Energy Engineering - Power Production from Renewable Energy

Multiple choice test

POWER PRODUCTION FROM RENEWABLE ENERGY AY 2022-23 30 th August 2023 Prof. Andrea Giostri Time: 1 hour NAME and SURNAME:.................................................................................................................................................. COURSE and CFU:............................................TYPE (Complete or Theory only):................................................. Instructions for the examination: 1) Clearly indicate your name on all the sheets you will deliver. 2) Give a synthetic and clear answer to as many questions as possible. The final score will be normalized according to the average results. 3) Talk with colleagues and / or cheating will cause the cancellation of the exam. OPEN QUESTIONS (points 18) 1. Consider a modern wind turbine (large in size) and draw an ideal Coefficient of Power (Cp)-speed curve highlighting and defining cut-in speed, rated speed and cut-off speed. Describe and explain the “active stall” control strategy. 2a. For Management Engineering students: Consider the collectors of a solar thermal power plant and highlight the differences between Fresnel and parabolic-trough technology. Explain the pros and cons of each solution. 2b. For Mechanical Engineering students: Discuss pros/cons of biogas. Describe qualitatively the process of biogas production reporting the main components of a plant to produce electricity. 2c. For Energy Engineering students: Consider a liquid dominant geothermal source and explain the reasons why the adoption of an ORC cycle can be beneficial. Draw the layout of a single pressure level ORC and report on a T-s diagram the streams conditions and represents the corresponding T-Q chart. Explain how to choose the ORC evaporation pressure and the reinjection temperature of the geothermal fluid. MULTIPLE-CHOICE QUESTIONS (points 12) Answer to the following 15 questions. Select the box with correct answer (0.8 points if correct, -0.3 points if wrong). Consider ideal Betz’s theory : v∞  undisturbed wind speed vd  wind speed at “disk” v1,v2  wind speed at inlet and outlet of stream tube The available wind energy is proportional to (v∞ )2 vd=(v1+v2)/3 Maximum Cp depends on v∞ and air density  true  true  true  false  false  false Consider the electricity generation from a CSP plant with a thermal energy storage (nominal capacity 50MW e): SM  Solar Multiple For a specific SM, TES capacity that maximizes electricity yield implies the lowest LCOE For a specific TES capacity, the solar-to-electric efficiency increases if SM increases For a specific SM, an increase of TES capacity always implies an increase of equivalent hours  true  true  true  false  false  false Consider a thermal collector mounted on a house roof (not tracking system): Evacuated tubes guarantee lower thermal losses and lower optical efficiency compared to flat panel The higher the incidence angle, the lower the optical efficiency Stagnation temperature implies no useful heat transferred to fluid  true  true  true  false  false  false Consider a photovoltaic (PV) module: Two-axis tracker allows maximizing the fraction of both diffuse and direct captured irradiation By-pass diodes are connected in series to protect module from overheating in partial-shaded modules Single crystal Si ensures a conversion efficiency approximately in the range 10-15 %  true  true  true  false  false  false For Mechanical and Management students (5 CFU) Consider two technologies (A, B) based on different renewable sources: EPBT  “Energy Payback Time” heq  Equivalent hours If EPBT A>EPBT B then LCOE A>LCOE B If heq,A