Small natural draft dry cooling towers for renewable power plants

Abstract

In thermal power plants using variety of heat sources, the redundant heat needs to be removed through cooling devices such as heat exchangers and cooling towers. Natural draft dry cooling towers (NDDCTs) feature no water loss and no parasitic power consumption during operation and are widely used in thermal power plants around the world, especially in arid areas. The Queensland Geothermal Energy Centre of Excellence (QGECE) is focusing on developing small- or medium-scale engineered geothermal system (EGS) geothermal and concentrated solar thermal (CST) power plants for Australia. The proposed renewable power plants are most likely to be located in arid remote areas where dry cooling is the only cost-effective option. These power plants may initially be introduced to supply remote communities away from the national grid. Such off-grid applications are expected to be relatively small reflecting the size of the demand. The aim of this Thesis project is to investigate whether natural draft dry cooling towers (NDDCTs) can be used for these small- or medium-scale renewable power plants. Crosswind is the most common factor affecting the cooling performance of natural draft cooling towers. But current NDDCT design procedures do not take the crosswind effect into account. It is probably not a critical impediment for large towers as the negative influence of the crosswind is negligible when the draft heights are above 100 m. On the other hand, in small NDDCTs with total heights less than 30 m, the crosswind effect could be substantial. A numerical study was carried out to investigate the thermal performance of the horizontally arranged heat exchangers in small NDDCTs under various crosswind conditions. In particular, a 15 m-high CFD NDDCT model was constructed and simulated to examine the crosswind actions in detail. It was found that at certain crosswind speeds the cooling tower performance can be considerably reduced. A very effective mitigation device, a tri-blade-like windbreak wall, has been found to dramatically improve the cooling performance of the small tower. The effect of the windbreak was sensitive to its orientation with respect to the crosswind direction. An experimental study was conducted with a 1:12.5 scaled natural draft dry cooling tower model in the wind tunnel. The thermal performance of the scaled tower was measured with and without the windbreak wall. The experimental results verified the numerical predictions for the small tower cooling performance and the effectiveness of the windbreak wall. The most important finding of this study is that the total heat transfer in a small natural draft cooling tower is a combination of the heat transfer by both natural convection and forced convection, with the contribution of the latter increasing at higher crosswind speeds. A simple correlation between the heat transfer and the crosswind speed was proposed to estimate the crosswind-affected thermal performance of natural draft dry cooling towers at different sizes and with horizontal heat exchanger arrangements. The study demonstrates the feasibility of utilising small natural draft dry cooling towers in renewable power plants. Crosswind could have a fatal effect on the performance of these towers without a proper design. However, by applying the mitigation method considered in the Thesis, crosswind can actually be converted to a beneficial effect for the cooling tower performance.