The Eastgate Centre in Harare, Zimbabwe, stands as an iconic example of sustainable architecture in Africa. Designed by Zimbabwean architect Mick Pearce, this shopping center and office block demonstrates an innovative approach to natural cooling, drawing inspiration from the ingenious temperature regulation systems of termite mounds.
Eastgate Building in Harare, Zimbabwe. Designed by Mick Pearce.
The Vision Behind Eastgate
In 1991, the multinational Old Mutual investment group approached Mick Pearce with an audacious assignment: to construct a retail and office complex called the Eastgate Centre in Harare. At 55,000 square meters, it was to be the country’s largest commercial building. However, Old Mutual wanted to avoid the high costs associated with air-conditioning such a massive space.
Pearce found inspiration in the termite mounds that dotted the savannas across Zimbabwe. These mounds, some reaching several meters in height, house millions of termites, fungi, and bacteria. Researchers, influenced by Swiss entomologist Martin Lüscher, believed that the mounds acted as natural air conditioners, maintaining a comfortable temperature, humidity, and oxygenation by continuously exchanging hot air from within the colony with cooler air from the surface. Lüscher's theory, which gained prominence in 1961, suggested that the mounds' height facilitated this convective exchange.
Imagining the mounds as air conditioners, Pearce designed a masonry-insulated building with large, open spaces, intricate ductwork, and tall, heat-exchanging chimneys. The ducts would channel air through the building, while the chimneys would vent heat from the occupants and machinery during the day, cooling the building after dark. Pearce even labored at the construction site, helping to cast and install masonry blocks to ensure the design was executed perfectly.
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Diagram of a termite mound, illustrating how it regulates temperature and air flow.
Biomimicry in Action
Eastgate was the first building in the world to mimic termite architecture in its cooling. Termite mounds are constructed like a chimney. The mound warms up in the sun during the day, and by nightfall the air inside the mound is warmer than outside.
Eastgate uses the same technique. Warm air is drawn up through distinctive brick chimneys on the roof, pulling in cool night air at the bottom. The cool air is captured in underground gaps and released into the building during the day through networks of pipes and tunnels.
Termites aren’t the only inspiration from nature. The architect, Mick Pearce, also looked at cacti. One of the ways that a cactus can resist high temperatures is that its many wrinkles, ridges and spikes increase its surface temperature. That makes it easier to disperse its heat at night.
Along with nature, Pearce also draws on traditional patterns and stonework to create a building that is uniquely Zimbabwean in its design. The interior uses the glass and steel of modern urban highrise buildings.
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Mick Pearce has a series of other buildings around Harare that demonstrate intriguing biomimicry ideas, as well as buildings in Australia and China.
Key Design Elements:
- Massive foundational concrete slabs act as heat sinks, storing thermal energy during the day and releasing it at night.
- Duct-laced atriums and soaring chimneys facilitate natural airflow.
- Building materials with high thermal capacity help regulate temperature.
The Eastgate Centre's design is a deliberate move away from the "big glass block". Glass office blocks are typically expensive to maintain at a comfortable temperature, needing substantial heating in the winter and cooling in the summer. They tend to recycle air, in an attempt to keep the expensively conditioned atmosphere inside, leading to high levels of air pollution in the building. Mick Pearce, the architect, therefore took an alternative approach.
To work well, the building must be very carefully designed. No direct sunlight must fall on the external walls at all and the north façade [direction of summer sun] window-to-wall area must not exceed 25%. They asked for a balance between artificial and external light to minimise energy consumption and heat gain. They said all windows must be sealed because of noise pollution and unpredictable wind pressures and temperatures, relying on ducted ventilation. To help with this last, the windows have adjustable blinds, but Pearce also used deep overhangs to keep direct sun off windows and walls.
Further, passive cooling systems are particularly appropriate for this part of Africa because, long before humans thought of it, passive cooling was being used by the local termites.
