Optimum design and performance analysis of heat exchanger coupling a flash dryer and parabolic trough collectors

Abstract

Responding to the climate change issue, the deployment of thermal solar energy has grown remarkably. However, its use for industrial applications remains limited due to techno-economic constraints, among which the choice and optimum design of the heat exchanger allowing a profitable integration. In this context, the design of a crimped spiral fin-tube heat exchanger (CSFTHX) used to couple parabolic trough collectors and flash dryer was conducted. For this purpose, a mathematical model of CSFTHX was developed based on (ε-NTU) method and fitted to experimental data obtained in this work. The sensitivity of the model to geometrical parameters and operating variables was evaluated which results permitted to qualitatively validate this model and identify the optimum ranges of key design parameters. Then, the optimal design was carried out applying the conjugate direction optimization method considering the minimization of total annual cost as the objective function. Results showed that, at low Reynolds number (<1000), all geometrical parameters presented a significant effect on the HX thermal and hydraulic performances. Regarding the operating variables, the oil temperature and airflow rate had the most important impact on the HX required area, thus conducting to define the design point. The HX optimum design was achieved with a 20% lowered annual cost and 44% improved HX thermal performance. The designed HX was integrated within a bench scale solar flash dryer and the effect of the solar radiation intensity on the drying conditions was investigated, showing the necessity to use the back-up heating and avoid functioning under broken-clouds.