Optimum selection of prime movers in combined heat and power (CHP) systems is of crucial importance due to the fact that inappropriate choices reduce the benefits of CHP systems considerably. In the selection procedure, the performance characteristics of prime movers as well as economic parameters should be taken into account. In this paper, a thermo-economic method for selecting the optimum nominal power and planning the operational strategy of gas turbine as the prime mover of a medium scale (500-5000 kW) CHP system is presented. Appropriate relations for estimating thermodynamic and economic parameters of the system in the context of net annual cost criterion are introduced. Three modes of operation have been considered, namely, two-way connection (TWC) mode, one-way connection (OWC) mode, and heat demand following (HDF) mode. In TWC mode, buying electricity from the grid and selling the excess electricity to the grid are allowed. OWC mode is a situation in which it is only possible to buy electricity from the grid. In HDF mode, buying electricity from the grid and selling electricity to the grid are allowed. HDF mode of operation is considered to have the minimum waste of energy due to the fact that prime movers work in a condition at which the excess produced heat is minimal. As a way of dealing with the environmental concerns, the impact of carbon tax has also been studied. The proposed method has been used for a case study. It was observed that the optimum nominal powers in TWC mode, OWC mode, and HDF mode are 3.5 MW, 3.4 MW, and 0.8 MW, respectively. Furthermore, in order to determine the sensitivity of results to parameters such as cost of electricity, cost of fuel, and carbon tax, a comprehensive sensitivity analysis was conducted. It is noted that the proposed method may be used for other types of prime movers (such as internal combustion engines) as well as various sizes of combined heat and power systems.
- carbon tax
- combined heat and power system
- gas turbine