After the working fluid expands and produces work. Therefore, the working fluid is still hot. To reuse it again, the fluid must be cooled back through leaving the heat to a heat Sink or in case of a heat engine a condenser must be used. In case of the removal of condenser
- The working fluid stays hot, with high volume.
- And as the fluid is already expanded and with high energy, it’s very tough to compress back to its initial state (as massive work done is required)
Let us take an example to analysis this
Here a simple heat engine is considered where it consists of a piston–cylinder device with two sets of stops.
Gas temperature is at 30 degree initially. Then from 100 Degree celsius heat source , heat applied to the system and 100 kJ of heat is transferred to the gas. Then the piston moved into the upper stop and the work done on the load during this expansion process is equal to the increase in its potential energy, say 15 kJ.
Now the excess energy (85 kJ) is not possible to transfer at 90 degree Celsius back to the reservoir at 100 degree Celsius.
So , the heat must be rejected in reservoir at lower temperature to bring back the cycle to initial condition and start the cycle again.
Conclusion: Every heat engine must waste some energy by transferring it to lower temperature reservoir in order to complete the cycle , even under the initial condition.