Design of an adaptive shading system for arid climates : theoretical, numerical and experimental analyses

INTRODUCTION:

What if? Two simple words, an ordinary question, yet this is one of the deepest and more important query for the whole human kind. This question is so mysterious and enigmatic for the man because it has not a unique or certain answer but it transcends the given reality as it is and tries to imagine an alternative present or future. This query is very often engendered by regret, regret for the past or regret for a wasted possibility that now can’t come back any more, regret for a future that could have been but never was.

All comes from the observation of what is happening in the Middle-East countries, especially in the Arabian Peninsula, when dealing with new buildings. The widespread habit nowadays is to build high-rise glass towers for the simple reason that the western world created them and uses them. This because in those cultures which have been economically inferior to the Western world for so long, all that comes from the West means power. The sheiks and Emirs who rule in those countries, in the last twenty years have oriented the architectural style on the European and American architecture simply because they intended it as a symbol of a newborn power, because they wanted to cast a message to the western world: “Now we’re powerful too”.

Aside from the political and economic reasons of these choices, which are not the matter of this study, there are some architectural implications which cannot be ignored. One of the main reasons for which western countries started to build glass buildings was because the deep buildings of the end of 19th century were very dark inside. In the first years of 20th century there was thus a great effort in trying to maximize the contribution of incoming daylight in the interior space of the buildings; with the technological development it was possible to create increasingly bigger windows

up to the “curtain wall”, a wall that, whether could be composed only by glass. As it always happens, an architectural solution is conceived, designed and tuned for carrying out a specific task in a specific place; this means that these technologies, developed in order to exploit all the possible useful daylight in latitudes where it was scarce over the year, were effective only in such places like Northern Europe and the Northern part of the U.S.A where daylight was low. From what’s been said so far it’s easy to understand that if glass buildings are copied from Europe or the U.S. and realized in the Middle-East exactly as they’re in the Western countries, without any modification that takes into account the differences in latitude, in the climate and in the conditions of daylight, the behavior of the building itself is completely different. In the matter in question a useful technology, able to bring inside the building a higher amount of daylight and reduce thus the energy demand for the lighting system is transformed into a technology not only useless, that loses completely its “raison d’être”, but even worse it becomes a device hostile to man. In fact the glass walls transform the building into a greenhouse and in the Middle-East, where the solar radiation is high over the whole year, the solar heating gains become enormous. To overcome this bad situation and make tolerable the indoor space of the building in terms of temperature, a huge demand of electric energy for the cooling system is necessary. It’s plain to see that all this is not only logically and concepe a great quantity of eunt of pollution thatly e more attentioblem in the design process of then the past thet could create and knew very well how to s og, create, even at tle ind

This last consideration and in estion betions, instead of abandoning the veeir traditions for adopting a foreign architeetelyhad canacular architecture had gone through the sat western architecture has experienced?

This of courbe answered in many different ways, and there’s not a right or wrong answer, but only a more or less liigation is huge, so the answer we trieblem: the issue of daylight. As already pointed out, in those climates daylight constitutes a problem for two main reasons: glare and overheating. Glare deals with the concept of visual comfort which is about the intensity and the quality of incoming daylight and aims to reach the best conditions for carrying out a specific visual task. Overheating instead deals with the concept of thermal comfort which here is about solar gains, i.e. the quantity of radiation that passes through the window pane, is absorbed by the interior surfaces and re-emitted at lower wavelength so that it can’t pass through the window pane any more and heats the indoor space. The goal is to create an indoor space in which thermal comfort is provided: at these latitudes this means blocking the direct solar radiation in order to have not too hot interior spaces. The vernacular Arabic architecture had developed a technology able to deal with this two issues at the main time: the Mashrabiya. The goal of this study is creating a novel mashrabiya device that, together with the most recent products of technology, is able to satisfactorily answer to the problems of contemporary architecture in terms of visual and thermal comfort.

But first of all let’s see more in detail what a mashrabiya is.

