Gas-filled Windows

ME 340 – Winter 2010

Kurt Hendricks & Nathan Toone

Problem

Most heat in a home is lost through the windows. Most heat in a home is lost through the windows.

Many efforts are made to insulate the windows to Many efforts are made to insulate the windows to decrease the rate of heat transfer, including decrease the rate of heat transfer, including double pane windows.double pane windows.

The air space between the two panes of glass is The air space between the two panes of glass is what provides the added insulation.what provides the added insulation.

Different gases obviously have different thermal Different gases obviously have different thermal conductivity coefficients. conductivity coefficients.

Objectives

Which gas will provide the Which gas will provide the most insulation: Argon, most insulation: Argon, Krypton, or Xenon?Krypton, or Xenon?

Is it cost effective to fill the Is it cost effective to fill the window with gas?window with gas?

Heat Transfer SetupWe will assume that heat is lost only by conduction We will assume that heat is lost only by conduction through the window.through the window.

The thickness of each glass layer and the air space in The thickness of each glass layer and the air space in between is 3/32 in. and 9/16 in. respectively. between is 3/32 in. and 9/16 in. respectively.

Each gas will have a specific thermal conductivity Each gas will have a specific thermal conductivity coefficient that is used to find the thermal resistance.coefficient that is used to find the thermal resistance.

The rate of heat transfer per unit area can easily be The rate of heat transfer per unit area can easily be found, as shown on the slide “Equations (1)”, by found, as shown on the slide “Equations (1)”, by assuming a temperature gradient between inside and assuming a temperature gradient between inside and outside air.outside air.

Cost Comparison SetupA hypothetical building with 3 large windows is chosen.A hypothetical building with 3 large windows is chosen.

The area of each window is calculatedThe area of each window is calculated

The cost to provide heat to the building is estimated The cost to provide heat to the building is estimated with a Dollars per Watt value.with a Dollars per Watt value.

The cost per watt for each gas is calculated and The cost per watt for each gas is calculated and compared to that of normal air as in the double pane compared to that of normal air as in the double pane window.window.

Savings are then compared to the cost of filling each Savings are then compared to the cost of filling each window with that gas.window with that gas.

CalculationsTotal Width L = 0.65625 inGlass Length L1 = 0.09375 in 0.00238125

  Conduc. k1 = 1.05W/mK

Air Length L2 = 0.5625 in 0.0142875

  Conduc. k2 = 0.025W/mK

Inside T1 = 70°F 21.11111111 294.2611111Outside T2 = 35°F 1.666666667 274.8166667

Window width = 5 ft 1.524  height = 4 ft 1.2192  AREA = 1.8580608m2# Windows N = 3  

Total Area = 5.5741824m2  

Price per Watt 0.08$/kW-h  

Gas k q/A kW-h/mo)/A Qout (W) (kW-h)/mo Cost/ATotal

Loss/moSavings

Air 0.025 33.75562307 24.30404861 188.16 135.4752 $1.94 $10.84 -Argon 0.016 21.66501139 15.5988082 120.7647252 86.95060215 $1.25 $6.96 $3.88

Krypton 0.00943 12.7953687 9.212665466 71.32371903 51.3530777 $0.74 $4.11 $6.73Xenon 0.00565 7.675549358 5.526395538 42.78491214 30.80513674 $0.44 $2.46 $8.37

Cost Savings with Window Area

Cost of Argon WindowsArea (sq ft) Extra cost to add argon

1 1.252 2.53 3.754 55 6.256 7.57 8.758 109 11.25

10 12.511 13.7512 1513 16.2514 17.515 18.7516 2017 21.2518 22.519 23.7520 25

The hypothetical window The hypothetical window used (4’ x 5’) would cost used (4’ x 5’) would cost $25 more when filled with $25 more when filled with argon than with airargon than with air

Savings per month were Savings per month were calculated to $3.88 for this calculated to $3.88 for this window, meaning the extra window, meaning the extra money spent would pay for money spent would pay for itself in about 6 ½ monthsitself in about 6 ½ months

ResultsFilling the windows with Xenon provides the Filling the windows with Xenon provides the most insulation as it yields the highest most insulation as it yields the highest thermal resistance.thermal resistance.

It is cost effective. The windows filled with It is cost effective. The windows filled with special gases cost much more than normal, special gases cost much more than normal, but the savings are quickly made up over the but the savings are quickly made up over the following months.following months.

The larger the windows, or the more windows The larger the windows, or the more windows there are, the more cost savings are there are, the more cost savings are available.available.

RecommendationsIt is clearly more effective to use windows filled It is clearly more effective to use windows filled with certain gases, rather than just air. Argon, with certain gases, rather than just air. Argon, Krypton, and Xenon are all more effective, but Krypton, and Xenon are all more effective, but Xenon provides the most insulation.Xenon provides the most insulation.

Though the windows are more expensive at first Though the windows are more expensive at first and appear to be an investment, the eventual and appear to be an investment, the eventual savings from heat loss are well worth the cost.savings from heat loss are well worth the cost.

These findings are most important for buildings These findings are most important for buildings with many large windows. Regular homes, with a with many large windows. Regular homes, with a few small windows, will not see significant cost few small windows, will not see significant cost savings.savings.

Equations (1)

Equations (2)