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Surface airier

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The recent AJ-sponsored Cool Space conference examined whether the mass of a building benefits thermal comfort

One of the key issues of the 'Cool Space' discussion, held at the RIBA in mid February, concerned the ability of a building to self-regulate its thermal performance. The question of using solar gain while ensuring the thermal comfort of the building user, still needs to be answered satisfactorily.However, the discussion revealed misconceptions of the practical value that mass structures have in storing and releasing heat.

High internal gains in modern, high-occupancy buildings can mean that little or no additional heating is required for much of the year. The issue facing designers is predominantly one of providing adequate cooling.

Passive thermal gain needs to be controlled without recourse to energy-intensive air conditioning plant, while at the same time avoiding pockets of unacceptable temperature fluctuations. Increasing the thermal capacity of a structure - the amount of heat which a structure can absorb - can mean that mechanical cooling may not be necessary or that cooling plant requirements can be reduced.

The principle of 'fabric energy storage', is straightforward: excess heat is absorbed and stored in the building fabric during the day and removed by overnight cooling. The cooling process can be fed back into a building management system to create a very efficient use of the otherwise wasted heat. Similarly, the exhaust system can be by natural stack effect of low energy or solar powered ventilation mechanisms. Operational energy and resulting carbon dioxide emissions can be reduced, as can capital, operational and maintenance costs.

If the mechanics of fabric energy storage is used effectively, the maximum internal temperatures within buildings can be reduced by 2degreesC or 3degreesC and the peak temperature delayed by several hours.

During the cooling period, fabric energy storage can significantly reduce air temperatures and radiant surface temperatures within buildings. These two temperatures are often combined as the dry resultant temperature (DRT), an indication of the temperature perceived by building occupants. Figure 1 compares the DRT in an office where use is made of the thermal capacity of the structure (exposed soffit with night cooling), with a similar office where no use is made of thermal capacity (hidden soffit without night cooling).

In the past, buildings which have sought to take advantage of fabric energy storage have often been designed to be physically heavy. However, this is misses the point. Research has demonstrated that relatively light steel-framed structures in association with composite or other types of floor slab can provide more than adequate thermal capacity. The maximum value of admittance - the ability to store and release heat - may be achieved in a naturally ventilated building with only 75-100mm of concrete. Since composite and precast floor slabs are significantly thicker than this, the lightest steel-framed buildings can generally achieve good performance in terms of fabric energy storage.

The primary objective, discussed at length at 'Cool Space', is to increase the area, rather than the mass, of the elemental surfaces. Indeed, Patrick Bellow, of Atelier Ten made great play of this in the labyrinthine underground heat exchangers he has designed for use in Doncaster's Earth Centre and Melbourne's Federation Square (see AJ 5.11.98 for a detailed analysis). In both, warm external air is brought through a major underfloor network of faceted concrete baffles, to cool the air entering the building.

The admittance concept is used to indicate the ability of a building element to store and release heat over a daily 24-hour (diurnal) cycle. Lightweight building elements, such as dry-lined partitions, have a low admittance, or a low ability to store and release useful amounts of heat. By contrast, exposed structural elements such as floor slabs have a high admittance.

In a naturally ventilated building the maximum value of admittance for a concrete slab exposed on one side may be achieved with only standard design thicknesses of concrete toppings (Figure 2). Furthermore, beyond the maximum value, admittance progressively decreases with increasing slab thickness because of the relative difficulty in extracting heat in the night cooling period.

However, increasing the surface area, by ribbing the decking or by introducing hollows within the slab, can greatly improve the thermal performance of the structure. Some examples are coffer slabs or hollow concrete planks.

Crispin Matson of Rybka Battle showed examples of cheap and straightforward technological devices currently on the market, such as fuelcells, solar air conditioning and micro-turbines. Rather than isolate one issue of energy efficiency at the expanse of others, he encouraged the audience to incorporate these costeffective technologies into schemes to assist other heat sink or thermal storage features.

The Building Services Research and Information Association has used dynamic thermal simulation modelling based on UK weather data for a typical year to compare common structural framing options for a typical office building. These included:

1. concrete topping on precast concrete on steel frame (type a);

2. composite steel and concrete construction (type b); and 3. in situ flat slab on concrete frame.

The graphs in figures 3 and 4 compare these three framing systems where the floor soffit is exposed.

They reveal very little difference between the systems in the amount of passive cooling provided.

An independent study carried out by Arup and Future Systems for the Ark Building at the Earth Centre in Doncaster concluded that 'the differential effect of using either steel or concrete roof structures on the winter energy use and summer-time comfort conditions is surprisingly small for this building. Other factors such as building occupancy patterns and insulation levels in the roof have a much greater influence on annual energy consumption.'

On grounds of structural efficiency excessive weight is normally positively avoided. Offering the same level of fabric energy storage, relatively light steel frames, and associated composite, or other, types of floor slab also minimise material usage, and offer efficient, environmentally positive solutions.

Thanks to John Dowling, manager of fire, environment and corrosion at Corus Construction Centre, Brigg Road, Scunthorpe, for assistance in preparing this article

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