吉布提法国中学丨吉布提丨R plus S ARCHITECTURE

Architect：R plus S ARCHITECTURE (Richard + Schoeller Architectes)

Location：Djibouti; | ;View Map

Project Year：2024

Category：Primary Schools;Secondary Schools

An architecture of wind, light and shadow.

1) Why the wind?

We had moved away to the west of Dikhil, near Bouh Barre, after crossing a dry wadi at this time of year. The sun, rising in the sky after an entirely starry night, embraced us with its torpor. Even though the summer season was not here, walking became difficult in May. Finding a stone hut, covered with a roof overflowing with sheet metal, we stopped in the shade. The thin thickness of this sheet appears providential to us. Under its shade the temperature had dropped by 10 degrees, without the sun’s rays hitting the skin. As we went around the cabin, the east wind which was blowing continuously suddenly diffused an unexpected freshness. We stood there thinking, sensitive to the slightest shadow, to the slightest breeze, considering that simple aspects of climate like wind can naturally ventilate a building, providing a comfortable indoor atmosphere without energy. And let’s look at how to adapt this thinking to Joseph Kessel High School.

In the same way, light and shadows can sometimes very simply allow partial self-sufficiency on certain primordial objectives of architecture.

2) Comfort

SHADE OF ROOFS AND THEIR INSULATION

In addition to our discussion on wind, we applied the principle of the roof which protects to the Joseph Kessel high school building in Djibouti. By the north-south orientation of the facades receiving practically no direct sunlight, by the rapprochement of the two parallel buildings which keep the space between them in the shade and the numerous systematic awnings along the facades which protect them , outdoor spaces are protected from direct sunlight.

This compact building with heavy inertia obtained by the insulated double wall facades provides freshness gained during nighttime periods by very specific insulation of the building.

Percentage of total annual radiation on roof and walls of a cubic structure location latitude 11°30’N

If we examine the diagram above we observe that almost 50% of the sun’s radiation hits the building on its roof.

The roof of the Joseph Kessel high school is protected by thermal insulation U of 6, 30cm thick, sheltered under

one on a white roof which protects from direct sunlight and accommodates the solar panels.

The west staircase at the back will go up to the roof via a separate access for maintenance of the exterior units and photovoltaic panels. A passage above the R+2 will allow access to the roof of the other building.

This arrangement will allow easy cleaning of the solar panels.

Only classrooms are isolated and cooled. This difference in treatment between ventilation cooled by solar panels in classes on very hot days and natural ventilation by wind towers and Venturi effects on the wind allows flexibility in the use of the building and significant savings.

LOWER BODY TEMPERATURE WITH WIND

CONVECTION COMFORT FELT

The air temperature is that measured with a thermometer one meter above the ground, sheltered from the wind.

The speed of the wind and its temperature influence the real temperature, called: felt temperature which changes according to the intensity of the wind. The correct term for this phenomenon is wind chill. It is the sensation of cold that the wind causes on a living organism which gives off heat without any change in the actual temperature of the air.

The temperature felt: a sensation on the surface of the skin

But what role does the wind play in this mechanism? this little insulating protection is perpetually blown away when it is windy! If the ambient air is colder , it is the skin which heats it. Heat exchange occurs by conduction. But if the wind blows, the layer of air is constantly replaced by cold air and heat exchange takes place by convection , a much more efficient phenomenon. The feeling of cold increases. Additionally, if the air is warmer, the body releases excess heat through sweating. Water, as it evaporates, heats and humidifies the air near the skin. If the wind renews this air, evaporation accelerates, dissipating more heat. In the absence of cool wind and if the air is humid, evaporation becomes more difficult: the feeling of heat increases.

USING WIND TO RENEW THE AIR AND COOL THE WALLS BY CONVECTION

In the same way as the skin, the cool wind, touching the exterior and interior walls of the building, cools the temperature by convection.

The spaces at the Joseph Kessel high school being almost all through, the wind can be used to renew the air for a large part of the year, particularly in the classrooms. Multiple vents, hooked towards the East on the building, protected from the sun, were planned to capture the wind towards the interior of these rooms, accelerate it by the Venturi effect and thus cool the atmosphere by keeping it within a range of hygrothermal comfort humidity. Thus, as planned in the program, the air renewal is optimized, natural and sustainable.

OPTIMIZE NATURAL LIGHT

Large openings are avoided on the East and West faces to prevent glare phenomena.

The glare of the sun in the East is entirely obscured by the same concrete panels as the bows window which brings sophisticated light.

