黑洞社区

• This 拢5.5 million office block on a business park in Leeds has achieved the highest ever BREEAM score, of 87.5%
• Precast concrete construction combined with a TermoDeck ventilation system cools air before delivering it to the office space.
• Sustainable accountability of building systems, construction and off-site manufacture of components taken into account.
• Details of waste heat use, water services, day lighting and sunpath modelling, as well as predictions for the building鈥檚 operation after occupation.

On an otherwise unremarkable office park to the east of Leeds city centre is an extraordinary new building: the Innovate Office. What makes this 拢5.5 million scheme so interesting is not that its carbon dioxide emissions are predicted to be more than 80% less than for a typical office building. It is not even that its environmental credentials are impeccable, having been awarded the highest ever BREEAM score of 87.5%. What is special about this 4500m2 project is that it is a prime example of what can be achieved, both environmentally and commercially, when engineering is allowed to lead the design process.

King Shaw Associates, the engineer responsible for the project, was involved in selecting construction components based on their contribution to the overall scheme. 鈥淲e worked from first principles to examine each element to assess its benefit to the overall building performance, rather than the practice of bolting on 鈥榮ustainable鈥� features to a conventional design,鈥� says Doug King, a founding partner of the practice. This gives a more holistic sustainable solution than is provided simply by adding 鈥済reen bling鈥�, he says.

This engineered solution nonetheless satisfied the client鈥檚 requirement for a cost-effective building, flexible enough to accommodate tenants in serviced office units. In fact, the building鈥檚 green credentials became a big factor in its commercial viability.

鈥淭he reduction in energy consumption represents a saving of approximately 拢1.30 per sq ft each year,鈥� explains King, temporarily slipping into letting agent speak. 鈥淲hen compared to the expected rental returns of 拢12-拢15 per sq ft, the energy savings will make a significant contribution to the scheme鈥檚 profitability.鈥�

Pursuit of excellence

King says the impetus to design a sustainable office building came initially from Rio Architects, which had already completed one office scheme for the client. When approached for this next project it decided to do 鈥渟omething exceptional鈥� with the design to make it sustainable.

When King Shaw was brought on board in autumn 2002 the aim was to achieve a BREEAM 鈥渆xcellent鈥� rating.

The engineering concept evolved over six months of intensive design and complicated computer modelling by IES Consulting. The scheme was then benchmarked against flagship low-energy schemes such as BRE鈥檚 黑洞社区 16, near Watford, and the Elizabeth Fry 黑洞社区 at the University of East Anglia.

The engineer鈥檚 involvement included an assessment of the site topography to decide the most effective location for the building, and orienting the building to make the most of solar gains and minimise overheating. King Shaw even contributed to designs for the building鈥檚 form: 鈥淲e discussed how to mass the building to make best use of daylight,鈥� says King.

Light touch

Daylight was a fundamental concern. 鈥淭his building is so well insulated it has virtually no heat demand, which means it is easy to control the heating but we had to work much harder to control the daylight levels,鈥� says King. The building comprises two parallel blocks 鈥� one two storeys high, the other three storeys 鈥� linked by a glazed roof to form an enclosed 鈥渟treet鈥�. The two wings are offset to create a simple glazed entrance to the south-west corner. 鈥淭he two-wing form with a central street was the result of the need to get enough daylight to the ground floor,鈥� King explains. .

The long axis of the two wings runs north to south, while the two-storey block is sited slightly farther to the east to reduce the building鈥檚 scale when viewed from the access road. More importantly, this orientation allows early morning daylight to penetrate deep into the heart of the building through the street鈥檚 glazed roof. King says the engineers did not want a south-facing elevation 鈥渂ecause the brise soleil would cut out too much daylight鈥�.

In total the window areas cover just more than 50% of the external elevations. Internal blinds are provided to control excess heat gains and to reduce glare. As a result of the engineer鈥檚 attention to daylight, the offices achieve a 4.5% average daylight factor. According to King, this means electric lighting will be required for only 20% of the working year. All fittings include a daylight sensor and presence detectors to ensure the lights do not remain on unnecessarily. .

Pre-cast concrete

The building鈥檚 concrete structure is also a significant factor in its environmental performance. 鈥淚 wanted as much thermal mass as possible,鈥� King emphasises. To enable the building to act as a thermal store King Shaw persuaded the structural engineer, Cameron Taylor, to opt for a panelised structural solution. The design therefore utilises 125mm thick, load-bearing, pre-cast concrete wall panels combined with pre-cast, hollow-core TermoDeck ceiling units (the ground floor is a suspended pre-cast concrete slab but without the hollow core).

TermoDeck is a fan-assisted ventilation system that pushes treated fresh air through a series of ducts formed within the individual concrete ceiling/floor slabs. As the air passes along the ducts the concrete warms or cools the fresh air before supplying it to the occupied space.

Each pre-cast unit incorporates four window units and the wall panels adhere to a 1.5m planning grid. Vertical fins between the windows provide additional solar shading. There is no fenestration to the offices on the north and south facades, which keeps solar gains and losses to a minimum, while the concrete walls and ceiling are exposed internally to give the interior a high thermal mass.

Sustainability has even been carried into the materials used in the offsite manufacture of the concrete wall units. The concrete incorporates pulverised fuel ash cement replacement and Lytag aggregates, and recycled steel reinforcement. The concrete shell is finished externally with a 250mm layer of polystyrene insulation with a white Sto-rendered finish, giving a U-value of 0.15W/m2/潞C, which substantially exceeds the 0.35W/m2/掳C required under the 黑洞社区 Regulations. 鈥淭he insulation makes the building behave just like the old-style electric storage heater,鈥� King explains.

