Any complex system comprises many subsystems some energy intensive, some not so intensive, and others least intensive. Energy efficiency optimization of the overall system may be achieved by either of two methods:

• Overall optimization of the total energy system data

• Synthesis of the individual subsystem data into one consolidated data point

A typical example is the energy consumption of a complex refinery, comprising many sections or subprocesses, with a specific process objective. Each unit consumes energy resources such as fuel, steam, and power.

By use of the operating data, the specific energy consumption of each unit may be determined. This may be synthesized to arrive at the overall energy consumption of the process. In the overall optimization method, all the energy cost centers will be taken into the data as input parameters, and a multivariable model will be developed to determine the total energy consumption of the process. The observed values are given in table 2-12. The advantage of individual unit-specific consumption is that it is possible to identify problem areas/units more precisely for remedial action.