
714 Principles of Sustainable Energy Systems
© 2008 Taylor & Francis Group, LLC
also produce products and wastes that move across the system boundary. Control system
theory deals with these externalities as material inputs and outputs to the physical system.
For the example of the cruise control system, the level of fuel in the tank is not a part of the
speed control system. However, the constant speed can only be maintained as long as the
fuel continues to ow to the engine.
Activities and technology can be changed in many ways, which we term disturbances,
even including innovation and technology development. At any given time, the existing