The mashrabiya is a traditional element of the vernacular Arabic architecture, also called Shanasheel or Rawshan. It is a latticework screen usually constituted by small turned wood elements assembled in geometric shapes, often quite complex. Windows, balconies and loggias are covered with the resulting latticework and are for this reason sometimes called with the same name for synecdoche. The technique itself, used also for the production of furniture, is likewise called in the same way. The name mashrabiya is derived from the triliteral root S-R-B, which means “to drink” or “to absorb”. The most common theory about this name is that it was initially used for a shelf where the drinking water pots were stored; in order to keep the water cool this shelf was enclosed by wood and located near the window. Later on it evolved until it became part of the room with a full enclosure, practically a completely different device but, despite the radical change in use, it retained the name.

There is not a precise date in history when mashrabiya appeared, but the first evidences of the use of this technology as it is today dates back to the 12th century in Baghdad, during the Abbasid period. Nevertheless the oldest original mashrabiya examples we can find today are three or four hundred years old, though they’re very few and the most part of the existing mashrabiyas was built during the 19th century and early-to-mid-20th century, when this technique almost completely disappeared.

Mashrabiya were mostly used in the Mashriq, i.e. the macroregion of the Arabic world composed by the Countries located to the East of Egypt, especially in Iraq, Levant, Hejaz and Egypt itself, even though some types of similar windows are also present in the Maghrib, i.e. the region of the Arabic world composed by all the Northern African countries located to the west of Egypt. Similar devices, though sometimes with different names, can also be found in places with an alike climate like former Arabic colonies

such as southern Italy, Portugal and Spain (very famous are the Alhambra mashrabiyas, in Grenade) or even in areas very far from the Arabic lands, like some regions of India and Pakistan (jaali,). The reasons of this widespread diffusion of mashrabiya in all the regions with desert climate lie in the fact that this latticework screen perfort the same time, the first of which is providing shade and protection from the hot sun while allowing the cool air to flow through. Blocking direct sunlight means to significantly reduce the internal solar gains and at the same time the cool air flowing through the geometric holes contributes to refresh the indoor space and create a agreable ambiance in terms of thermal comfort. The second task is that reen eliminates glare and visual discomfort: in fact it blocks the intense desert solar radiation and, due to its shape and its depth, it transform the strong incoming direct sunlight into a soft and very pleasant diffuse light. The third task mashrabiya accomplishes is a social function strictly connected to the Arabic culture: the need for privacy. In fact it allows the women, which in Arabian culture must be concealed from the glances of strangers, to have a good view of the outside without practically being seen and preserves the private interior without depriving the occupants from a vista of the public outer space. For this last reason it’s usually used in private buildings such as houses and palaces, although sometimes we can find it public buildings like hospitals, inns, schools and government buildings. For the same reason mashrabiyas are mostly present in urban settings and rarely in rural areas.

The idea:

As already briefly explained the idea of this study comes from the observation that glass buildings realized in Middle-East countries have enormous problems in terms of visual comfort and excessive solar gains. Most of the solutions proposed so far are mediocre and inadequate because either they don’t really overcome these problems or they manage to solve them only at the cost of creating other problems. For example some shading systems manage to block the direct solar radiation, but they create dark interior spaces and a high amount of electric energy for the lighting system is required. Another example is provided by those kinetic systems which, with fine adjustments based on the solar incident angle, are able to screen the direct solar radiation without completely blocking the incoming daylight, but in the first place they’re very expensive in terms of electric energy necessary to allow their movement and, most important, they’re totally unsuitable for the desert conditions.

The design of a solar screen providing visual comfort and preventing excessive solar gains is particularly challenging in arid regions such as the Middle-East because the abundance of solar radiation requires a very efficient shading system while the combination of wind, sand and corrosion due to prevalent condensation creates harsh environmental constraints. On one hand, the static vernacular solution of mashrabiya is well adapted to these constraints but fails to meet our contemporary needs for visual comfort due to insufficient daylighting and strong reduction of outside view. On the other hand, a kinetic shading system like Venetian blinds meets the requirements for efficient shading and for visual comfort (minimal glare, adequate daylighting and maximal outside view). However, in order to avoid excessive solar gains, such a shading system must be placed outside of the window, where it cannot withstand the harsh local environmental conditions. More sophisticated contemporary technologies embedded in the window, like electrochromic glass, are in principle unsuitable for these climates due to their propensity to absorb solar radiations resulting in excessive solar gains. Therefore the challenge is to design a kinetic shading system that manages to solve the problems exposed at the beginning of this paragraph and at the same time can cope with the desert harsh environmental conditions.

