ES220117 test WHG

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Wind, Hydro and Geothermal Power Generation AY 2021-22 17th January 2022 Prof. Paolo Silva Time: 90 minutes 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. 1. Considering a three-bladed horizontal axis wind turbine, explain why the pitch angle of the blade has to vary along the radius (the blade is said to be "twisted"). Draw the velocity triangles re- spectively at the hub, the average radius and the tip of the blade, graphically showing a situation where the pitch angle is negative. (5 points) 2. Consider a direct geothermal plant exploiting a liquid-dominant source. Explain the reason why a lamination valve before the demister can be convenient by an energy point of view, despite the irreversibility introduced. What are the consequences of lamination in terms of O&M issues? (5 points) 3. Draw the plant scheme of a hydro-pumped storage plant based on binary groups. Describe its start-up procedure in the pumping mode, discussing the hydraulic and electrical connections of the groups. (5 points) 4. Explain the definition of “grid parity” for a renewable power plant, distinguishing between the concept of grid parity in terms of price and cost. How can the growing share of electricity from renewables affect respectively the average cost of kWh and the balancing costs of the electric grid in a long-term perspective? (5 points) Exercise (10 points) A three-bladed horizontal axis wind turbine is equipped with a variable speed system. The turbine has a rotor diameter of 132 m and a rated wind speed of 11…