PERFORMANCE: Energy & Cost


According to the Department of Energy's 2010 Buildings Energy Data Book, commercial buildings represent just over 18% of U.S. energy consumption of 96.3 Quads. Office, retail, and educational facilities represent about ½ of commercial sector energy consumption. Windows are responsible for 1.88 Quads of energy for heating and 3.86 Quads of energy for cooling.

Windows have a dominant influence on a building's appearance and interior environment, yet windows can be one of the most important components impacting its energy use, peak electricity demand, and environmental consequences. Heat gain and heat loss through windows can represent a significant portion of a building's heating and cooling loads. By providing natural light, windows can reduce electric lighting loads by using design strategies such as dimming controls, automated shading, and light redirection.

Tools such as the Facade Design Tool and COMFEN demonstrate the environmental impacts of various design scenarios—allowing for decisions to be made early in the design process.

This image illustrates annual energy use per square foot of heating, cooling, fans and lights for 4 scenarios in Chicago, south orientation, 40% window area, and 4 unshaded glazing types.


This image illustrates monthly zone energy due to heating, cooling, fans and lights for the same 4 scenarios as in the previous illustration using the Facade Design Tool. COMFEN can show the energy use annually, monthy for the zone, or monthly for the facade.

Site and Source Energy
Site energy is the amount of heat and electricity consumed by a building as reflected in utility bills. Site energy may be delivered in one of 2 forms: primary and/or secondary energy. Primary energy is the raw fuel that is burned to create heat and electricity. Secondary energy is the energy product (heat or electricity) created from a raw fuel. Primary and secondary energy consumed at a site are not directly comparable because one represents a raw fuel while the other represents a converted fuel. In order to assess the relative efficiencies of buildings with varying proportions of primary and secondary energy consumption, conversion these 2 types of energy into equivalent units of raw fuel consumed to generate that one unit of energy consumed on-site is necessary. To achieve this equivalency, the Environmental Protection Agency (EPA) uses the convention of source energy.

The EPA has determined that source energy is the most equitable unit of evaluation of energy consumption of commercial buildings. Source energy represents the total amount of raw fuel that is required to operate the building incorporating transmission, delivery, and production losses. This allows for a complete assessment of energy efficiency in a building by providing equitable rating and consumption in a single common unit.

When primary energy is consumed, the conversion to source energy accounts for losses that are incurred in storage, transport and delivery of fuel to the building. When secondary energy is consumed, the conversion to source must account for losses incurred in the production, transmission, and delivery to the site. Source energy comparisons generally reflect energy costs and carbon emissions more accurately than site energy. The factors used to restate primary and secondary energy in terms of the total equivalent source energy units are called the source-site ratios.


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