Property developer Basil Walker had a problem. He needed to be allocated 330 kilovolt-amperes (kva) of power by the authorities to be able supply his proposed office development in Ferndale, Randburg. But in the era of the energy crunch officialdom was prepared to make only 150kva available.
This meant he would have had to build to just half of what was planned or implement a set of energy-efficiency measures that would mean that 150kva would be sufficient to power the building.
Interventions included using special glass that limits the amount of heat going into the building, shades that keep the rays of the sun off the windows, occupancy sensors that switch off the air conditioner and lights if a room is unoccupied, super-efficient lights, daylight sensors that moderate the lighting and an ice maker that uses relatively cheap night-time electricity.
The centrepiece of the energy systems, though, is a rock store. This is a structure about three storeys high with two sides of rocks encased in wire cages or gabions.
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Between a rock and a hard place
The rocks are packed 2,5m thick on each side of the structure, which also has an internal passageway. Large rocks are placed next to the wire. The inside is filled with 38mm rocks, about the size of the stones used on railway tracks. The key energy requirement of the building, an office block, is for cooling.
Walker says that each human occupant generates 100 watts of energy, with desktop computers generating 180 watts each. This can make the inside of the building uncomfortably warm. Walker says that although we have cool night-time air on the Highveld, we do not use it, preferring to use coal-fired electricity to cool buildings during the day.
The rock store has two fans, one which extracts air from the store during the night. This causes cool air to flow in, cooling the rocks. During the day a second fan pulls air over the cool rocks and circulates it through the building.
In March the temperature leaving the rock store during the day was measured at seven degrees cooler than the ambient daytime temperature. But the rock store does not fully meet the cooling requirements of the building; hence the ice maker.
Walker reckons that based on what has been learned from constructing this unit, he would be able to introduce further efficiencies in future units. These include optimising the size of the rock store, refining its design and placing it underground at lower cost.
The Ferndale rock store includes the ability to warm the air, principally by using the afternoon sun. Walker, a quantity surveyor, sees the potential for sourcing warm air trapped in ceilings to heat rock stores during the day. He says there is the potential for the heating and cooling needs of Highveld buildings to be more or less entirely met using natural sources.
The rock store was budgeted to produce a 10% return on investment at a cost of R350 000. The actual cost was R450 000 and the calculations were made before Eskom’s last price hike. With more hikes predicted, the investment will pay for itself.
Walker says electricity costs for the building are low, at about R9 000 a month compared with R21 000 for a similar building across the road. The maximum electricity used to date has been 110kva, one-third of what was originally specified.
Anton Fylinck of Spoormaker and Partners designed the mechanical equipment for the air conditioning.