Gas-filled Windows

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

7. Cost Savings with Window Area

8. Cost of Argon Windows • 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)