CIBSE Case Study: Natural Ventilation used in BREEAM Outstanding WWF HQ
CIBSE Case Study: Natural Ventilation used in BREEAM Outstanding WWF HQ
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Article from the May 2014 edition of the CIBSE Journal written by Andy Pearson.
An innovative natural ventilation strategy, featuring underground air ducts and thermal cooling, enabled Atelier Ten to achieve a net zero -carbon headquarters building for WWF with minimal mechanical heating and cooling. Atelier Ten's Patrick Bellew explains the BREEAM Outstanding design.
A public car park in Woking, Surrey, is not the most obvious location for a headquarters building. For conservation charity WWF, however, the site was perfect. The organisation wanted its new headquarters building to have a positive impact on the local environment, so placing it on a raised platform above a municipal car park was ideal.
The building's site is not the only unusual aspect of this project. WWF wanted its £13m Hopkins Architects designed home to show others that, through the smart use of design, materials and technology, it is possible to make the building an exemplar of low -carbon office design. To this end, the client's brief was for the office to be certifi ed BREEAM Outstanding, and to be net zero carbon in operation.
The brief was made even more challenging by WWF's requirement for the designers to optimise their selection of components used in the build, based on minimising embodied carbon and whole -life emissions over a 60-year period. "In addition to cost, carbon was made one of the main value systems for the project," says Patrick Bellew, principal of Atelier Ten, the scheme's building services and environmental engineer.
Despite the challenging brief, Bellew describes the scheme as evolutionary rather than revolutionary. In particular, it is an evolution of the solution Atelier Ten developed with Hopkins Architects for Yale University's Kroon Hall. This was a LEED Platinum scheme, completed in 2009, which makes extensive use of timber, and which has won numerous design and sustainability awards. "We already had some of the shorthand resolved: we were looking for a building with high thermal mass, with,principally, underfl oor air-type solutions, and as much thermal storage as we could achieve," he explains.
References to Kroon Hall are apparent in Hopkins Architects' design for WWF's HQ. Like Kroon, the building features an arched, timber roof as part of the architectural statement. In Woking, the huge 80m -long, curved timber grid shell roof is the building's most striking feature, and it defi nes the construction. The roof springs from inclined external steel columns to span the entire 37.5m width of the concrete podium that holds the two-storey building above the car park. Beneath the roof, the building's walls and end elevations are fully glazed. The advantage of being one floor up is that workers on the lower level are at the height of the tree canopy that borders the adjacent Basingstoke Canal. Double doors along the length of the building lead out, either side, onto five-metre-deep balconies, to allow occupants to take in the view. These balconies are lined with planters, and incorporate rainwater-retention tanks for irrigation.
The second floor of office space is tucked under the arched roof. It is formed of two concrete platforms, raised on columns and set either side of a central atrium. There are no glazed walls at this level; instead, the occupants have views of the sky, through three rows of skylights running the length of the building. These are fitted with automated blinds that are controlled by the BMS, based on daylight levels. "We were keen to maximise daylight for health and wellbeing, and to cut down lighting loads, so we pulled apart the first floor to let light flood the lower level," explains Bellew.
The roof overhangs the fully glazed eastand west-facing end elevations to provide shade. Additional protection against glare is provided by angled wooden louvres, tucked beneath the arch of the roof. The louvre blades are inclined at a different angle for each elevation, in response to the angle of the early morning and late evening sun. With so much glazing, the office is a bright, light-filled space, with a daylight factor of four or more over 90% of its floor plate.
The light and airy feel to WWF's new openplan office is in stark contrast to the charity's previous home, a cellular office space on an industrial estate in Godalming, Surrey. The new office has far fewer desks for its 300 staff than its former home, after surveys revealed only about 40% of desks were occupied at any one time; a figure that increased to a maximum of 60% at peak times.
To optimise the use of space in its new 3,700m2 office WWF has adopted a hotdesking strategy based on 200 workstations. "If you had designed the office to fit all the employees it would have been at least 2,000m2 bigger," Bellew explains.
The result of this hot-desking strategy is a more densely populated office and a greater reliance on an effective mixed-mode ventilation strategy. "We wanted the building to be naturally ventilated," says Bellew. However, modelling showed that this would only be possible for about eight months of the yearr, mechanical ventilation is used for the remaining four months.
On a day when the office is naturally ventilated, air will enter the building through small, manually openable windows, set above the glazed doors lining both sides of the lower floor. The air is drawn across the deep-plan office and upwards, around the chamfered edges of the two floating first-floor platforms and into the central atrium. Four roof-mounted cowls, made from recycled aluminium, expel the air. "We modelled the building to show that natural ventilation would function effectively in such a deep-plan building," says Bellew.
The effectiveness of the natural ventilation system is enhanced by the heat-absorbing properties of thermal mass, cleverly incorporated into what, initially, appears to be a predominantly wooden building. The exposed concrete soffits beneath the first-floor office bays are one such area. Less obvious are the panels of Du Pont's Energain phase change material, attached to the underside of the roof behind both the solid timber and acoustic panels, to add thermal mass. "It helps keep overhead temperatures cooler, so means radiant temperatures are lower," explains Bellew. The thermal mass is recharged at night using a night-cooling strategy, with air supplied through the earth ducts and underfloor air system, and expelled via the roof cowls.
