Experimental investigation on spray cooling using saline water for natural draft dry cooling towers

Abstract

In recent times concern has grown over the effects of climate change on the environment and interest has grown in improving renewable resource technologies. To reduce the impact of thermal power plants on fresh water resources, Natural Draft Dry Cooling Towers (NDDCT) can be utilised to reject heat to the environment.Previous research has shown that NDDCTs are an effective cooling technology, which can be used in most thermal power plants. However, these power plants experience significant reductions in power generation capacity during periods of high ambient temperatures. One proposed method is to reduce the impact of high ambient temperatures is spray cooling. Water spray is employed to pre-cool the inlet air to improve the performance of dry cooling towers in hot weather conditions.nIn other studies, water spray has been used to pre-cool the inlet air of the NDDCT due to its simple design and lower initial cost, it also increased power plant efficiency and drastically reduced the amount of water consumed when compared to wet cooling towers or other evaporative cooling methods. Spray cooling technology has been successfully applied in gas turbines however; little attention has been given to applications involving small scale NDDCTs. Other researchers had proposed that saline water from sources such as Coal Seam Gas (CSG) provided increased benefits when used in spray cooling systems. To determine the suitability of replacing fresh water with saline water for spray cooling an experimental setup was designed in the current study.Three different water sources were examined in the current study n fresh, saline and Coal Seam Gas water. A comparison between nozzles using fresh and saline water was created by developing 3D models and testing in a wind tunnel. The results showed an increase in cooling efficiency if saline water is selected as the water source, however cooling efficiency could be increased more significantly by selecting the more appropriate orientation of the nozzle with regards to the airflow. Provision was made at the end of the wind tunnel to capture solid particles. The presence of solid particles was found. These particles would increase the fouling and possibly the corrosion rate of heat exchanger surfaces.In addition to the models developed to determine the suitability of using spray cooling for NDDCTs, this study also considered the possibility of using NDDCTs to achieve other goals of preserving the environment. In particular the dual use of evaporative media, as an evaporative cooler for the NDDCT and a wet scrubber for a Direct Air Capture system, was examined. Renewed interests are focusing on developing new technologies to directly capture CO2 from the atmosphere. Traditional designs of Direct Air Capture contactors require significant amounts of electrical energy to process the large volumes of air. Since natural draft dry cooling towers (NDDCT) used in thermal power plants (which work by means of the stack effect) draws large amount of atmospheric air for power plant cooling without consuming electricity, the device (NDDCT) could be used at the same time for CO2 capture without additional cost. This study is to determine if it is possible to utilise NDDCT operating in a power plant to provide the required airflow for a Direct Air Capture contactor component. A proposed design is included by adding evaporative cooling media around the base of a NDDCT. A 1D model was developed in MATLAB to calculate the airflow through a NDDCT considering the additional pressure losses due to the added resistance from evaporative cooling media. Based on the analysis for a 200MW power plant cooling system, a total of about 27000 t CO2 can be captured per year without any negative effect on the power plant performance.This thesis has developed an improved understanding of the suitability of saline water usage for spray cooling in NDDCTs. This study shows that saline water can be utilised in spray cooling to preserve fresh water resources. This thesis also shows that NDDCTs can be utilised to capture carbon dioxide from the air by using a sodium hydroxide solution.n