Under pressure

Given Nottingham's reputation as a centre for learning about building services, and its published environmental policy, it is fitting that the avant garde services design should underline energy efficiency. The new buildings also underscore an important anniversary in the university's history.

The campus marks fifty years since Nottingham was awarded a Royal Charter as a university. The new buildings are just a mile from the existing campus, and line the north bank of a new lake. They will look out onto views over the lake and a developing wildlife belt beyond.

Scanning from north to south across the campus, its eight buildings comprise: a block of postgraduate accommodation, three faculty buildings and a central teaching block, with a 'learning resource centre' or library to the west, and two undergraduate halls to the east (see site plan).

All but the undergraduate halls were designed by architect Michael Hopkins & Partners, with structural and services engineering by Ove Arup & Partners.

Hopkins and Arup won the contract in a competition in 1996, beating five other design teams. John Berry from Ove Arup says that the low energy, environmental design was a critical aspect of the winning entry, but the architect was unwilling to talk about its work on the campus.

The client actually awarded the contract in 1996: a management contract, with Bovis Europe appointed construction manager. Design work ensued through 1997 and work on site began at the end of that year.

The total cost of the main buildings will be £25.5 million. Of this, about £0.75 million has come from an EC 'Thermie' grant intended to support technologies that are too expensive to meet commercial viability.

The brief and scheme

The brief was encapsulated in the design competition. It dictated that buildings should be naturally ventilated "wherever possible" and low energy. Berry says that, "it was not too prescriptive – a good thing because there is no single set of criteria you can apply to all buildings to end up with a low energy building." The campus occupies a 30 acre site on the western edge of the city – once the home of Raleigh's bicycle factory. The architect had to resolve the tension between large warehouses and gas-works to the north of the site, and terraced housing to the south.

Hopkins & Partners designed the main buildings to butt up against the new lake. The three faculty buildings and the central teaching block consist of three wings, connected by full height sloping atriums and open courtyard gardens.

Landscaping, the lake and the wildlife area are very important aspects of the scheme, and tie into the University's conservation policy. The planting and habitat for wildlife even extends to the roofs of the faculty buildings, which have been planted with a grass-like tundra of moss and lichen.

These grass-covered roofs also add thermal mass to the top floor – often the most vulnerable to temperature swings – and so stabilise temperatures. Structurally, the grass roof is no heavier than a conventional roof, as only 50 mm of earth is required for the tundra to take root.

The greenery blends well with the cedar cladding system used on the faculty buildings. This was sourced from certified sustainable forestry in Canada. While this may not have been the closest source of cedar, Berry says it was "probably the most economically advantageous".

The three faculty buildings – education, computer science and management and finance – presented the main opportunities for low energy services, and therefore formed the focal point for Arup & Partners' design. They have three floors and fairly narrow plans of between 12 m and 18 m. Interiors have 2·7 m high ceilings, and about 50% of the facades are glazed.

The central teaching block has three progressively large lecture theatres stacked on top of each other. They seat 100, 200 and 300 students respectively. Beside the lecture theatres are two blocks housing large seminar rooms, and in the shadow of the theatres is a glazed break-out courtyard with shops.

Between the school of computer science and the faculty of education is a large, double-glazed atrium to be used for catering. This attractive 18 m wide space will form one of the hubs for social activities on the campus.

The buildings' walls are insulated with Warmcell – made from recycled paper – while polystyrene is used to insulate the floors and roof. The earth roof offers additional insulation against solar gains and stabilises the roof slab. Resultant U-values beat Building Regulations requirements by nearly 50%: from 0·22 W/m²K (roof) to 2·4 W/m²K (windows).

Ventilation strategy

Arup built on its previous experience from three pieces of work, all with Hopkins and Partners as architect. First, the Inland Revenue building, which is also in Nottingham. Second, it took lessons from the Parliamentary building – which uses an elementary low pressure ventilation system – and from Saga Group headquarters on the South coast.

Third, Arup used conclusions from its EC-funded research into solar and wind energy, including wind tunnel testing of different shapes of wind turrets.

Taken together, explains Berry, these suggested that a low pressure mechanical system tied to heat recovery would give better energy performance than natural ventilation. There was another reason for steering towards mechanical ventilation for the Jubilee Campus: it seemed unlikely fresh air would penetrate three atrium-linked wings without help.

Air is introduced directly into the roof-mounted air handling units, where it passes through an electrostatic filter. From here it is blown down through vertical air shafts, into traditional floor voids and on into teaching rooms via low pressure floor diffusers.

