240212 Problems ECT Martelli

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Politecnico di Milano Energy Conversion Technologies prof. E. Martelli - Academic Year 2023-24 page 1 of 2 Written Exam of 12 February 2024 - Problems Time: 1:20 hours 1) Exam is open book, but computers and cell phones are NOT allowed. Talking with colleagues and/or copying will lead to the immediate cancellation of the exam. 2) Answer clearly ONLY to the questions posed by the problem sets. Even if correct, additional considerations and/or calculations will NOT be considered. 3) In addition to the points indicated for each problem, a bonus of max 1 point may be given based on whether the solutions are presented with clear calligraphy and procedure. 4) The exam must be written in English Energy Conversion Technologies Problem 1 (14 points) A boiler burns natural gas (assumed pure methane) and generates flue gases with O2 molar concentration of 5%. The stack temperature is 80 °C (above the water dew point temperature), the inlet air temperature is 10 °C, the specific heat capacity Cp of air is 1.0 kJ/kg-K, the Cp of flue gases is 1.07 kJ/kg-K. 1) Calculate the percentage excess of air. (4 points) 2) Calculate the flue gas composition. (5 points) 3) Calculate the boiler thermal efficiency assuming that the unburned fuel losses and the thermal losses are 1% of the fuel chemical power each (5 points) Politecnico di Milano Energy Conversion Technologies prof. E. Martelli - Academic Year 2023-24 page 2 of 2 Problem 2 (20 points) The geothermal binary power plant shown in the figure exploits a low temperature geothermal heat source to produce electricity with an organic Rankine cycle (ORC) using R134a as working fluid. Geothermal water is taken from the reservoir at Tw,in = 100 °C and is cooled down in the primary heat exchanger (PHE). The superheated working fluid exits the…