School of hard rocks

Harare, Zimbabwe's capital of just over a million inhabitants, now has a thriving business district. Manhattan-style air conditioned buildings predominate – evidence of Zimbabwe's urge to copy the west rather than rely on more indigenous and thereby appropriate forms of architecture.

For despite the glitz and glamour of western corporate architecture, Zimbabwe has neither the technical know-how nor the money to run such complex buildings. As for a maintenance philosophy – there isn't one. Add to that the unreliability of the nation's power supplies and the 200% import duty on spare parts, and passive solar design is clearly the only sustainable way forward.

Enter British consulting engineer Mike Rainbow of Ove Arup & Partners, whose Harare office has been involved in designing low energy buildings capable of being constructed to Zimbabwe's limited infrastructure and demanding climate. Ove Arup has worked at both ends of the scale, from high-rise offices like Eastgate, Harare's largest commercial development, to a school for expatriate American schoolchildren.

The American-owned Harare International School was completed in the summer of 1998. The building had to satisfy a number of key requirements, not least the expectation of the expatriate schoolchildren for a tolerable internal climate. Not easy when the climate is sub-tropical and there is a 14°C diurnal swing.

As mentioned, electrical supplies are not the most reliable. Power is imported from the Congo, where Zimbabwe is currently indulging in military action. Construction skills are best described as rudimentary, although on the plus side there is no shortage of labour or building materials.

The designers opted for a simple, rectangular, brick building with a pitched roof and a verandah for shading against low latitude sun. Harare is between 17-18° south of the equator, so the sun is pretty much directly overhead most of the year.

The difficult part was controlling the internal temperature. Rather than go down the route of using split air conditioning units (the norm for Zimbabwe – chilled water circuits being rare) Ove Arup offered to condition the building's supply air using thermal rock stores. Largely sold to the client on the basis of their educational value, the rock stores are externally located, protected from direct solar radiation beneath the Verandah, and adjacent to the classrooms.

Air is drawn down into the rock stores via a ventilation system located in a storeroom. This pumps air to a builderswork duct running parallel to the building, which gives access to steel cages filled with granite pitching stone – each classroom being served by a pair of rock stores.

Early experience shows supply air temperatures 8°C lower than external ambient, and environmental temperature about 4-5°C lower than outside. That, though, is only half the story. What is more interesting is how Ove Arup arrived at this happy situation.

There is little guidance available on the dynamics of rock stores and the rate of heat transfer for different materials, at different air supply temperatures, supply air volumes and velocities. It's often a question of trial and error, and once Arup's had the idea of controlling temperature using thermal storage, it set out to test which materials would provide enough cooling capacity to satisfy the school's hours of occupation, set at 08·00 – 14·30 h every weekday.

The project engineers designed a rock store testbed, in which a variety of thermal storage materials could be tested. This comprised a 100 mm insulated case containing a 1 m3 void, intake and outlet ducts, two fans and temperature recording equipment.

Water-filled cartons worked fairly well, having the virtue of being half the density of rock with four times the specific heat capacity. Leaving aside the required void fraction between the cartons, Arup's estimated that they could get away with half the size of the storage compartment needed for rock.

However, the cartons released their coolth rather too quickly, with heat transfer being mostly by convection rather than conduction. Air was also finding its way through the cartons without actually connecting with them. Time was running out at that stage with the contractors wanting confirmation of what material was going to be used to fill the stores, leaving the designers unable to test other forms of water container. So that was that.

Arup's engineers then tried 40 mm gravel, which again tended to store and release its heat quickly. This would require more store partitions and a system of dampers to switch airflow between them.

The trade-off was clearly between surface area and the heat transfer coefficient. While rounded river stones performed better, dense angular granite proved to have the highest thermal conductivity and the least propensity to turn to sand over 30 years. It was also locally available and very cheap.

Once this was decided the local builder set to dig the rock stores. These are simple 2 m deep brick basements, rather deeper than was required "but the guys just kept on digging," said Arup's Mike Rainbow.

These were then fitted with steel cages and filled with granite prior to being topped off with a concrete lid. Small access hatches give access to the rock stores for any maintenance that might be needed, while the builderswork duct feeding the classrooms with the tempered fresh air doubles as an access corridor. The supply air enters the classrooms through low level displacement grilles. Pressure loss through the rockfill is said to be very low, at less than 10 Pa/m² at an airflow density of 0·25 kg/s/m² (figure 2).

In operation the classrooms are consistently 3-5°C cooler than external temperature, although Mike Rainbow suspects that there is a lot of air leakage in the structure and through the fan, which means the rock stores are not delivering their maximum capacity. The school operates on a night purge cycle which dissipates the heat stored in the rocks and the building fabric.

Although the school only cools down at the very end of the night, that is not normally a problem as external temperatures early in the morning are usually in single figures. As the temperature of the rock store only falls to 20°C, the school gets 4-5°C preheating for the first couple of hours, after which there is a neutral period before cooling is required in the early afternoon. Electric heater batteries have been provided, but they only tend to operate early in the morning, if at all.

There were a few niggles which compromised the rock store's performance. For example, the builder put the insulation between the joists not under the roof tiles, so the air in the pitched roof void tended to get hot and preheated the supply air before it got to the fans in the roof space. The insulation has been repositioned and the supply ductwork has been lagged.

The supply fans are also only achieving 60% of their duty, with appreciable leakage. Preliminary tests show about 2·5 ac/h, but the designers would like to get that up to 3·5 ac/h. Supply air temperatures are being logged, and the designers suspect that higher flowrates will increase the swing in supply air temperatures while producing a greater net cooling input to the classrooms.

Rock thermal storage offers great potential to other similarly resource-strapped countries. Climate is the big decider. Systems like this work best in a continental climate, with enough altitude to ensure clear skies at night and a diurnal swing of about 11K. Mike Rainbow estimates that regions within plus or minus 30° latitude offer the best opportunities, this includes Johannesburg and Pretoria in South Africa, Lusaka in Zambia, Nairobi in Kenya and Addis Ababa in Ethiopia.

Outside Africa suitable conditions exist in many places in Central America, including Mexico City and Guatemala City. Tehran and Kabul are also close to the 30° limit.

Ove Arup, meanwhile, is taking rock stores a step further in Harare in the design of Zimbabwe's new Constitutional Court. Here a series of flap dampers will switch the supply airflow between three rock stores in order to maintain a constant air supply temperature during the day. Figure 3 details exactly how the system will work in cooling mode after midday.

The design and construction of rock stores are well within the capability of the local community, although Mike Rainbow concedes that more research is needed before a passive rock store can be designed without going through all the maths. There are crude rules of thumb – such as one cubic metre of rock for every 10 m² of floor area – but these need to be refined and expressed in different ways for the local builder.