2023 Exams collection

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ENERGY CONVERSION Note: you can keep this text for self-evaluation. For uploading the scanned written exam follow: Exam 17/01/2023 Please, write your person code and surname-name on all sheets you hand in. Exercise 1 (10 points) A small-scale air-cooled saturated steam Rankine cycle operates burning 36 kg/h of wood (lower heating value of 15 MJ/kg, stoichiometric air-fuel ratio 10). The cycle has the following non-conventional features: • the boiler is in cocurrent configuration (exhaust and working fluid in the same direction), • there is one low-pressure direct preheater deriving the steam from the inlet of the expander, • there are no high-pressure preheaters, • there is no air preheating. Knowing that the evaporation temperature is 150 °C, please: i. motivate clearly the solution procedure and highlight neatly the assumptions (1 point); ii. sketch the process flow diagram, the T-s diagram, and T-Q diagram of the boiler (2.5 points); iii. calculate the second-law efficiency loss due to heat transfer at the stack (1.5 points); iv. estimate the gross electric power output (3 points); v. comment without calculating how the power output changes if the boiler is designed as countercurrent (2 pts). Exercise 2 (12 points) Consider an air conditioning system that cools indoor air from 28 °C to 24 °C exploiting a flow of hot air at 200 °C. The mass flow of air is 20 kg/s. This system rejects heat at 40 °C to the environment at 30 °C through the condensation of the working fluid. The saturated liquid is split then into two streams: (i) the first is pressurized through a pump, heated and evaporated exploiting the hot flow of air; next it is expanded in a turbine and sent to the condenser; (ii) the second stream is throttled, evaporated by cooling the indoor space ; then the…