TermoDeck proved to be the best solution to a number of design challenges. There was concern that passive design would not sell, so using TermoDeck has enabled the offices to be marketed as conditioned space. From King鈥檚 perspective, TermoDeck also made sense because the offices鈥� internal heat gains are too high, at the design occupancy density, to be met through passive means alone. 鈥淥ur initial calculations showed a demand of about 150kW of cooling,鈥� he says, 鈥渂ut when you take into account the night cooling using TermoDeck, the cooling load can be reduced to about 50kW, which is well within the reach of an absorption chiller.鈥�

Using waste heat

The idea of using an absorption chiller led to the concept of utilising waste heat from a CHP engine. 鈥淚t brought into play the idea of having a summer-time heat sink for a CHP system,鈥� says King. In fact, two chillers have been installed 鈥� the absorption chiller does about 35kW of cooling while a 20kW electric chiller makes up the remainder of the load because, King says, 50kW absorption chillers were not available.

The principal summer cooling strategy is to use passive night-time cooling to chill the TermoDeck slabs and then to use the building鈥檚 thermal mass to store the additional cooling energy provided by the chiller, which is run 24 hours a day, at periods of peak overheating. 鈥淲e developed this idea of using an optimiser, and a controls strategy similar to that used for an ice-storage cooling system, to run the absorption chiller continuously during periods of likely overheating,鈥� says King.

Understanding TermoDeck鈥檚 performance was critical to the scheme鈥檚 success. IES Consulting developed software specifically to model the heat-exchange process within the slabs. The simulation modelling also allowed King Shaw to derive a novel operating strategy for the building management system based on the detailed analysis of hourly thermal data from the simulation modelling.

鈥淲e worked then with the controls specialist to come up with a strategy to maximise the use of CHP,鈥� says King. CHP does not modulate down; it is either on or off. When the system is on, the entire heat load generated by the CHP engine has to be utilised or the system will shut down. This problem is exacerbated when there is a small demand for heat.

鈥淭he CHP will fire up for 30 seconds, put a slug of water into the system and then shut down,鈥� King says. 鈥淏ut if you wind the system back so that the CHP engine does not keep switching on and off, then the system鈥檚 boiler is likely to fire to meet the heat load 鈥� and so the CHP hardly runs.鈥�

King鈥檚 solution to maximise the use of the CHP has been to put in a wide dead-band (of +/- 1.5C) on all the control set-points. 鈥淭here is a clever little bit of programming that looks at the simultaneous demand right across the building while all settings are in their dead-band,鈥� he explains.

As soon as a zone starts to call for heating, the BMS adds up the available capacity within the dead-band by totalling the heat requirement of all zones that are within that band, to try to create enough of a heating load to warrant starting the CHP engine. Once the CHP has fired up, it will run until all the zones are at the top of their control dead-band before shutting down the system. 鈥淚 estimate the CHP will give nearly 6000 hours of operation because the building has so much thermal lag,鈥� says King.

In winter, the air system is put on full recirculation overnight to preheat the TermoDeck slabs prior to the offices being occupied. The heat recovery air-handling units collect the heat gains from people and computers and store them in the concrete. This heat is then transferred to the incoming fresh air.

Fresh air to the offices is supplied from four roof-mounted air-handling units with fully modulating speed control. These units are housed within fibre-reinforced shrouds, which look like fuel tankers parked in the four corners of the roof. Service risers from these units drop down at both ends of the office wings to supply air to the TermoDeck panels.

Each floor is zoned east to west. This allows part or all of a floor to be shut down depending on occupancy. The 3掳C control dead-band allows variations in office temperature to be accommodated across each zone. And if the occupants need more fresh air, they have the option of opening the windows.

Water works

Water services, too, have been designed with sustainability in mind. The building uses rainwater harvesting, coupled with a vacuum drainage system, to eliminate the use of treated mains water for flushing toilets almost entirely. The vacuum drainage system resembles that used on ships. The vacuum takes waste away so the treated rainwater is used only to wash the bowl.

鈥淭his solution reduces by 75% the overall sewage volume discharged,鈥� says King. An additional advantage of the system is that it has allowed the designers to run pipework uphill and over partitioning.

The building鈥檚 design exploits the site鈥檚 natural gradient. Originally its owner proposed scraping 70,000m3 of topsoil from the site to level it. King says he managed to put a stop to this with just days to spare. As a result the design team has used the area鈥檚 natural fall to their advantage: permeable paving, a lined pond and a wetland area allow storm water to be retained on site instead of discharging into the offsite drainage.

The building鈥檚 significant level of sustainable features soon made it clear the design would get a high BREEAM score. How high became obvious only once the designers had spoken to the BREEAM assessor, who suggested the scheme had the potential to top the league of BREEAM-assessed buildings.

To ensure its success, the obligatory cycle racks were added in the car park, and the scheme鈥檚 mechanical and electrical contractor, Goodmarriott & Hursthouse, donated 10m lengths of ductwork to ensure that the air handling unit鈥檚 supply and exhaust duct were separated by that distance, which gained an extra BREEAM point.

With the project complete, the first tenants are starting to occupy the offices. As a result of the engineering-led design, King predicts that the C02 emissions from the comfort-cooled building will be substantially lower than for a typical air-conditioned office and on a par with the best naturally ventilated buildings. 鈥淎nnual emissions from the building services are predicted to be less than 22kgC02/m2, a reduction of more than 80% compared with a typical office building, saving more than 350 tonnes of C02 a year,鈥� says King.

Such claims help to explain why the scheme won Environmental Initiative of the Year at the 黑洞社区 Services Awards 2007. To see if this engineering-led design solution lives up to those claims, Leeds University will monitor the building鈥檚 performance for the next two years. We鈥檒l keep you posted.