Direct evaporative cooling from wetted surfaces: Challenges for a clean air conditioning solution

Evaporative cooling has a major role to play in fighting climate change and in achieving a low‐carbon economy. As it helps to reduce energy demand for air conditioning, it is gaining attention in terms of improving energy efficiency in buildings. Evaporative cooling from wetted media can enhance water–air contact, thereby improving heat and mass transfer further and avoiding aerosols. Wetted media are commonly called evaporative cooling pads and are widely used in greenhouses, intensive livestock farming, and industrial facilities. However, a deep understanding of evaporative cooling pad performance can enhance their application to indoor occupied spaces such as residential or commercial cooling, or in hybrid air conditioning systems. Most studies analyze pad performance mainly in terms of pressure drop and saturation effectiveness. However, some studies propose alternative cooling efficiency parameters and others provide insights into key aspects such as power requirements and the coefficient of performance, water consumption, risk of water entrainment, material decay, and air quality, as well as the effect of water temperature and salinity, solar radiation, or wind speed. Existing results on these less studied performance issues are reviewed, and we identify the gaps in the literature in addition to highlighting the main challenges encountered, in an effort to guide future researchers in the field and enhance the application of direct evaporative cooling.