Evaporative-Radiant Cooling on Superhydrophilic Boehmitized Aluminum Surfaces

Abstract

Buildings, which account for 40% of US energy consumption, use most of the energy they need just for heating and cooling. In conventional air conditioning systems, which cool and distribute air throughout the building, significant energy is wasted in moving air (powering fans) that is not needed for building ventilation. Furthermore, these systems often fail to accomplish their goal: providing a comfortable environment for building occupants. Instead, they often cause drafts and poor indoor air quality.

The goal of this project is to explore and evaluate a high-efficiency alternative to conventional air conditioning systems. The prototype design combines the principles of radiant cooling and indirect evaporative cooling. Radiant cooling uses cooled surfaces such as ceiling panels to effect radiant heat exchange with room occupants, eliminating the need to move high volumes of cooled air, but in humid conditions condensation can develop on the cooled panels and water drips onto the room below. Using evaporative cooling in this context is potentially advantageous because evaporative cooling, in addition to being highly efficient, is thermodynamically limited to temperatures above the dew point. This would eliminate the need for precise (and costly) control of the radiant panel temperature to avoid condensation. Such a technology, if it can be implemented as efficiently as expected, would have the potential to significantly decrease the energy and carbon costs of cooling buildings while providing superior comfort.

By designing and testing a prototype evaporative-radiant panel and comparing experimental results with a theoretical model, this research seeks to demonstrate the technology's viability and understand how design and operation would impact its performance. Throughout the process of prototype design, different hydrophilic materials and water flow configurations were explored with the goal of achieving a stable thin film of water to enable effective evaporative cooling. The final prototype, which used hydrophilic boehmitized aluminum surfaces to support the thin water film, was able to cool the radiant surface to 16C in ambient conditions of 21C and 35% relative humidity. Although difficulties in measuring air temperature within the prototype and evenly wetting the evaporative surface prevent a rigorous confirmation of predictions made by the model, the measured temperatures of the radiant surface agreed within 1C with model results. The design lessons learned in this process and the qualitative affirmation of the model are encouraging first steps in demonstrating the potential of evapo-radiant systems and provide insights for future work.