Evaporative cooler

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An Egyptian qullah, set in drafts to cool interiors. Porous pottery and coarse cloth maximize the area for evaporation.






A traditional air cooler in Mirzapur, Uttar Pradesh, India




Physical principles[edit]




Other types of phase-change cooling[edit]





Before the advent of modern refrigeration, evaporative cooling was used for millennia, for instance in qanats, windcatchers, and mashrabiyas. A porous earthenware vessel would cool water by evaporation through its walls; frescoes from about 2500 BCE show slaves fanning jars of water to cool rooms. Alternatively, a bowl filled with milk or butter could be placed in another bowl filled with water, all being covered with a wet cloth resting in the water, to keep the milk or butter as fresh as possible (see zeer, botijo and Coolgardie safe).[9]

California ranch house with evaporative cooler box on roof ridgeline on right


Psychrometric chart example of Salt Lake City


Evaporative cooling is most effective when the relative humidity is on the low side, limiting its popularity to dry climates. Evaporative cooling raises the internal humidity level significantly, which desert inhabitants may appreciate as the moist air re-hydrates dry skin and sinuses. Therefore, assessing typical climate data is an essential procedure to determine the potential of evaporative cooling strategies for a building. The three most important climate considerations are dry-bulb temperature, wet-bulb temperature, and wet-bulb depression during a typical summer day. It is important to determine if the wet-bulb depression can provide sufficient cooling during the summer day. By subtracting the wet-bulb depression from the outside dry-bulb temperature, one can estimate the approximate air temperature leaving the evaporative cooler. It is important to consider that the ability for the exterior dry-bulb temperature to reach the wet-bulb temperature depends on the saturation efficiency. A general recommendation for applying direct evaporative cooling is to implement it in places where the wet-bulb temperature of the outdoor air does not exceed 22 °C (72 °F).[7] However, in the example of Salt Lake City, the upper limit for the direct evaporative cooling on psychrometric chart is 20 °C (68 °F). Despite the lower temperature, evaporative cooling is suitable for similar climates to Salt Lake City.

Evaporative cooling is especially well suited for climates where the air is hot and humidity is low. In the United States, the western and mountain states are good locations, with evaporative coolers prevalent in cities like Albuquerque, Denver, El Paso, Fresno, Salt Lake City, and Tucson. Evaporative air conditioning is also popular and well-suited to the southern (temperate) part of Australia. In dry, arid climates, the installation and operating cost of an evaporative cooler can be much lower than that of refrigerative air conditioning, often by 80% or so. However, evaporative cooling and vapor-compression air conditioning are sometimes used in combination to yield optimal cooling results. Some evaporative coolers may also serve as humidifiers in the heating season. In regions that are mostly arid, short periods of high humidity may prevent evaporative cooling from being an effective cooling strategy. An example of this event is the monsoon season in New Mexico and central and southern Arizona in July and August.


Other examples[edit]






Evaporative cooler illustration


Direct evaporative cooling[edit]

Direct evaporative cooling



Passive direct evaporative cooling can occur anywhere that the evaporatively cooled water can cool a space without the assist of a fan. This can be achieved through use of fountains or more architectural designs such as the evaporative downdraft cooling tower, also called a "passive cooling tower". The passive cooling tower design allows outside air to flow in through the top of a tower that is constructed within or next to the building. The outside air comes in contact with water inside the tower either through a wetted membrane or a mister. As water evaporates in the outside air, the air becomes cooler and less buoyant and creates a downward flow in the tower. At the bottom of the tower, an outlet allows the cooler air into the interior. Similar to mechanical evaporative coolers, towers can be an attractive low-energy solution for hot and dry climate as they only require a water pump to raise water to the top of the tower.[14] Energy savings from using a passive direct evaporating cooling strategy depends on the climate and heat load. For arid climates with a great wet-bulb depression, cooling towers can provide enough cooling during summer design conditions to be net zero. For example, a 371 m2 (4,000 ft2) retail store in Tucson, Arizona with a sensible heat gain of 29.3 kJ/h (100,000 Btu/h) can be cooled entirely by two passive cooling towers providing 11890 m3/h (7,000 cfm) each.[15]

For the Zion National Park visitors' center, which uses two passive cooling towers, the cooling energy intensity was 14.5 MJ/m2 (1.28 kBtu/ft;), which was 77% less than a typical building in the western United States that uses 62.5 MJ/m2 (5.5 kBtu/ft2).[16] A study of field performance results in Kuwait revealed that power requirements for an evaporative cooler are approximately 75% less than the power requirements for a conventional packaged unit air-conditioner.[17]

Indirect evaporative cooling[edit]

The process of indirect evaporative cooling

Indirect evaporative cooling (closed circuit) is a cooling process that uses direct evaporative cooling in addition to some heat exchanger to transfer the cool energy to the supply air. The cooled moist air from the direct evaporative cooling process never comes in direct contact with the conditioned supply air. The moist air stream is released outside or used to cool other external devices such as solar cells which are more efficient if kept cool. This is done to avoid excess humidity in enclosed spaces, which is not appropriate for residential systems.

