Unit Seven Homework Solutions, October 26, 2010
1 A 600 MW steam power plant, which is cooled by a nearby river, has a thermal efficiency of 40%. Determine the rate of heat transfer to the river water. Will the actual heat transfer rate be higher or lower than this value? Why?
From the basic
definition of cycle efficiency,
, we can compute
. From the cycle relationship between |Q
H|, |Q
L|, and |W|, |Q
H| = |Q
L| + |W|,
we can write, for heat rates and power,
. Thus we find that the
heat rejected to the river is 
=1500 MW – 600 MW =
900 MW.
In the actual power plant there will be other sources of heat loss. These include heat transfer to the surrounding air from the power plant and leaks of the working fluid. In addition, power plant efficiency is usually determined as the power output divided by the equivalent heat from the fuel. There is a significant contribution to the inefficiency from the exhaust gases.
