Analysis of small size combined cycle plants based on the use of supercritical HRSG

An interesting feature in the design of Combined Cycle Power Plants (CCPP), as the gas turbines become larger and the exhaust temperatures rise, is the use of Heat Recovery Steam Generators (HRSG) with supercritical (SC) steam properties. The paper is orientated at assessing the potential of using supercritical steam cycles in combined cycle power plants. The interaction between gas turbine and HRSG thermodynamic is mainly focused. The analysis is based on minimizing total exergy losses in order to optimise the performance of the HRSG-steam turbine system. In the paper the alternative design of combined cycle power plants in which a supercritical (SC) HRSG is present has been analyzed. The analysis includes a comparison of three different supercritical HRSG configurations with single (SC1RH) and double reheaters (SC2RH) with simple HRSG single pressure configurations and advanced double and triple pressure HRSG structures. The HRSG structures (steam pressure, maximum temperature and mass flow rate) are designed considering as objective function the minimization of the total exergy losses in the system composed by HRSG and steam turbine (ST). The optimization procedure is based on combination of random-search and simplex method. The evaluation shows that even if the use of supercritical HRSG is non particularly convenient in the perspective of efficiency increase it can be a valid technical solution aiming to the development of medium size (50–120 MW) combined cycle power plants. Four different commercially available gas turbine cycles have been tested for this specific purpose obtaining efficiency level approaching in particular cases the state of the art results with simpler technical solutions.