Evap·o·rate, to pass off in vapor or in minute particles.

All evaporative cooling rely on the energy required for the evaporation of water to absorb heat from the air and lower the temperature. This is due to the very high enthalpy of vaporization of water, the phase transition between the liquid and the gaseous state requires in fact a large amount of energy (which is more properly called enthalpy) that is taken from the air in the form of sensible heat (which is the temperature, something we feel with our skin and determines our comfort) and it is converted into latent heat (which is an energy “hidden” in the vapor component of the air). The result of this adiabatic process is a drop in the temperature of air and an increase in its humidity, therefore it’s clear that this cooling system is particularly effective in dry and hot climate zones where the higher humidity and the lower temperature can be both seen as advantages. Clearly the evaporating process is a key also for some convective cooling processes (that we treated here) but they rely also on the reduced buoyancy of cooler and more humid air to obtain the final effect while evaporative cooling techniques only rely on the evaporation of water.

Although primitive evaporative techniques were used in ancient times (in combination with convective and ventilation devices like windcatchers and qanats in iran) and porous water jars are still used in many hot areas in combination with Mashrabiya other ventilation apertures to naturally cool down the interior of buildings the use of evaporation to cool down outdoor spaces is very recent. Evaporative cooling depends largely on how effective we are able to evaporate water, and a basic physical variable plays a big role in this case: surface-area-to-volume ratio, the more surface area we are able to expose the more energy we are able to exchange.  There are basically  two ways to proceed nowadays to maximize the surface area, evaporative pads and misting. Evaporative pads are generally used in evaporative cooling machines oriented to indoor cooling, these pads are cheap and effective but they are relatively fragile, require continuous maintenance and are most effective in controlled environments where the airflow can be adjusted and controlled, the “wetpads” are made of porous materials that have to be maintained wet while air passes through. The peculiar structure of these materials offers the largest possible surface area to the passing air which is then humidified and pushed into the building or the room. This technique can’t be used for outdoor cooling clearly because of the required control to the ariflow that is necessary.

Misting is instead widely used nowadays to lower temperatures both in buildings and open spaces. The use of water mist to generate passive cooling in closed buildings is strictly related to passive (or mechanical) evaporation towers and therefore to what we have been explaining in the convective technique post in open spaces the use of water jets and mist is instead very efficient (of course depending on specific climate conditions) and cost effective.

Although it is not strictly designed to be a bioclimatic public space, the Miroir d’eau designed by Michel Corajoud in 2006 in Bordeaux is one of the most successful examples of water evaporation usage in public space design. In this case a large square, just in front of the famouse Place de la Bourse, is designed to be a large water mirror where hundreds of water nozzles spray water from the floor either in the form of a fountain or of a mist cloud. In the first case, where tall gushes are produced, water evaporation is limited and the playful atmosphere dominates the large plaza, but when short mist clouds are produced the evaporation rate of the water is greatly increased and a cooling effect is produced, although in Bordeaux climate conditions are quite mild, and hot days are limited to few occasions during summer the square is very popular.

Vaporizing water coming from the floor is a quite common and effective mean to condition large open spaces, the effect that everybody has noticed of a slight refreshment when passing by a fountain in a square or, even more, while staying close to a waterfall is due to the very same thermodynamic principle, the small drops of water that the are created when water breaks while falling to the ground or splashing into more water dramatically increase the surface-area-to-volume ratio favoring a faster evaporation, the nebulized microscopic drops evaporate instantly causing a sudden temperature drop that can be magnified by the wind or other design inventions. In the Sevilla 1992 EXPO this effect was widely used, large fountains and water basins were placed all around the EXPO along all the main paths and squares to increment climatic comfort, in some areas even vertical walls of water were designed to expose the visitors to an even more effective cooling device, but the most common strategy was the use of conventional fountains and mist nozzles integrated in the many green shading roofs.

The design of these spaces has to be developed with special care, the effectiveness of the strategies used in Seville for example varied much depending on the surrounding conditions, evaporative cooling could be very effective if combined with the right design of protective and shading elements, with a correct sun and wind exposure and material use but could be also nullified simply by not considering the wind variation. Even if water vaporization is widely used in many terraces, bars, public venues, etc. because of its low cost, obtaining an effective bioclimatic effect is harder to achieve. Ecosistema Urbano employs evaporative cooling in one of their seminal project, in the Vallecas ecoboulevard, the Ludic and the Media Tree are not equipped with evaporative towers but with water spraying nozzles that are oriented towards the circular public space beneath them.

