2021 Exams collection

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ENERGY CONVERSION A 2020-21 Note: you can keep this text for self-evaluation at the end of the exam; in case you withdraw, you shall return it. Exam 11/01/2021 Please, write your person code and surname-name on the sheets you hand in. Exercise 1 (10 points) A small-scale concentrated solar power plant comprises (i) a solar field and (ii) a power island. The solar field employs parabolic troughs, with a total surface of 38000 m2, generating steam directly from the solar radiation, with a nominal irradiance of 900 W/m2. Moreover, the efficiency of the solar field is 75%. The power island employs a superheated steam Rankine cycle with (i) one low-pressure feedwater preheater, (ii) the deaerator, (iii) one high- pressure preheater and no reheat; bled steam for the high-pressure preheater is taken from turbine inlet. At nominal conditions, liquid water enters the solar field at 9.0 MPa and a subcooling temperature difference of 15 °C, while steam exists at 8.0 MPa and 500 °C. In the superheated region, the specific heat at constant pressure can be taken equal to the mean value of 3.8 kJ/(kg K) for simplicity. Please: i. motivate clearly the solution procedure and highlight neatly the assumptions (1 point); ii. sketch the cycle T-s diagram, the plant process diagram, and the heat transfer diagram of the solar field (2 points); iii. calculate the mass flow rate of the generated steam as well as the mass flow rate of the bled steam serving the high-pressure feedwater preheater at nominal conditions (2.5 points); iv. estimate the second-law efficiency loss due to the heat transfer in the solar field at nominal conditions (1.5 points); v. if during an off-design condition steam exists the solar field at 520 °C, motivate without calculating how the mass flow rate of steam varies,…