Exam 20220623 Eng

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ENERGY CONVERSION Note: you can keep this text for self-evaluation at the end of the exam. Exam 23/06/2022 Please, write your person code and surname-name on all sheets you hand in. Exercise 1 (10 points) An existing single-pressure combined cycle power plant is being modified to improve its efficiency. In the modified configuration, natural gas from the pipeline to the gas turbine is preheated to 180 °C deriving hot liquid water properly from the heat recovery steam generator. The water stream is then returned properly to the steam cycle. The gas turbine in combined cycle configuration without fuel preheating is characterized at nominal conditions by: • electric power output: 7660 kW, • electric efficiency: 33%, • compressor pressure ratio: 16, • exhaust gas mass flow rate: 27.0 kg/s, • turbine inlet temperature (TIT): 1300 °C Knowing that the natural gas mean specific heat at constant pressure is 2.5 kJ/(kg K), please: i. motivate clearly the solution procedure and highlight neatly the assumptions (1 point); ii. decide and motivate where the liquid water is derived and returned in the modified configuration (2 point); iii. sketch the process flow diagram of the entire combined cycle in the modified configuration (2.5 points); iv. compute under simplifying assumptions the gas turbine electric efficiency in the modified configuration (2pts); v. comment without calculating how the condenser heat load changes in the modified configuration (2.5 points); Exercise 2 (12 points) Pumped Thermal Energy Storage (PTES) systems exploits low-cost electricity to store thermal energy at high temper- ature through a heat pump (charge phase). The stored high-temperature heat is then exploited through a power cycle to produce electrical power (discharge phase). The Round Trip…