The concept of exergy is defined and explained with the intention of its direct application toward pipeline simulation. Exergy is the actual useful work that can be obtained from any system, while irreversibility can be viewed as the lost opportunity to do work. All pipelines experience heat transfer and frictional pressure drop to varying degrees, resulting in entropy generation and a corresponding decrease in exergy, leading to less economical transport. An "exergetical" analysis can lead to design changes to improve overall efficiency, such as reducing pump or compressor power requirements while addressing environmental goals, such as avoidance of the melting of permafrost surrounding arctic pipelines, or lowering of carbon dioxide emissions. This paper intends in the first part, to provide the background of exergy and entropy generation and introduce the analysis of pipeline systems using the second law of thermodynamics. In the second part, an actual crude oil pipeline simulation is described where exergy destruction and carbon dioxide emissions are compared for cases of differing pipe insulation thickness.
Current practice for pipeline design consists of conducting a thermal-hydraulic analysis (Menon & Menon, 2013) followed by an economic feasibility study. From a thermodynamic perspective the thermal-hydraulic calculations embody the conservation laws of mass, motion and energy, fluid properties and the transfer of heat. The conservation of exergy is the "first law". However for any process to actually occur the second law of thermodynamics dictates that this occurs in a certain direction. A process is said to be reversible if both the system and surroundings can be returned to their original conditions. The reality is that there exist irreversibilites during any real world process and the original conditions cannot be restored. These irreversibilities are typically friction and heat transfer and lead to the concept of entropy generation and the second law of thermodynamics which states that entropy generation is always positive for an irreversible process.