In Australia, most potential sites for Engineered Geothermal System (EGS) geothermal power plant are located at arid areas, where are characterized by the scarcity of water, high ambient temperature and strong solar radiation intensity. Local environments make wet cooling unpractical owing to the shortage of water. It seems dry cooling system is the only solution, either natural draft or mechanical draft. However, mechanical draft dry cooling systems inevitably lead to considerable parasitic losses owing to power consumption by fans, especially during hot periods in summer. Therefore, natural draft dry cooling tower (NDDCT) may offer a cost-effective solution for these power plants. The disadvantage associated with all dry cooling systems is the low cooling efficiency when the ambient temperature is high, which is usually the period of peak electricity demand.
At the potential EGS sites in Australia discussed above, high ambient temperature accompanies high solar intensity. This high solar intensity could be used to compensate the net power losses due to the low cooling efficiency of a natural draft dry cooling system in hot periods. A new cooling technology, named the Solar Enhanced Natural Draft Dry Cooling Tower (SENDDCT), was developed in this thesis. It includes a tower in the middle and solar collector around the tower, which is similar to the layout of solar chimney power plant. Heat exchangers are placed vertically along the outer edge of the solar collector. The total frontal area of heat exchangers may be less than the area offered at the outside perimeter of the solar collector. If this is the case, the rest of collector entrance needs to be blocked.
SENDDCT uses solar energy to increase the suction through the tower so that more airflow is achieved through heat exchangers therefore the better cooling performance. Based on the governing equations of conventional natural draft dry cooling tower and solar chimney, a one-dimensional model of SENDDCT was developed. Using this one-dimensional model, a comparative analysis between SENDDCT and NDDCT was performed and the results showed that SENDDCT has substantial advantages over NDDCT as well as standalone solar chimney power plants.
A three-dimensional model was developed to explore the optimal structure design for SENDDCT, and to investigate the effect of introduced partial blockage on the cooling performances of SENDDCT. Three-dimensional simulations showed that for a fixed heat-exchanger area, to increase solar collector size for greater solar enhancement, the original design with an introduction of partial blockage at collector entrance is more beneficial to the cooling capacity of SENDDCT than that with a reduction in the height of heat exchangers, which ensures heat exchangers always fully covers the collector entrance. Besides, it was found that unlike solar chimney power plant design, tilted sunroof would not bring obvious additional benefit to SENDDCT.
Further three-dimensional analysis showed the increasing solar collector size is limited by the corresponding blockage ratio which could not be larger than 50%.
Furthermore, a detailed cost model of SENDDCT was proposed including the capital, labour, maintenance and operation costs of each component. Based on this cost model, an optimisation scheme for the design of SENDDCT has been proposed. As a case study, a SENDDCT was optimally designed to meet the cooling demand for a 50MW geothermal power plant with EGS technology. Results from this case study indicated that the economic viability of SENDDCT highly depends on the cost of solar collector and the sale price of the electricity. The economic assessment in this case study showed a considerable additional benefit can be obtained with the application of this new cooling technology for a 50MW EGS power plant.