The Humid Threshold: Cooling hot, humid climates via membrane dehumidification

PLA plastic model used for the support of membranes in experiment

by Pamela Lucia Cabrera Pardo (MDes ’19) — Recipient of Daniel L Schodek Award for Technology and Sustainability

The thesis centers on the design of a membrane material for cooling hot, humid climates via dehumidification, by selecting water vapor out of humid air. While membrane selectivity is a well-known field of study, found throughout nature and applied across industries, architecture has only regarded membranes as barriers. However, membranes have the potential to separate substances through diffusion, a passive process that is isothermal, and therefore a membrane selectivity system for dehumidification can be less energy intensive than typical vapor compression systems (Yang, Yuan, Gao, & Guo, 2015) (Woods, 2014) (Mahmud, Mahmood, Simonson, & Besant, 2010). Membrane dehumidification research has been developed over two decades for the enhancement of air mechanical units, but it has not yet been proposed as a building material. This thesis investigates the possibility of using membranes as a building screen material to dehumidify incoming air as it is drawn into a building. This application could lower the latent heat that drives air conditioning demand in humid climates, and thus increase natural ventilation potential and other passive dry-bulb cooling strategies.

Psychrometric chart showing three dehumidification processes for cooling. (blue) conventional vapor-compression air- handling unit; (red) desiccant; (green) vacuum-based membrane dehumidification. The red shaded area is the dangerous and extremely dangerous zone defined by the Humid Index (HI) and Humidex. The grey shaded area can be mitigated with passive cooling strategies such as: thermal mass, evaporative cooling and natural ventilation.

In comparison with desiccants and condensation processes used for dehumidification, the membrane system extracts water vapor as a gas, without a phase change, meaning that there is no heat byproduct from the process. Through a mixed-method study that includes experimentation, design prototyping, and simulation, the thesis proposes a Miura geometry membrane arrangement to deploy as a building screen. The geometry is intended to increase surface area and induce flow turbulence for higher air-membrane contact. Two membrane materials are tested under different form configurations, a dry membrane (PVA with LiCl) and a supported liquid membrane (PTFE and CA with PEG400). The results show that the proposed design has a higher impact on the performance of the membrane than the difference in material permeability. A new passive form of latent cooling through openings in a building envelope could reduce the need for vapor compression mechanical systems, reduce the operational energy of buildings and create more resilient and healthier spaces by allowing natural ventilation design strategies.