Provided that the mashrabiya is the only vernacular device able to survive to arid climates conditions, we tried to develop it in order to make it answer in a satisfying way to all the problems highlighted so far.

The solution we came up with for the issues underlined relies on a siategy to deal with abundant solar radiation applicable to these specific climate conditions. With clear sky conditions prevailing throughout the year in these regions, strong direct sunlight irradiating a window must be uncompromisingly blocked. We believe that fine adjustment of the shading system with solar incident angle are not absolutely required, and not even desirable when striving to minimize solar gains. W assumption, the shading system is either closed in presence of direct sunlight, or open when skylight dominates. The sufficiency of such a binary function facilitates the design of a simple and robust kinetic shading system potentially capable to cope with harsh climate conditions. Another important motivation for using a kinetic shading system with binary positions is the possibility to obtain a solar responsive system by exploiting a novel passive actuator, i.e. a device able to carry out the movements of the shading system without the need of electric energy.

Applied as such, our approach would provide insufficient daylighting when blocking direct sunlight. This important limitation, which is commonplace, independently on climatic conditions, in most shading systems (missing or not optimized), forces the occupant to use, quite paradoxically, electric lighting despite abundant daylight availability. This happens, for instance, with fully closed Venetian blinds.To tackle this important limitation, we devised a shading system that incorporates this very desirable daylighting function, i.e. exploit natural light when blocking direct sunlight.

The device we created is made of three identical perforated opaque shields that can move relative to

each other so as to switch between an open and a closed configuration. A shield structure consists of a bi-dimensional assembly of identical perforated motives, each covering a square area, designed taking inspiration from the traditional mashrabiya pattern.

The first shield, the one next to the window pane, is always motionless. In the open configuration, which is used when skylight dominates, the shields are exactly superimposed and nearly in contact with each other.

In the closed configuration, activated in presence of direct sunlight, the second and third shields move along the lateral and vertical dimensions by half of the motif length, in order to fill all the openings and block most of the direct sunlight. Moreover these shields simultaneously move along the width dimension so as to create a gap of length between themselves in order to allow multiple scattering reflections between them.

The result of this operation is the transformation of the direct sunlight into scattered light which is then used for diffuse indoor lighting. As previously mentioned, we plan to produce the force necessary to the movement of the device by means of a solar passive actuation and detection system based on a combination of custom optics and Phase-Change Material (PCM).

Since such a solar responsive system is essentially restricted to binary actuation, it lends itself well to moving our device. With this simple solar responsive system (no detector, motor or electronics), whose description is outside of the scope of this work, and this simple mechanical structure, we expect our device to be extremely robust. In the end, to summarize the working principle, a solar responding system allows switching our device, depending on the sun’s position, between an open and a closed configuration, which consists of a three dimensional structure blocking most incident sunlight while transforming a fraction of it into scattered light used for indoor daylighting.

Once formulated the idea of how to solve the problems expressed above, it was necessary to establish some criteria with the function to direct the design and study process. These were practically a few key features of different nature (Technological, Functional, Architectural) we wanted our shading and daylighting system to incorporate and which can be expressed by the follong high-level requirements:

-Ability to switch in a timely manner between an open and a closed configuration, respectively in absence and presence of direct sunlight

-Maximal Daylighting and outside view in the open configuration

-Efficient shading in the closed configuration while minimizing solar gains

-Incorporation of an efficient daylighting function, i.e. transformation of a fraction of the blocked incident dect sunlight into diffuse light (for indoor lighting)

-Ability to withstand the harsh local climatic conditions

Potential for integration into a flat glass façade and coupling of our solar responsive system

-Preservation of local architectural character, i.e. mashrabiya inspired design to both the open and closed configuration

As of now, to highlight the kinetic features of our shading and daylighting system, and the specific architectural attribute imposed by our last requirement, we will designate it as Shape Variable Mashrabiya, abbreviated SVM.

After this brief overview on the idea that underlies this project and the requirements we want to meet, let’s see in detail all the steps of the path that, starting from the design process and passing through all the performance tests, led us to the creation and optimization of this novel mashrabiya technology.