The heat island phenomenon rises the temperature of cities, increases the energy demand for cooling and deteriorates comfort conditions in the urban environment. To counterbalance the impact of the phenomenon, important mitigation techniques have been proposed and developed.
The use of water in reducing ambient temperature is known from the traditional architecture. To evaporate water in the atmosphere, latent heat is used that contributes to decreasing the ambient temperature and probably improves the indoors and outdoors thermal comfort conditions, (1). The latent heat of evaporation is so high that the evaporation of 1 kg of water may decrease the temperature of 2000 cubic meters of water by 1 K (1). Apart of the evaporation processes, the water surface may be of several degrees lower temperature than the surrounding environment and contribute additionally to cooling the ambient air through convective processes.
Additionally, to the natural urban water bodies, numerous evaporation technologies and techniques are designed and implemented in cities to decrease the ambient temperature. Pools, ponds and fountains and a variety of other passive evaporative systems are usually integrated in urban public spaces for decorative and climatic reasons (7). In parallel, active and hybrid systems like evaporative wind towers, sprinklers and water curtains have been designed, developed, implemented and tested in numerous urban public spaces, (1).
The cooling and mitigation potential of evaporative and water-based techniques has been thoroughly analysed through studies investigating the temperature distribution and patterns in cities surrounded by lakes, rivers and other water reservoirs (2-5).
Experiments have shown that urban wetlands contribute to creating ‘Urban Cooling Islands’ characterized by a very considerable reduction of the urban ambient temperature. The mitigating potential of wetlands depends on several parameters like the shape and the landscape characteristics around the water body and the wetland proximity to the city. Research has shown that the cooling potential of urban wetlands is between 1–2 K (6).
The performance of the passive and active evaporative mitigation systems depends on the local climatic conditions and the landscape of the urban area as well as on the geometric and physical characteristics of the system. Climatic parameters like the ambient humidity and temperature, wind speed, turbulence and solar radiation are highly determining the mitigation potential and the capacity of the water-based technologies and techniques.
The most common and well-known water mitigation system is the use of pools and ponds. The heat transfer mechanisms that contribute to reducing the ambient temperature are a) the evaporation of the water through the pond surface and b) the convective heat transfer between the ambient environment and the low water surface temperature of the pond. Detailed numerical tools to assess the mitigation potential of water ponds are given in (1).
The cooling capacity of the ponds and pools is determined by the capacity of the atmosphere to include additional water vapor, while convective heat transfer is a function of the temperature difference between the water surface and the ambient air. Water reservoirs and ponds of low thermal capacitance when exposed to solar radiation may present a higher surface temperature than the surrounding ambient environment. Given that cooler air is transferred from the wind, the cooling effect of the pools and ponds is more important in the leeward space zone of the water body. Experimental data have shown that the average drop of the ambient temperature in the surrounding area varies between 0.1 and 1.9 K, while the maximum temperature drop varies between 0.4 and 7.1 K, with an average maximum value close to 2.8 K, (8).
Pools and Ponds
Figure 1 : An urban pool- Fontana di Trevi Rome – Copyright : Fontana Di Trevi Rome - Bing images
As concerns the cooling capacity of fountains, their evaporation potential is primarily determined by the initial and final water drop radius and initial and final drop temperatures (1). It is found that average ambient temperature drop caused by fountains is close to 1 C, while the maximum temperature drop is between 1 C and 4 C, (8).
Fountains
Figure 2 : Fountain in Paris : Copyright : fountains paris - Bing images
Sprinklers are used to supply water directly into the air. Evaporation takes place while the cool air is descending, providing a decrease of the ambient temperature. The cooling potential of sprinklers depends on the size of the droplets, the type and the size of nozzles, the local climatic conditions like the wind speed and direction, ambient humidity, and temperature. As a function of the pressure on the nozzles, water droplets of various size are created. In general, higher pressures improve the evaporation efficiency by generating smaller droplets (9). The potential use of the nonproper nozzles may generate droplets at a size that may not contribute to any significant reduction of the ambient temperature (10-11). Experimental data shows that the average ambient temperature drop at the local level is between 3 and 4 C, however such a temperature drop is considered very high and probably the data are not fully accurate.
Sprinklers
Figure 3 : Outdoor Misting Systems. Copyright: outdoor misting systems for cooling - Bing images
In evaporative or cooling towers, water is sprayed in the upper part of the tower while air is induced to the tower by mechanical or natural means. Because of the evaporation the air becomes cooler and heavier and descends to the lower part of the tower and then is distributed to the ambient environment. The system is well analysed and additional information may be found in (12).
Evaporative Towers
Figure 4: Evaporative Towers installed in Sevilla, Spain. Copyright : cooling towers Sevilla - Bing images
Research has shown that the cooling capacity of the evaporative systems and techniques is improving with increasing ambient temperatures as higher ambient temperatures raise the saturation capacity of the ambient air and the evaporative potential of the systems.
MATURITY:
Passive Evaporative Cooling systems are very mature mitigation techniques known from the ancient times. Active Evaporative Cooling systems are industrial components tested and well optimised for outdoor use and can support the improvement of the urban microclimate and counterbalance the impact of urban overheating.
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