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Considerations with Distributed Generation: Source vs. Site Efficiency

With increased emphasis on efficiency, many government agencies are viewing source energy consumption with increasing interest. This calculation compares alternative technologies on a fuel cycle basis, accounting for the inefficiencies of converting a fuel at the source to a useful energy form at the end user. For example, coal generated electricity typically entails a 1% loss to extract the coal from the ground and transport it to the power plant, a 70% loss to convert the coal to electricity and another 10% to transmitt the electricity from the power plant to the end user. These inefficiencies can be combined into a fuel cycle efficiency (FCE). In the example just described:

FCE = (1-.01) x (1-0.70) x (1-0.10) = .267

Source energy consumption is determined by dividing site energy consumption by the FCE. For the same example, a total of 3.7 kW (1 /.267) of energy is required for every 1.0 kW consumed .

An FCE of 0.9-0.95 is typically calculated for natural gas as compared to .25-.35 for electricity. When source energy consumption is one of the criteria for system selection, natural gas engine driven equipment can have an advantage over electrically powered motors.