Integrated systems for cogeneration and district energy

Abstract

Systems that integrate cogeneration and district energy are described. Included are descriptions of district energy (i.e., district heating and cooling), showing how it builds on and combines technologies for district heating and district cooling. Cogeneration-based district energy is defined and many facets of it are discussed, including possible variations of cogeneration-based district energy. Cogeneration-based district energy is compared with alternative and conventional systems for electrical and thermal energy, which constitute the main competing technologies. A general model for cogeneration-based district energy suitable for engineering and thermodynamic assessments is presented and illustrated and various operation modes for it identified. A detailed thermodynamic description of a combined cogeneration-based district energy system is given, along with corresponding energy and exergy balances and efficiencies. Alternative measures of system efficiency are also provided. The need for proper measures of merit for systems for cogeneration-based district energy is emphasized, especially when they include chiller systems, since useful, meaningful and logical measures are lacking. Finally, systems for integrated cogeneration-based district energy are described, by drawing on the material presented for both cogeneration and district energy. It is explained how district cooling systems using absorption chillers often complement district heating systems when both use heat supplied from a cogeneration plant because the demand for heat in a district heating system is lower in summer than in winter and heat-driven district cooling, which requires heat mainly in the summer, can help to balance the seasonal demands for cogeneration-derived heat.