Optimizing the amount of natural light in the building is an important factor in comfort for occupants, under the intense light of Djibouti, while also saving energy. Here the facades are integrated into the climate by shelves of lights and shadows, fixed perpendicular and horizontal surfaces along the windows which allow direct protection from direct solar radiation by allowing rays that do not directly strike the work surfaces to penetrate. allowing you to work without lowering the blinds.

To complete the solar and thermal protection process, the glazing thus protected from the sun is of the anti-solar type FS 40% 16 mm argon > 85 %.

3 ) Djibouti extreme conditions

The Joseph Kessel high school in Djibouti clarifies in a concrete way the potential of a passive architecture thanks to its orientation, the preliminary study of the climate, the materials used, the choice of methods of energy supply by solar panels and an environment in a wooded and organic living environment.

THE SITUATION IN DJIBOUTI

The location of the city of Djibouti is close to the parallel at 10° above the equator. (the exact geographic coordinates for Djibouti are 11.589° latitude, 43.145° longitude and 6 m altitude.) So the city is located in the Northern Hemisphere. The days are longest in summer and the shortest in winter. But the difference between the winter solstice and the summer solstice is only one and a half hours. (around seven hours in Europe) The sun follows the orange line in the diagram below and the temperature oscillates between 20° and 45° felt 48 to 50° when the Khamsin hot wind blows which comes from the desert from the west loaded with sand.

THE WIND IN DJIBOUTI

The climatic risk of extreme heat is therefore significant and the project takes particular account of this. It meets this essential condition in a frugal way by using, among other things, the fresh wind from the sea. The majority of the year it is the trade winds which blow from the subtropical zones where high pressures are located, towards the Equator. These are constant winds, which blow with the same intensity at an average speed of 20 km/h all year round. In Djibouti these trade winds are accentuated by sea breezes. These are winds resulting from the thermal contrast between different regions of the earth’s surface. To better understand what a breeze is, we can cite the one that generally occurs on the coasts: the land heats up more than the water during the day, then it heats the air which moves upwards. This then causes a drop in pressure which is reminiscent of sea air, it is the sea breeze. The sea surface temperature in Djibouti varies between 24° and 30°C, providing constant coolness. At night, the phenomenon is reversed, the land cools faster than the sea and the air above the water becomes warmer than that above the land. The hot air rises and the drop in pressure brings back the cool air from the earth, this is the land breeze.

THE DESIGN OF THE HIGH SCHOOL

The Joseph Kessel French high school has three distinct poles which are designed on the same orientations in relation to the winds and their accentuation by the Venturi effect:

- high school,

- the elementary

- Kindergarten.

Simulation of the speed of current winds on the Lycée Joseph Kessel model Urbawind modeling. The blue zones are those where the wind speed is the greatest.

These provisions are as follows:

- The opening of buildings towards the trade winds in the North, North-East which also bring the sea breeze.

- The search for Venturi effects on the latter by their arrangement in the form of bows, their shape and the addition of fins stretched towards the South between the buildings at the bow facing the wind.

- The use of wind towers and “sills” to allow significant intrinsic natural ventilation providing comfort against excessive heat in circulation areas and classrooms.

The repetition of these similar shapes intrinsic to the capture of the wind on the three poles makes it possible to establish an identity to this ensemble which forms the French high school.

VENTURI When wind speed varies, a pressure difference occurs, which causes air to move from an area of higher atmospheric pressure to an area of lower pressure. This is based on Bernoulli’s theory and is called "Venturi action", which explains that as the speed of a moving fluid increases, the pressure decreases.

The example constructed today of the secondary school allows us to affirm the effectiveness of this system.

It is built with an optimal orientation of the facades towards the North and the South to avoid solar radiation with its architecture close to the equator. The sun being always vertical during the day, the North and South facades rarely see the sun, it is the East and West arrangements which are synonymous with discomfort due to glare over a great depth in the morning and evening of the spaces thus oriented.

The Joseph Kessel high school, through this exposure, does not receive any direct solar emissions and avoids solar rays grazing from East and West. It nevertheless needs additional protection for the light which is so intense that it can become annoying due to its too much brightness.

We’ll expand on the wind capture essential to the process below.

4) The wind

CAPTURING THE HORIZONTAL WIND

The geometry of the project took into account the pleasant dominant flow of the North-North-East sea breeze. This breeze corresponds to the fresh trade winds which blow all year round with constant speeds of 5 to 15 knots.

The project organizes all the courtyards, courtyards and circulation under the flow of this stable wind, cooler than the ambient temperature.