Maisotsenko cycle[edit]

One indirect cooler manufacturer uses the Maisotsenko cycle (M-Cycle), named after inventor and Professor Dr. Valeriy Maisotsenko, employs an iterative (multi-step) heat exchanger made of a thin recyclable membrane that can reduce the temperature of product air to below the wet-bulb temperature, and can approach the dew point.[18] Testing by the US Department of Energy found that a hybrid M-Cycle combined with a standard compression refrigeration system significantly improved efficiency by between 150-400% but was only capable of doing so in the dry western half of the US, and did not recommend being used in the much more humid eastern half of the US. The evaluation found that the system water consumption of 2-3 gallons per cooling ton(12,000 BTUs) was roughly equal in efficiency to the water consumption of new high efficiency power plants. This means the higher efficiency can be utilized to reduce load on the grid without requiring any additional water, and may actually reduce water usage if the source of the power does not have a high efficiency cooling system.[19]

An M-Cycle based system built by Coolerado is currently being used to cool the Data Center for NASA's National Snow and Ice Data Center (NSIDC). The facility is air cooled below 70 degrees Fahrenheit and uses the Coolerado system above that temperature. This is possible because the air handler for the system uses fresh outside air, which allows it to automatically use cool outside ambient air when conditions allow. This avoids running the refrigeration system when unnecessary. It is powered by a solar panel array which also serves as secondary power in case of main power loss.[20]

The system has very high efficiency but, like other evaporative cooling systems, is constrained by the ambient humidity levels, which has limited its adoption for residential use. It may be used as supplementary cooling during times of extreme heat without placing significant additional burden on electrical infrastructure. If a location has excess water supplies or excess desalination capacity it can be used to reduce excessive electrical demand by utilizing water in affordable M-Cycle units. Due to high costs of conventional air conditioning units and extreme limitations of many electrical utility systems, M-Cycle units may be the only appropriate cooling systems suitable for impoverished areas during times of extremely high temperature and high electrical demand. In developed areas, they may serve as supplemental backup systems in case of electrical overload, and can be used to boost efficiency of existing conventional systems.

The M-Cycle is not limited to cooling systems and can be applied to various technologies from Stirling engines to Atmospheric water generators. For cooling applications it can be used in both cross flow and counterflow configurations. Counterflow was found to obtain lower temperatures more suitable for home cooling, but cross flow was found to have a higher coefficient of performance (COP), and is therefore better for large industrial installations.

Unlike traditional refrigeration techniques, the COP of small systems remains high, as they do not require lift pumps or other equipment required for cooling towers. A 1.5 ton/4.4kw cooling system requires just 200 watts for operation of the fan, giving a COP of 26.4 and an EER rating of 90. This does not take into account the energy required to purify or deliver the water, and is strictly the power required to run the device once water is supplied. Though desalination of water also presents a cost, the latent heat of vaporization of water is nearly 100 times higher than the energy required to purify the water itself. Furthermore, the device has a maximum efficiency of 55%, so its actual COP is much lower than this calculated value. However, regardless of these losses, the effective COP is still significantly higher than a conventional cooling system, even if water must first be purified by desalination. In areas where water is not available in any form, it can be used with a desiccant to recover water using available heat sources, such as solar thermal energy.[21][22]

Theoretical designs[edit]

In the newer but yet-to-be-commercialized "cold-SNAP" design from Harvard’s Wyss Institute, a 3D-printed ceramic conducts heat but is half-coated with a hydrophobic material that serves as a moisture barrier.[23] While no moisture is added to the incoming air the relative humidity (RH) does rise a little according to the Temperature-RH formula. Still, the relatively dry air resulting from indirect evaporative cooling allows inhabitants' perspiration to evaporate more easily, increasing the relative effectiveness of this technique. Indirect Cooling is an effective strategy for hot-humid climates that cannot afford to increase the moisture content of the supply air due to indoor air quality and human thermal comfort concerns.


Hybrid designs[edit]



Evaporative + conventional backup[edit]



Traditionally, evaporative cooler pads consist of excelsior (aspen wood fiber) inside a containment net, but more modern materials, such as some plastics and melamine paper, are entering use as cooler-pad media. Modern rigid media, commonly 8" or 12" thick, adds more moisture, and thus cools air more than typically much thinner aspen media.[24] Another material which is sometimes used is corrugated cardboard.[25][26]

Design considerations[edit]

Water use[edit]

In arid and semi-arid climates, the scarcity of water makes water consumption a concern in cooling system design. From the installed water meters, 420938 L (111,200 gal) of water were consumed during 2002 for the two passive cooling towers at the Zion National Park visitors' center.[27] However, such concerns are addressed by experts who note that electricity generation usually requires a large amount of water, and evaporative coolers use far less electricity, and thus comparable water overall, and cost less overall, compared to chillers.[28]



