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Floors and flexibility

technical

The Boots Library at Nottingham Trent University was taken to planning permission stage and the exterior designed by James McArtney Architects. Here we examine ecd Architects' working out of the environmental agenda, focused on the library interior.

It is becoming a truism that you cannot have a green building without a green client. It can certainly make a big difference. In this case the university had a green commitment, for example adopting several items from the breeam agenda. And it was interested in life-cycle costing, prepared to make some investments within a typical library budget of £10.8 million for 9000m2, to accrue savings over a 25-30-year time horizon.

It had decided that because of the busy roads bordering the site and the noise of vehicles from the local fire station, the building could not be reliant on opening windows for ventilation. To avoid conventional air conditioning, client and designers settled on TermoDeck, a system based on hollow concrete floor slabs through which fan-driven ventilation air is delivered. The slab soffit is left exposed. With the close contact and hence effective heat transfer between air and concrete, the floor's heating and cooling capacity is much greater than the thermal mass of an ordinary exposed concrete floor soffit.

The system has been used four times before in the uk, for example at the Elizabeth Fry building at uea, designed by John Miller & Partners working with Fulcrum Engineering (aj 11.8.93, 15.6.95). The client was involved enough to visit examples in Sweden where TermoDeck systems have been running successfully in offices, hotels, etc, for some years.

Mixing modes

The section drawing at the atrium shows what looks much like the common passive office design. The plan is less conventional, arranged facing roughly north and south, but with only some of the 10m-deep spaces adjacent to the teardrop-shaped atrium. In spatial terms the atrium provides some light and openness, a circulation stair, plus a sense of focus and connection between spaces.

In energy terms the atrium is not very important. With no shading at roof level there will be some daylight contribution, but lights are likely to be on full-time anyway. As the TermoDeck air system runs near full- time, the building is largely independent of whether people open windows or not (the system should respond to any beneficial natural ventilation cooling of the slab). Ventilation air is delivered at ceiling level and returned via a bulkhead at the atrium perimeter. While cross-ventilation between windows is also possible, the atrium does not have the common passive role of promoting stack-effect ventilation, though it will act as a sink for warm air escaping from the floors around it.

As with any low-energy design, the first step is to get the building right. Fabric insulation values are good (especially in the rooftop flat- roof area above some of the TermoDeck slabs at U=0.15). Window glazing is low-emissivity double glazing with an integral blind, giving an acceptable rather than exceptional U=2.0. The window is a standard unit from Samson. Attention was also paid to building airtightness to limit heat loss, particularly because the building is running at positive pressure. A whole-building pressure test was part of the contract. Advice was taken on air-sealing, especially at window-to-wall joints.

TermoDeck as a system

TermoDeck is more a system than a product. The extruded hollow-core precast concrete structural floor slabs can come from any manufacturer (here Richard Lees). Slabs are drilled and plugged on site to connect the central three of the five cores into one snaking tube and to provide an air inlet and outlet to this at either end. It is the changes of direction in the tube which cause air turbulence and so where most of the air-concrete heat transfer takes place. Slabs are pressure-tested on site. Tubes are vacuum- cleaned and should be checked every year. With good maintenance of plantroom filters, etc, cleaning may only be needed every 10 years.

On the curved part of the plan the space between slabs is made up with in-situ concrete. The soffit finish is white paint.

Air was to have been ducted from the ceiling down the walls to provide displacement ventilation from the floor. This was a victim of cuts (as were many of the lighting controls), leaving air inlets at ceiling level.

The floor slabs are connected to heat exchangers, an integral part of the system. It has ingenious mechanised baffles which can reorganise air streams within the exchanger to produce a surprising variety of modes of operation relatively simply:

free cooling, using cooler outside air to supply the building, or for night cooling of the slab

recirculation of air without heat exchange, for use in unoccupied periods when evening out heat levels between spaces, or for taking heat from slabs prior to occupancy, or when co2 sensors indicate acceptable freshness in existing air

recovering heat from exhaust air to pre-warm 100 per cent fresh ventilation air

cooling 100 per cent fresh air where it is warmer than building exhaust air

completely shut off.

Generally the TermoDeck system is relatively simple. Faults with existing systems have been 95 per cent with controls. It is important to have good temperature and humidity sensing, and control of heat recovery modes, to maximise free cooling and avoid running up a large energy bill for fan energy.

