An Application of Integrated Thermal and Electrical Energy Cogeneration Optimization

Abstract

The savings associated with operations optimization of power generation and cogeneration facilities are large, and readily justify the hardware and software costs required for implementation of Energy Management Optimization Systems (EMOS). The objective of such systems is to minimize the total energy operating costs for specified power and steam load profiles, including the purchase of external power and/or steam, and the use of internal self-generation equipment. The EMOS may require online operation using current measurements (e.g. flow, powers, temperatures, etc.), and calculating optimum energy purchase and equipment dispatch within time periods consistent with changing ambients, loads and/or purchase energy price conditions. The automatic recognition of changes in equipment status and system operating configuration may be required. The EMOS may also consider the electrical distribution system to minimize losses, and to ensure that tbe optimum thermal power dispatch may be reliably delivered to the loads under tbe existing distribution configuration within electrical equipment operating limits. Automatic generation dispatch may also be required. A system which incorporates the requirements of the above specification and more, has been designed, installed and is operational at a large industrial cogeneration facility. A description of the specifics of this entire system is beyond tbe scope of this paper, however, a discussion of selected system features will be given. This application involves the simultaneous optimization of energy supply for in-plant power and process steam from many highly integrated system components. Cogeneration plants, as shown in Figure 1, are generally characterized by multiple sources of energy, various types of prime movers (e.g. boilers, waste heat recovery, steam and gas turbines, etc.), and varying requirements for process heat and electrical power, particularly if bulk power is being purchased, or dispatched to a utility grid as in the case of Independent Power Producers. In addition, the operating characteristics of tbe equipment and loads are continuously changing due to outage of equipment, changes in process steam and electrical demands, ambient conditions and performance deterioration. The ability to coordinate and optimize the simultaneous operation of the various components to meet all the energy requirements at minimum cost is a formidable task. In addition to the thermal optimization of boilers, gas turbines, and various types of condensing and autoextraction steam turbines, the system also considers the electrical distribution system, where changing bus configurations, power and voltage control impose additional constraints and limits which are solved in the optimum dispatch. The application incorporates automatic closed loop control of many process set points with a sophisticated system of permissives and automatic generation control features. Since a high on-line operating factor is essential, many design features are incorporated for signal validation and malfunction identification, and to make the system robust to instrument failure and drift. The system can be used as an on-line or off-line supervisory program. For on line implementation, closed loop response, fail safe operation and interfacing with process control systems are key closed loop implementation considerations. The system involves the interaction of several modules. The following will describe selected modules and how they interface to satisfy existing loads at minimum cost.