Numerical study on the power generation capacity of a solar chimney system with diffuser-type chimney and low-grade waste heat

Abstract

The solar chimney system for power generation - also known as Solar Chimney Power Plant (SCPP) - is an alternative solar thermal power technology. The buoyancy-driven airflow - as a joint product of the greenhouse effect from a greenhouse-like collector and the stack effect from a gigantic chimney - is generated in the system to drive wind turbines for electricity generation. The SCPP’s simple working principle brings advantages through low requirements for construction, operation and maintenance, giving it great application potential in less-developed regions that have large amounts of available land and abundant solar radiation.

To date, the biggest challenge to the commercialization of a SCPP is its drawback of low power-generation capacity, which is a crucial problem being studied for years. To solve this problem, this thesis raised a new formulation of the driving potential of a SCPP. The new formulation predicted that a chimney of expanding shape can be a better solution than a cylindrical chimney for a SCPP. Hence, the present study proposed diffuser-type chimneys, comprising fully-divergent, divergent-inlet and divergent-outlet chimneys, to heighten the power output of a SCPP. CFD modelling was used to evaluate the performance of diffuser-type chimneys and the major findings are as follows: • Numerical simulations showed a significant enhancement effect of diffuser-type chimneys on power output. The (fully-) divergent chimney achieved the best performance in the examined chimneys, with output that, optimally, was 13.5 times higher than a cylindrical chimney. • The enhancement effect of diffuser-type chimneys was dominated by both aerodynamic (i.e. geometric parameters) and thermodynamic (i.e. temperature rise in the system) factors. • A guide-wall structure at the chimney base was also employed to improve the chimney performance by reducing the loss from the flow direction shift. The improvement in power output reached 40% and 9% in a cylindrical and divergent chimney, respectively.

Another solution proposed in this thesis to enhance the power-generation capacity of the SC system is to incorporate solar energy with low-grade waste heat. The hybrid system was named Solar-and-Waste-Heat Chimney Power Plant (SWHCPP). A new CFD model for the SWHCPP was first established with the compressible-air assumption and validated with the experimental results of Manzanares SCPP. Then, the hybrid system was numerically examined with the following observations. • Under the cylindrical-chimney scenario, the presence of waste heat resulted in improvements of 50% in temperature rise and 67% in power output. The contribution to power output reached ~40% from the waste heat. • Divergent chimneys were also examined in the SWHCPP. The power output was significantly improved, but the chimney performance was dramatically deteriorated by the exhaust-gas injection. • The negative impact of the exhaust-gas injection on divergent chimneys was eliminated through optimizing its layout by taking various parameters into consideration, including injection height, injection angle, and pipe size.

In conclusion, this study explored both geometric and thermodynamic innovations to enhance the power-generation capacity of the SC system. The outcomes should provide preliminary references for potential practical applications of the new technical concepts and point to further research in this field.