Most offices and shopping centers around the world look the same- glass facades with steel frameworks. These designs use an elaborate HVAC arrangement that is expensive to construct and maintain. They keep recycling air, leading to high levels of air pollution within the premises. While this is true everywhere in the world, a country like Zimbabwe encounters additional problems like import of the system, spare parts, and squandering of foreign exchange reserves. Hence, when asked to design an office in Harare, Mike Pearce, the architect decided to tread the road not taken. Despite being in the tropics, Harare has a temperate climate that oscillates between 10 to 14 °C, making it suitable for mechanical or passive cooling systems. The office and shopping complex, therefore, plays away from the average glass fortress and operates at a level of vernacular architecture that responds to its environment.
Planning of Eastgate center
The mid-rise building has 5,600 m² of retail space, 26,000 m² of office space, and parking for 450 cars. Its style defines “old meets new”, where the traditional stone architecture of Zimbabwe is combining with the modern brick and glass construction. It employs the 19th-century modernist technology of lattice steelwork; glass suspension bridges and glass roof, along with localized stone masonry techniques. It is made up of two buildings connected through a glass roof. The atrium in-between is composing of steel bridges and lifts. The lifts are suspending on cables from the steel lattice beams that run across them.
The lifts are associated with a suspended glass skywalk that spans the atrium at level 2. in succession, the middle portion of the skywalk is hitched to the street level by escalators, where the street leads to the city.
Both buildings have a gable roof, that is lined with 48 brick chimneys to funnel out the exhaust air from the offices below. There is a mezzanine plant room under the office floors. It is present behind the cross-chevron screen along with 32 low and high-volume fans that draw air from the atrium through filters.
Shading strategies at Eastgate center
After computer simulation and analysis, the engineering firm Ove Arup gave Pearce a set of rules. One of the major requirements was to provide shading against direct sunlight. Additionally, the windows need to seal to protect against noise pollution and unpredictable wind pressures and temperatures. They needed to act as light and noise filters, control glare, and offer security to the tenants. Pearce came up with a comprehensive solution to solve them all. Deep eaves (a common feature in Africa) were used for shading from the high summer sun, all the while permitting the low winter sun rays to mellow the insides.
The windows furnishing with adjustable binds and deep precast concrete overhangs regulate sunlight in the interiors. The protruding stone elements (overhangs) also increase the external surface area for increased heat loss at night and minimize heat gain during the daytime. The precast concrete is the brush to expose the granite aggregate underneath to match the lichen-covered rocks of the wild landscape of the region. The horizontal projecting ledges are interrupted by steel columns that support green vines.
Why is Passive Cooling benefitting Africa?
In a hot country like Zimbabwe, cooling a building using artificial means is over-price and impractical. Moreover, the termites of Africa are the supposed masterminds behind the passive cooling strategies we use today. The tall mounds include conducts which release air from the top and sides, while the mound itself is designed to capture the breeze. As the wind blows, hot air from the dominant chambers underground is drawn out of the edifice, aided by termites modulating the tunnels to control airflow. Ergo, it is only appropriate to use the knowledge gained from the termites to the benefit of humans.
Passive cooling Policy
It is a perfect example of biomimicry in architecture where the passive cooling system has been inspired by the ‘self-cooling termite mounds’ found in the continent. Simply put, “Passive cooling works by storing heat in the day and discharging it throughout the night”.
At the beginning of the day, the temperature of the building remains cool. As the day progresses, heat from the sun, machines, and people is assimilating by the fabric of the building. The temperature inside does not rise significantly as the materials have a high heat capacity. As the temperature begins to drop outside. The warm internal air rises and is let out through chimneys on the roof utilizing fans. As a result, cool air is brought via the bottom of the building. This activity extends through the night until an ideal temperature has reached.
Challenges at Eastgate center
- The system, though imperfect is continuously monitoring and refining by a logger set up by Ove Arup. They have identified two areas of improvement namely-
- A) Fresh control systems that take advantage of the uncertain air temperatures prevailing outside.
- B) Promote further research into the designing of concrete floor slabs that optimize heat transfer.
The chart above demonstrates that the Eastergate Centre consumes less than 50 % of the energy used in regular air-conditioned buildings. Further, the control system in the building has deducted. Eastgate uses 35% less energy than the average consumption of six regular complexes in the area. Numbers aside, Eastgate’s true success is its ability to continue operations. When other buildings couldn’t due to reasons such as power shut down or poor maintenance of HVAC systems. The naturally cooled building provides comfortable internal environs for about 52 weeks a year. Such passive cooling through natural convection systems has enabled a 10% ($3.5million) saving on the total cost of the building. This, in turn, leads to 20% lower rents than the surrounding buildings. Therefore, the Eastgate Centre at Harare is better for both mother earth and the people in it.