The claimed energy savings over conventional air-conditioning systems are quite considerable. A typical fan-coil air-conditioning system can consume around 225 kWh/m2 per annum for heating, ventilating and mechanical cooling. Using the earth duct approach Atelier Ten anticipates the Butterfield complex might get down to 45 kWh/m2 per annum. If Phase 2 keeps its thermal wheels (hopefully with bypass), then energy consumption might get down to 28 kWh/m2 per annum.
The office floor plates themselves are very simple 13 m-deep zones. They have unpainted exposed soffits for their thermal benefits, and well-engineered façades with openable windows and motorised internal blinds.
The extract path from the open plan offices became subject to value engineering, a process which led to the omission of extract ductwork that otherwise would run down the central spine of the offices, removing polluted air near the source. The ducts were replaced by simple air return-grilles located at high level in the walls next to the plantrooms. The approach will still work, as the air volume rates are the same no matter where the point of extract, but how effectively it works at relieving stuffiness when the partitions start going up is another matter.
The building's facades are really the only highly detailed element of the whole installation - and rightly so. The thermal insulation is about 30 percent better than the 2002 Part L requirements, according to Patrick Bellew, and therefore about equal to the 2006 requirements.
Window bays comprise insulated panels at each end, so the actual glass area is only about 40 percent. A PIR-based lighting system has been adopted, with daylight linking control for the two rows of T5 fluorescent lights nearest the windows.
One in five of the clerestory windows on each elevation are motorised under the control of the building management system. The control strategy is to relieve peak summer conditions and to augment the mechanical ventilation system.
There is always a temptation, particularly with design and build it seems, to reach for the simplest and cheapest methods of solar and glare control, and to also value-engineer the ventilation controls to the point where they become marginal and risky. The Office Village has suffered a bit of this, in that the external shading was cut on cost grounds (except for the central wing where it was really needed). This placed more emphasis on the quality of the blinds.
The design team approached this with a commendable attention to detail. The venetian blinds on the east and south-facing elevations go up and down in the usual fashion, but the occupants can also rotate the blades in the lower section of the blinds independently to those in the upper section. This means daylight can enter at the top, while the users can cut out glare at the bottom.
The German-made blinds are also motorised on the east and west elevations so that the BMS can set them to limit solar gains early in the morning (the system also resets the blinds from the previous day). Atelier Ten's modelling simulations had showed significant risk of overheating between 06:00 h and 09:00 h.
Motorising anything in an occupied space is a risky business. People get easily annoyed and distracted by things whirring or grinding into action, so fine-tuning of the blinds will inevitably be necessary. Furthermore, occupants might adjust them to get a view of outside, only for the BMS to close them if it thinks the light in the space is too bright. Those kinds of conflicts will need managing during occupation.
Completed in October 2007, Phase 1 of the Butterfield Office Village is only partially occupied, so there's not enough data to confirm the site's sustainability credentials.
The buildings have yet to experience peak summer conditions, and a cold and wet 2008 may not provide it. However, the client is sufficiently happy with the system to adopt it for Phase 2, a 9000 m2 mixed industrial and office development including a 4100 m2 warehouse.
The only mechanical cooling is that put in by tenants for communications and server rooms, the heat rejection equipment already being located rather inelegantly by outside walls. If only the thermolabyrinth approach of ribbed concrete panels could be used in server rooms as thermal soakaways, with mechanical or stack ventilation doing the rest. Heat from plug-in power may not be a consequence of design, but it's all carbon dioxide, no matter how it's generated.
Everyone involved learned how not to dig the trenches for the earth ducts. "The idea was to carry on building the buildings at the same time the trenches were excavated," recalled Bellew, "but by the time we'd criss-crossed the site with trenches it all became a quagmire and caused a few problems. They also started at both ends and arrived at the middle, which meant they ended up with a big bog. The next time we do this we'll probably work outwards from the site."
Theoretically, earth ducts are robust enough to outlive the buildings they serve, and could therefore be used to serve the next generation constructed on the site. This makes them more sustainable than other renewable energy systems. Even high efficiency air-conditioning kit will be replaced within 15 years.
"Here, we have a small fan in an air handling unit and that's it - no chillers," said Bellew. It will be interesting to see if cooling coils end up being installed, particularly as summer temperatures begin to climb during the following decades.