Design for operational efficiency
Energy consumption in buildings accounts for nearly half of all energy used in the UK. Government’s Energy White Paper has set out a target to reduce CO2 emissions by 60 per cent by 2050. 50 per cent of these savings are expected to come from greater energy efficiency.
Operational energy efficiency in buildings is important from the point of view of:
- Whole-life operational cost
- Depletion of non-renewable resources, e.g. oil, gas and coal
- Global warming from the burning of carbon-based fossil fuels
Energy efficiency in buildings is achieved by:
- Reducing primary heat losses through the façade
- Reducing cooling loads
- Introducing energy saving measures in the operation of the building
- Installing energy creation systems, such as wind turbines and photovoltaics
It is widely recognised that the operational energy consumption of most building types far outweighs their embodied energy. Research undertaken by the Steel Construction Institute has found that for an air-conditioned office building over a 60-year design life, the ratio of embodied energy to operational energy is around 1:12.
Light steel framed construction
Highly efficient cladding systems can be designed in steel with U-values less than 0.2 W/m2 ºC although the current Part L of the Building Regulations only requires 0.25 W/m2 ºC for roofs of industrial buildings and 0.35 W/m2 ºC for walls in England and Wales. There are two main types of envelope for industrial buildings; twin skin built up systems and composite or sandwich panels.
These consist of a weathering sheet, insulation and a liner sheet. The sheets are held apart by spacer systems which also have to transfer the external loads back to the supporting structure. The outer sheet is typically 0.7 mm thick with a variety of organic coatings applied to galvanised or aluzinc substrate. The sheets are supplied in a wide variety of colours and coating types suited to the intended building usage. Insulation thicknesses vary according to the insulation material and designed U-value but are typically 180 mm for 0.25 W/m2 ºC.
Composite panels are again formed from two steel sheets separated by insulation. However, in this case the insulation is bonded to both sheets and contributes to the structural performance. No spacer systems are required. The insulation is usually either polyurethane or polyisocyanurate foam or mineral wool. With rigid insulation the panel acts as a whole and there is no need for either of the sheets to be profiled other than for aesthetic reasons. The same range of coatings is available as for built-up systems.
As U-values have improved, so air leakage is an increasingly significant factor in energy wastage. Buildings over 1000 m2 have to be tested and must not exceed air leakage of 10 m3/m2/hr at a pressure difference of 50 pascals to comply with the 2002 Building Regulations. More stringent rules are likely in the next revision due in 2005/6. This requirement is easily achievable using steel cladding systems but compliance depends on both good detailing of the joists and in the sheets and interfaces between the cladding and openings. It is also dependent on good quality construction.
As with all forms of construction the regulations for domestic buildings are increasingly stringent. The use of light steel framing for external walls provides a system which can easily accommodate these requirements. Warm frame construction, with the structural frame on the inside of the insulation, is normally adopted and this provides a system which is largely independent of the insulation thickness. The insulation is generally a foil faced rigid foam board which is applied, as part of the frame erection process, to give rapid weather protection to enable construction to proceed independently of progress with the brick outer skin. Performance in excess of the requirements for energy conservation in the Building Regulations can comfortably be achieved.
Fabric energy storage can be used to absorb excess heat inside buildings during the day which can then be removed by night time ventilation. Used effectively it can mean that mechanical cooling can be reduced or even eliminated. The amount of heat that a structure can absorb is its thermal capacity.
High internal gains in highly serviced, modern commercial buildings can present designers with the challenge of providing adequate cooling to achieve optimal working conditions throughout much of the year. This is particularly the case in busy urban settings and high-rise buildings where opening windows during the day, to provide natural ventilation, is not possible.
Cooling loads are four times more energy intensive than heating and therefore in many commercial and other highly serviced buildings, energy consumption for cooling can exceed consumption for heating.
Fabric energy storage (FES) can be used to help moderate internal temperature gains in buildings thereby reducing the requirement for mechanical cooling and reducing operational costs.
What is fabric energy storage?
All materials have the capacity to absorb, store and release heat.
By controlling the rate at which energy is absorbed and released by the building fabric, internal temperature gains can be moderated and the need for mechanical cooling reduced or even eliminated.
Control is achieved by providing sufficient building fabric to absorb energy and designing the building to enable heat to be easily transferred into (and out of) the fabric. The simplest way of doing this is to expose the maximum area of those building elements that have the greatest thermal capacity. This is usually the underside of the floor slabs.
By allowing the fabric of the building to absorb energy during the hottest part of the day and then releasing it by cooling the fabric down, usually overnight, daytime peak internal temperatures can be moderated.
FES can reduce peak internal temperatures by 3-5°C, shifting the ‘midday peak’ to later in the day after the building occupants have left.
There are two basic design strategies for utilising fabric energy storage in buildings.
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Passive systems in which the building fabric, usually the floor soffit, is exposed to allow heat exchange to take place between the floor slab and the air. The typical night cooling performance of an exposed flat slab in the UK is 10 to 20 W/m2.
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Active systems in which heat removal from the building fabric is enhanced through the mechanical supply of air through, or over, the floor slab. Cooling of 20 to 30 W/m2 of floor area are achievable using active systems.
How much thermal capacity is required?
The parameter used to measure the ability of a building element to store and release heat is admittance. The higher the admittance value, the greater the ability to store and release useful amounts of heat.
In a naturally ventilated building, the maximum value of admittance for a concrete slab exposed on one side, can be achieved with only 75-100mm of concrete. Furthermore, beyond the maximum value, admittance progressively decreases with increasing slab thickness because of the relative difficulty in extracting heat.
Fabric energy storage in steel buildings
Historically, buildings designed to take advantage of fabric energy storage were specifically designed to be ‘heavy’. This is a myth. Work undertaken by Oxford Brookes University has proven that optimum levels of FES can be achieved in relatively light, structurally efficient buildings that are constructed from fewer resources.
Dynamic thermal modelling has been used to compare the FES performance of common structural framing options for a typical office buildings over a typical year. No differences were observed in the amount of passive cooling provided by steel and concrete-framed buildings. A number of fabric energy storage systems are compatible with steel-framed buildings. These include:
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Passive systems in which the slab soffit is exposed or in which thermally permeable suspended ceilings are used.
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Active systems that include:
- Raised floor voids through which air is passed either via natural of forced ventilation
- Air cores systems such as Termodek and Airdek in which air is passed through voids in the slab. The Termodek system comprises a hollow-core pre-cast concrete slab supported on universal or Slimflor beams. The Airdek system uses a steel liner to form a narrow airway along ribs on the underside of Slimdek floor construction
- Water-cooled slabs in which water is passed through plastic pipes cast within the concrete slab
