IntroductionWith the aim of reducing CO2 emissions and dependency from fossil fuels, in various countries the sharp increase of electrical RES (Renewable Energy Sources) as a result of policies of substantial subsidies is creating problems to an efficient and reliable operation of the electrical power system.
Both transmission and distribution systems have to face an increasingly variable production of intermittent RES with special reference to wind and photovoltaic; the problem is not however limited to T&D, but it calls for enhanced performances of the thermal fleet of a country which is required of a “flexible operation” (start-up time, ramps of power increase / decrease, reduced hours of operation) by far beyond technical, economic and environmental limits of the great majority of the present plants in service. Bottlenecks in transmission and distribution are further aggravating the problems.
It is quite clear how in this view energy storage systems could play a fundamental role in the evolution of smart grids (both super grids and sub transmission and distribution systems), but interventions on the flexibilization of thermal plants could be an economical solution or a parallel one and it is being investigated in Italy where the present 15 GW of   PV plants and 7 GW of wind ones are changing the electrical market behavior. This is true especially in the evenings (sunset and stop of PV production) and over weekends in the vacation periods, when the load is getting close to the potential non-programmable RES power generation. Considering the indispensable backup thermal hot power, necessary to warrant system stability and security, the possibility of operating thermal units at “very low technical minimum load” is becoming a key asset together with the capability to have adequate speed in the evening ramps. The enhanced flexibility of thermal units allows to avoid the need for sharp reduction of the NTC (Net Transfer Capacity) in import, with the consequent market fragmentation, and to better optimize the operation of the hydro plants, particularly for the run-of-river units avoiding waste of renewable energy.Practical applications to power plants
In general, with flexibility of power plants we denote their ability to perform frequent start/stop cycles, to withstand large and rapid load variation from nominal value to their minimum operating load, to take part in the regulation of the network frequency and more generally to provide ancillary services (i.e. all those services necessary to ensure the safety of the entire electrical system) and finally to allow the reliability and repeatability of the maneuvers and of the start-up times.
More in detail, one of the major facts limiting the flexibility of power plants in terms of load ramp rates and startup times, namely of combined cycles but also of traditional fossil fuel boilers, are the temperature and pressure transients on the steam turbine and on the steam generator.  For the combined cycles this fact penalize the fast startup capability of the gas turbines requiring waiting times compared to simple open cycle startups. Dynamic mathematical models has demonstrated good capabilities in evaluating the pressure and temperature profiles of the thick metal wall components above mentioned where the thermal stress is higher during the transients. Hence with these tools the best startup strategy and load ramp rate can be studied in advance to minimize the life consumption of the most stressed components and these results can be transferred with high benefits to the normal operation procedures.
Control system tuning is another important task that can be undertaken. Talking about ancillary services, it is of basic importance to have a well-tuned control system to be able to provide these services to the electric grid with adequate regulating band and promptness. On field control system tuning should be considered a prerequisite for the achievement of the optimal plant flexibility of whatever industrial plant.

 As for the combined cycles, it could be requested to improve the frequency regulation or to exploit this service in those conditions where the gas turbine cannot supply it. This can also be done by the steam turbine (which normally doesn’t take part to the frequency regulation) or by means of other techniques temporary exploiting the thermal energy of the steam generator. All these techniques can be tested in advance by means of mathematical models and then implemented on the plant reducing the risk of not adequate performances.
 Reducing the minimum operating load is another important goal related to the flexibility enhancement of fossil fuel power plants.  While for combined cycles this can be done by the manufacturers of the gas turbines acting on the design of the burners to reduce the pollutants at low loads, for the fossil fuel boilers it is a matter of temperatures and flow rates on the furnace metal walls. Studies on this subject have been performed on once through power plants. The final results have been transferred by the owners to the plants with a significant reduction of their minimum operating load.
 As a final remark, according to the experience of CESI’s consultants and thanks to the specific tools developed for the analysis, CESI can play a significant and strategic role in support of the power plants owners willing to improve the flexibility of their plants. Analyzing the plant status, suggesting the possible improvements on the basis of cost/revenue evaluations, testing in advance the proposed solutions by means of simulators and acting as the technical link between the producers and the manufacturers are the main tasks that can be pursued to improve power plants flexibility.