16 07 20sol

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Politecnico di Milano Department of Energy School of Industrial and Information Engineering Course Energy Systems LM prof. S. CONSONNI - Academic Year 2019/20 Pag. 1 Written Exam of 16 July 2020 – INDICATIVE ANSWERS TO QUESTIONS Q1) Describe the expected impact on power output and efficiency of injecting liquid water into the combustor of a gas turbine (real open gas cycle) assuming that the combustor o utlet temperature is kept constant by adjusting the excess of air. Is water injection beneficial or detrimental to net power output? And to the efficiency? Why? In order to maintain the same combustor outlet temperature, if some liquid water is injected into the combustor the excess air must decrease. Thus, less power is needed by the air compressor. As for water, it must be compressed as liquid to be injected into the combustor. Then, the same water expands through the turbine as vapor, thus increasing turbine work. Since the specific volume of vapor is much greater than the specific volume of liquid water, the extra‐work produced by the turbine is much greater than the work needed to compress liquid water. In summary, the net power generated by the gas turbine [(expansion work of flue gases+water vapor) – (air compression work) ‐ (water compression work)] INCREASES. Efficiency is the ratio between net power (numerator) and thermal power generated by the fuel (denominator). As explained above, the numerator increases. Likewise the denominator, because extra fuel is needed to evaporate water. Thus, at a first glance one cannot say whether efficiency increases or decreases (or stays constant). To understand the impact on efficiency, one may consider that t he water‐injected cycle is a “weighted average” of the gas cycle and the steam cycle ca rried out by the flow of…