8-10C What is the second-law efficiency? How does it differ from the first-law efficiency? 8-11C Does a power plant that has a higher thermal effi- ciency necessarily have a higher second-law efficiency than one with a lower thermal efficiency? Explain. 8-12C Does a refrigerator that has a higher COP necessar- ily have a higher second-law efficiency than one with a lower COP? Explain. 8-13C Can a process for which the reversible work is zero be reversible? Can it be irreversible? Explain. 8-14C Consider a process during which no entropy is gen- erated (Sgen = 0). Does the exergy destruction for this process have to be zero? 8-15 The electric power needs of a community are to be met by windmills with 10-m-diameter rotors. The windmills are to be located where the wind is blowing steadily at an average velocity of 8 m/s. Determine the minimum number of windmills that need to be installed if the required power output is 600 kW.