Mathematical Modeling – The Impact of Cooling Water Temperature Upsurge on Combined Cycle Power Plant Performance and Operation

This paper presents the mathematical modeling analysis on cooling system in a combined cycle power plant. The objective of this study is to get the impact of cooling water upsurge on plant performance and operation, using Engineering Equation Solver (EES™) tools. Power plant installed with total power capacity of block#1 is 505.95 MWe and block#2 is 720.8 MWe, where sea water consumed as cooling media at two unit condensers. Basic principle of analysis is heat balance calculation from steam turbine and condenser, concern to vacuum condition and heat rate values. Based on the result shown graphically, there were impact the upsurge of cooling water to increase plant heat rate and vacuum pressure in condenser so ensued decreasing plant efficiency and causing possibility steam turbine trip as back pressure raised from condenser.


Introduction
In combined cycle power plant, energy produced from the conversion gas heat and steam expansion through turbine into mechanical shaft coupled to electric generator. The magnitude of generator power depends on expansion process where gas, steam and its condensate release their potential energy (named enthalpy). Heat and steam enthalpy would change into blades rotation (mechanical energy), exhaust heat and cooling medium (sensible heat). As the energy sources generally come from natural gas and oil, then its combustion exhaust gas recovered into steam in steam generator equipment. Cooling water plays important role to give the condenser the optimum vacuum pressure or enthalpy drop in turbine system. With principle applying heat balance in turbine and condenser, it could be known and calculated the amount cooling water and its outlet temperature for the determined output power and turbine inlet steam. When demand of output power increased, thus condenser must be able to achieve the suitable vacuum condition to expand steam optimally. Conversely if inlet temperature of cooling water increased, it could reduce the latent heat for condensation and for compensating this, steam consumption to be magnified.
The cooling water temperature upsurge is the important parameter that effects plant performance as well its reliability. Anozie, A. N. and O. J. Odejobi (2011). developed calculation model for simulation of a thermal plant at various circulation water flowrate and saturation pressure , to determine the optimum condenser cooling water flowrate for the process. The influence of circulation cooling water on the efficiency thermonuclear plant also conducted by Gañán, J., et al. (2005). to improve plant performance with appropriate cooling system. The important role of cooling system for power plant as well was concerned by Wu, J., et al. (2001) to study simulation of cooling water discharge from power plant to give contribution in feasibility studies of engineering options that increase the cooling capacity of the waterbody. This parameter is one of the most influential that affect operation variables such as condenser pressure, heat rate, fuel consumption, and cycle efficiency which had interested Basically in combined cycle power plant, there are two sections of energy conversion i.e. combustion heat and steam power to mechanical power. Combustion convert chemical energy (contained in fuel) into combustion heat and its high pressure utilized to rotate the turbine blades. Meanwhile at the second one, the exhaust gas from the gas turbine recovered in heat steam generator to convert into steam that used to produce power through steam turbine.

Figure 1. Principle of Energy Conversion in Combined Cycle Power Plant Modeled
System cycle efficiency indicated by the parameter of heat rate that is the ratio of the amount input energy to output power achieved. Output power depends on the turbine enthalpy drop, which determined by condenser performance for rejecting heat to cooling medium. This paper presents the study of the cooling water upsurge to power plant performance and operation using mathematical modeling heat balance on turbine and condenser system

Methods
Mathematical Modeling for analyzing the impact of cooling water temperature rise to plant performance and its operation, was built through principle of mass and heat balance calculation in control volume bounded at condenser and steam turbine. For this modeling, assumptions taken are steady state, heat transfer parameters at condenser as surface area, global heat transfer coefficient is constant. As the variables of analysis are operation parameters such as pressure P, temperature T, flowrate of fuel G F , steam G s and cooling water G CW also generator power from gas and steam turbine W GT and W ST , those collected from the plant design, performance test/and or operation data record. Firstly it had to be identified the cycle process of gas system and steam flow to turbine and condenser as well power generator to understand the thermodynamic state and cycle of power generated.
Output power generator W ST is the result of energy balance upon steam turbine and condenser. Enthalpy drop (ΔH) defined by the difference of total enthalpy inlet turbine H inturb and condensation enthalpy H cond . These functions are defined in the following equations based on the energy balance. And W GT obtained from measured value from performance test/and or operation record.
As the performance value determined by the Net Plant Heat Rate (NPHR) based on the amount of total heat from combustion (HHV and LHV basis) and steam divided to the net total output power generator at gas and steam turbine after subtracted auxiliary power. For combustion heat calculated by variables fuel consumption and calorific value.

NPHR LHV = (Q LHV + H inturb )/(W GT + W ST -P aux ) (11)
Where cp is specific heat of cooling water in KJ/kg. o C ; all enthalpies in KJ and W in kW units. Subscripts HP and LP indicates high pressure and low pressure state conditions whereas hot and cold explained temperature for outlet and inlet from and into condenser of cooling medium. Notation η ST is turbine efficiency. For h cond is specific enthalpy for condensation at saturated pressure P cond . Calculation model need to be verified to ensure or know the bias (error), compared to the performance test/and or operation record. As the parameters for verification are the input variables and calculated variables. Model that has been verified, then for the analysis variable is the rise of cooling water temperature concerning to the vacuum pressure condenser and output power changes.

Result and Discussion
The combined cycle power plant comprises 2 blocks (block#1 and block#2) with gas turbines, HRSGs, and steam turbines system that identified from plant design/process flow diagram. Each   To clarify the hypothesis that the raised cooling water would decrease the plant performance and operation, previously it was important to analyze the operation data. Figure 3 (from model recalculation) shows that with referring to plant design there are the most influential parameters of condenser pressure and cooling water, which values subsequently are 0.083 bar.abs and 29 o C for cold water-in for mass flow 16300 ton/hr. But from observation when steam turbine pressure, temperature and steam flow operated at 62.99 bar; 510 o C; 276.48 kg/s (HP) and 4.72 bar; 313 o C; 69.55 kg/s (LP), the condenser actually operates at average values of pressure 10 kPa.abs and temperature below 30 o C thus output power turbine produces more than 120 MWe. Conversely when temperature cold-in higher than 30 o C corresponds to the output power below 100 MWe.
It is clearly shown that the cooling water upsurge has the impact upon output power (related to plant performance) and possibility the change of condenser pressure.
For inlet cooling temperature changing from 30 ÷ 35 o C, the condenser pressure and output power will vary with corresponding to condenser inlet heat. Inlet heat of condenser depends on thermodynamic state of steam, correlating to condensation pressure or temperature at condition of steam out from turbine. It could be seen that the higher cooling water temperature causing output power more higher, this is because latent heat of condensation at correspond pressure saturated more higher. This modeling shows averagely that the condenser pressure is still far from the limit design pressure -520 mmHg, nevertheless this bring the potential problem turbine could be tripped by the raised back pressure.