1. Gas-filled Windows ME 340 – Winter 2010 Kurt Hendricks & Nathan Toone

2. Problem Most heat in a home is lost through the windows. Many efforts are made to insulate the windows to decrease the rate of heat transfer, including double pane windows. The air space between the two panes of glass is what provides the added insulation. Different gases obviously have different thermal conductivity coefficients.

3. Objectives Which gas will provide the most insulation: Argon, Krypton, or Xenon? Is it cost effective to fill the window with gas?

4. Heat Transfer Setup We will assume that heat is lost only by conduction through the window. The thickness of each glass layer and the air space in between is 3/32 in. and 9/16 in. respectively. Each gas will have a specific thermal conductivity coefficient that is used to find the thermal resistance. The rate of heat transfer per unit area can easily be found, as shown on the slide “Equations (1)”, by assuming a temperature gradient between inside and outside air.

5. Cost Comparison Setup A hypothetical building with 3 large windows is chosen. The area of each window is calculated The cost to provide heat to the building is estimated with a Dollars per Watt value. The cost per watt for each gas is calculated and compared to that of normal air as in the double pane window. Savings are then compared to the cost of filling each window with that gas.

6. Calculations Total WidthL =0.65625in GlassLength L1 =0.09375in0.00238125 Conduc. k1 =1.05W/mK AirLength L2 =0.5625in0.0142875 Conduc. k2 =0.025W/mK InsideT1 =70°F21.11111111294.2611111 OutsideT2 =35°F1.666666667274.8166667 Windowwidth =5ft1.524 height =4ft1.2192 AREA =1.8580608m2 # WindowsN =3 Total Area =5.5741824m2 Price per Watt0.08$/kW-h Gaskq/AkW-h/mo)/AQout (W)(kW-h)/moCost/A Total Loss/mo Savings Air0.02533.7556230724.30404861188.16135.4752$1.94$10.84- Argon0.01621.6650113915.5988082120.764725286.95060215$1.25$6.96$3.88 Krypton0.0094312.79536879.21266546671.3237190351.3530777$0.74$4.11$6.73 Xenon0.005657.6755493585.52639553842.7849121430.80513674$0.44$2.46$8.37

7. Cost Savings with Window Area

8. Cost of Argon Windows Area (sq ft)Extra cost to add argon 11.25 22.5 33.75 45 56.25 67.5 78.75 810 911.25 1012.5 1113.75 1215 1316.25 1417.5 1518.75 1620 1721.25 1822.5 1923.75 2025 The hypothetical window used (4’ x 5’) would cost $25 more when filled with argon than with air Savings per month were calculated to $3.88 for this window, meaning the extra money spent would pay for itself in about 6 ½ months

9. Results Filling the windows with Xenon provides the most insulation as it yields the highest thermal resistance. It is cost effective. The windows filled with special gases cost much more than normal, but the savings are quickly made up over the following months. The larger the windows, or the more windows there are, the more cost savings are available.

10. Recommendations It is clearly more effective to use windows filled with certain gases, rather than just air. Argon, Krypton, and Xenon are all more effective, but Xenon provides the most insulation. Though the windows are more expensive at first and appear to be an investment, the eventual savings from heat loss are well worth the cost. These findings are most important for buildings with many large windows. Regular homes, with a few small windows, will not see significant cost savings.

11. Equations (1)

12. Equations (2)