Part-load analysis of gas turbine & ORC combined cycles

Organic Rankine Cycles (ORC) are considered to be effective solutions for low and medium temperature heat recovery for power production. In such applications where the temperature of waste heat is in the 300-450 degrees C range, they stand as a more interesting option to enhance system performance than the more complex steam cycles typically used in heavy duty combined cycle plants. In this paper, the rated and part-load performances of combined cycles formed by a topping gas turbine and a bottoming ORC are analysed. To this aim, five commercial gas turbines are used in combination with an ORC bottoming cycle in lieu of a conventional steam cycle, with the goal of maximising the overall efficiency of the plant at part-load. These operating conditions are typical of power stations operating in load-following mode. During the analysis of part-load operation, different control strategies are studied in order to recuperate as much heat as possible from the gas turbine exhaust. It is observed that keeping the live vapour conditions in the bottoming cycle as constant as possible produces maximum power at any operating conditions, compensating for the poor performance of the gas turbine engine at part-load. The interest of this work is twofold. First, it confirms the interest of merging gas turbines and ORC units for efficient power generation under variable operating conditions. Second, the work is based on commercial gas turbines, thus trying to analyse the real potential of state-of-the-art technology in the application proposed. (C) 2011 Elsevier Ltd. All rights reserved.