Impact of WWR on Building Energy Consumption

The buildings sector accounts for about 76% of electricity use and 40% of all U. S. primary energy use and associated greenhouse gas (GHG) emissions, making it essential to reduce energy consumption in buildings in order to meet national energy and environmental challenges (Chapter 1) and to reduce costs to building owners and tenants.

2014 Residential and Commercial Building Primary Energy Use (Quads)

2014 Electricity Sale for Buildings

Source: energy.gov

There are recommendations for WWR in building code compliance such as ASHRAE 90.1, ECBC, etc. It is due to the fact that WWR can significantly change the energy consumption if we compare the energy use in eQuest(energy simulation software) by keeping all the input parameters the same and change the window to wall ratio on each floor. We can have some comparable results.

How to calculate W.W.R?

Window to wall ratio is the ratio of total glazing area to the gross exterior wall area.

Building Energy Consumption w.r.t WWR

Window to wall ratio can affect the Building energy consumption in many ways. For example the more the window surface area, more is the solar heat gain through it. And there is another factor, Daylighting, if we have installed daylight controls installed in the buildings, the more window area would mean more daylight and less lighting power.

In this example, different WWR in the same building is considered to compare the Annual building energy consumption, modeled in an energy simulation software called eQuest. Constant values taken in all models are as follows:

• Building Type: Office Building
• Location: Gurugram, India
• Climate: Composite

• U-value of glass: 1.57 W/m2 K
• SHGC of glass: 0.25
• VLT of glass: 0.8

Building Envelope with 10% WWR

Building Envelope with 20% WWR

Building Envelope with 30% WWR

Building Envelope with 40% WWR

Building Envelope with 50% WWR

Building Envelope with 60% WWR

Building Envelope with 70% WWR

Building Envelope with 80% WWR

Building Envelope with 90% WWR

Following are the results showing the energy consumption in kWh/year with different WWR.

Results

Results show that energy consumption is lowest for 20%-30% WWR.
Contrasting results: Energy Consumption for 10% WWR case is closest to the 90% WWR case but their window to wall ratio is different by 80% . This is due to the daylight sensors being used, 10% WWR case doesn’t give much daylight and hence the lighting load is more in this case. While in 90% WWR case, the surface area for the windows is a lot which has the most solar gains, hence the overall energy consumption.