A changing climate for thermal mass

Exploiting thermal mass within a building can eliminate the need for air conditioning. Tom De Saulles explains how.Recently published Met Office data for 2006 continues to indicate a warming climate, both around the world and especially in the UK. In central England, the highest average yearly temperature was recorded since records began, and, perhaps more significantly, extended hot periods have also broken previous records. At the same time the use of air-conditioning in the UK is growing by around 8% each year. However, this growth has not been at the expense of passive cooling techniques, which are also seeing greater uptake — often in combination with air conditioning to reduce chiller loads. Along with a greater emphasis on suitability, key drivers are increasing energy costs and changes to Part L, which now includes strict overheating limits for buildings without air conditioning. Exploiting thermal mass Effective solar shading and a good ventilation strategy may be the only measures needed for relatively undemanding environments. However, most projects require more cooling effect, and where practicable, the use of thermal mass and night cooling is proving to be an increasingly popular option. This approach generally takes the form of concrete floor slabs with an exposed soffit, which can absorb internal gains and provide a radiant cooling effect, helping to stabilise the operative temperature during the warmest part of the day. In a typical office environment, thermal mass can delay the onset of the peak internal temperature by some six hours, and this peak usually occurs after occupants have left the building. Night ventilation is then used to purge heat stored in the slabs, and is relatively effective as the diurnal temperature swing in the UK is rarely less than 5 K — the minimum temperature difference needed to get a worthwhile level of heat flow in and out of the concrete. System options There are a number of ways of exploiting the inherent thermal mass of upper-floor slabs. These are summarised below, along with their approximate cooling capacity. 1. Exposed slab with natural ventilation. A flat or profiled slab combined with wind-driven, or a combination of wind and stack ventilation. Cooling from thermal mass is 15 to 25 W/m2. 2. Exposed slab with mechanical underfloor ventilation. In addition to an exposed soffit, the raised floor is used as a ventilation supply plenum, maximising exposure of the thermal mass by allowing convective heat transfer with the top of the slab before air enters the space via floor-level diffusers. This system typically forms part of a mixed-mode approach to take advantage of natural ventilation, while providing good control of ventilation rates both at night and during the heating season. Cooling from thermal mass is 20 to 35 W/m2. 3. Exposed hollowcore slab with mechanical ventilation (Termodeck). The mechanical supply is ducted through the cores of exposed, hollowcore concrete slabs, providing good convective heat transfer, helped by the turbulence created by the serpentine path the air must follow before entering the space through soffit-mounted diffusers. Heat transfer is also provided by the exposed soffit. Cooling from thermal mass is 40 to 60 W/m2. 4. Exposed water-cooled slab with (Thermocast). Polybutylene pipework, is embedded in the concrete to provide a highly effective means of heating and cooling the slab. This approach can take advantage of natural water sources during the summer, such as aquifers and lakes. Conventional chillers can also be used and will be able to take advantage of free cooling for much of the year due to the relatively high chilled-water temperature that is used. Cooling can also be supplemented by conventional night ventilation. Cooling from thermal mass/water is 60 to 80 W/m2. 5. Chilled beams with exposed or partially exposed slab. Chilled beams are suspended directly below the concrete soffit, which may be fully exposed or partially concealed behind a permeable ceiling. Cooling is provided by a combination of the beams and the thermal mass, allowing a passive approach to be fully exploited whilst still being capable of meeting relatively high cooling loads. Cooling from thermal mass and chilled beams is up to 190 W/m2. Integrating the services The use of exposed soffits presents new challenges in terms of locating services normally found in the ceiling void. However, there are a number of simple solutions that can be adopted. • Integrated service modules.
• Multi-service chilled beams.
• Surface mounted and suspended luminaires.
• Perimeter bulkheads for extract grills and lighting distribution etc.
• Corridor distribution.
• Permeable ceilings (allowing conventional high level services).
• Slab located services (rebates/ducts cast into slab for luminaires, smoke detectors, extract ducts etc.). Conclusions The use of thermal mass can do much to simplify the design of buildings and their services. However, the need to model system performance is essential as the thermal environment will be closely influenced by external conditions. For design briefs that do not permit the occurrence of overheating, the addition of a modest chiller will limit peak internal temperatures and can be easily integrated into mixed-mode systems and water-cooled slabs. Using a chiller in this way can also provide greater flexibility in commercial developments, whilst still offering a more sustainable building with lower running costs and a pleasant working environment. Tom De Saulles is senior manager for building sustainability with The Concrete Society/British Concrete Association. For more information on system design, a new guide has been published entitled ‘Utilisation of thermal mass in non-residential buildings’. It is available from The Concrete Centre at the web site below.

Related links:

• www.concretecentre.com