The buildings are, however, closed on the west side in order to avoid the penetration of hot winds from the Khamsin during the summer and carrying sand.

Closing the wind towers with steel shutters and the circulation areas with doors helps protect the buildings from sand during Khamsin. The Khamsin wind luckily corresponds to the month of July and August, during which the high school is closed.

The height of eight meters of the volume of the high school on the west side makes it possible to repel the wind beyond the nursery and elementary classes according to the diagram above.

Wind rose of Djibouti.

1 knot = 1.852km/h

Winds in Djibouti

1- PERIOD FROM BEGINNING SEPTEMBER TO END JUNE: Trade winds

During this period the wind is of the trade wind type which blows throughout the country, practically every day, in the same East/North East sector with constant speeds of between 5 and 15 knots.

1.1- In the city of Djibouti

Most often a thermal effect swells this trade wind between 12:00 p.m. and 4:00 p.m.

Roughly at the peak of the thermal effect, in this period from September to June, we have approximately 20% of days with a wind of less than 6 knots, 35% with a wind between 6 and 10 knots, 35% with a wind from 10 to 12 knots and 10% with a wind of more than 12 knots.

It also happens 2 or 3 times in the period between December and March, through the passage of a depression, that this trade wind strengthens to reach a maximum of 20 knots.

The 2 extremes of this period (September and June) correspond to the end and the beginning of the period of the Khamsin wind oriented completely in the opposite direction (West/South-West).

The afternoon Thermal often “wins” its fight over the still timid morning khamsin and the wind suddenly shifts from North/North East around 2:00 p.m./3:00 p.m. to blow up to 12/14 knots.

2- START PERIOD FROM MID JUNE TO EARLY SEPTEMBER

- In Djibouti city

The Khamsin wind rises early in the morning or sometimes at dawn, strengthens until around 10:00 a.m. and falls slowly from 1:00 p.m.

The first blows of Khamsin (read " ramcine " which means fifty in Arabic) in reference to the duration in days of the approximate period of activity of this wind which starts between June 20 and July 15 while we can still find Khamsin until the end of August. The strength of Khamsin and its frequency are quite variable from year to year.

Basically over the 2 months of wind activity, you should expect around 40% of days with less than 12 knots, 50% of days with 12 to 20 knots and 10% of days with more than 20 knots. The khamsin can go up to 35 knots. The strength of the Khamsin proportionally influences the temperatures: it is on the high days of Khamsin that we reach 45 degrees in the city of Djibouti.

All of the high school buildings and their facade openings, courtyards, courtyards and circulation areas are organized under the movement of this constant wind. The trade winds are very low on the Beaufort scale and do not cause any discomfort.

The general shape of the project is split into two parallel East-West volumes over sixty-five meters long, while remaining resolutely closed on the West side. These two volumes are joined by three perpendicular aerial walkways.

This arrangement of the built volumes allows an in-between where the sea breezes penetrate inside the central square space already protected from the sun by the height of the buildings.

DRAWING

This orientation system open to the dominant sea breezes is deliberately accentuated at the same time:

- by the front angle shape on the east side of the two parallel buildings which captures the trade winds as widely as possible

- by the concrete wings which join these two volumes at the bow of the building on the east side, thus forming a wind accelerator sensor device.

This system of parallel horizontal strips and opening facing the wind accelerates the sea breezes which bring freshness through the Venturi effect.

The wind glides along the passageways and circulation areas which are all protected from the sun.

DRAWING

Venturi effect when the wind passes through a narrower space

The interior space located behind this mechanism has been planted with trees and plants to better refresh it and accentuate the phenomenon of well-being from the breezes.

Vegetation is also used according to the bush/tree/air inlet logic and the Venturi effect.

Thus this space between the two North and South buildings remains protected from the sun, generously sized, it creates a cool space at the heart of the project.

Entering the space between the building which keeps the shade, these breezes accelerated by the Venturi effect roll over the footbridge which joins the two parallel lines in the center and are brought down towards the ground. The west side being completely closed, they emerge by crossing the two courtyards, along the horizontal circulations of the R+1 and R+2 installed along the facades or by entering the buildings on either side of the central space refreshing by their passage these places of rest and conviviality of the students.

Thus a good part of the horizontal circulations and staircases arranged in the same North South orientation parallel to the North building benefit greatly from the sea breeze while being protected by the rhythmic horizontal and vertical sunshades of this heart of the main space.

The North building is served by exterior passageways and shaded by their superposition. Two staircases parallel to the passageways from top to bottom and a vertical sunshade five meters and fifty high which begins on the R+1 direct the wind towards the passageways or gives it the direction which follows them and thus cool the facades.

b) CAPTURE THE VERTICAL WIND.

