Entropy generation versus transition time of heat exchanger during transient processes

Abstract

The flexibility of thermal power plants plays a critical role to accommodate high penetration of fluctuant renewable energy. Heat exchanger is the key device restricting the flexibility and energy efficiency of thermal power plants owing to its large thermal inertia. Therefore, evaluation of the additional entropy generation and transition time for the heat exchanger during transient processes is important, which are quantitative measures of the efficiency and flexibility. Consequently, dynamic models of a heat exchanger are developed. The dynamic behaviors of the heat exchanger are analyzed using numerical simulation and entropy generation analyses. The relationship between additional entropy generation and transition time is determined. Results show that: when flow rate decreases and temperature increases, this relationship is synergetic for hot fluid, metal wall and heat exchanger. The opposite correlation occurs when flow rate increases and temperature decreases. However, the cold fluid has the contrary tendency. The augmentation in heat transfer resistance reduces the additional entropy generation and transition time, whereas the effect of increasing the range of the boundary disturbances, such as temperature and flow rate, is on the contrary. For a heat exchanger, the additional entropy generation versus the transition time can be described by an exponential function.