Determination of the real power loss for a steam turbine

All real processes generate entropy and the power/exergy loss is usually determined by means of the Gouy-Stodola law. If the system only exchanges heat at the environmental temperature, the Gouy-Stodola law gives the correct power loss. However, most industrial processes exchange heat at higher or lower temperatures than the actual environmental temperature. To calculate the real power loss it is not correct to use the actual environmental temperature in the Gouy-Stodola law. The first aim of this paper is to show through simple steam turbine example cases that the previous statement is true. The second aim of the paper is to define the effective temperature to calculate the real power loss of the system from the Gouy-Stodola law and apply it to turbine examples. Turbine examples reveal that the sum of the power loss and real power production does not give the correct maximum power production if the real environmental temperature is used in all cases in the Gouy-Stodola law. Example calculations also show that the correct power loss can be defined if the effective temperature is used instead of the real environmental temperature.