Thin films for solar control applications

Abstract

The excessive use of heating systems in cold climates and air conditioning systems in hotter climates is resulting in the extensive use of electricity in order to maintain such systems. This in turn leads to the greater use of fossil fuels and higher emissions of carbon dioxide and other pollutant gases. The growing amount of carbon dioxide emissions is contributing to the problem of global warming, hence increasing the need for alternative technologies to heating and air conditioning systems.One such alternative is the production of thin films which can be used as window glazing coatings to construct 'smart windows'. These windows have the greatest use within constant climates. In cold climates, windows with high solar transmittance and low thermal emittance are needed; this allows sunlight into the building to brighten the room but stops heat from escaping thus warming the room. In constantly hot climates, materials that are transparent in the visible region but reflective in the infrared, such as thin metallic coatings, can be used to ensure that the inside of the building remains cool.These solar control coatings, however, pose a problem in varying climates such as in northern and central Europe. For these cases, materials that have altering properties owing to external surroundings could be the solution. These 'chromatic' materials include several categories, such as photochromic glasses and polymers, thermochromic metal oxides and electrochromic materials.This review will begin with a discussion as to the chemistry behind the solar control coatings and chromic materials including ambient radiation and the ideal of a blackbody object. It will then look in depth at each class of these chromic materials considering experimental results and theoretical insight as well as production techniques and applications.

Variation of Thermochromic Glazing Systems Transition Temperature, Hysteresis Gradient and Width Effect on Energy Efficiency

“…In cold environments it is necessary to produce a material that will trap thermal energy with in the building. An example of this is low emissivity (Low-E) glazing, which reflects black body radiation thus reducing the loss of internal heat [3]. Conversely, in high temperature environments there is a need for heat to be able to leave a building while also preventing heat from entering from the outside, this is achieved using absorbing glass that prevents the solar thermal energy from entering the building [3].…”

“…An example of this is low emissivity (Low-E) glazing, which reflects black body radiation thus reducing the loss of internal heat [3]. Conversely, in high temperature environments there is a need for heat to be able to leave a building while also preventing heat from entering from the outside, this is achieved using absorbing glass that prevents the solar thermal energy from entering the building [3]. However, many cities are in environments where the climate varies from hot to cold throughout the year for which a material with variable properties is desirable.…”

Developments in the synthesis of flat plate solar selective absorber materials via sol–gel methods: A review

“…Other methods of SSA production such as CVD and mechanical grinding also have their plus points and drawbacks. In general, the CVD method has good potential in industrialscale production but there are difficulties in ensuring the stoichiometry of the metal oxides produced [39]. Mechanical grinding is a simple and cost-effective method of SSA production but the selectivity of the absorber material is low [11,40].…”

The Effect of Transition Hysteresis Width in Thermochromic Glazing Systems

“…The rutile material is metallic in nature and reflects a wide range of solar radiation, whereas the monoclinic phase is a semiconductor and transmissive across the same range of solar radiation. This dynamic switching behaviour is in contrast to existing commercial approaches which rely on glazing with static behaviour such as heat mirrors, absorbing glass or Low-E coatings [7]. For vanadium dioxide to be effective as an intelligent window coating it is highly desirable to lower the transition temperature from 68˚C to nearer room temperature in order to maximize the energy demand reduction properties [1].…”

“…The advantage of a thermochromic system based on VO 2 compared to these systems is that there is no change in the visible portion of the spectrum, only in the infrared portion [7].…”