Modular Design Technique for an Adaptive Cooling and Daylighting Roof Aperture System

Abstract

Cooling and heating systems are a major source of energy consumption in residential and commercial buildings globally. More than 60% of total building energy consumption is attributed to heating, ventilation, and air-conditioning (HVAC). Existing experience has shown that passive cooling techniques with natural ventilation and evaporative-cooling provide excellent thermal comfort, together with very low energy consumption. One of the oldest passive cooling strategies in use today is the windcatcher system. This roof structure has openings toward the prevalent wind direction to catching the airflow for downdraft cooling. Also, the system’s function at night is like a chimney that pulls the interior space heat out. Windcatchers are typically made with masonry and thermal mass materials that help the radiant cooling during the cold night of desert climate. The main objective of the windcatcher is to keep interior spaces at the proper static temperature while relying on dynamic outdoor wind forces. The adaptive roof aperture is an advancement on prior windcatcher technologies to provide dynamic response to the external wind directions and patterns to modulate the thermodynamic functions. This system provides either downdraft airflow at daytime or nighttime radiation by changing its geometry. Other prior research demonstrates the effectiveness of superporous polyelectrolyte hydrogels for water sorption and diffusion. In addition, the hydrogel provides multifunctional environmental response for evaporative cooling, natural daylighting, and heat capacitance with radiative cooling because of the material structure and optical characteristics. This research addresses the adaptive functionality for a windcatcher through integrating unique materials such as hydrogel and phase-change materials (PCMs) into modular units for localized adaptive response. These materials provide high heat capacitance and emissivity while also transmitting natural daylight. The proposed integration of the modular units into the windcatcher form will accommodate evaporative and radiant cooling, natural daylighting, and water recuperation. Evaporative cooling is enabled for the daytime downdraft when the hot-dry airflow streams interface with a hydrogel membrane embedded at the top part of the windcatcher. During the night, the hydrogel membrane at top of the structure will remain unsaturated to assist stack-ventilation, night-flush cooling. The hydrogel membrane may also provide daylighting based on saturation states. A selection of modular units that have optimum sky-exposure incorporate PCMs to provide thermal storage during the day and radiative cooling at night. The project also explores the potential for rain-water harvesting through the overall geometry and form to provide the system’s required water. Different environmental analyses were conducted to determine the optimum overall geometry. Solar radiation analyses were conducted with the Rhino-Grasshopper Ladybug plug-in to determine optimal material integration to increase the radiant cooling and decrease the heat exchange. The daylight analyses were conducted with Rhino-Grasshopper Honeybee plug-in to determine the amount of illumination based on the different saturation levels of the hydrogel. The solar radiation and daylight analysis were performed for three times of day (8 am, 12 pm, and 4 pm) at three times of the year (spring equinox, summer solstice, and winter solstice). The Computational Fluid Dynamics (CFD) simulations were conducted to analyze airflow morphology through different spatial conditions. CFD simulations are performed with the Rhino-Grasshopper Butterfly plug-in accessing the OpenFOAM analysis platform. To identify the spatial factors affecting the airflow behavior through the unique geometry, parametric variations are defined for the windcatcher height and aperture diameters and placement. Initial results provide insight into the optimal relationships between windcatcher geometries for inducing adequate airflow for passive cooling and a self-shading geometry to gain less radiation during the day. At the next step, within a computational design process, the modules all over the shape were designed for holding the thermal mass. During the design process of modules, the defined parameters such as depth of each module, distribution of them, and the opening size of each module for sky cooling were linked to the solar radiation analysis. All these processes were performed to prevent the thermal mass materials to gain high radiation during the day and being as much as possible exposed to the sky during night time. Ultimately, the analyses were conducted again on the final geometry to determine the efficiency of the design proposal. The full-scale prototype will be built within a research group in Tucson, AZ to compare the simulation results and validate them. The information about illuminance, air velocity, temperature differentials, water harvesting, and moisture diffusion rates will provide insight into how the design will be successful for both energy and water conservation potentials.