The "wasted" heat that thermal power plants reject to
the surroundings is rejected at a temperature only
slightly above ambient. A steam turbine generator
has an exhaust steam condenser which operates at a
vacuum, where the steam condenses at only a few degrees
Fahrenheit above the ambient temperature. There is no
significant temperature difference available for the
new device to operate with.
While thermal power plants do reject over half the fuel
energy consumed to the surroundings, it is a myth that this
rejected heat can be effectively used. The rejected heat
is available at a low temperature, only slightly above ambient,
therefore little effective use can be made of it.
This is the penalty that the laws of thermodynamics
impose on the conversion of heat into work.