Evaporative cooling is a process that uses the effect of evaporation as a natural heat sink. Sensible heat from the air is absorbed and it is used as the latent heat necessary to evaporate water. The required latent heat for water evaporation process is 2256 kJ/kg at standard condition, which is 500 times lesser than what it is needed in water heating. The amount of absorbed heat depends much upon the amount of water which can be evaporated. In which, it means that the more water that has been evaporated, the lower the temperature it could reduce. It is also important to emphasize that the building and surrounding air are not cooled by water itself, but by the latent heat flux when water evaporates.
Evaporative cooling is close to the nature than what we might have expected. It is considered to be adiabatic, due to the fact that no additional heat or energy is required for the water evaporation. Only sensible heats from the surrounding and water body are used, and it result a temperature drop of the surrounding and water body.
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There are different variables which largely affect the process of evaporation:
The surrounding temperature
The sun radiation
Difference in vapor pressure of water vapor mixture in the interior air and water vapor pressure at the exterior surface
The velocity of exterior air flow/ wind speed
The air humidity of exterior air flow
The size of the contact surface
The mass transfer coefficient
These variables could be largely influenced by method of dehumidification. For example the sice of contact, such as droplet size, running tap water, sprinkler, or water misting or even the water fog by a metal diaphragm vibrating at an ultra or infrasonic frequency could produce different result of temperature drops.
Basically the evaporative cooling process can be subdivided into two, which is direct evaporative cooling and indirect evaporative cooling. Both of the processes are almost similar, but indirect evaporative cooling uses some type of heat exchanging water resistant wall, which the indoor relative humidity will not increase to unacceptable levels.
Direct evaporative cooling
The underlying concept of direct evaporative cooling is based on the conversion of sensible heat to latent heat. As from the diagram that has shown, the surface temperature is reduced when the evaporation process of the water in the air stream takes place. The latent heat follows the water vapor and diffuses into the air. The absolute humidity of this air stream increases during the process. When the moisture level of the air reaches to the maximum extent, the evaporation will stop to takes place and the temperature level will reach a constant. Hence, the cooling effect might not sufficient especially in a very humid climate. To encounter it, the incoming air has to be dehumidified by forcing it through a desiccant to improve the cooling efficiency. z
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Indirect evaporative cooling
In the indirect evaporative cooling process, there will a primary and secondary air stream involved. A primary hot air stream will be humidified by the wet surface and is cooled due to the release of latent heat; while, a secondary air stream is used as the supply air of the building interior. This secondary air stream is cooled due to the absorption of heat by the heat exchanging area. As the supply air stream does not make any direct contact with the wet surface, the absolute humidity will not increase.
One of the integrated building applications of this indirect evaporative cooling concept is the porous roof. Porous layer on the roof able to retains a significant amount of rainwater. When the process takes place, the temperature of the surrounding air and porous layer will decrease. The heat flux from the roof slab would also be reduced greatly. In this case, the building envelope is the heat exchanging area.
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The comparison of other building integrated application on evaporative cooling
Besides water wall, there is also many others building integrated evaporative cooling system that applies the same underlying theory.
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One of the building integrated applications is the passive evaporative cooling wall (PECW), which is constructed of porous ceramics. Porous ceramics possess the characteristic of having the capillary force that able to soak water and allowing winds penetration at the same time, thus reducing its surface temperature by means of water evaporation. The main advantage is that no additional pumps are needed for a circulating water flow, so the system is totally passive. However, the disadvantage is that, the water height are limited by capillary suction, and it only works if the wind pressure on the wall is sufficient to allows cool air coming into the building, or else the humidity level in the building will be relatively high.
Vegetation or green wall is one of the applications that can be applied both inside and outside. The green wall can reduce the buildingâ€™s energy consumption by reducing the internal air temperature through a process called evapotranspiration. During the photosynthesis and dissimilation process of a green plant, the excess of water in the leaves can evaporate to cool the surrounding air and the green plants itself also gives shades and reduce radiation. However, the disadvantage of the green wall is the non-transparency of the application, in which sun lights are unable to penetrate into wherever space that requires day lighting. There is also a constraint over the limited amount of evaporation and the increase in absolute humidity in the large glass-covered space which cost to a difficulty in building maintenance.
From the recent studies and invention, as to overcome the main disadvantage of non-transparency problem, the super-hydrophilic photo catalyst (TiO2) coating technology has made it possible to maintain thin film of water on all external surfaces of glazed buildings. The surface coated with TiO2 becomes highly hydrophilic due to the irradiation of UV light from the sun. This invention gives a large step in minimizing the amount of sprinkling water or running water to form a thin water film. The main advantage is that the coatings are applicable to most of the building materials, either vertical or horizontal surface, and especially on a glazed faÃ§ade. Most importantly, the cooling function is achieved by only sunlight and rainwater.
External building surfaces coated with TiO2 , which utilize the whole building as a heat exchanging area.
To weight the different criteria of these different building integrated applications, Kesselring method are used to examine the efficiency of these method by assessing both their functioning and realization. The criteria examine under functioning requirements included it humidifying process, cooling capacity, energy use, water use, water supply requirements, hygine and multi functionality. On realization of the method, it weighs the integration level, noise reduction , serviceability and sustainable materials.
Below are the relative score by Kesselring Method presented in S-diagram.