Combining pinch and exergy analysis for process modifications

The strength of pinch analysis is that system information can be represented using simple diagrams (e.g. composite curves, grand composite curves) and thus targets for the system under consideration can be readily obtained prior to design. In contrast, the power of exergy analysis is that it can identify the major causes of thermodynamic imperfection of thermal and chemical processes and thus promising modifications can be determined effectively. By combining the strengths of both methods, the proposed method can represent a whole system, including individual units on one diagram, which helps to screen the promising modifications quickly for improving a base case design. It has been realised that the major limitation of pinch analysis is that it can only deal with heat transfer processes, not the processes involving changes in pressure or compositions,;which are very common in power and chemical processes [1, 2]. To overcome this limitation,a generic diagram is introduced, which is the so-called Omega-H diagram, where Omega indicates the energy level and H states the amount of energy. Both energy and exergy balances for a whole system can therefore be represented simultaneously on this diagram. Using this diagram, the major advantages of both pinch and exergy analysis are combined since the diagram enables one to view the performance of a system and set targets for improvement. Meanwhile, the inefficient processes and equipment can be identified and thus promising modifications can be obtained by revealing the actual potential, which is determined by applying the concepts of inevitable and avoidable exergy losses. The analysis of avoidable exergy losses indicates the maximum potentials (modification targets) which are achievable in current technical and economic conditions. A combined-cycle power station is used to demonstrate the effectiveness of this new method. Copyright (C) 1996 Elsevier Science Ltd.