Question

Combined cycle power plants are common because of their high efficiency and scalability. They typically combine a gas turbine cycle (the Brayton Cycle) with a steam turbine cycle (the Rankine Cycle) [1]. The goal of this project is to determine the operating efficiency and profitability of a realistic combined cycle power plant. The power plant is as follows: 1. A natural gas-fired Brayton cycle with mÛ air,Br = 1.25 kg s−1 . (a) Ambient air at 1 bar and 300 K is taken in. (b) A compressor with an inlet-to-outlet pressure ratio of 1:9. (c) Regeneration between the compressor and the combustor, increasing the temperature to 800 K (d) Combustion at constant pressure to 1600 K. (e) A two-stage turbine system with reheat between the stages. Reheat occurs at 3 bar and raises the temperature to 1400 K. (f) Discharge to a heat exchanger at 1.2 bar, where waste heat is used to warm steam for the Rankine Cycle. The outlet temperature from this heat exchanger is 1000 K. (g) From that heat exchanger, discharge to the regeneration heat exchanger in part 1c. 2. A water-steam Rankine Cycle with mÛ water,Ra = 1.40 kg s−1 . (a) A two-stage turbine system, with reheat between the stages. The first turbine inlet is at 560 ◦C and 160 bar. Reheat occurs at 40 bar up to 560 ◦C. The secondstage turbine outlet is 1.5 bar. (b) Cooling at constant pressure in a condenser via heat exchange with ambient air to saturated liquid. The air used in condenser cooling must not exceed 400 K when it is released from the power plant. (c) A pump from the low-side pressure to the high-side pressure. (d) Heating in a boiler at constant pressure, using the waste heat from the Brayton Cycle first and natural gas combustion second to reach the turbine inlet temperatures. All turbines are 80% isentropically efficient. All turbines are used to spin electrical generators that are 95% efficient - 95% of work done on the generator is converted to electrical power. For the Brayton cycle, airstandard analysis may be used. Compressors and pumps have isentropic efficiencies of 70%. Combustion is 80% efficient - that is, 80% of the fuel’s heating value is delivered into the working fluid. The heat of combustion of natural gas is 50 MJ kg−1 . 1 Thermodynamics Combined Cycle Project 2 Results The results needed are: 1. The net work produced by the power plant. 2. The mass flow rates of cooling air across the condenser. 3. The mass flow rate of natural gas required. 4. If natural gas costs $5.00 per 28 m3 with a specific volume of 0.0022 m3 kg−1 , the cost to operate the power cycle per hour. 5. If electricity is sold at $0.15 per kW h, the hourly profit (sales minus operating cost) of the power plant.

Answer 1

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