Corporate Green

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Hamilton Associates has designed an office park with a Roman-style ventilation system. It's remarkably innovative, finds Hattie Hartman

A speculative office park on a greenfield site is not where you'd expect to find a radical approach to heating and cooling. Indeed, the first phase of the Easter Group's 35-hectare Business and Technology Park at Butterfield, Luton, by Hamilton Associates may look like no more than conventional corporate architecture, but its predicted annual CO2 emissions are a respectable at 27.5kg/m2.

This low figure is the result of an intelligent combination of off-the-shelf materials, a carefully considered natural ventilation strategy, and an innovative system that includes up to 80m runs of underground concrete pipes which replace conventional air conditioning. Paul Hartley of Hamilton Associates insists this system is nothing new. 'The technology is very mundane,' he says. 'It's basically what the Romans were doing 2,000 years ago.'

Phase 1 of the scheme, entitled The Village, was completed in August and comprises approximately 1,700m2 of office space in five buildings. It has since achieved a BREEAM 'Excellent' rating. The environmental agenda for the site was led by the client and its service engineer, Atelier Ten, who spent a year researching and costing various options for the site, which included photovoltaics, wind turbines and a CHP plant. In the end, capital intensive renewables were ditched in favour of a concrete structure for thermal mass, and natural ventilation.

Air is pulled into the plant rooms from concrete ducts and delivered to the offices

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The decision to naturally ventilate the offices was controversial and was taken before sustainability became a hot topic. 'This was a brave move for Easter Group two to three years ago,' Hartley explains. The biggest hurdle was overcoming commercial suspicion of non-air-conditioned buildings. Atelier Ten wanted to counter the frequent tendency of tenants to fit out naturally ventilated buildings with air-conditioning after the first hot summer. To promote their vision, Easter Group hosted breakfast meetings for letting agents, where Atelier Ten explained the strategy.

The buildings have a concrete structural frame with exposed concrete ceilings for thermal mass. The width of the buildings was kept at 13.5m to ensure good crossventilation. Operable windows were installed along the top sections of the aluminium curtain wall system and feature automated sensors that open them at night to draw cool air across the underside of the slabs. The windows also have blinds which are automated on the south and east facades to control solar gain.

The most unusual aspect of the project is the use of external concrete earth ducts, which Bellew refers to as 'de-coupled or non-room based' thermal mass. 'We have a lot of experience with thermal mass – exposed slabs would not have been enough to control peak summer temperatures,' explains Bellew. 'Earth duct and earth tunnel installations are very common in Germany, and it seemed to me that there was no reason why they shouldn't be applied here.' Bellew first learned of the systems from Stuttgart-based engineer Thomas Auer of Transsolar, who is a fellow teacher at the Yale School of Architecture.

On the Butterfield site, the team located a system of 900mm-diameter ducts at 1.2m below ground, where the earth maintains a relatively constant temperature throughout the year. Fans in the two-storey stacked plant rooms located on the ends of the buildings pull in fresh air from free-standing ventilation shafts located in the car park. The air is then distributed into occupied spaces through floor diffusers from belowfloor plenums.

The curtain walling incorporates windows operated by automatic sensors to optimise natural ventialtion. Blinds on the south and east facades are automatically operated to control solar gain

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In summer, cool air from the concrete ducts is introduced at night until the building reaches its target temperature of 20°C, eliminating the need for mechanical cooling. In winter, air in the ducts is warmed by the earth, which can be 12-16°C warmer than outside, before passing over auxiliary heaters and being distributed into the interior.

Design simulations by Atelier Ten estimate that the energy consumption of the buildings will be about 80 per cent less than comparable air-conditioned offices. Other sustainable aspects of the scheme include the use of recycled aggregate in the concrete and low-energy light fittings on automated sensors throughout. Surface water was a major issue, as no outflow of water from the site was permitted, so careful site engineering by Waterman Burrows Crocker – using a series of open ditches, attenuation ponds, and rills and swales – was a key aspect of the masterplan.

Four out of the five Phase 1 buildings are now occupied and the fifth is committed. Phase 2 will break ground in mid-2008 with a combination of concrete ducts and ground- source heat pumps because space constraints and ground conditions limit the applicability of a concrete duct system.

The Easter Group is already funding a post-occupancy study at the University of Brighton to monitor occupant comfort and energy performance – a signal of the developer's commitment to the strategy. Despite its uninspiring architecture, this project is noteworthy because it has taken its environmental agenda seriously, from initial site planning to post-occupancy. It is proof that green buildings are no longer one-off demonstration projects but are becoming part of the mainstream.