Low Emissivity Coating for Cold Climate Windows
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Solution Overview
Problem
Conventional low emissivity coatings for architectural windows are not suitable for cold climates, as they block too much solar heat in winter, failing to provide adequate heating while maintaining low heat transfer.
Innovation Solution
A low emissivity coating with a higher solar heat gain coefficient (SHGC) and a low overall heat transfer coefficient (U factor) is developed, comprising multiple phase adjustment layers and metal functional layers, allowing more solar heat to enter while keeping heat inside, and optionally providing high visible light transmittance and UV radiation blocking.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If conventional low emissivity coatings are applied to block solar heat, then the U factor is reduced (heat transfer is reduced), but the SHGC becomes too low (solar heat gain is insufficient) for cold climates
Solution Approach 1:
The coating is divided into multiple functional layers including dielectric layers (e.g., silicon oxide, silicon nitride) and metal layers (e.g., silver, aluminum), each segment performing a specific function: dielectric layers provide thermal insulation and control emissivity, while metal layers reflect infrared radiation. This segmentation allows independent optimization of SHGC and U factor by adjusting individual layer properties and thicknesses.
Solution Approach 2:
The coating uses composite material structures combining different materials with complementary properties: dielectric materials (silicon oxide, silicon nitride) combined with metals (silver, aluminum) in specific configurations. This composite approach enables the coating to simultaneously achieve low emissivity for heat retention and high solar heat gain by leveraging the unique optical and thermal properties of each material.
2Loss of energy
If low emissivity coating blocks solar infrared energy to reduce cooling costs, then SHGC is reduced, but visible light transmittance may be compromised
Solution Approach 1:
The coating exhibits local quality by having different optical properties at different wavelengths: it is highly reflective to solar infrared radiation (blocking heat) while remaining transparent to visible light. The dielectric and metal layers are specifically designed to create this wavelength-selective behavior, allowing the coating to block unwanted thermal radiation while maintaining good visible light transmission for natural illumination.
Solution Approach 2:
The coating achieves wavelength-selective properties by controlling parameters such as layer thickness, material composition, and refractive index. By adjusting these parameters, the coating can be tuned to reflect specific infrared wavelengths while transmitting visible light, thus resolving the contradiction between heat blocking and light transmission.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The coating effectively allows more solar heat into buildings in cold climates for heating while maintaining low heat transfer, enhancing energy efficiency and reducing the need for artificial lighting, with aesthetic appeal and UV protection.
Implementation Method 1
Low emissivity coatings act as thermal barriers that decrease the emission of radiant infrared (IR) energy, particularly thermal infrared energy
Implementation Method 2
A high SHGC allows more solar heat to pass into the building to heat the interior of the building
Implementation Method 3
The low emissivity coating comprises a plurality of phase adjustment layers; a first metal functional layer; and a second metal functional layer located over and spaced from the first metal functional layer
Implementation Method 4
Low emissivity coatings act as thermal barriers that decrease the emission of radiant infrared (IR) energy
Data Source
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AI summary
A low emissivity coating (30) includes a plurality of phase adjustment layers (40, 50, 62); a first metal functional layer (46); and a second metal functional layer (58) located over and spaced from the first metal functional layer (46). A ratio of the geometric thickness of the first metal functional layer divided by the geometric thickness of the second metal functional layer is in the range of 0.6 to 1. The low emissivity coating (30) provides a reference IGU summer/day SHGC of at least 0.4 and a reference IGU winter/night U factor of no greater than 0.4 BTU/hr-ft2-°F (2.27 W/m2-K).