Mejora de la eficiencia energética de sistemas de climatización mediante enfriamiento evaporativo

Abstract

This doctoral dissertation has been presented in the form of thesis by publication. The national and European legislative framework has, among its objectives, the improvement of energy efficiency of air conditioning equipment. Hence, the need to explore alternative air conditioning systems aimed at reducing energy consumption compared to existing systems. In this regard, systems capitalizing on the benefits of evaporative cooling represent one of the most effective solutions. This thesis aims to investigate two approaches to achieve this goal. The first involves the use of cooling towers, which are based on this principle but have the drawback of causing significant environmental impact due to the water droplets emitted during the process. The other method involves precooling the incoming air of conventional air conditioning equipment using ultrasonic atomisers, a technique that lacks references in the literature. This thesis addresses the experimental characterization of the thermal performance and emission levels of a novel cooling tower prototype that has been designed and patented to prevent the reléase of suspended particles into the atmosphere. The aim is to reduce the environmental and health impact typically associated with such evaporative cooling systems. The experiments were conducted in a pilot plant built ad hoc for this purpose. In the environmental impact assessment (drift emissions), the sensitive paper method was used. A comparison between the obtained results and those found in the literature for similar cooling towers indicates that performance of the inverted cooling tower in terms of emissions is remarkable, with a drift rate of 1.47 ⋅ 10−6 kg/s (0.00015% of circulating water). This value is up to 13 times lower than the limits imposed by several international standards and involves a reduction in terms of emissions ranging from 40.21% to 82.54% compared to commercial towers. Regarding the size of the droplets escaping from the tower, the results were also promising, with a maximum diameter of 50 μm and a Sauter Mean Diameter of the ensemble of droplets of 31.42 μm. Concerning thermal performance evaluation, the studied tower is classified as a mechanical forced draft, counterflow-parallel flow cooling tower. For this reason, the influence of the analysis method (Merkel and Poppe) and the arrangement of flow between the water and air streams (counterflow, parallel flow, and counterflow/parallel flow) on the tower's performance under specific operating conditions has been discussed. The main novelty lies in the use of the Poppe model combining two different flow arrangements: parallel flow and counterflow. Additionally, the adaptation of the Poppe model for parallel flow, which is an approach not commonly found in the literature. From this analysis, it is concluded that the most appropriate method for assessing the thermal performance of this innovative prototype is one that uses the Poppe theory and combines counterflow and parallel flow arrangements to evaluate thermal performance. This approach not only it provides the best predictions for the outlet water and air temperatures (0.44°C difference for the water prediction and 0.74°C difference for the air) but it is a good approximation of the complex underlying physics of the problem. The study of thermal performance has been completed by investigating two key aspects in the design of a cooling tower: the fill length and the nozzle arrangement (position and hydraulic characteristics). The objective was the experimental optimization in terms of thermal performance of this new prototype of inverted cooling tower. The results indicate that spraying from an upper position (only parallel flow) presents results 24% better than spraying from an intermediate position (mixed and parallel flow) and 37% better than spraying from a lower position (equal to the intermediate but with greater distance to the fan). This is because better nozzles were installed in the upper position. However, based on the results, the chosen nozzles should be those installed at the upper position, although possibly, those installed at the other positions would present better results. This is because the combination of higher hydraulic resistance of the sprayers and a larger surface area of Exchange (lower position) could result in the best configuration. Regarding the fill, it was observed that it similarly influences the operation of all spraying positions since, in almost all of them, all cooling is done in a parallel flow arrangement, i.e., the section where the fill is located. It was observed that the performance for a fill length of 1.6 m is 25.5% better than for the other two lengths tested. The main conclusion regarding this is that installing a large amount of fill clearly improves the tower's performance, but installing a small amount is not always comparable to not installing anything, due to the added pressure loss. For all results, it was also confirmed that the Ashrae correlation along with the Poppe method successfully predicts the thermal performance of the cooling tower, with an average difference between experimental and predicted results of 0.37°C for water outlet temperature. Moreover, it is also capable of predicting air outlet conditions with an average difference of 4.93% (1.35°C). Regarding the use of ultrasonic atomisers for air pre-cooling, previous work of the research group focused on an ultrasonic mist generator, which obtained promising results but had limitations such as the inability to control droplet distribution and its high electrical consumption. To address this, the use of ultrasonic atomisers capable of overcoming these limitations is proposed. A numerical model of this innovative system has been developed and validated using experimental data obtained in a wind tunnel. A parametric analysis was conducted with the obtained data, considering key variables in the cooling process: injected water mass flow rate, required distance for complete droplet evaporation, cooled area, and atomiser power consumption. The results regarding the required distance for complete droplet evaporation to prevent them from reaching the condenser are promising, as this only occurs for water-to-air mass flow ratios greater than 1.7 10-3 with humidities of 0.5 and 1.2 10-3 for 0.7. With the ultrasonic mist generator, this did not occur for any working range. An optimization analysis was also performed, revealing that the optimal operating ranges for overall performance are water-to-air mass flow ratios below 1.8 10-3 for a relative humidity of 0.5 and below 8.1 · 10-4 for 0.7. Within these conditions, water spray is more evenly distributed throughout the control section, facilitating a homogeneous and efficient evaporative cooling process. The maximum evaporative coefficient of performance achieved across all simulations was 30.49. This value is four times higher than that of previously studied equipment, demonstrating that this innovative system surpasses the limitations of the previous systems and enhances its performance.