Understanding Second Law Efficiency — Lesson 4

This lesson covers the concept of second law efficiency in thermodynamics, using practical examples of non-flow, steady flow, and cyclic processes. It explains how to calculate the second law efficiency for different processes, such as a piston-cylinder mechanism, a non-flow process with displacement work, and a constant volume process followed by a constant pressure process. The lesson also emphasizes the importance of exergy and entropy in evaluating the efficiency of these processes. For instance, it uses an example of air compression to illustrate how second law efficiency can guide the selection of the best design among various strategies.

Video Highlights

00:05 - First example: piston cylinder assembly containing 2 kg of air undergoing a polytropic process with a polytropic index of 1.27
03:58 - Second example: 5 kg of saturated r134a in a rigid vessel
09:42 - Third example: piston cylinder mechanism
14:49 - Inefficiency of the process due to the large temperature difference between the heat source and the system

Key Takeaways

- The concept of exergy is crucial in evaluating the efficiency of non-flow processes.
- The second law efficiency can be calculated by comparing the actual work required with the work that would have been required had the process been carried out reversibly.
- In non-flow processes with displacement work, the actual useful work is the total work minus the work interaction with the atmosphere.
- Processes that involve supplying heat across a large temperature difference tend to have high external irreversibility, leading to high entropy generation and exergy destruction.