System performance should be very stable. For example tests at bre (reported in Building Services January 1993) showed that with 40degreesC input air the average slab temperature rose from 17degreesC to 22degreesC over 10 hours. Air is typically delivered to the occupied space within 0.5degreesC of average slab temperature. The system can be very effective in ironing out heat peaks and troughs.

The other side of the coin is slow system response to short-term peaks. One compensating measure available is called Switch Flow. Cooled air is supplied to slabs where a damper has closed down the tube in the slab to only one core. Supply is near-direct rather than chilling the slab to cool the space.

At Nottingham there is provision for active chilling in addition to the fan-driven system with its night cooling. This active chilling of supply air raises the cooling capacity overall from 45 to 65-75W/m2. TermoDeck says this is five or six times the capacity to be expected of an ordinary exposed slab with passive night cooling.

There is also air heating provision, though this is not expected to be used with current occupancy until the outside temperature gets close to 0degreesC. Having around 1000 occupants is not uncommon, peaking at around 1500.

Space temperature targets are 19-22degreesC in winter, 22-24degreesC in summer, rising to 25degreesC or beyond for 5 per cent of the time. As the slab is typically a little cooler than room air, the air temperature should be able to rise a bit more than normal without discomfort (thermal comfort involves a balance of radiant and air temperature). Accepting this temperature fluctuation, the TermoDeck system is providing otherwise similar performance to air conditioning. TermoDeck suggests that air conditioning would cost £110- 115/m2 compared with £98/m2 here. The main savings come on energy performance and maintenance costs.

TermoDeck's heating/cooling system has a total energy target of around 50kWh/m2/y (gas 24, chiller 1, fans and pumps 23; also 37 for lighting). This would give co2 emissions of 42kg/m2/y. The building was occupied in January so has not yet experienced significant hot weather.

At the Elizabeth Fry building, initial performance was 120kWh/ m2/y, but subsequent tuning has brought it down to 85kWh/m2/y (60 gas, 25 electricity). TermoDeck is providing a back-up member of staff for queries and for regular quarterly meetings over two years for these first system installations. As with other buildings, whether this building remains as-commissioned over 5-10 years remains to be seen.

Apart from the low predicted energy usage, advantages of the system are seen to include minimising chiller plant, being able to use all fresh air, heat recovery and free cooling without draughts, flexibility of capacity and lower maintenance costs. At Elizabeth Fry, system maintenance costs £1.40/m2/y compared with a suggested £3/m2/y for air-conditioning plant.

One possible downside of a system where the climate is 'always spring' is some loss of contact with outside conditions, often valued in passive designs. Not completely, of course, since windows could be opened and lights switched off.

David Turrent of ecd sees the system as most challenging to quantity surveyors - asked to shift money around between fabric and service budgets - and services engineers - asked to see the fabric as part of the building servicing design.

Flexibility and passive design

Flexibility is increasingly required of buildings, due to changing work practices or buildings being sold for other uses - is it an office, an apartment building, a learning resource centre? Such changes are testing for any building. Low-energy design strategies can be particularly tested. Looking at the library from this viewpoint, a management department was introduced on the third floor during construction, creating a more office- like use with tighter temperature control required. And the university explicitly sees the library building as real estate, capable of selling on, say, as urban offices.

The following are some of the testing conditions for a low-energy design that changes in use can bring:

space use (and technology) intensification, increasing cooling loads locally or throughout

a future occupancy with less heat-intensive it, moving the design focus from cooling to heating

24-hour working, undermining many approaches to passive night cooling

partitioning - cellular or around the atrium edge - with its inhibiting effects on cross- and stack ventilation

a requirement for tighter environmental standards, closer to those of air conditioning

installing air conditioning

subletting the building (here there are two plant rooms with heat exchangers per floor, for north and south)

a quieter acoustic (here, the atrium balustrades are acoustic-absorbent panels. Ceiling-hung baffles were cut from the budget. Lighting is aligned parallel to windows to allow their future installation, but the existing air flow would be inhibited so floor-based displacement ventilation would be needed)

individual environmental control

centralised environmental control

the longer commissioning period often needed for a wholly or partially passive design to tune the building for all seasons.

No building can do everything, and flexibility comes at a price. Against the above criteria, the Nottingham design looks robust. Obviously the capacity to increase heating or cooling supply is not limitless, and could reach a point where there are more efficient ways to provide it.

Apart from those sitting near the window there is no personal control. On the other hand control systems that offer many alternatives for local and central control have a well-established reputation for falling over.

What stands out is the apparent simplicity, both of energy design concept and built implementation, giving a stability of performance that promises both comfort and low energy consumption.

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