windows

Energy Efficient Windows

windows

Image source: www.manor.net.au

The latest trends in ecology have led to development of energy efficent product and among them are also energy efficient windows. The windows are made to save energy thus their production creates less pollution.

Basics:

Homes are insulated to prevent heat loss. A wall insulating value of R-19 is usual while any window will be less. It makes little sense to insulate walls well only to lose excessive heat through windows. One square foot of glass can lose as much heat as ten square feet of wall.
Climate plus heat loss/gain is the basis for the sizing of the heating cooling system. Excessive heat loss/gain requires a larger heating/cooling system to compensate while maintaining a comfortable temperature inside the dwelling. This is extra expense on a heating/cooling system which could go toward better windows or into ones pocket. [1]

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 ab- ove 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. [2]

The objective of this study is to increase the visible transmittance of a low-emittance (low-e) glazing as much as possible by antireflection treatment. This has been carried out by depositing thin porous films of silicon dioxide, SiO2, on both sides of a commercial glazing with a pyrolytic low-e tin oxide-based coating. SiO2 was chosen because its refractive index makes it suitable for antireflection treatment of both the uncoated glass side and the side of the tin oxide coating. The deposition of the antireflective films was performed with a dip-coating method, where the substrate was dipped in a sol–gel of silica. Two different silica sol–gels were used, one was manufactured in the laboratory and the other one was a commercial solution with a higher porosity. An increase of the integrated visible transmittance (Tvis) by 9.8% points up to 0.915 was achieved for a coating produced with the commercial solution. Calculations of U value, g value and Tvis for window configurations were also performed. [3]

Thin films of hafnium oxide were deposited by electron beam evaporation. The films were characterized using X-ray diffraction, X-ray photoelectron spectroscopy and normal incidence transmittance. The films were amorphous, stoichiometric, and transparent down to a wavelength of 300 nm. The optical properties of the films, including the refractive index, the absorption index and the bandgap, were determined. The refractive index, in the visible, was relatively high (1.89). The direct bandgap was found to be 5.41 eV. Absorption was insignificant for wavelengths above 250 nm. A heat mirror was built based on the hafnium oxide/silver/hafnium oxide/glass system. This heat mirror was found to be transparent in the visible with an average transmittance of 72.4%, and reflective in the near infrared (wavelength = 700–2000 nm) with an average reflectance of 67.0%. Such a heat mirror can be used in applications involving energy-efficient windows. [4]

This article presents results from a pilot study comparing condensation patterns on small glass samples with different surface properties. Experiments were carried out on three commercial glass samples (clear float, TiO2-coated and SnO2-coated) to see how water condensed on the different surfaces. The experiments were carried out under a clear night sky in Uppsala, Sweden. It was found that the pane with the low-emittance coating stayed clean of condensation longer than the other two. In the morning, the water layer on the TiO2-coated sample was smeared out so that it was possible to see through that pane, while the view through the other two was still blurred. The TiO2 coating does not prevent condensation, but makes it easier to see through the water layer. These simple tests indicate noticeable differences between different surface materials and also that these effects can be studied by exposing small samples to a clear night sky without having to perform full scale tests. [5]

Heating and cooling energy lost through windows in the residential sector (estimated at two-thirds of the energy lost through windows in all sectors) currently accounts for 3 percent (or 2.8 quads) of total US energy use, costing over $26 billion annually in energy bills. Installation of energy-efficient windows is acting to reduce the amount of energy lost per unit window area. Installation of more energy efficient windows since 1970 has resulted in an annual savings of approximately 0.6 quads. If all windows utilized existing cost effective energy conserving technologies, then residential window energy losses would amount to less than 0.8 quads, directly saving $18 billion per year in avoided energy costs. The nationwide installation of windows that are now being developed could actually turn this energy loss into a net energy gain. Considering only natural replacement of windows and new construction, appropriate fenestration policies could help realize this potential by reducing annual residential window energy losses to 2.2 quids by the year 2012, despite a growing housing stock. [6]

Conclusion:

Energy efficient windows are made to save a lot of energy and they are successfully doing that. Their price is not big when the amount of saved energy is taken into consideration. Their popularity is growing and hopefully all households will have them soon.

References:

[1] http://www.greenoptions.com/a/energy-efficient-windows
[2] ”Evaporated Sn‐doped In2O3 films: Basic optical properties and applications to energy‐efficient windows” by: Hamberg, I., Granqvist, C. G.
[3] ”Antireflection treatment of low-emitting glazings for energy efficient windows with high visible transmittance” by: Elin Hammarberg, Arne Roos
[4] ”Optical properties of hafnium oxide thin films and their application in energy-efficient windows” by: M.F. Al-Kuhaili
[5] ”Condensation tests on glass samples for energy efficient windows” by: Anna Werner, Arne Roos
[6] ”Savings from energy efficient windows: Current and future savings from new fenestration technologies in the residential market”


Josip

AUTHOR: Josip Ivanovic

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One Comment

  1. Lyndsay Goltz
    March 23, 2012 at 6:15 am ·

    Aw, this was a very nice post. In concept I would like to put in writing like this moreover – taking time and actual effort to make an excellent article… however what can I say… I procrastinate alot and in no way seem to get something done.

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