By Tim Shay Chemical Engineering Student

 Goal: Reduce our need of fossil fuels  How? ◦ Renewable resources  Solar  Biomass  Wind  Hydro  Geothermal ◦ Energy Efficiency  Conversion losses  Automobiles  Heating/cooling

 Heat flow follows Newton’s law of cooling ◦ U = Heat transfer coefficient ◦ A = Area ◦ ΔT = Temperature difference  Low-end window U-factor = 1.20 BTU/(ft 2 hr°F)  Home wall U-factor ~ BTU/(ft 2 hr°F)  Windows can lose about times more heat per area

 The science behind the different technologies  Modeling  Testing  Quality Assurance  Consumer view point

 Conduction ◦ Glass  Gas fill  Multi-plane windows ◦ Spacer  Structure ◦ Sash  Material  Design  Convection ◦ Weather Strip  Radiation ◦ Low-E coatings ◦ Heat mirrors y-statistics.htm

 Generic diagram of heat flow with various materials Transport Phenomena, Bird, Stewart and Lightfoot ◦ Fluids cause large change in temperature ◦ Reduces heat transfer

 Thermal conductivities of gas greatly affects overall heat transfer ◦ Can generally treat as being stagnant Gases used to fill windows GasThermal Conductivity (W/mK) Air0.03 Argon0.016 Krypton0.009

 Gas inside isn’t stagnant ◦ Movement increases heat transfer  Heating and cooling causes convection ◦ Taken into account in FEA models

 Spacers are what seal the gas in and separate the panes  Spacers come in many different designs and materials  Can have a large affect on ◦ Strength of window ◦ Conduction of heat

 Temperature/pressure changes stress the system ◦ Window deflection Concave Shape Normal Shaped IG unit Cold Temperatures High Atmospheric Pressure Hot Temperatures Low Atmospheric Pressure Convex Shape

 Intercept spacers ◦ Can bend more ◦ Let less heat through  Box Spacer ◦ More rigid ◦ More heat transfer  Allows more conduction

 Tri-pane is the highest generally seen in the market  Increases resistance ◦ More surfaces to transfer heat through ◦ Generally thicker

 Made from various materials ◦ Wood ◦ Vinyl ◦ Aluminum ◦ Special materials  Andersen’s Fibrex  Consumer choice also plays a key role in selection  Each has it’s own conductivity

 Non solid frames have complex structures ◦ Designed to maximize efficiency and for strength ◦ Makes use of heat transfer between barriers

 Air flow will find any open gap between the exterior and interior ◦ Create tight seal  Problems ◦ Wear and tear  Extensive testing performed ◦ 10,000 cycle test weatherstripping-options.html

 The sun emits radiation to the Earth ◦ This radiation is generally classified two ways for window manufactures  Solar heat gain  Visible light  Light has three options on surfaces ◦ Transmittance ◦ Absorbance ◦ Reflection

 Coatings of silver and metal oxides can be used to help reflect heat ◦ More layers reflects more heat ◦ Can go up to 3 layers

 Placed on second face ◦ Sheltered from elements ◦ Most efficient location  Coating other surfaces ◦ Must be resilient to the elements ◦ Must be safe  Tri-pane windows allow more surfaces to apply coating to

 More heavy coatings will prevent transmission of solar heat, but also visible light

Structural-Sealant-Glazings  Operates the same as low-e coatings  Made of a thin clear film  Also operates as a divider ◦ Creates 2 gaps ◦ Decreases conduction  Pricier

The Vdara hotel in Las Vegas

 Sunbathers have been severely burned in as little as 10 minutes  Reflected light is becoming an increasing issue  Affecting buildings that already exist

 Low end window U factor ~ 1.2  High end window U factor ~ 0.20  Example ◦ 1 window that is 10ft 2 ◦ ΔT = 70°F ◦ ΔQ = ΔU*A*ΔT  700 BTU/hr ◦ Over a 1 day span  16,800 BTU/day = Therms/day  1 Therm ~ 1 $