Advances in the application of passive down-draft evaporative cooling technology in the cooling of buildings

A passive down-draft evaporative cooling (PDEC) tower is a component that is designed to capture the wind at the top of a tower and cool the outdoor air using water evaporation. While several different types of this particular system exist, PDEC tower with spray was studied in that it is flexible, efficient, and prevalent. Sustainable feasibility has been known as the main benefit of PDEC towers, leading to significant energy savings, improvement of indoor environmental quality, and reduction of environmental cost. In contrast, PDEC towers have not been considered in some circumstances as an alternative to conventional air conditioning systems due to strong climatic dependency, insufficient cooling capacity, and huge water consumption when they could be successfully integrated. In addition, suitable methods that can resolve problems associated with PDEC towers and improve the performance of this particular system have not been presented in the literature. This study was thus designed to present the solutions that overcome these problems with PDEC towers so that they can be widely used in many types of buildings and climatic regions. Computational process modeling was carried out to understand fundamentals of down-draft evaporative cooling processes. This study developed a computational model using a commercial CFD code FLUENT, and this model was validated against experimental data. The model then explained physical phenomena occurring within the effective area of PDEC towers, so that conditions of the air were accurately predicted in different weather conditions as well as PDEC tower conditions. In addition, parametric study with this computational model defined critical factors that significantly impact the cooling performance of PDEC towers, and the importance of various factors. As a result, a practical design guide to droplet size and tower height was presented, which is applicable to most circumstances where PDEC towers could be integrated. Regression analysis using general-purpose statistical software Minitab was then followed to formulate mathematical models that provide accurate conditions of PDEC air flows. Two dependent variables, temperature and velocity, were considered. An individual sample was created by the computational model developed in this study. Correlation analysis determined independent variables that have significant relations with each dependent variable. A preliminary sampling process collected reasonable numbers of samples dealing with wide ranges of weather conditions and PDEC tower conditions. Additional samples were added using forward sampling methods so that minimum number of samples, which explain certain relations between dependent and independent variables at the lowest cost, can be appropriately determined. As a result, linear relations between dependent and independent variables were found and mathematical forms of regression equations obtained was presented. Dynamic simulations, using a whole building energy simulation program EnergyPlus, employing new mathematical models developed in this study were performed to investigate actual impacts of PDEC towers in various situations. A short-term simulation analysis demonstrated problems with current PDEC towers operation, as well as impacts to indoor thermal environment. Various alternatives to typical PDEC towers operation were analyzed, so that water flow control in conjunction with primary cooling system was determined as a reliable solution that overcomes those problems defined in this study. In addition, energy performance and various impacts to indoor thermal environment were analyzed in a long-term simulation. Consequently, PDEC towers are considered as a feasible component in various types of buildings and climates. The findings using the methods in this study demonstrate that typical PDEC towers are inefficient in energy performance and indoor thermal environment. The cooling performance of PDEC towers should thus be properly controlled to be an energy-efficient system. In addition, PDEC towers can be considered as a secondary cooling system that meets a portion of cooling loads in a space in order to accomplish low-energy goals as well as a comfortable indoor thermal environment. Furthermore, the performance of PDEC towers is strongly dependent on each critical parameter described in this study. Efforts should thus be made to find the most efficient design conditions for main parameters corresponding to the local environment. Moreover, PDEC towers are viable in various climates rather than a hot-dry climate, achieving almost the same level of energy savings with lower water consumption