8-16 One method of meeting the extra electric power demand at peak periods is to pump some water from a large h=75m FIGURE P8-16 body of water (such as a lake) to a water reservoir at a higher elevation at times of low demand and to generate electricity at times of high demand by letting this water run down and rotate a turbine (i.e., convert the electric energy to potential energy and then back to electric energy). For an energy storage capac- ity of 5 x 10 kWh, determine the minimum amount of water that needs to be stored at an average elevation (relative to the ground level) of 75 m. Answer: 2.45 x 1010 kg 8-17 Consider a thermal energy reservoir at 1500 K that can supply heat at a rate of 150,000 kJ/h. Determine the exergy of this supplied energy, assuming an environmental temperature of 25C. 8-18 A heat engine receives heat from a source at 1500 K at a rate of 700 kJ/s, and it rejects the waste heat to a medium at 320 K. The measured power output of the heat engine is 320 kW, and the environment temperature is 25C. Determine (a) the reversible power, (b) the rate of irre- versibility, and (c) the second-law efficiency of this heat engine. Answers: (a) 550.7 kW, (b) 230.7 kW, (c) 58.1 percent 8-19 S Reconsider Prob. 8-18. Using EES (or other) software, study the effect of reducing the temper- ature at which the waste heat is rejected on the reversible power, the rate of irreversibility, and the second-law efficiency as the rejection temperature is varied from 500 to 298 K, and plot the results. 8-20E A heat engine that rejects waste heat to a sink at 530 R has a thermal efficiency of 36 percent and a second-law effi- ciency of 60 percent. Determine the temperature of the source that supplies heat to this engine. Answer: 1325 R TH Heat engine 7th=36% Tg=60% 530 R FIGURE P8-20E