Exam 20230615 Eng

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ENERGY CONVERSION Exam 15/06/2023 Please, write your person code and surname-name on all sheets you hand in. Exercise 1 (10 points) A research team is developing a water Rankine cycle using the combustion of hydrogen in oxygen as a source of energy . In particular, hydrogen and oxygen are stored in high -pressure cylinders; they are burnt in stoichiometric quantities and the resulting steam is injected directly into the feedwater to evaporate it completely. Moreover, the cycle is saturated and non-regenerative for simplicity. • hydrogen higher heating value: 142 MJ/kg, • hydrogen lower heating value: 120 MJ/kg, • feedwater flow rate: 1 kg/s, • evaporation pressure: 0.8 MPa. Please: i. motivate clearly the solution procedure and highlight neatly the assumptions (1 point); ii. sketch the overall process flow diagram and the T-s diagram of the steam cycle (2 points); iii. write the proper energy balance for the determination of the hydrogen consumption (2 points); iv. estimate, with simplifying assumptions, the net power output of plant (3 points); v. propose a method to avoid accumulating water inside the cycle (2 points). Exercise 2 (12 points) Consider a power plant exploiting a flow of exhaust gases at 650 °C. The plant uses ambient air as working fluid. This air is compressed and then preheated in a recuperator; next, heated further by the flow of exhaust gases to a temperature of 560 °C in a heater. Moreover, there is one reheat in the process. The turbines are designed in order to have same expansion ratio and same turbine inlet temperature. The hot air is then cooled down in the mentioned recuperator before flowing to the stack. Ultimately, consider two alternative configurations for the heat exchange b etween air and exhaust gases: in configuration A the reheater…