Performance improvement of natural draft dry cooling towers using wetted-medium evaporative pre-cooling

Abstract

A cooling tower is a heat rejection device which extracts waste heat to the atmosphere. A Natural Draft Dry Cooling Tower (NDDCT) is an alternative cooling method when large quantities of water are not available. Examples of the proposed applications are the enhanced geothermal and concentrated solar thermal (CST) power plants in Australia and the rest of the world, most of which are expected to be constructed in dry climates. Unfortunately, the performance of a NDDCT is severely reduced when the ambient air is hot, which is because the NDDCT is driven by buoyancy effect and relies solely on air to cool the working fluid.Reduced cooling tower performance lowers the efficiency of the thermal power stations they are serving. The conventional solution is to use wet cooling towers but this solution consumes large quantities of water. The present project introduces inlet air pre-cooling using wetted media, which limits water consumption only to the periods when the system is used at high ambient temperatures. However, wetted-medium cooling introduces extra pressure drop which reduces the air flow passing through the tower and thus impairs the tower heat rejection. To this end, this project takes into account the trade-off between the wettedmedium cooling and the extra pressure drop, aiming to optimize the performance of a proposed NDDCT when evaporatively pre-cooled using different types of wetted media.An extensive literature review was conducted and the promising wetted media were selected for further study. Four wetted media, i.e., two film media (Cellulose7060 and PVC1200) and two trickle media (Trickle125 and Trickle100), were selected and were experimentally studied in UQ Gatton wind tunnel under different working conditions that represented NDDCT's operation. The medium performance correlations, i.e., the cooling efficiency, heat transfer coefficient and pressure drop, were obtained based on the experimental study. The test results were then used to simulate the operations of pre-cooled NDDCTs to investigate the alterations of tower performance (A MATLAB program was developed to fulfil this task).Finally, the most promising wetted medium together with its performance characteristics were recommended for future pre-cooling application.The main findings of the thesis are:(1) Choosing a suitable wetted medium for a specific application requires knowledge of different working parameters. Not only the balance between the cooling potential and the extra pressure drop, but also the consideration of medium cost, service life, type of process to be cooled, environmental conditions, water quality, space availability, locations and economic requirements are absolutely necessary, and some trade-offs have to be made.(2) The experimental studies of the four selected media found that: (a). Film media (e.g., Cellulose7060 and PVC1200) offer higher cooling efficiencies and pressure drops when compared with trickle media (e.g., Trickle125 and Trickle100). Cellulose7060 is found no water entrainment, however, care must b...

A review of the crosswind effect on the natural draft cooling towers

“…Unlike the mechanical or fan assisted cooling towers where the airflow is generated by mechanical fans, such towers with prominent hyperboloid structures generate the airflow not using an external source, but through density difference between the ambient air and the hot air inside the cooling tower. Their advantages include power saving, low environmental impact, lack of mechanical noise, safe operation, low maintenance costs and no recirculation as the plume is rejected at high level [3,[11][12][13].…”

“…For natural draft cooling towers, there are two big challenges: hot ambient temperature and the crosswind. Both have negative effects on the thermal performance of the cooling tower [3,11,13,15,[23][24][25]. Compared with high ambient temperatures, crosswind effects are much more complex and harder to predict.…”

Numerical and experimental study of small natural draft dry cooling tower for concentrating solar thermal power plant

“…Such towers with prominent hyperboloid structures generate the airflow not using an external source, but through density difference between the ambient air and the hot air inside the cooling tower. Their advantages include power saving, low environmental impact, lack of mechanical noise, safe operation, low maintenance costs and no recirculation as the plume is rejected at high level [4,6,24,25].…”

“…Both have negative effects on the thermal performance of the cooling tower [4,6,24,26,34,35]. Compared with high ambient temperatures, crosswind effects are much more complex and harder to predict.…”

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