Instead of aiming at a particular volume of air per occupant or a specific air change rate, the system supplies 2·5 litres/s for each square metre of room space.

Exhaust air leaves the rooms through the corridor extract path – using a low pressure, attenuated flow path designed to preserve acoustic separation. Air then rises through the stairwell, to return to the ahu for heat recovery or evaporative cooling, finally being jettisoned outside through the cowl.

The design hinges on efficient mechanical ventilation which circulates 100% fresh air all year. The ventilation is efficient as the design team put so much effort into keeping down pressure drops, so reducing the fan power required to move air through the buildings.

The engineers also used extra large air paths to feed the low pressure ventilation system around the buildings. Builders work ducts from the ahus carry air with velocities as low as 1·5 m/s. These ducts lead into floor voids which, at 350 mm, are again larger than standard systems. These inevitably carried a cost penalty, which was covered by the Thermie grant.

A sophisticated sequence of bypassing plant when it is not required also helps cut pressure losses in the ventilation system. Some pieces of plant, like an evaporative humidifier, create a pressure drop of 70 Pa, says Berry. So why burden the ahu by running air through a humidifier constantly when it is actually used for only 50 hours a year? Instead of using a heating coil in the system, which again would create a pressure loss, Arup designed a balanced flue boiler in the air chamber – working off-stream, like an electric fan heater.

The designers' success is clear from the pressure drop figures. While a typical system might handle 1200 Pa to 1600 Pa, here the fans deal with from 280 Pa to 340 Pa, according to the season.

Although the engineers anticipate buildings will be mechanically ventilated for most of the year, they have included a control mode for genuine natural ventilation: using windows alone to introduce fresh air, then relying on the stack effect to pull it up through the stairwell chimney.

This may be an option for mid-season, when external temperatures are between 18°C and 25°C. However, Berry harbours doubts about whether the building will ever actually be used this way.

The central teaching facility has similar air handling plant, but air is cooled and delivered beneath the seating. For the atriums, meanwhile, ventilation is independent and unpowered: they use natural ventilation only.

Apart from the catering facility, all atriums are single-glazed and unheated. They have profiled nose-shaped south facades, intended to funnel more air inside.

There are no plans to test airtightness of the buildings, although ducting and floor voids have been pressure-tested. Berry explained that careful detailing and materials choice should keep infiltration rates low.

The cowls’ charm is undeniable, but their green message outweighs the effect on fan power.

Air handling with the wind

At the heart of the ventilation system are eight modular air handling units, purpose-made by ABB Flakt. These are located atop the stairwells of each of the faculty buildings and the central teaching facility.

Externally each ahu looks identical, however internally the sizes of components differ according to the volume of air required to serve the spaces. This proved to be a "very economical" way to manufacture the units, says Berry.

Revolving steel cowls sit above the ahus, using the wind to extract exhaust air. These 3·5 m turrets were custom-made by Gill Air Ventilation, and turn in the wind so that the exhaust vents always face downwind.

The cowls were tested in a low-speed wind tunnel, and found to revolve with wind speeds of as little as 2 m/s. These tests also revealed that the cowls were stable in winds of 40 m/s and over.

Berry describes the cowls as "architectural engineering": the designers were as conscious of the aesthetic side as they were its technical characteristics.

He acknowledges that "the reality of wind in suction mode provides only a very small force indeed". Indeed, his figures show that the fan energy saved by using the cowls is less than 1% of the total fan power.

"It would never be justified on financial grounds for the suction power alone," he says. Here, though, the cowls act as attractive lids for the stairwells, which presumably helped win a thumbs-up from the quantity surveyor.

Heating and cooling

Heating and cooling systems in the building are integrated – both make use of energy stored in the building fabric, and both use the thermal wheel to regain energy that would ordinarily be lost outside.

The campus's requirement for heating is delivered by condensing gas boilers on the roof of each building. These feed standard radiators with thermostatic valves, sized to ensure that return temperatures are low enough for the boilers to function in condensing mode.

Incoming fresh air is heated to 188C – mainly by passing through large thermal wheels which are claimed to recover 84% of the energy in exhaust air. When external temperatures fall below 2·38C, this is supplemented by a 30 kW gas-fired boiler mounted on the wall of the ahu. Arup's thermal modelling, using weather data for Nottingham, suggested that this boiler would only be used for 10% of occupied hours (7.00 am to 6.00 pm).