The main innovation in the use of evaporation in this case is due to the form of the designed public space, because, as we already said, there is not much to innovate about the nozzles technology itself. Actually the most important issue is the control of the water flow and pressure as it has to be correctly regulated depending on the actual dry-bulb and wet-bulb temperature, relative humidity etc. in the case that those variables are considered, evaporation should be instantaneous without any dripping nor condensation. In the case of the Media Tree temperature and humidity sensors regulate the flow and the pressure of the water flowing to the spraying nozzles constantly adapting it to the weather conditions. In this case the design is particularly effective not only because of the cooling technology but mostly because of the shadow provided by the “trees” themselves and the protective design of the ground section that allow the cooled air to linger in the “inhabited” space and not being immediately dispersed.

But misting has a close bound with atmosphere and space, being one of the few atmospheric phenomena that we can directly observe fog and mist have been used also to define spaces, these new approaches, even though not directly related with bioclimatic architecture, open the door for future developments. In one of their most famous, and paradoxically iconic, works Diller+Scofidio designed a “formless, massless, colorless, weightless, odorless, scaleless, featureless, meaningless” that was basically made of mist and nothing else. Their explication for the work was open-ended, blur-building was not only the name they gave to it but also a factual assertion: the definition of it was also blurry. This event contributed to redefine, or to destroy, the meaning of building and the separation between what is a building and what is environment, up to even questioning what is architecture, for the first time the space was not defined by walls or windows or any stable solid material but was only an undefined mutating cloud made of vaporized water.
But this wasn’t in fact the first building that used mist water to blur its edges (although that they are all curiously related to universal expositions, more about expos here), the Pepsi pavilion in Osaka was the result of the fructuous cooperation between engineers and artists within the Experiments in Art and Technology  group and it was constantly covered with a thick layer of fog that partially hid it. In this case the building was still present and firm, a concrete entity with an interior and exterior form and a “conventional” space inside but the fog sculpture, designed by the japanese artist Fujiko Nakaya who spent her life working with fog, contributed to the creation of a memorable innovative pavilion.

At the Seville EXPO in 1992 the so called “bioclimatic sphere” was also one of the main attractions of the whole exhibition and surely one of the most iconic ones. A tubular sphere was placed in the middle of one of the most important boulevards of the exhibition rounded by fountains and water basins as a part of the bioclimatic design of the open space of the exhibition. Although being highly symbolic and recognizable this sphere as reported in the follow-up publications about the Expo was not really contributing to any bioclimatic effect on the square or the boulevard, this depended basically on the fact that the device was placed in an open space and the diffusion of mist was not controlled in any way (a very interesting publication about the follow up of the climate conditioning in the EXPO 92 has been published by the same engineers that contributed to the design of the project and a short extract can be found here).

In 2016 also the famous artist Olafur Eliasson started working with fog and misting, naturally he is not concerned with the bioclimatic function of fog but more about the terms of landscape and vision and interaction between the user and the fog itself. Placed in the Versailles garden, “fog assembly”, is a ring emitting a swirling mist that involves the objects around and changes appearance depending on the site conditions. The user is invited to interact with the installation, crossing it and begin part of the fog it is producing, in this sense, this artwork can be easily assimilated to a public space generating a connection with the theme of this research.

Ven·ti·late. The natural or mechanically induced movement of fresh air into or through an enclosed space.

Natural ventilation was widely used in traditional architecture to improve the bioclimatic comfort of tents first, and then rooms and whole houses, before the advent of air conditioning, natural ventilation was one of the few techniques available to lower the temperature of a closed space exploiting the cooler winds blowing outside or just the movement of cooler air.
The first and most important examples of architectures using the wind as a cooling medium to improve indoor environmental conditions are found in Persian traditional architecture, the badgir (or mulqaf in arab) is an extraordinary piece of spontaneous design, using only the natural flow of the wind – often combined with many other bioclimatic arrangements like thick insulating walls, very packed constructions, small apertures, etc. – it is capable of cooling and improving the climatic comfort of a house in the torrid deserts of Iran and the Arabic Peninsula.

