass. The alignment and superimposition of these strips are designed in relation to the maximum angles of incidence of radiation during the winter and summer solstices in the façade plane, corrected by the refraction index of the glass material (Fig. 2). The exact pattern for every situation is deduced using trigonometry calculations, the details of which are outside of the scope of this paper. The end objective is to achieve the same passive protection effect as that produced by an eave over a façade oriented towards the midday sun: in the northern hemisphere it provides protection from radiation in the summer, but enables much of the radiation to enter the building in the winter. The key is the geometric arrangement of the strips compared to the angle of incidence of the sun during the winter and summer solstices, or in other words, the angle of incidence at the equinox plus the angle of declination in the summer (+23.4°) or minus the angle of declination in the winter (−23.4°). To achieve this effect at any orientation, the arrangement of the strips on the plane of glass is determined by the geometric intersection between the façade and the plane of the sun’s path at the equinox. It will vary depending on the orientation of the glass and the latitude of the site. Thus, the best pattern for a southern orientation (in the northern hemisphere) will be comprised of horizontal strips, whilst in an eastern or western orientation the strips should slope with an angle of inclination equal to the co-latitude (Fig. 3). The offset between the strips in the different layers is determined by the angles of declination in summer and winter, with respect to a geometric reference plane perpendicular to the façade and to the alignment of the protective strips.

The width of the strips, the distance between them and the offset in alignment are determined by the number and thickness of glass layers in the glazing unit. As more superimposed layers are used, the strips can be separated by a greater distance (Fig. 4). In the practical cases that were studied, a minimum of three layers created by a reflective and semi-transparent coating of metal oxides on the surface of the glass was considered [11].

The first results obtained with calculation tools confirmed the passive seasonal discrimination behaviour of the FB720 glazing. The solar factor value for the angle of incidence of solar radiation at the summer solstice is 50 % of the solar factor value for the angle of incidence of solar radiation at the winter solstice for Barcelona’s latitude (Fig. 9). These results were compared with experimental tests carried out in the laboratories of the University of Zaragoza’s Department of Applied Physics. A UV–VIS–IR spectrometer was used to measure the values of the transmittance and reflectance factors simultaneously in the range of 200–2500 nm light frequency, which is typical of solar radiation. Measurements were made for different degrees of incidence of the light beam (Figs. 8, 9, 10; Table 3).

Using the values obtained, we could determine the variation in solar factor and light transmittance of the samples according to the angle of incidence. The results coincided with the estimations of theoretical behaviour with an acceptable level of accuracy. Therefore, the simulation tool was validated.

On the basis of these preliminary values, several additional simulations were carried out to evaluate the energy behaviour of the FB720 glazing. The theoretical energy demand for heating, cooling and lighting in a standard office module was compared for different façade systems using “Tas Building Designer” and “eQUEST” software. The simulation is not described in detail here, as it is outside the scope of the paper. However, preliminary results show that the use of FB720 glazing in a geographical situation such as the Iberian Peninsula may lead, in some cases, to a 30 % lower energy demand than standard solar control glazing (Fig. 11). The glazing was most efficient in the most extreme summer climate conditions (high solar radiation, clear skies and a high average outside temperature) and when the use of the building involved a high internal thermal load, for example, when it was used for administrative or teaching purposes. In buildings with these uses, it may be particularly useful to have a façade that is passive, has low maintenance requirements, does not need to be handled by the user and users do not need to be trained to achieve the best results. FB720 glazing is also more efficient when the orientation of the façade is similar to the orientation of the sun at midday. For example, in the northern hemisphere the best use of the capacities of FB720 glazing is obtained in façades with southeast to southwest orientations. The viability of FB720 glazing for practical applications has been verified. At the time of writing this paper, the first real building with FB720 glazing in its façade had been completed (Fig. 12).

The FB720 glazing for protection of solar radiation has numerous qualitative and quantitative advantages over other conventional solar control glazing systems used in temperate climates (external blinds, external reflective layers, coloured glass, etc.):

FB720 glazing can be combined with other glazing technologies (double glazing, low emissivity, airborne sound, etc.) to achieve more complex envelope systems.