2022 Exams collection

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ENERGY CONVERSION Note: you can keep this text for self-evaluation at the end of the exam; in case you withdraw, you shall return it. Exam 25/01/2022 Please, write your person code and surname-name on all sheets you hand in. Exercise 1 (10 points) Originally, a micro gas turbine operating with natural gas (lower heating value 46 MJ/kg) is characterized by: • breathed air mass flow rate: 1.3 kg/s, • compressor pressure ratio: 4.0, • maximum cycle temperature: 900 °C, This micro gas turbine is modified inserting an additional burner, between the turbine outlet and the recuperator hot side inlet, which operates with biomass (lower heating value 18 MJ/kg), while keeping the maximum cycle temperature constant. When burning 0.005 kg/s of biomass, the natural gas rate is reduced by 0.0018 kg/s with respect to the original configuration. Neglecting all pressure drops in the micro gas turbine components, please: i. motivate clearly the solution procedure and highlight neatly the assumptions (1 point); ii. sketch the process flow diagram and the cycle on the T-s diagram before and after the modification (2 pts); iii. compute the natural gas flow rate and the net electric power output in the original configuration (2.5 points); iv. estimate the net electric efficiency after the modification (2.5 points); v. indicate without computing how the second-law efficiency loss at the stack of the modified configuration compares to the original one (2 points). Exercise 2 (12 points) Consider a cogenerative combined cycle with an exhaust gas flow of 800 kg/s at 580 °C. The exhaust gas is cooled to 200 °C in a single-pressure heat recovery steam generator (HRSG) with one reheat. The exhaust gas is further cooled to 110 °C by heating a heat transfer fluid from 80 °C to 150 °C. The plant is…