Evaporative cooling without adding humidity

Scientists at the University of Harvard have designed a prototype evaporative cooling system called Cold-SNAP that effectively cools air at low cost for both dry and humid climates.

According to the International Energy Agency (1), as average global temperatures continue to rise, global demand for air conditioning is expected to triple by 2050 without action to address energy efficiency. As temperatures rise, so does the demand for air conditioning, which requires huge amounts of energy.


Throughout history, humans have found innovative ways to cool air around them. One of the most common ways to cool warm air is to bring it into contact with water, which absorbs heat from the air as water evaporates, a process called evaporative cooling (EC).


However, because evaporative cooling adds moisture to the air, it only works well in hot, dry climates such as the Middle East and the southwestern United States. It is not the case in humid regions such as the tropics, where nearly half of the world's population is concentrated.


However, a variant of EC called indirect evaporative cooling (IEC) was introduced around the same time that mechanical vapour compression was invented in the early 20th century. Like evaporative cooling, IEC also cools buildings by evaporation of water, but IEC systems contain a heat exchange unit that isolates the evaporating water from the air that is directed into the building, allowing the heat to be removed without adding moisture to it. The reason why IEC systems have lagged far behind the market-dominant mechanical vapour compression units is that they are systems that, despite requiring very little energy to operate, are difficult to manufacture because of the complexity of the heat exchange unit, making them expensive and their performance difficult to optimize.


Two Harvard scientists (Jack Alvarenga, M.S. and Jonathan Grinham, D.Des) are working to infuse 21st century technology into IEC systems, allowing them to effectively and inexpensively cool air in wet and dry climates. According to their work, their technology, called Cold-SNAP (short for Cold Superhydrophobic Nano-Architectured Process) is claimed to use up to 75% less energy than mechanical vapour compression air conditioners.

As  mentioned in another news of this newsletter "The benefits of 3D printing in refrigeration and air conditioning", different substances can be used in 3D printing: metal powder, thermoplastics, composites, glass but also ceramics ; Thus Cold-SNAP achieves its high performance through the integration of old and new : ceramics, one of the oldest, cheapest and most widely used building materials, and a new surface coating recently developed in the laboratory of Joanna Aizenberg, a member of the Wyss University of Applied Sciences. The nanoscale roughness of the coating makes it super water-repellent and, when applied to a ceramic tile with high water absorption, it produces a highly efficient heat exchange unit that can effectively isolate evaporating water from cooled air. Because ceramic is very malleable, it is possible to produce a complete heat exchanger by extrusion or 3D printing in one piece, and its shape can be adjusted to maximize the surface area available for heat transfer and evaporation. The hydrophobic coating is then selectively applied to the components that will manage the dry air flow, coupled with a water pump, fan and controls.


According to the scientists, preliminary studies have indicated that the cold-SNAP system can be up to four times more efficient than conventional air conditioners.


M. Grinham, said: "The impact that Cold-SNAP can have globally is two-fold: on the one hand, its low projected cost will enable people in poorer regions to afford efficient cooling; on the other hand, its low energy consumption will help reduce overall electricity consumption as people change or upgrade their ageing air-conditioning systems, thereby mitigating future temperature increases.


The team at Havard university is currently exploring different manufacturing techniques and is preparing for a pilot study this summer to see how the system performs in real-world conditions of heat and humidity.


(1) https://www.iea.org/reports/the-future-of-cooling

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