Taking Thermals Masses

Air-conditioning can be a major contributor to the operational CO2 emissions of a building. Utilisation of a building’s thermal mass can significantly reduce or even remove the need for air-conditioning and therefore its CO2 emissions. New technical design guidance from the Concrete Centre explains how.

AS SUMMERS get hotter due to climate change, the use of air-conditioning is increasing. In the UK the uptake of air-conditioning is rising by 8% per year. This could result in an extra six million tonnes of CO2 a year by 2020. To counter this, the 2006 edition of Part L2A of the Building Regulations sets tough targets for CO2 emissions, making the installation of full air-conditioning system harder to justify especially in the face of rising energy costs.

Building Regulations now include clear overheating limits for non air-conditioned buildings to future-proof against the impact of climate change. Against this background, passive cooling techniques are being increasingly applied in non-residential buildings.

At the heart of low-energy design is the thermal mass provided by the building fabric and the way in which it interacts with the internal and external environment. Here, the unrivalled, inherent thermal mass of concrete plays an important role in ensuring a comfortable internal conditions. The use of exposed concrete to provide passive cooling can achieve significant savings in terms of capital and operating costs through avoiding or minimising the need for air conditioning.

The basic approach is to expose the soffit of the floor slabs, which can then absorb heat gains during warm weather and stabilise the internal temperature. Typically, night air is then used to ventilate and cool the building in readiness for the following day. This cycle of heating and cooling using the thermal mass of a building is often referred to as Fabric Energy Storage (FES).

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For a typical office, FES can help delay the onset of peak internal temperature by some six hours, usually after the occupants have left the building. At this point the FES cycle is reversed. As evening progresses, the external temperatures drops making night ventilation an effective means of removing the accumulated heat from the building fabric in preparation for the next day. The UK variation in diurnal temperature rarely drops below 5°C making night cooling relatively effective. As an alternative or addition to night ventilation, water cooling may be used. This can offer improved flexibility and control of slab cooling. There are a number of generic FES systems. These include natural ventilation with exposed soffits, which are flat or profiled floor slabs used in conjunction with natural ventilation. This may be wind-driven, or a combination of wind and stack ventilation.

Underfloor ventilation with exposed soffits can also be used to heat and cool buildings. The void created by a raise floor is used as a plenum for mechanical ventilation maximising exposure of the thermal mass by allowing heat transfer with top of the slab in addition to the soffit on the underside. Air enters the occupied space through floor diffusers. This system is often used in conjunction with an exposed, profiled slab and with openable windows to provide a mixed-mode solution.

Precast hollowcore concrete slabs with mechanical ventilation via the cores, can provide good convective heat transfer between the air and concrete. Further heat transfer is provided by the exposed slab soffit. The system is typically referred to by the trade name of “Termodeck”.

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Precast or cast in-situ slabs, with water cooling provided by embodied polybutylene pipework, can be used in conjunction with a night ventilation strategy. The precast option is trademarked as “Thermocast”.

Chilled beams with exposed or partially exposed soffits can also be used. These take the form of flat or coffered concrete soffits with chilled beams suspended directly below. A permeable ceiling may be used, or the soffit left exposed. FES is provided in the usual way, using natural and/or mechanical ventilation, with additional cooling from the chilled beams.

For many developments, FES design will be combined with mixed-mode ventilation, and/or air-conditioning. This is due to the need for enhanced cooling performance for demanding office environments and in order to address issues of security and occupant control of windows.

A high thermal mass building will provide a high quality, well-ventilated space in which occupants are empowered to take control of their environment. The building operator will benefit from lower operating costs and everyone will benefit from reduced CO2 emissions.

There is already a significant number of FES designed bespoke buildings and increasing evidence that FES is being used for speculative office developments. This is being driven not only by awareness of the need for greater sustainability, but also by developers and building occupants recognising the long-term cost benefits of high thermal mass buildings. This may well result in FES becoming the norm for 21st Century commercial design and construction.

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