Natural ventilation can be increased by opening the rows of double doors leading onto the balconies. Discreet green and red lights above the doors tell the occupants whether or not it is a natural ventilation day, so they know when the doors and windows can be opened. This was a solution Atelier Ten pioneered at Kroon Hall, and which, according to Bellew, has been very successful. In addition to the traffic lights, occupants also get a message sent to their computer to inform them when the building is switching to mechanical ventilation mode.
A displacement system provides the mechanical ventilation. This will operate primarily in winter, when heat recovery is beneficial to energy demand, and in warm weather, when the supply air will be cooled.
Air is drawn into the building through six, 80m-long earth ducts, constructed from 900m-diameter concrete drainage pipes buried in the ground beneath the car park, thus connecting the building back to the earth. The tempered air passes through one of two heat-recovery air handling units (AHU), situated in the car park beneath the office, and the primary distribution ductwork, concealed within a 500mm-deep floor void. Variable air volume (VAV) boxes control the quantity of air supplied to floormounted swirl diffusers. Air is returned to the AHU through grilles in the ceiling, above a series of first-floor meeting rooms. "The system works because the office is so open plan," Bellew says.
The ventilation system runs all year round to supply fresh air to a series of first-floor meeting rooms and a 150-seat conference room on the entrance level. However, the VAV boxes supplying the main office floors are programmed to switch off when the building is in natural ventilation mode, and the speed of the supply air fans will wind down accordingly. Heating and cooling to the AHUs is provided by a Groenholland ground-source heat pump connected to 20, 100m-deep boreholes. The heat pump uses two separate plate heat exchangers, one for heating and one for cooling, to deliver heating and cooling to the space. "In winter we draw heat out of the ground, in summer we use the ground as the heat exchanger condenser," explains Bellew.
The building's heat demands are minimal; the large areas of glass, however, mean that heat gains have to be carefully controlled to keep cooling loads to a minimum. Solar gains on the glazed southern elevation are kept low by a projecting shelf, and by the louvred brise soleil on the end elevations. The engineers were helped in their mission to create a comfortable internal environment by nature: the tree canopy provides shade in summer to keep heat gains low; in winter, when leaves have fallen, the trees allow passive solar gain.
The lighting load is only 5.5W/m2 installed capacity, Atelier Ten made extensive use of LED lamps linked to movement sensors, and perimeter daylight-dimming controls.
Power for the scheme is from Thamesway Energy's CHP plant. It also has photovoltaic (PV) panels: 410 covering 510m2. The two rows of panels are positioned between the skylights on the curved roof, which is on an east-west axis, meaning some of the PV panels are orientated to face north. Nevertheless, the PVs have an estimated peak output of 55kW, and are predicted to provide about 20% of the building's regulated energy needs from fans, pumps and lighting. Any unused PV-generated electricity is sold back to Thamesway.
The PV panels were one of 1,700 components, including 271 M&E elements, that were assessed in terms of embodied carbon by Sturgis Carbon Profi ling. To pass muster, a component's embodied carbon had to be lower than the RICS standardised fi gure for that particular material. For example, the metal-and-timber fl oor was selected because it had the lowest embodied energy of any raised -fl oor system. "If there was a variation in the design, it was assessed in terms of cost and carbon savings," says Bellew.
One of the biggest carbon savings was in the cement used to back-fill the piles dug to house the ground-source loops. Normally a Gunite-type mixture of dry, loose sand and cement is used. However, by changing the specifi cation of the cement, the team has managed to achieve significant carbon savings.
Another major carbon saving was achieved by substituting a natural polymer for synthetic glue in the glulam beams . Bellew says the team saved 42% of embodied carbon on the whole project, which equates to 5,400 tonnes CO2e, at no additional cost.
According to WWF, the basic building cost £2,609/m2 and the services £1,039/m2. Not bad for an exemplar building, which refl ects the WWF's values by treading as lightly as possible on the planet, and car park. CJ
The Turkish Contractors Association wanted its new Ankara headquarters building to be a showcase for construction, and to demonstrate progressive environmental design. It also had to achieve LEED Gold.
Engineers Atelier Ten, working with Avci Architects, developed a solution for the sixstorey building based on exploiting thermal mass, to take advantage of the large diurnal temperature range experienced, year round, in the Turkish capital. There, August temperatures can soar to 34 degrees C during the day, but drop to a chilly 10 degrees C at night as a result of the city's elevation and inland location.
The scheme incorporates a giant labyrinth built into the ground beneath the building's basement to act as a thermal store. During the day, fresh air passing through the labyrinth will be cooled by its contact with the ground. At night, the labyrinth is purged using cool night air to pre-cool it before the start of each day.
Building Services Engineer: Atelier Ten
Architect: Hopkins Architects Partnership
Cost consultant: Gardiner & Theobold
Structural engineer: Expedition Engineering
Whole life carbon assessment: Sturgis Carbon Profiling
Contractor: Willmott Dixon
WWF, founded in 1961, is a global conservation charity, operating in more than 100 countries. It aims to find ways for people and nature to share the Earth's resources fairly. www.wwf.org.uk