Example 5.1 A central power plant, rated at 800,000 kW, generates steam at 585 K and discards heat to a river at 295 K. If the thermal efficiency of the plant is 70% of the maximum possible value, how much heat is discarded to the river at rated power? Solution 5.1 The maximum possible thermal efficiency is given by Eq. (5.7). With TH as the steam-generation temperature and Tc as the river temperature: 295 Carnot=1 - = = 0.4957 and 585 n = (0.7)(0.4957) = 0.3470 CHAPTER 5. The Second Law of Thermodynamics where n is the actual thermal efficiency. Combining Eq. (5.6) with the first law, written W=-QH-Qc, to eliminate QH, yields: (-800,000) = -1,505,475 kW Qc = (1-7)w = (1-3477) (- W= This rate of heat transfer to a modest river would cause a temperature rise of several °C.