As summer arrives, the rising demand for air-conditioning in offices calls developers, designers and occupiers to examine the potential of heavyweight construction to reduce bills and carbon emissions.
“The irony of air-conditioning is that its use contributes to the hotter summers that are predicted as a result of climate change. The hotter the summer the more that air-conditioning is turned-up,” said Steve Elliott, chairman of the British Association of Reinforcement (BAR). “There is a simple alternative. Use the thermal mass of heavyweight concrete construction to provide energy-free cooling.”
The Carbon Trust estimates that by 2020 that some 40% of commercial floor space is expected to be fully air-conditioned compared with only 10% at the end of 1994*. The continued energy-intensive growth in the use of air-conditioning undermines government policies to try to reduce carbon emissions – the main contributor to global warming. BRE meanwhile calculates that air-conditioning equates to 10% of UK energy consumption.
Steve has called for a greater realisation of the potential of concrete thermal mass which, when used in combination with night time and cooling, he says can reduce or even eliminate the need for air-conditioning. Often referred to as Fabric Energy Storage (FES), the basic approach is to expose the soffit of concrete floor slabs which can then absorb heat gains during warm weather and so reduce the internal temperature. The use of cooler night-time air ventilation or embedded water-cooling cools the soffits in readiness for the following day. The best level of thermal mass is provided by heavyweight construction. Lightweight construction such as steel and timber structures do not offer a comparable level of heat absorption.
He said: “FES thermal mass, especially when used as part of an integrated passive design solution that includes building orientation, shading and natural ventilation, can reduce significantly the reliance on air-conditioning and in return reduce the level of carbon emissions. Given the considerable amount of energy that air-conditioning uses, designers and developers should actively embrace the potential of heavyweight construction.”
He added that building occupiers should choose heavyweight construction over lightweight construction particularly when, according to the Carbon Trust, the energy costs and association carbon emissions of typical air-condition building are 30% higher than a naturally ventilated building. The air-conditioned building is also more likely to have increased capital and maintenance costs.
The use of concrete construction does raise questions concerning the level of embodied CO2 when compared to other structural materials. “Some passively cooled buildings will have an initial higher embodied CO2. However, that will be quickly offset by the reductions in ongoing operational CO2 which can range from just one to six years,” explained Steve. “Already this year parts of the UK have reached 29oC. Turning up the air-conditioning is not environmentally or economically the answer.”
*Carbon Trust; Air conditioning – maximising comfort, minimising energy consumption; 2012