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Eastgate comprises two buildings side by side linked together by a glass roof. Below this, steel bridges and lifts suspended on cables from steel lattice beams span over the atrium below. The lifts connect with a suspended glass skywalk which runs the length of the atrium at level 2. Along the ridge of the red tiled roof are 48 brick funnels topping internal stacks which pull the exhaust air out of the seven floors of offices below. Under the office floors is a mezzanine plant room behind the cross chevron screen where 32 banks of low and high volume fans draw air from the atrium through filters.
This air is pushed up through the supply section of vertical ducts in the central spine core of each office wing. From the duct the air is fed through the hollow floors to low level grilles under the windows. As it is warmed by human activity it rises to the vaulted ceiling where it is sucked out via the exhaust ports at the end of each vault through a system of masonry ducts to the exhaust sections of the central vertical stacks. The sandwich of the vaulted ceiling and the voided floor above acts as a heat exchanger. The cold night air passing through the void festooned with concrete teeth removes the heat of the previous day and on the following day warm external air is cooled about 3°C by the same teeth before entering the room.
Normally the high volume fans run at night to give ten air changes per hour and low volume fans run during the day giving two air changes per hour. By timing the change over from low to high air velocities the optimum use of the diurnal swing of the biosphere can be utilized.
The engineers, Ove Arup & Partners, have installed a data logger which continuously records air temperature at five critical positions.
Eastgate is emulated by London's Portcullis House (2001), opposite the Palace of Westminster.
Diagram of the natural cooling system in Eastgate, inspired by termite mounds.
Performance and Impact
Eastgate uses 35% less total energy than the average consumption of six other conventional buildings with full HVAC in Harare. The saving on capital cost compared with full HVAC was 10% of total building cost.
The buildings performance has been equal to or slightly better than that originally predicted by Ove Arup the Engineers.
The data logger graph shows that in average conditions covering ten months of the year 3°C of cooling between outside and inside temperature is achieved. Optimum cooling is achieved when the external night temperature falls below 20°C. The peak internal temperatures recorded at 4:00pm on occasions during 2 to 3 weeks of the whole year have been up to 27 - 28°C on certain days within that period. Usually during the rains in November onwards cloudy nights are followed by cloudy days and the internal office peak temperatures remain below 26°C.
People and machines add 1.5°C to the internal temperature each day which if not flushed out each night tends to accumulate until the week ends when the building cools down sufficiently. However, by altering our control system we have reduced this week day build up.
The difference between low level and high level temperatures in the room is 1.5°C due to stratification in a 3m high room.
The $35 million building saved 10% on costs up-front by not purchasing an air-conditioning system.
Eastgate Centre Performance Data
| Metric | Value |
|---|---|
| Energy Consumption | 35% less than conventional buildings |
| Capital Cost Savings | 10% of total building cost |
| Average Cooling | 3°C difference between outside and inside |
Awards and Recognition
Eastgate's innovative design has garnered international recognition, including:
- International Council of Shopping Centres Certificate of Merit 1997
Eastgate in Harare is an expression of two architectures; the new order of brick and reconstructed stone and the old order of steel and glass. The new order moves away from the international glamour of the pristine glass tower archetype towards a regionalized style that responds to the biosphere, to the ancient traditional stone architecture of Great Zimbabwe and to local human resources.
In the new order massive protruding stone elements not only protect the small windows from the sun but also increase the external surface area of the building to improve heat loss to space at night and minimize heat gain by day. These are made of precast concrete, brushed to expose the granite aggregate that matches the lichen-covered rocks in Zimbabwe’s wild landscape. The horizontal protruding ledges are interrupted by columns of steel rings supporting green vines to bring nature back into the city.
The old order comprises the lattice steel work, the hanging lift cars, the glass and steel suspension bridges and the glass roof. It is the architectural expression of the technology brought to Zimbabwe by the mineral hungry settlers in the late 19th century.
How Termite mounds Cool Itself | Biomemetic architecture: Zimbabwe Eastgate center
Sustainable architecture must satisfy the needs of present users without diminishing the prospects of future generations. It must also be embedded in its natural and social environment.
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