Looking to the other side of the temperature band, when external temperatures rise above 248C, night cooling is expected to meet the bulk of the cooling need. In night cooling mode, the fans move double the normal volume of air: 5 litres/m²/s.

The cooler outside air lowers the temperature of the building fabric as it passes along the slabs in floor plenums. This helps to cool the exposed soffit ceilings found in most teaching areas. These cool surfaces subsequently make the rooms more comfortable the following day.

Evaporative humidifiers in the ahus cut in for additional cooling – initially with an internal setpoint of 228C. The thermal modelling predicted that this 'adiabatic' cooling will be needed for just 0·8% of occupied hours, and will result in a temperature drop of 48C on the hottest days (see 'Arup's charming models').

Some areas of the central teaching facility will have to cope with a high density of occupants and considerable IT loads, so Arup decided some form of mechanical cooling would be needed. Rather than using a reverse cycle heat pump to dump heat into the lake – an energy efficient way to resolve the problem – designers opted for conventional fan coils linked to a packaged chiller unit, which is sited in one corner of the ground floor.

Electrical engineering

The ring main supply from East Midlands Electricity brings electrical power to the Campus. The 11 kV supply is metered at high voltage in the first substation, then extended on the client's side to serve two more substations located on the site perimeter.

Power is transformed to 415 V three-phase electricity in each substation, then fed via a switchboard to busbar risers in individual buildings. Transformers are housed in the substations to save space in the main buildings. Each floor of the faculty blocks has its own distribution board with a 63 A miniature circuit breaker – the board sized to provide a minimum 15% spare load capacity.

In addition to lv electricity from substations, the busbars take power from photovoltaic cells. The cells are wired in series as a string, feeding into a rectifier/switchboard that plugs into the busbar. Arup calculated how much electricity would be used by the ventilation fans over the course of the year at 51 000 kWh. The designers found that 450 m² of BP's high efficiency monocrystalline cells would meet this energy requirement entirely.

These cells are the most durable – and most expensive – type of pv. They would not normally have been economically viable. Fortunately, the full cost was entirely covered by the Thermie grant.

Batteries of ten pv cells are sandwiched between two sheets of glass in the roofs of four of the campus's atriums. The 54 kW peak capacity – too little to sell into the grid – is all used on site.

Clearly, most of the pv-generated energy is available in the summer months – when the university may be only lightly occupied – while the ventilation plant runs throughout the year. The timing of fan demand and pv supply will not, therefore, match very well and the pv energy will actually be used wherever it is required on campus.

Let there be light

The buildings welcome natural light into spaces where it is needed, while deterring too much light from entering when it's inappropriate. Monodraught light tubes, for example, are used to guide daylight into the centre of top floor seminar rooms.

Fixed horizontal louvres covering the top half of east and west-facing windows help to reduce internal gains and cut glare. The louvres' white surface works in tandem with internal light shelves to bounce natural light deeper into teaching rooms.

The south facades of the buildings also feature motorised awnings which project from the building to cut glare and reduce solar gains in summer. Berry expects top-ranking professors to grab these prime locations with good views.

Inside the teaching rooms is a secondary defence against glare that manages to penetrate the louvres: manually-controlled motorised blinds.

Control for electric lighting, meanwhile, comes from a stand-alone intelligent lighting management system linked to passive infra-red movement detectors in every room. In addition, custom-made luminaires lining the perimeter of glazed rooms have daylight sensors that dim or turn off lights when sunlight allows. There are no manual light switches.

The twin 36 W luminaires were made by Teison and have high-frequency ballasts. They are suspended from the soffits and spill a little light onto the ceiling, while the majority goes below. Ove Arup claims they result in maintained illuminance of 420 lux using only 8 W/m² of electricity.

The lighting distribution was designed around a flexible grid of cables cast in the concrete slabs. Unused conduit boxes are available around a 6 m x 6 m grid, allowing the lighting to be adjusted as the University's needs change – without unsightly cabling crawling over the exposed soffits.

Viewed as a whole, Arup has produced what looks like a very efficient scheme – assuming the buildings are as airtight as the designers hope. Calculations suggest the faculty buildings will use less than 85 kWh/m² annually – comparing favourably with Econ 19's good practice figure of 112 kWh/m² pa for a naturally ventilated cellular office. Careful design has allowed Arup to trim pressure drops to a fifth of normal levels, resulting in dramatic savings in fan power.