Low Emissivity
   HOME

TheInfoList



OR:

Low emissivity (low ''e'' or low thermal emissivity) refers to a surface condition that emits low levels of radiant
thermal A thermal column (or thermal) is a rising mass of buoyant air, a convective current in the atmosphere, that transfers heat energy vertically. Thermals are created by the uneven heating of Earth's surface from solar radiation, and are an example ...
(heat) energy. All materials absorb, reflect, and emit
radiant energy Radiant may refer to: Computers, software, and video games * Radiant (software), a content management system * GtkRadiant, a level editor created by id Software for their games * Radiant AI, a technology developed by Bethesda Softworks for '' ...
according to
Planck's law In physics, Planck's law describes the spectral density of electromagnetic radiation emitted by a black body in thermal equilibrium at a given temperature , when there is no net flow of matter or energy between the body and its environment. A ...
but here, the primary concern is a special wavelength interval of radiant energy, namely
thermal radiation Thermal radiation is electromagnetic radiation generated by the thermal motion of particles in matter. Thermal radiation is generated when heat from the movement of charges in the material (electrons and protons in common forms of matter) is ...
of materials. In common use, especially building applications, the temperature range of approximately -40 to +80 degrees Celsius is the focus, but in aerospace and industrial process engineering, much broader ranges are of practical concern.


Definition

Emissivity is the value given to materials based on the ratio of heat emitted compared to a perfect
black body A black body or blackbody is an idealized physical object, physical body that absorption (electromagnetic radiation), absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence (optics), angle of incidence. T ...
, on a scale from zero to one. A black body would have an emissivity of 1 and a perfect reflector would have a value of 0. Kirchhoff's law of thermal radiation states that absorption equals emissivity opaque for every specific wavelength/frequency (materials often have quite different emissivities at different wavelengths). Therefore, if the asphalt has an emissivity value of 0.90 at a specific wavelength (say wavelength of 10 micrometers, or room temperature thermal radiation), its thermal absorptance value would also be 0.90. This means that it absorbs and emits 90 percent of radiant thermal energy. As it is an opaque material, the remaining 10 percent must be reflected. Conversely, a low-''e'' material such as aluminum foil has a thermal emissivity/absorptance value of 0.03 and as an opaque material, the thermal reflectance value must be 1.0 - 0.03 =0.97, meaning it reflects 97 percent of radiant thermal energy. Low-emissivity building materials include window glass manufactured with metal-oxide coatings as well as house wrap materials, reflective thermal insulations, and other forms of radiant thermal barriers. The thermal emissivity of various surfaces is listed in the following table.


Low-emissivity windows

Window glass is by nature highly thermally emissive, as indicated in the table above. To improve thermal control (insulation and solar optical properties) thin-film coatings are applied to the raw soda–lime glass. There are two primary methods in use: pyrolytic
chemical vapor deposition Chemical vapor deposition (CVD) is a vacuum deposition method used to produce high quality, and high-performance, solid materials. The process is often used in the semiconductor industry to produce thin films. In typical CVD, the wafer (subst ...
and magnetron sputtering. The first involves the deposition of fluorinated
tin dioxide Tin(IV) oxide, also known as stannic oxide, is the inorganic compound with the formula SnO2. The mineral form of SnO2 is called cassiterite, and this is the main ore of tin. With many other names, this oxide of tin is an important material in ...
at high temperatures.
Pyrolytic coating Pyrolytic coating is a thin film coating applied at high temperatures and sprayed onto the glass surface during the float glass process. Advantages *Relatively durable coating. * Can be tempered after coating application. * Can be used in single ...
s are usually applied at the
float glass Float glass is a sheet of glass made by floating molten glass on a bed of molten metal, typically tin, although lead and other various low-melting-point alloys were used in the past. This method gives the sheet uniform thickness and very flat sur ...
plant when the glass is manufactured. The second involves depositing thin silver layers with antireflection layers.
Magnetron sputtering Sputter deposition is a physical vapor deposition (PVD) method of thin film deposition by the phenomenon of sputtering. This involves ejecting material from a "target" that is a source onto a "substrate" such as a silicon wafer. Resputtering is re ...
uses large vacuum chambers with multiple deposition chambers depositing 5 to 10 or more layers in succession. Silver-based films are environmentally unstable and must be enclosed in
insulated glazing Insulating glass (IG) consists of two or more glass window panes separated by a space to reduce heat transfer across a part of the building envelope. A window with insulating glass is commonly known as double glazing or a double-paned window, ...
or an Insulated Glass Unit (IGU) to maintain their properties over time. Specially designed coatings may be applied to one or more surfaces of insulated glass. One type of coating (low-e coatings) reduces the emission of radiant
infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from aroun ...
energy, thus tending to keep the heat on the side of the glass where it originated while letting visible light pass. This results in glazing with better control of energy - heat originating from indoors in winter remains inside (the warm side), while heat during summer does not emit from the exterior, keeping it cooler inside. Glass can be made with differing thermal emissivities, but this is not used for windows. Certain properties such as the iron content may be controlled, changing the thermal emissivity properties of glass. This "naturally" low thermal emissivity is found in some formulations of
borosilicate Borosilicate glass is a type of glass with silica and boron trioxide as the main glass-forming constituents. Borosilicate glasses are known for having very low coefficients of thermal expansion (≈3 × 10−6 K−1 at 20 °C), m ...
or Pyrex. Naturally, low-e glass does not have the property of reflecting
near infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...
(NIR)/thermal radiation; instead, this type of glass has higher NIR transmission, leading to undesirable heat loss (or gain) in a building with that type of window.