Mechanical systems[edit]




Different types of installations[edit]

Typical installations[edit]



Evaporative (wet) cooling towers[edit]

Large hyperboloid cooling towers made of structural steel for a power plant in Kharkіv (Ukraine)


Misting systems[edit]

Mist spraying system with water pump beneath


Misting fans[edit]







= direct evaporative cooling saturation efficiency (%)
= entering air dry-bulb temperature (°C)
= leaving air dry-bulb temperature (°C)
= entering air wet-bulb temperature (°C)

Evaporative media efficiency usually runs between 80% to 90%. Most efficient systems can lower the dry air temperature to 95% of the wet-bulb temperature, the least efficient systems only achieve 50%.[31] The evaporation efficiency drops very little over time.






Some examples clarify this relationship:



Because evaporative coolers perform best in dry conditions, they are widely used and most effective in arid, desert regions such as the southwestern USA, northern Mexico, and Rajasthan.


Comparison to other types of air conditioning[edit]

A misting fan

Comparison of evaporative cooling to refrigeration-based air conditioning:


Less expensive to install and operate

  • Estimated cost for professional installation is about half or less that of central refrigerated air conditioning.[32]
  • Estimated cost of operation is 1/8 that of refrigerated air conditioning.[33]
  • Can be operated on home power inverter during power cuts. This is particularly useful in areas that experience frequent power outages.[34]
  • Newly launched Air coolers can be operated though remote control.[35]

Ease of installation and maintenance


Ventilation air

  • The pad itself acts as a rather effective air filter when properly maintained; it is capable of removing a variety of contaminants in air, including urban ozone caused by pollution,[citation needed] regardless of very dry weather. Refrigeration-based cooling systems lose this ability whenever there is not enough humidity in the air to keep the evaporator wet while providing a frequent trickle of condensation that washes out dissolved impurities removed from the air.






Water use

  • Evaporative coolers require a constant supply of water.

Maintenance frequency


Health hazards


See also[edit]


  4. ^ Scott Landis (1998). The Workshop Book. Taunton Press. p. 120. ISBN 978-1-56158-271-6. evaporative cooler squirrel cage southwest popular.
  10. ^ Bonan, Gordon B. (13 June 2008). "Forests and Climate Change: Forcings, Feedbacks, and the Climate Benefits of Forests". Science. 320 (5882): 1444–9. Bibcode:2008Sci...320.1444B. doi:10.1126/science.1155121. PMID 18556546. S2CID 45466312.
  19. ^ Coolerado Cooler Helps to Save Cooling Energy and Dollars: New Cooling Technology Targets Peak Load Reduction; Robichaud, R; 2007; http://www.osti.gov/biblio/908968-coolerado-cooler-helps-save-cooling-energy-dollars-new-cooling-technology-targets-peak-load-reduction
  20. ^ "Coolerado Chills Snow & Ice Data Center". Data Center Knowledge. August 12, 2011.
  21. ^ Maisotsenko cycle based counter and cross flow heat and mass exchanger: A computational study. 2017 International Conference on Energy Conservation and Efficiency (ICECE). Rasikh Tariq ; Nadeem Ahmed Sheikh. Publication Year: 2017, Page(s): 44 - 49
  22. ^ Maisotsenko cycle: technology overview and energy-saving potential in cooling systems. Journal of Energy and Emission Control Technologies. 6 March 2015 Volume 2015:3 Pages 15—22. Emmanuel D Rogdakis, Dimitrios Nik Tertipis. Faculty of Mechanical Engineering, National Technical University of Athens, Athens, Greece
  23. ^ "cold-SNAP: Eco-friendly air conditioning". Wyss Institute. 27 September 2019.
  24. ^ (PDF) http://www.pge.com/includes/docs/pdfs/myhome/saveenergymoney/savingstips/evap/eedectechsheet.pdf. Missing or empty |title= (help)
  27. ^ Torcellini, P; Long, N; Pless, S; Judkoff, R (February 2005). Evaluation of the Low-Energy Design and Energy Performance of the Zion National Park Visitors Center - Technical Report NREL/TP-550-34607 (PDF). p. 88. Retrieved 9 June 2020.
  30. ^ [1] Archived May 18, 2007, at the Wayback Machine
  34. ^ "Air Cooler or Air Conditioner - Which is Better & Why?". energyconversiondevices.com. Retrieved 2021-07-03.
  35. ^ "Latest technology stylish looks and compact build". bonafideresearch.com. Retrieved 2021-08-04.
  36. ^ "A brief note on the NID Cooler" (PDF). Government of India - National Centre for Disease Control. Archived from the original (PDF) on 10 October 2017. Retrieved 22 November 2013.

External links[edit]