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Model demonstration project - Butterfield Business ParkOctober 2008

Roderic Bunn visits The Village at Butterfield Business and Technology Park near Luton to find air conditioning ditched in favour of ground-coupling.

One of several air intakes that are dotted around the car park at the Office Village
Environmental responsibility, natural ventilation, openable windows, passive cooling - not the sort of words one would have found in a developer's promotional literature for high quality office space a few years ago. How things have changed.

And indeed, how fast things are changing. A problem with rapid change is a tendency to grab the solution that offers the biggest bang for the buck, not necessarily a solution that proves the most robust, simple to build and easy to maintain.

So on the one hand we have biomass boilers and heat pumps running off boreholes - renewable energy, yes, but also risky from many standpoints - while on the other hand there's the approach pioneered at Butterfield Business and Technology Park: ground-coupled ventilation ducts made from concrete drains.

It doesn't get much more complicated than that, really. But of course using the stable temperature of the ground to heat and cool incoming ventilation air requires a good knowledge of thermodynamics, plus a lot of computer modelling to get it right.

Property developer Easter Developments opted to use earth ducts for the first phase of its business and technology park on the outskirts of Luton. The development is a gaggle of two-storey 400-800 m2 office blocks designed by Hamilton Associates and consulting engineer Atelier Ten.

The initial success of the earth duct approach will see the system used for subsequent phases, with incremental improvements in the way the system is configured and installed based on lessons learned.

Earth duct principles

Phase 1 of the Butterfield Office Village during its late construction phase. First tenants include the Royal Bank of Scotland and the University of Bedfordshire. Copyright Easter Group.
Atelier Ten has built up a lot of experience with exploiting thermal mass to temper ventilation air, having used thermolabyrinths - serpentine airways created through subterranean concrete undercrofts - for buildings as diverse as the Davies Alpine House at Kew Gardens and Federation Square in Melbourne, Australia.

Thermolabyrinths, though, can be expensive to excavate and to construct. Being usually underneath a building and thermally coupled to it, thermolabyrinths can also build up heat that needs to be periodically discharged.

A simpler way is to use earth ducts radiating into a building from the surrounding area. This involves a concrete or steel-lined ventilation tube, buried about metre underground, that is used to bring a building's ventilation air into contact with the thermal mass of the earth.

Off-the-shelf products offer the most sustainable way of building an earth duct. Concrete drain sections provide excellent thermal mass in themselves, are non-corroding, and can be sealed against water ingress using proprietary gaskets. The slightly porous nature of concrete also helps to control humidity.

Ribbed steel metal tubes can also be used. They're lighter, easier to manoeuvre, and have a higher surface area that encourages turbulent airflow and thereby better heat transfer. The downsides are a risk of corrosion and the relative difficulty of sealing joints.

Atelier Ten went for the concrete option. "We used 900 mm diameter prefabricated concrete pipe for buildability reasons," explained Atelier Ten's Patrick Bellew. "One of our main worries about the clay soil was water ingress, so the pipes are laid to a slight fall back to each end. That way we'll see when water does get in."

The intakes are prominent circular devices dotted around the site. They have heavy metal mesh to prevent creatures from getting access to the system. Hatches at each end enable maintenance people to gain access for inspection.

As mentioned, heat transfer is best with turbulent flow, something that smooth concrete pipes don't encourage. For this reason, Atelier Ten's thermolabyrinths are often constructed from ribbed concrete panels. At Butterfield, the solution was to put the earth pipes through three or four ninety degree bends to encourage heat transfer and non-laminar flow.

This is similar to Termodeck, where research found that heat transfer occurs mostly at the bends. "Our latest computer modelling suggests that using slightly higher velocities and more bends is a better way to go than low velocity and fewer bends," said Patrick Bellew.

Atelier Ten originally modelled the earth ducts using Tas software, which at the time was the only suite to have an earth duct program. The consultant now uses IES with a modified weather file based on ground temperatures. "Modelling the effects of varying the airflow rates is the tricky part," said Bellew, "but we have a little macro that can handle that."

The supply air is drawn from the air intakes dotted around the site, then passed through the concrete pipes to air handling units in each wing of the office development. The air is heated if needed (but not mechanically cooled) by some recirculation and a lphw heater battery. (Thermal wheels were omitted on cost grounds, but have been reinstated for the phase 2 development).

Supply air is injected into the floor void and thence into the office zone from swirl floor diffusers. Design air volumes range from about 1.5 ac/h in winter to about 4 ac/h in peak summer conditions.

The concrete drain sections being placed in the ground. The ducts are about one metre down, lying on about a metre of foundation hardcore. Copyright Easter Group.
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 fa├žades 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.

Facade engineering

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.

Lessons learned

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.

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