Thermoeconomic Simulation of 27 MW Campus Cooling Heating Power (CHP) Plant

This paper describes the modeling of a 27 MW combined cycle cogeneration plant with 10,000 tons of cooling made available as chilled water at the central cooling facility that was designed and is currently operated to provide heating, cooling and electricity to the University of Maryland campus. The topping cycle of the combined cycle cogeneration plant consists of two gas turbines each producing 11 MW of electric power at full load. The energy of the exhaust gases from these gas turbines is then utilized to generate steam in two heat recovery steam generators. The heat recovery steam generators have supplemental duct firing using natural gas to meet the peak steam load. In the bottoming part of the combined cycle, the steam from the heat recovery steam generators is expanded in a backpressure steam turbine to supply steam to the campus at about 963 kPa, generating an additional 5.5 MW of electric power in this process. There is no condenser wherein the campus acts as a sink for the steam. The central cooling facility is designed to supply 10,000 tons of cooling as chilled water out of which 3800 tons is supplied by two steam driven centrifugal chillers, which utilize a part of the steam supplied to the campus and the remaining by the centrifugal electric chillers. The combined cycle cogeneration plant along with the central chilled water-cooling facility is modeled using a commercially available flexible cogeneration software package. The model is built based on the design data available from design manuals of gas turbines, heat recovery steam generators, backpressure steam turbine and centrifugal chillers. Two energy or cost savings opportunities were evaluated using the cogeneration software model. The first is adding inlet air-cooling using either an absorption or electric chiller to increase electrical power output during hot weather. This assessment included estimating kWh savings over a range of ambient temperatures. The second opportunity is using economizers to provide free cooling and reduce the usage of the electric and steam driven chillers. Detailed results of the thermal energy savings as well as the electrical and natural gas cost savings by employing these technologies are discussed in this paper.