An Investigation on the Exergo-Economic Performance of an Evaporator in Orc Recovering Low-Grade Waste Heat

Abstract

Based on the perspective of exergy recovery (profit), an exergo-economic performance evaluation model of evaporator in ORC recovering low-grade waste heat is established. Selecting dry fluid R600a as a working fluid, an exergo-economic criterion was defined to estimate the effects of operating parameters and tube length on the performance of evaporator when the boiling temperature of evaporator is fixed. The optimal flow rate of organic working fluid G_t (flow rate of exhaust flue gas G_s), the flow rate ratio of exhaust flue gas to organic working fluid y, and tube length l have been discovered to maximize the annual net profit value NPV. Furthermore, there exists the critical values of G_t (G_s), y, and l which make NPV = 0. The results also show that, with the increase of the flow rate ratio of exhaust flue gas to organic working fluid, the maximum annual net profit value first increases sharply, and then decreases slowly, while the optimal tube length of evaporator reduces monotonously. Furthermore, the evaluation method from purely thermodynamic viewpoint is imperfect, and the economic considerations change the results obtained with only the maximization of recovered exergy.

Keywords:

• Low-grade waste heat
• Exergy recovery
• Evaporator
• Organic Rankine cycle (ORC)
• Exergo-economic performance

Acknowledgments

This work was financially supported by the National Basic Research Program of China (973 Program) (Project No. 2011CB710701).