Study on the Effect of a Cogeneration System Capacity on its CO2 Emissions

Abstract

With the global warming problem aggravation and subsequent application of the Kyoto Protocol, CO2 emissions are becoming an important factor to regard when verifying the applicability of cogeneration systems. Considering that, the purpose of this work is to study the influence a cogeneration system capacity has on its CO2 emissions under two kinds of operation strategies: one focused on exergetic efficiency and another on running cost. The system meets the demand pattern typical of a hospital in Japan, operating during one year with an average heat-to-power ratio of 1.3. The main equipments of the cogeneration system are: a gas turbine with waste heat boiler, a main boiler and an auxiliary steam turbine. Each of these equipments was characterized with partial load models and the turbines’ efficiencies at full load also changed according to the system’s capacity. Still, it was assumed that eventual surplus of electricity generated can be sold. The main results show that for any of the capacities simulated, an exergetic efficiency focused operational strategy always resulted on higher CO2 emissions reduction than a running cost focused one. Furthermore, the amount of reduction in emissions decreased when the system capacity decreased, reaching a value of 1.6% when the system’s capacity was 33% of the maximum electricity’s demand with a heat-to-power ratio of 4.1. When the system operated focused on running cost, the economic savings increased with the capacity and reached 42% for a system capacity of 80% of maximum electricity’s demand and with a heat-to-power ratio of 2.3. In such conditions however, there was an increase in emissions of 8.5%. Still for the same capacity, an exergetic efficiency operation strategy presented the best balance between cost and emissions, generating economic savings of 29% with a decrease in CO2 emissions of 7.1%. The results found show the importance of an exergy focused operational strategy and also indicated that lower capacities resulted in lesser gains on both CO2 emissions and running cost savings.