A window upgrade is considered for an apartment building from double-pane metal frame windows (UE = 4.61 W/m^2.°C) to double-pane with low-e film and wood frame windows (UR = 2.02 W/m^2.°C). The total window area to be retrofitted is 200 m^2. The building is located in Nantes

Question

A window upgrade is considered for an apartment building from double-pane metal frame windows (UE = 4.61 W/[latex]m^{2}[/latex].°C) to double-pane with low-e film and wood frame windows (UR = 2.02 W/[latex]m^{2}[/latex].°C). The total window area to be retrofitted is 200 [latex]m^{2}[/latex]. The building is located in Nantes (France) and is conditioned 24 hours/day, 7 days/week throughout the heating season. An electric baseboard provides heating and a window AC provides cooling (EER = 8.0).Assume that the cost of electricity is $0.10/kWh.

Accepted Answer

To determine the energy savings due to the addition of insulation, we assume that the annual heating and cooling degree-days before and after the retrofit remained unchanged (this assumption is justified by the fact that the window contribution to the BLC is relatively small) and are, respectively, [latex]DD_{H}[/latex] = 2,244°C-day/yr and DDC = 255°C-day/yr. The energy savings during heating (assume that the system efficiency is 1.0 for electric heating) are calculated to be:

[latex]ΔE=24.200 m^{2}*[(4.61-2.02)W/m^{2}°C]*27244°C.day/yr=27,897kWh/yr[/latex]

If an EER (energy efficiency ratio) value of 8.0 is assumed for the AC system, the energy savings during cooling are estimated as follows:

[latex]ΔE=24.200 m^{2}*[(4.61-2.02)W/m^{2}°C]*2755°C.day/yr *1/8.0=396kWh/yr[/latex]

Therefore the total energy savings due to upgrading the windows is 28,293 kWh which corresponds to about $2,829 when the electricity cost is $.10/kWh. The cost of replacing the windows is rather high (it is estimated to be $150/[latex]m^{2}[/latex] for this project). The payback period of the window retrofit can be estimated to be:

[latex]Payback=\frac{200m^{2}*150\$/m^{2}}{28,293kWh/yr*0.10\$/kWh}[/latex]=10.4 years

Therefore, the window upgrade is not cost-effective based solely on thermal performance. The investment on new windows may, however, be justifiable based on other factors such as increased comfort within the space.

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