Question

Why must a power cycle release heat at some point

Answer 1

It is necessary for a power cycle to loose heat at some point in order to increase the thermodynamic efficiency of the process.

Lets understand it with an example of Thermal power plant.The superheated steam after falling on turbine is sent to the condenser to exchange some heat and then again it is fed to the boiler to produce superheated steam.Most of the times people think that this heat exchange in the condenser occurs in order to recover heat in that stream before discharging .Yeah that might be true but that is not exact answer.Since is a cyclic process,its thermodynamic efficiency will be given by Carnot's Theorem as efficiency= [1-T(2)/T(1) ] where T(2) is the temperature of stream exit from condenser and T(1) is the temperature of superheated steam falling on the turbine.Now we can increase efficiency of the process,either by reducing T(2) or increasing T(1).Increasing T(1) much high is not possible from the economy point of view but we can decrease T(2) and that's what we actually do in order to increase the efficiency of the thermodynamic process by using condenser.