Energy, Exergy analysis and performance evaluation of a vacuum evaporator for solar thermal power plant Zero Liquid Discharge Systems

Abstract

Water scarcity and environmental impacts of blowdown within steam power plants are among the important growing concerns. In order to solve these problems, applying a zero liquid discharge (ZLD) system for treating the brine of the power plants and reusing this water is crucial. In this study, the process of a ZLD system is evaluated by using energy and exergy analyses. The ZLD system was designed to recover brines to demineralize water, which consists of four main parts including vacuum evaporator, roots pump, heat exchanger, and circulation pump. The effects of the dimensional and operating parameters on the freshwater flow rate, exergy efficiency, and the consumption power are investigated. When volume of the evaporator is 7 m3, with increase in total evaporation time from 1 to 3 h, total power consumption decreased from 106.16 to 99.52 kW h and freshwater production reduced from 5914.62 to 2048.52 L h−1. The amount of produced freshwater flow rate is independent of the recirculating flow rate and is a function of the evaporator’s volume. Therefore, in volumes of 3, 5, and 7 m3, the produced freshwater flow rate is constant at about 1300, 2200, and 3070, respectively. Also, the results showed that when increasing the concentration of the brine in the range of 2000–30,000 ppm, the flow rate of the produced freshwater decreases from 3377 to 2911 L h−1 and the total power consumption reduced from 113.28 to 96.42 kW h. Moreover, by increasing the volume of vacuum evaporator, freshwater flow rate rises. Increasing the freshwater flow rate has a dramatic influence on the early working cycles. Since the evaporation is a cyclic process, the exergy efficiency of the roots pump and heat exchanger improves, while the exergy efficiency of the vacuum evaporator decreases versus increasing working cycles.