Criticism of low-E windows

It has been suggested that the high reflectivity of low-E windows can contribute to a concentration of solar radiation which can potentially cause damage to their surroundings; damage to the sidings of homes and to automobiles has been reported not only in news stories, but may cause legal issues as well. Low-e windows may also block radio frequency signals. Buildings without
distributed antenna system A distributed antenna system, or DAS, is a network of spatially separated antenna nodes connected to a common source via a transport medium that provides wireless service within a geographic area or structure. DAS antenna elevations are general ...
s may then suffer degraded cell phone reception.


Reflective thermal insulation

Reflective thermal insulation is typically fabricated from aluminum foil with a variety of core materials such as
low-density polyethylene Low-density polyethylene (LDPE) is a thermoplastic made from the monomer ethylene. It was the first grade of polyethylene, produced in 1933 by Imperial Chemical Industries (ICI) using a high pressure process via free radical polymerization. I ...
foam, polyethylene bubbles,
fiberglass Fiberglass (American English) or fibreglass (Commonwealth English) is a common type of fiber-reinforced plastic using glass fiber. The fibers may be randomly arranged, flattened into a sheet called a chopped strand mat, or woven into glass cl ...
, or similar materials. Each core material presents its own set of benefits and drawbacks based on its ability to provide a thermal break, deaden sound, absorb moisture, and resist combustion during a fire. When aluminum foil is used as the facing material, reflective thermal insulation can stop 97% of radiant heat transfer. Recently, some reflective thermal insulation manufacturers have switched to a metalized polyethylene facing. The long-term efficiency and durability of such facings are still undetermined. Reflective thermal insulation can be installed in a variety of applications and locations including residential, agricultural, commercial, aerospace, and industrial structures. Some common installations include house wraps, duct wraps, pipe wraps, under radiant floors, inside wall cavities, roof systems, attic systems, aircraft fuselage systems, space probe systems, and crawl spaces. Reflective thermal insulation can be used as a stand-alone product in many applications but can also be used in combination systems with mass insulation where higher R-values are required.


Military applications

Low emissivity coatings have found applications in stealth technology, reducing the thermal infrared emissions from military equipment in the short-wave, mid-wave and long-wave infrared portions of the electromagnetic spectrum. Kelly, Lance C. (2020). ''Low-Emissivity Topcoats for the Reduction of Thermal Infrared Emissions from Military Platforms'' https://www.paint.org/coatingstech-magazine/articles/low-emissivity-topcoats-for-the-reduction-of-thermal-infrared-emissions-from-military-platforms/].


See also

* Selective surface


References


External links

* {{cite web , url=http://efficientwindows.org/lowe.php , title=Window Technologies: Low-E Coatings , publisher=The Efficient Window Collaborative Website maintained by a trade organization. Windows Thermal protection Glass coating and surface modification