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Structures: Smart and cool

The new Centre for Medicine at the University of Leicester will be more than six times as energy efficient as its predecessor. Dave Parker reports.

Construction of the UK’s most energy-efficient large building is well advanced on the University of Leicester’s main campus. The new £42M Centre for Medicine, built to the onerous Passivhaus standards for energy conservation, will replace an existing building and is designed to reduce current annual energy bills by more than 80%.

University of Leicester estates division project manager Dave Vernon acknowledges there is an initial premium to be paid for such high performance levels. “But the university is committed to reducing its overall carbon emissions by 60% by 2020,” he says. “The current Maurice Shock building is energy hungry, so the decision was taken to decant into a much improved alternative next door.”

Leicester Centre for Medicine

Pioneering: When completed next year, the new Centre for Medicine will be four times larger than any other Passivhaus in the UK

Construction of the UK’s most energy-efficient large building is well advanced on the University of Leicester’s main campus. The new £42M Centre for Medicine, built to the onerous Passivhaus standards for energy conservation, will replace an existing building and is designed to reduce current annual energy bills by more than 80%.

University of Leicester estates division project manager Dave Vernon acknowledges there is an initial premium to be paid for such high performance levels. “But the university is committed to reducing its overall carbon emissions by 60% by 2020,” he says. “The current Maurice Shock building is energy hungry, so the decision was taken to decant into a much improved alternative next door.”

Passivhaus targets are intimidating at first glance. To meet them, the project team was ­aiming for a total energy consumption of 80kWh/m², compared to the Maurice Shock’s 500kWh/m², which will slash annual energy bills from £360,000 to less than £58,000. Reductions like these begin with the building envelope: roof, walls and ground floor slab are highly insulated, all achieving U-values of 0.13W/m²k.

“In summer chilled water can be run through the pipes at night, so the chilled soffits can keep the building cool during the day”

Carl Standley, Couchperrywilkes

Airtightness is critical. The building is designed to have a permeability of less than 1m³/m²/h under a pressure differential of 50Pa, a testing target on such a large curtain walled project. High performance triple glazing is used throughout, and there will be a 150m² photovoltaic array on the roof.

Less obvious options were also adopted. Associated Architects project architect Warren Jukes says the orientation of the building was not ideal. “It’s about 28° from the optimum north/south alignment. The constraints on this restricted site from nearby listed buildings meant we had little choice in this, although it made only a relatively small impact on the ability to manage solar gain.”

A design featuring two tower blocks maximises the use of natural daylighting, and an “intelligent façade” with active external blinds will provide essential shading in summer.

The Ramboll UK-designed concrete structural frame looks utterly conventional at first glance, but environmental building service engineer Couchperrywilkes associate director Carl Standley says its thermal mass is one of the key factors in minimising heating and cooling energy demands. “It stabilises the internal temperature profile and reduces internal temperatures in summer.”

Cooling coils

Night time purging is not enough for deep plan internal areas on the ground and first floors, Standley continues. “Cooling coils are embedded in the floor slabs and the soffits are exposed. In summer chilled water can be run through the pipes at night, so the chilled soffits can keep the building cool during the day.”

Precast concrete cladding panels faced with brick slips also contribute valuable thermal mass. Carbon fibre brick ties were specified to minimise thermal bridging, one example of the attention to every detail that achieving Passivhaus standards demands.

Heat recovery from ventilated air is achieved with high efficiency thermal wheel (rotary heat exchanger) technology. Some 30% of the air entering the building, winter and summer, will pass through Rehau “earth tubes” - otherwise known as ground air heat exchangers.

These take advantage of relatively stable subsoil temperatures to pre-cool the incoming air in summer and preheat it in winter, with significant energy savings as a result.

Early earth tube installations were prone to mould growth inside the plastic tubes, Standley says. But the Rehau cross linked polyethylene pipes have a biocidal nanosilver lining.

“We did have a potential problem with the air intakes, however,” he says.

Noxious emissions

These were originally located low down at the front of the building - just about where any exiled smokers would congregate, and there was a risk that their noxious emissions might be drawn into the building’s ventilation system.

“However, half the columns on the front of the building are non-structural and hollow, just there to maintain architectural harmony with surrounding buildings,” says Jukes.

“So we moved the intakes 3m up these columns, which should solve the problem.”

Space heating when needed will be supplied from the university’s existing district heating network. Unlike some earlier, not entirely successful low energy buildings, the Centre will allow occupants to modify the internal environment, with each room having individual controls for heating, lighting and the external blinds.

There are still some uncertainties about how the new occupants will adapt to the building - and vice versa - and what impact the transferred medical equipment will have on the total energy demand. A three year “soft landing” process has been scheduled for the transfer from the Maurice Shock building, and the soft landing team is already in place. Construction is due to finish in August next year.

 

Ticking off the challenges faced by the construction team

Leicester Centre for Medicine

Schematic: Passivhaus standards require technically advanced design

Willmott Dixon operations manager Paul Nesbitt says the biggest construction challenge on the £42M project was not the very constricted site, hemmed in by other university buildings.

“Yes, space is so tight one of the two tower cranes had to be set up within the building’s footprint,” he says. “Its piled foundation had to be carefully co-ordinated with the building’s foundations, but in the event there were no real problems.

“And there’s virtually no room to store materials on site, but we got round that by setting up a yard nearby and organising a just in time delivery system.”
Nor is the structural frame the main challenge. This is a reasonably conventional combination of 400mm square reinforced concrete columns and post tensioned concrete floor slabs spanning up to 6m, all sitting on 450mm diameter, 15m deep continuous flight auger piles.

The clay underlying the site posed no problems in itself: but it was the piling that really had the construction team members scratching their heads.

Nesbitt explains: “The earth cooling tubes, all 1.6km of them, have to be installed to falls 3m to 6m deep. Ideally, you would get the tubes into the ground first, backfill, then bring in the piling rig.

“But the tubes couldn’t take the transferred loads from the piling rig, so we had to think again.”

The solution turned the construction sequence on its head. Piles were installed first, then excavation for the cooling tubes took place around them, leaving the upper 3m or so of piles effectively freestanding until tube installation was complete and backfill could go ahead. Up to 130mm of Styrofoam-A expanded polystyrene insulation was specified for the underside of the ground floor slab and the 1.5m by 750mm by 1.2m deep pilecaps. “The problem here was that we needed very large quantities of the insulation, and it was quite hard to source enough,” Nesbitt reports.

A conventional C50 mix with 100% CEMI Portland cement was adopted for the structural frame to achieve early formwork striking times. Supplies came from readymixer Hope Construction Materials. Post tensioned floors using steel tendons are a well established option for longer spans, but even so, particular care is needed to ensure the tendon ductwork is installed to the specified tolerances. Floors on this project are a typically slender 350mm deep - but construction is significantly more complex than the norm.

More than 7km of 16mm diameter cross-linked polyethylene soffit cooling tubes has to be installed on top of the bottom layer of reinforcing mesh without displacing the ducts, and curtain walling brackets slotted in around the slab periphery. “Design co-ordination was really the key factor,” Nesbitt comments. “It’s a real spaghetti junction in there.”

Accurate placing of the sophisticated curtain walling brackets is essential to meeting the onerous Passivhaus air tightness standards.

Standard Passivhaus practice is to install a separate airtight membrane inside the cladding. On this project, however, the high tech cladding panels from Schuco form the airtight seal, and have to be installed to much tighter tolerances than permitted for the slab construction.

“The answer is brackets that allow installation to plus or minus 1mm,” says Nesbitt.

“The only penetration through the panels are the power supplies to the external blinds - and these are very carefully sealed.”

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