OTROS ASPECTOS DE LA SOCIEDAD PITIUSA DE LOS AÑOS TREINTA DEL SIGLO

Low e (low emissivity) glass is a technology used to increase a glass pane insulating performance by using transparent conductive oxides. These nano-coatings act as a conductive layer to increase radiant surface temperature to enable decreased cooling demands in building interiors. The application of these thin film materials has thermally improved glass pane technology in maximization of transparent properties, solar gain control and day light control. This coupled with movement away from

single glazing pane application that achieved U-values in a range of U ~ 0.6 W/m2K. Introduction of Argon or Kryton gas filled glazing cavities have increased the performance technological potential, as shown in Figure 2.9.

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Figure 2.9 Glazing Unit Configurations

The progression of this thermally advanced multi pane glazing application is driven by, Figure 2.9, of double, triple, gas filled cavities and low e nano coatings. Low emissivity , low-e coatings refers to a low emissivity over the long wavelength portion of the light spectrum, (Taylor & Kerr, 1941) . Low e coating ( in colours of gold, silver and copper) gives solar control through this thin film coating. The thickness of the coating 1 µm is comprised of 3 layers; a metal layer that is embedded

between two dielectric layers, ASHREA, 2005. These coating are either hard or soft that is deposited on a flat glass pane and thin plastic film.

This integration of low-e gazing into multiple leaf glass configurations allows significant reduction of radiative surface to surface heat transfer between glass panes. It can be seen the U - value of a clear glazed unit reduced by 32-53% compared to a single pane counterparts ( Bahaj et al, 2008). Increasing the depth of multi-pane windows by a third and four layer improved the insulation further (Eicker et al, 2008). However air filled cavities are subject to air flow movement within the cavity space due to buoyant induced air currents. The air absorbs solar gain that is carried to the top of the window that consequently creates a cooler air pool at the lower window inner pane section (Manz, 2003).

Replacing the air filled cavity with krypton enhances better thermal insulating properties and reduces overall window depth for a thinner section profile. Combining this glass filled element with low-e coatings, the radiative surface to surface heat transfer can be reduced to below 0.1 W/m2 K , (Manz, 2008). The introduction of a vacuum to replace air and gas filled cavities will further reduce the depth separation between glass panes. This narrow vacuum separation space is typically 12mm that will require spacers to maintain this dimension by an array of pillars, spacing that is 25-40mm (Fang, 2007). The integration of low-e thin film to the inner panes (for radiant heat transfer) with a vacuum cavity, a thermal transmittance of 0.4 W/m² is achievable, within a double -vacuum glazing (Manz, 2008). However manufacturing of an effective vacuum seal to resist wind pressure at cavity edges, for long term durability remains an issue for this multiple panes solution.

The emerging developments of this technology in focused on materials in thin multi- layering insulated glass unit configurations for U value performance, weight reduction and reducing costs of glazing unit (Jelle, et al., 2012). Transparent insulation materials aerogel have been integrated into glazed cavities as they present greater insulation properties for glazed unit configurations. Aerogel material is light transmitting (comprising of 95% air) in nano-sized pores, that inhibit heat transfer, have been developed. This material has thermal and transparent properties to enable solar transmittance. However, the transmitted light does have a tendency to scatter and reduce day light potential.

The progressions of multi-layered configurations are methods of spectrally glazing selection to control visible and near IR spectrum wavelength by thin nano technology coating. This transparent technology achieves 90% of visible light and less than 10% of near IR spectrum transmitting wavelength. These glazed pane configurations represent fix transmission losses that have led to decreased thermal transmittance. However, these glazing configurations have to comply with the following constraints in moderate climatic regions.

• _{Limit glass area to 1 /5 of floor area.}

• Solar orientation positioning southeast over South to Southwest and Northeast over North to Northwest in the Southern Hemisphere.

• Extensive glass oriented East over South, (North in the Southern hemisphere) to west will risk overheating of internal building spaces in springtime, summer and autumn. Cooling demand would be required in a moderate climatic region. (Hens, 2011)

This performance is conceptually acceptable for colder climates where daylight admittance is important. However, in warmer climatic regions of strong direct sun light, regulation and moderation of solar radiation to control thermal transmittance and day lighting, these glazing solutions are mechanically weak. Radiant glazing without day lighting controls are mechanically suited to cooling climatic regionalization. In climatic regions of high thermal transmittance and solar glare, active or passive solar shading are required. However, despite performance driven design in optimization of these systems, artificial lighting efficiency has an impact in energy consumption efficiency on net energy demand. This represents significant challenges in the development and refinement of these systems as an integrated transparent façade approach, as in the New York Times Headquarters (Fernandes, 2013).