25 06 18t

Page 1

ENERGY CONVERSION A 2017-18 Name/Lastname:__________________________________________ Code n.___________ Exam 25/06/2018 Exercise 1 (10 points) A geothermal well provides a two-phase water flow at: temperature of 150°C, liquid quality of 60% and total mass flow rate of 100 kg/s. The flow pressure is reduced by 2 bar employing a throttling valve and, subsequently, the vapor phase is separated from the liquid phase. The separated vapor is expanded in a turbine and condensed in a condenser. The previously-separated liquid and the condensed liquid are pumped to the well pressure, mixed together and injected underground. Please: i. motivate clearly the solution procedure and highlight neatly the assumptions (1 point); ii. sketch the overall plant layout and the cycle on the Ts diagram (2 points); iii. compute the maximum obtainable power from the two-phase flow as well as the lost fraction due to not ex- ploiting the liquid phase (1 point); iv. determine the net electric power assuming adequate performance parameters for the equipment (2 points); v. list all the irreversible processes occurring within the system and compute exclusively the lost power due to the throttling of the flow and to the mixing of the liquids (2 points); vi. calculate the volume ratio of the expansion across the steam turbine and sketch the velocity triangles of the last stage at the mean diameter for a turbine rotating at 3000 rpm (2 points). Exercise 2 (12 points) A 250-MW subcritical steam power plant is fired by low-Sulphur fuel oil, which is characterized by a lower heat- ing value of 35 MJ/kg. In order to favor oil atomization as well as to reduce NOx emission during the combustion process within the boiler, the oil is pumped to 0.6 MPa and, in addition, steam is bled from the turbine and mixed…