Evaluation of Repowering Cycles

There are three primary methods by which an aging steam electric unit can be repowered by incorporating a gas turbine-generator. These are: heat recovery steam generator repowering, Feedwater Heater repowering, and Windbox repowering. Evaluation of these options can be very difficult due to the fact that the initial capital cost may be the smallest cost. The present worth of future fuel and maintenance costs can far outweigh the initial cost. An additional complexity is the fact that the repowered unit may have the best unit heat rate within the system. It will certainly be improved, which means the capacity factor will be increased. As the capacity factor increases, the cost of fuel becomes more significant. For a base loaded fossil unit, the cost of fuel is the largest single annual cost. Prediction of the future capacity factor is complex, involving many variables and uncertainties. Selection of the best method requires an evaluation of future energy demand and currently incurred capital costs as well as future fuel and maintenance costs. Determination of an optimum balance between present and future costs has always been the problem of the utility planner. The intent of this paper is to outline an evaluation method to determine the most suitable repowering option. Comments regarding each option will be made. In some cases, a selection can be made by intuition or by observing the obvious. In other cases, the selection will not be obvious. The utility will always benefit from the execution of a well thought out evaluation plan. Following an orderly procedure will enhance the thought process. In addition, a documented procedure will be available for re-evaluation when changes occur and will always provide documentation for future reference.