The badgir, in arid and dry climates is often combined with the use of water to implement evaporative cooling improving even more its cooling capacity and generating cool breezes even without the presence of winds outside, in this case the thermodynamic effect is not based only on ventilation but also on convection. The windtower, or literally windcatchers, can be found as a traditional element in most of the modern Islamic world area with few regional variations, its usage has been consistent through the ages but in the western gulf region it almost disappeared due to the rapid urban growth and modernization of the cities, in Bahrain, for example, only one ancient badgir remains.

The traditional usage of windcatchers has been nowadays reinterpreted in many ways using both natural and mechanical aided ventilation, the great Egyptian architect Hassan Fathy used it widely in his buildings, but for sure, one of the best practice (at least if we consider this research public-space oriented) is the Qatar University Campus designed by the Egyptian architect Kamal el Kafrawi (with the collaboration of Ove Arup) and opened in 1985.

In this groundbreaking project the use of windcatchers is systematic and characterizes the whole campus. Based on an octagonal and square plan geometry, the low rise concrete modules the projects makes large use of natural light and natural ventilation through the hundreds of windtowers that top every module and mashrabiyas to protect the classrooms from sunlight and permit the air circulation. The aggregation of the modules juxtapose classroom modules, halls and rest spaces enriched with vegetation and constantly ventilated through the roof.

In contemporary architecture, and specifically in the climatic improvement of public space, the use of ventilation devices, especially in high-humidity environments is quite a new thing and mechanical ventilation is generally used as a cost-efficient way to overcome tropical humidity especially in southeast Asia. Two projects are to be considered references in this case, one is Will Alsop’s Clarke Quay in Singapore and the other is Ecosistema Urbano’s Air Tree for the Shanghai Expo 2010.

In this 2006, project, Alsop is called to regenerate the Clarke Quay riverfront and the market with the objective of drawing tourists and locals back to the old Singapore’s waterfront. The most interesting thing of this bold design is certainly  the bioclimatic intervention in the market, refusing to create a closed shopping mall the architect designed a mitigated semi-external space, protected from frequent rains and with improved environmental conditions.

The market intervention is composed by two main parts, the roof and the ventilation devices.
The roof is constituted by giant umbrella-like structures covering the internal streets of the market, the ETFE canopies covering the streets offer protection from both the rain and solar radiation that in this climate are equally detrimental for the use of public spaces. This roof maintains the temperature in the central square and the four streets of the market at around 28º Celsius when outside temperatures can rise up to a mean of 31ºC. But the most important and innovative feature are the “whale-tail” shaped ventilators placed in the vertical supports of the roof structure. These big fans have a fundamental role in maintaining good environmental conditions in the market streets, considered the high relative humidity level(year average 84%) ventilation is the only way to make the air tolerable. Using slow rotation fans these sculptural objects blow a constant breeze in the lower part of the market favoring the natural evaporation cooling of the skin.

The Air Tree that Ecosistema Urbano realized in Shanghai for the 2010 Expo is a prototype of an intervention in contemporary urban space. It is conceived as a new kind of public space, a technological urban furniture, which also serves as a virtual node of connectivity where users can actively interact. Its different technical layers enables multiple final configurations and a myriad of intermediate positions (opaque, translucent, transparent, bright, interactive, open, etc.). Different textiles for video projections allow an unlimited combination of scenarios adaptable to citizen needs. Its appearance can be transformed over the daily cycle, as well as through the different seasons. By sensors it is connected in real-time with the climatic conditions of Shanghai, constantly adopting the optimal physical and energy consumption configuration to generate climatic comfort for the citizens.

To improve climatic and environmental conditions, that in Shanghai basically have to deal with high temperature during summer and high relative humidity during all year, a 7.3 m diameter fan suspended by a tensegrity structure in the center of the space, at a height of 11.5 m provides air flows inside the space. Through a telescopic system the fan can be lowered several meters to come closer to the ground. The exact position and speed at each moment is determined according to the instant climatic conditions of the environment, real time monitored in the surroundings of the structure. Together with the variable configuration of the tree’s skin the flow of air generated with the fan can effectively improve the environmental conditions inside the tree.