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 wetted-medium 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). (b). The pressure drop ranges of Cellulose7060, PVC1200, Trickle125 and Trickle100 are 1.5−101.7 Pa, 0.9−49.2 Pa, 0.7−50.0 Pa and 0.6−41.6 Pa, respectively, depending on the medium thickness, air velocity and to some extent on the water flow rate. The cooling efficiencies are 43.0−90.0%, 8.0−65.0%, 15.7−55.1% and 11.0−44.4% for Cellulose7060, PVC1200, Trickle125 and Trickle100, respectively, depending on the air velocity and the medium thickness. (c). The correlations for cooling efficiency, heat transfer coefficient and pressure drop of the studied media are obtained. (d). The Cellulose7060 is found no water entrainment, however, care must be taken in the use of PVC1200, Trickle125 and Trickle100 as water entrainment happens even at relatively low air velocities.
(3) The simulations revealed that: (a). The NDDCT can benefit from wetted-medium pre-cooling when the ambient air is hot and dry, but the improvement declines quickly at increasing ambient humidity. (b). There is a critical ambient temperature below which the tower performance does not benefit but is hindered by wetted-medium pre-cooling. This critical temperature depends on many factors, such as the tower specifications, air relative humidity, medium type and thickness. (c). The wetted media with high (e.g., Cellulose7090 and Cellulose5090) or low (e.g., PVC1200, Trickle125 and Trickle100) cooling efficiencies and pressure drops produce less performance enhancement for the studied NDDCT when compared with the media with middle (e.g., Cellulose7060) cooling efficiencies and pressure drops. (d). The Cellulose7060 with the pressure drops of 28.6−272.1 Pa/m and the cooling efficiency range of 44.7−88.5% is the most promising medium for the pre-cooling enhancement of the studied NDDCT.
Overall, the Cellulose7060 with almost no water entrainment and with the pressure drops of 28.6−272.1 Pa/m and the cooling efficiency range of 44.7−88.5% (at air velocity range: 0.41−3.15 m/s; medium thickness range: 0.10−0.30 m; water flow rate range: 31−62 l/min/m2), is the most promising medium for the pre-cooling enhancement of the studied NDDCT (Height: 120 m, designed heat rejection rate: 296 MW). The tower performance enhancement can go up to 105% by increasing the tower heat rejection rate from 45.0 MW to 92.3 MW at relative humidity of 20% and ambient temperature of 50 oC. However, whether this pre-cooling proposal will bring potential increase in the revenue for the owner of the power plants employing NDDCTs requires further study.
The current work provides useful information for tackling the challenges to water scarcity and low performance of NDDCTs during hot seasons.