Theoretical performance assessment of a multi-effect distillation system integrated with thermal vapour compression unit running on solar energy

Abstract

Abstract Mathematical models can be successfully applied to predict the multi-effect distillation system integrated with thermal vapour compression unit (MED-TVC) performance under different operating conditions. In this work, a one-dimensional (1-D) mathematical model was developed to perform the theoretical analysis of a small-scale parallel feed MED-TVC desalination system driven by solar heat. A parametric study was conducted allowing the ejector motive steam temperature, first effect temperature, between effects temperature difference and ejector compression ratio to vary within an expectable range. The results showed that higher motive steam temperatures, had a positive impact on the water yield up to an optimal point. Any additional increase of driving steam temperature reduced water yield. Gain output ratio ($GOR$) was maximum at the highest considered motive temperature. It was found that the calculated specific heat transfer area does not depend on the motive temperature variation. Water yield, $GOR$ and specific heat transfer area decrease with the ejector compression ratio. The temperature difference between the evaporator effects had no effect on the water yield and $GOR$. It only decreases the specific heat transfer area. First effect temperature should be as low as possible with respect to operational constraints. Based on the parametric study, the optimal design operating conditions, corresponding to the maximum water yield is identified for a small-scale three-effect desalination system. The unit can produce up to 204 l/h drinking water with a gain output ratio of 8.6 and specific heat transfer area of 575 m2/kg/s using about 22 kW of solar heat.