Blowing in the wind

At one time the Dome was little more than a huge Bedouin tent. Admittedly a 50 m high Bedouin tent with considerable structural integrity, but nevertheless more a weatherscreen than a sealed, conditioned environment.

Inevitably, the increasingly elaborate exhibits and structures began to open up to the protected environment offered by Buro Happold's stunning fabric roof. Equally inevitably (and very quickly) the Dome changed from being a simple umbrella to a semi-controlled environment.

So, how to control a volume of 2 100 000 m³? Not surprisingly Buro Happold's senior partner Tony Mclaughlin reached for computation fluid dynamics (cfd) for an initial assessment of the Dome's loads, airflows and temperature profiles. This process was somewhat hampered by the lack of information on the location of exhibitions and a constantly developing brief for the central arena, but at least it gave some background data on how the macro-climate of the vast space was likely to perform.

The designers anticipated a constant level of high heat gains from occupancy and electrical loads in the exhibitions, along with gains and losses caused by solar radiation, air temperature, humidity and wind.

The full 360° cfd simulations (run on a super computer at the Atomic Energy Authority – the only computer powerful enough to cope with something as big as the Dome) enabled the designers to visualise air movement and temperature profiles in slices. This not only informed the powered ventilation rates, but also confirmed that the natural ventilation openings were adequate.

The ventilation strategy

The design philosophy is not to condition the entire space under the Dome, but to provide air movement of around 1-2 m/s. Enough to give a strong perception of air movement and some evaporative cooling during the summer period.

In the early stages it was decided to install inverter controlled extract fans in the Dome's steel support masts. Each fan unit is capable of handling 45 m³/s to give the Dome a nominal one air change per hour.

Fresh air is supplied into the Dome by large ahus located in each of the core buildings. The air is coarsely conditioned (heated or cooled), filtered, and then injected into the outer perimeter at 25 m³/s through two huge 2 m diameter ducts located on the core building roofs. This equates to around 8 litres/s/person at peak occupancy.

The Dome ahus have quite high attenuation to ensure that NR 45 is achieved at 6 m from the discharge nozzle, which essentially equates to the nearest balcony. NR 45 has also been specified at the perimeter wall. The total system pressure drop is about 400 pa.

The six core buildings are themselves equipped with three air handling units (one per floor) to supply and extract filtered (EU6) and conditioned air to and from the catering areas, corporate hospitality suites and public toilets. Generally speaking, the ground and second floors have supply and extract, and the first floors have supply only – the floor being open to the Dome.

Inboard of the raised pedestrian promenade is a central performance arena, this will be ventilated by air handling units which will recirculate and recondition the Dome air. The ahus for this area are being installed in six plantrooms located under seat decking which encircles the performance arena.

Three of these plantrooms serve the upper seating deck, with air supplied through Gilbert slot diffusers in the step threshold, while the remaining three ahus will blow air from under lighting towers which will illuminate the central performance area.

Each extract fan is capable of handling 45 m³/s to give the Dome a nominal one air change per hour

As the Dome ahus only provide around 0·5 ac/h (half the extract rate of the mast extract fans) natural ventilation has been provided to handle the remainder. The air path begins with a 500 mm gap above the Dome's 10 m high perimeter wall which gives a total free area of around 500 m². This is then matched by a similar free area created by 36 motorised louvres encircling the Dome's apex.

The vents modulate on a screw jack mechanism, the motors of which are controlled by a simple timer to provide closed, intermediate and fully open settings. Under normal conditions the louvres are either fully closed or fully open (60°). In the open position an automatic override is linked to a rain sensor (one sensor per three louvres).

This override commands the units to close if rain is detected. After a predetermined time the louvres are then instructed to open to 20°, but if rain is still detected the louvres will close again. This process will repeat until the units return to the fully open position.

The vents are backed up by twelve 5 m³/s extract fans, also located in the dome cap. These will be operated manually to remove humid air from the central performance arena and thereby reduce the risk of condensation on the fabric, when the louvres are closed.

As already mentioned, the design process of the Dome can best be described as fluid, and it is to the credit of the entire Dome operation that design, development and construction could often successfully run in parallel. An example on the ventilation is the increase of the electric heater battery capacity in each ahu from 216 kW to 414 kW. This is to cater for the central open arena, which was originally to be an enclosed exhibition.

Operation strategy

The designers have broken down the seasonal operation strategy into summer, winter and exhibition hours and non-exhibition hours.

During winter (and outside exhibition hours) the ahus will only be on when internal temperature around a core building is less than 8°C. The fans will run on recirculation, again until the local environmental temperature rises to 10°C.

At other times of the year (and during exhibition hours) the operation of the system will be achieved using a single control strategy. Depending on conditions, the cold day strategy will be to run the ahus on full fresh air and staged heater battery heating, with recirculation up to 50% as temperatures drop. Recirculation will only be used if supply temperatures cannot be achieved by heating alone, the right amount of fresh air being the designers' primary consideration.

On hot and hot, rainy days, when the Dome temperature is around 23°C, the system will adopt 100% cooling via a cooling coil. As the chilled water is a constant temperature circuit, the cooling coils will not operate within the temperature band of 19-22°C. Instead, this will be handled by a free cooling cycle.

Although there is provision for recirculation during winter, there is no other means of heat recovery – a function of time, budget and poor payback given the Dome's anticipated design life of only two years. The same reasons apply to the heater batteries which are electric – gas-fired boiler plant was simply uneconomic.

In control terms, the designers have kept things as simple as possible. There is no complex building energy management system controlling the air handling units, nor any of the other hvac services. Instead the Dome's operation will rely on a team of facilities engineers and a monitoring system with a comprehensive graphics front-end.