Energy Engineering - Power Production from Renewable Energy

Full exam

POWER PRODUCTION FROM RENEWABLE ENERGY AY 2022-23 19 th June 2023 Prof. Andrea Giostri Time: 1.5 hours NAME and SURNAME:.................................................................................................................................................. COURSE and CFU:......................................................................................................................................................... Instructions for the examination: 1) Clearly indicate your name on all the sheets you will deliver. 2) The score refers to exercises done in a comprehensive manner with exact numerical results. Numerical results correct but not accompanied by explanations will not be taken into account. The final score can be normalized according to the average results. 3) Answer briefly and clearly only to the asked questions. Calculations and explanations which do not respond to the questions will not be considered for evaluation even if correct. 4) Talking with colleagues and / or cheating will cause the cancellation of the exam. 5) All the needed data for the resolution of exercises lies on this paper. It is NOT ALLOWED to use material other than this (e.g. books, clipboard etc.). Exercise 1 (15 points) A CSP plant based on solar tower adopts molten salts as Heat Transfer Fluid (T min =300°C – T max =555°C) and a steam cycle for the conversion of thermal power into electricity. The power plant and the heliostat field design characteristics are reported in the table below: Calculate (i) the solar receiver thermal efficiency and the solar multiple (4 points). Compute (ii) the receiver average external temperature knowing the receiver specific thermal losses (see above table) and assuming the convective/radiative heat transfer coefficient with the environment equal to 75 W/m 2K (3 points). Compute (iii) the volume of the molten salts TES with a capacity of 7 hours (points 3) (Average specific heat of molten salts is 1.52 kJ/kgK and the average density is equal to 1804 kg/m 3). Assuming an average annual DNI of 710 W/m 2, compute (iv) the yearly electricity yield (3 points) and (v) the average sun-to-electricity efficiency of the plant (2 points), assuming the average values reported in the following table (Discuss any simplifying hypothesis used in the calculations): Average optical efficiency 0.59 Average Cleanliness factor of mirrors (reduction of mirror reflectivity) 93 % Solar field working hours 4000 Exercise 2 for Energy Engineering students (15 points) A geothermal source has the following characteristics at the wellhead: pressure 22 bar and 0.1 quality. CASE A Consider a single flash steam cycle, based on a cooling tower condenser with a condensation pressure of 0.07 bar. The flash pressure is 11 bar. Design the lay-out of the plant (i) with all the components and identify qualitatively on T-s diagram the fluid streams (2 points). Determine (ii) the vapor fraction at the outlet of the steam turbine (2 points). Neglecting the presence of non-condensable gases, calculate (iii) the mass flow rate of the geothermal Steam cycle net power 30 MW Single h eliostat reflective surface 120 m2 Efficiency of power block 39 % N° of heliostats 2550 Ambient temperature 35 °C Optical efficiency 66 % Cylindrical Receiver diameter 8 m Receiver specific thermal losses 35 kW/m 2 Cylindrical Receiver height 11 m Design DNI 800 W/m 2