Optimization of the cooling water mass flow rate under variable load of a power unit

As the contribution of renewable energy sources in the energy balance is increasing, traditional thermal power units have to be more flexible over a full and thus extended range of load conditions, among other requirements. Furthermore, the efficiency of power units should not significantly decrease under lower load conditions which will often occur. The paper analyzes the process of determining the cooling water mass flow rate under the power unit load varying between 100% and 40% with the aim of achieving the maximum efficiency and power output. A mathematical model of a 225-MW power unit was developed using Ebsilon software and validated based on measurement data. The efficiency characteristics of the boiler, turbine and cooling water pump in off-design conditions as a function of load were entered in Ebsilon. As part of the calculations, the optimal cooling water mass flow rates were determined. The analysis shows that reducing the cooling water mass flow rate may seem reasonable in the case of power units operating at less than 60% load. Assuming that the power unit operates under 40% load for 1000 h per annum, if the cooling water flow rate is reduced from the nominal value down to the optimal one, the 225-MW power unit would produce an additional net power output of about 137 MWh.