In known methods and systems for optimal operation of a power plant, plant operation is controlled by control values generated by a control system and is optimised in order to minimise fuel costs while achieving nominal required output values for produced power and process steam.  This is done by, at a given time, determining future values of control values and simulating a behaviour of the plant up to a given future time.  From the simulation, fuel costs are determined and an objective function comprising the fuel costs and costs for buying power from another source is computed.  In order to determine optimal control values, the control values are varied and the simulation is repeated until a minimum of the objective function is found.  The above procedure is also called “Unit commitment problem” (UCP).  A mathematical treatment is given in “The Generalised Unit Commitment Problem”, Ross Baldick, IEEE Transactions on Power Systems, 10(1):465-475, February 1995.  The optimisation is constrained by the fact that predetermined operating limits of the plant may not be exceeded. 
“Optimal-Maintenance Modelling on Finitic Time with Technology Replacement and Changing Repair Costs”, Jason W. Rupe, IEEE 2000 Proc.  Annual Reliability and Maintainability Symposium, shows a method to determine when to replace or maintain components in a system with limited lifetime. 
“Optimal Generation Scheduling with Ramping Costs”, C. Wang, and S. Shahidehpour, IEEE Trans.  on Power Systems, Vol.  10, no.  1, pp 60-67.  February 1995, shows how to include an effect of fatigue stress and a resulting steam turbine rotor depreciation in scheduling algorithms for minimising operation cost.