In the same spirit, the wind towers located on the three schools provide two important natural ventilation effects that are economical, environmental and sustainable:

These wind towers located above the stairs and circulation areas of the south building allow natural ventilation not only of the circulation areas but also of the classrooms through frames which can open onto the circulation.

Two functions of these wind towers allow the building to be cooled: the wind catcher effect and the chimney effect.

The operation of a wind tower is governed by the 2 physical principles of natural ventilation: the effect of the wind and thermal draft. These 2 principles strongly depend on external conditions of wind direction and speed, as well as the temperature difference between the interior and exterior of the building. They allow fresh air to be brought in either from the top or from the bottom.

For wind speeds less than 2m/s, the thermal draft will dominate, beyond that it is the wind draft.

1) In the presence of wind, the system captures the wind thanks to the small pressure difference between the base and the top inside the column. As wind penetrates through the top the pressure difference helps to lift warm air up to the top and bring cool air down the column.

2) In the absence of wind, the air at the top of the tower heated by the sun heats up. Due to convection, it rises, escapes through the top of the tower and creates a draft of air which ventilates the horizontal and vertical circulations located below.

The wind towers located above the stairs and circulation areas of the south building allow natural ventilation not only of the circulation areas but also of the classrooms through frames which can open onto the circulation. Openings in the slabs covered with gratings in the circulation areas allow the passage of air from the ground floor drawn upwards and widen the mechanism.

These air movements naturally ventilate the high school spaces. Its passive comfort is produced by the implementation of wind trap devices in the upper part which cool the air in the event of wind and by the natural convection of the air taken in the lower part and attracted by the slots in the roof.c) HANG THE HORIZONTAL SLIDING WINDThe “gills” or roughness of the facadesOn the facades of the entire high school,project extensions capture the east wind and provide additional natural ventilation to the through classrooms.The unmarked rooms are all through,allowing natural ventilation accentuated by the projections in the direction of the wind between the windows on the exterior facades and the vertical frames and the doors opening onto the passageways or interior circulations.This natural ventilation is intensified by the projections in the direction of the wind: “the gills” allow the use of classrooms without cooling for a good part of the year.Only classrooms located in parallel strips perpendicular to north-south are cooled by mechanical means during the summer months. The circulations are cooled by the wind movements inherent in the wind towers. This minimization process allows the building to be self-sufficient in energy with solar panels most of the year.On all facades, the windows are either protected by a horizontal steel sunshade, Corten shade, 160 cm wide, or placed behind a concrete bow window projecting 75 cm open completely to the East to capture the North East wind.. This thick concrete shutter protects from the strong light of Djibouti in the same way as the horizontal sunshades but also serves as a wind sensor element, these are the elements that we call “the gills”.

6 ) Conclusion Does it work?

a) WIND AS THE IDENTITY OF THE CAMPUS

All these repetitive and functional elements give the identity of the Lycée français de Djibouti. Although autonomous and different, the three distinct poles have the same architectural expression giving an intrinsic unity to the Lycée français Joseph Kessel de Djibouti as a uniform campus adapted to the climate.

The same repetition of projecting elements making it possible to capture the East North wind and to ventilate the rooms is found in the nursery and elementary schools with wind towers which make it possible to amplify the natural cooling process.

As on the high school, an over-roof which accommodates the photovoltaic panels on the insulated roof makes it possible to block more than 60% of the sun’s rays. The photovoltaic panels will be sized for the entire cold supply in autonomous units powered by photovoltaic sensors and part of the lighting.

In building design, the effect of wind, the venturi effect and the effect of wind towers combine.

By coupling these three phenomena so that they reinforce each other, we arrive at the design of free cooling * by natural cooling complemented by increased solar protection of facades, sunshades, projections, and roof coverings which move forward, protected passageways along the buildings, and the creation of in-between spaces which create shaded spaces.

* Passive cooling technique where fresh air ventilates a room or building, in addition to hygienic ventilation (with or without mechanical support). Additional fresh air is introduced into the building and the feeling of cooling due to air circulation is increased.

Team:

Architects: R plus S ARCHITECTURE (Richard + Schoeller Architectes)

Structure: batiserf structure

Photographer: Sergio Grazia

Materials Used:

Facade cladding: stone egyptian

Flooring: tiles Marazzi

Doors: wwod Dorigo Italy

Windows: steel and aluminiumMingley ( china)

Interior lighting: LED OPPLE

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