Are energy consumptions decreased with the addition of a double-skin?
Introduction
The double-skin facade is an architectural phenomenon driven by an aesthetic desire for an all-glass facade.
Transparency is often seen as the main architectural reason for a double-skin facade, because it allows close contact with the surroundings. From the client's point of view, physical transparency may appear to indicate a transparent organisation with a large degree of openness [1].
This “emerging technology” of heavily glazed facades is also often associated with buildings whose design goals include energy efficiency, sustainability, and a “green” image.
So there has been an increase in the numbers of this type of building. The success of these facades also lies in the fact that they admit a large amount of daylight, exhibit a uniform exterior, and have attractive aesthetics.
The costs of double-skin facades are higher than those of normal facades, but claims of energy and productivity savings are used to justify some of these increased costs [2]. However, this argument must be taken with many precautions. The efficiency of the double-skin depends indeed on many factors.
The advent of computers and other electric office equipment has increased the internal heat gain in most offices. Highly glazed facades, often with poor shading, have become very common. This, together with the extra heat gain from the electric lighting made necessary by deep floor plans, and the widespread use of false ceilings, has increased the risk of overheating [3], [4].
In the 1990s, concern about global warming resulted in a resurgence of interest in natural cooling strategies, including natural day and night ventilation and solar protections use [5], [6], [7].
There is also an increasing demand for high-quality office buildings. The occupants and developers of office buildings require healthy and stimulating working environments [8]. This is usually provided by an air conditioning system. But in many cases, with some effort to reduce internal heat gain (well chosen equipment), solar protection and natural ventilation may be sufficient to ensure good comfort levels for the buildings’ occupants.
In that case, air conditioning system will not be necessary, which will result in considerable energy and cost savings. It will also indirectly reduce the burden on the environment, since the use of energy is always associated with the production of waste materials [9].
Double-skin facades are assuming an ever-greater importance in modern building practice. They are already a common feature in architectural competitions in Europe; but there are still relatively few buildings in which they have actually been used, and there is too little information on their behaviour in operation [10], [11].
There are many unknowns: optical and thermal modelling of these systems is not routine and coupling heat transfer and air flows from an isolated facade system to the whole building is complex. A variety of thermal coupling strategies must be simulated [12], [13], [14], [15].
Moreover, although subjective claims abound in the architectural literature, it is extremely difficult to find any objective data on the actual performance of buildings with double-skin facades.
Results of simulations show that heating loads are decreased in an office building with a double-skin facade. Indeed, the temperature of the air layer in the double-skin is more important than the outside temperature and so the cavity protects the building from the cold. Moreover, double-skin hot air can be recovered to heat the coldest zones of the building [16], [17].
On the other hand, results of simulations show that cooling loads are increased in an office building with a double-skin facade. Indeed, the hot air layer becomes an obstacle with the cooling of the building. Application of natural cooling strategies becomes still more important in the building with double-skin than in building without double-skin [18], [19], [20], [21].
To undertake the study, we chose an office building with various level of thermal insulation. We compare consumption of heating and of cooling in a building with or without double-skin when the heating and cooling natural strategies are or are not used, according to the level of insulation and the orientation of the double-skin.
Simulations were performed with the thermal program TAS.
Section snippets
TAS program
TAS is a software package for the thermal analysis of buildings. It includes a 3D modeller, a thermal/energy analysis module, a systems/controls simulator and a 2D CFD package. There are also CAD links into the 3D modeller as well as report generation facilities. It is a complete solution for the thermal simulation of a building, and a powerful design tool in the optimisation of a building's environmental, energy and comfort performance [22].
The building
The simulations where undertaken using the building
Well-insulated building, no use of heating and cooling natural strategies
The addition of a double-skin decreases the heating loads of 11.3–13%. The greatest reduction is observed for the northern double-skin: indeed, the southern zones benefit fully from the solar profits which are not filtered by the double-skin and the zones north are protected by a buffer space.
When the sky is clear, southern double-skin induces overheating in southern offices, even in winter period. Since the strategies are not implemented, the double-skin air is not used to provide hygienic
Impact on the CO2 emission
The CO2 emissions are proportional to the primary consumption of energy and depend on the type of fuel:
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Natural gas: 0.198 kg CO2/kWh;
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Fuel oil: 0.264 kg CO2/kWh;
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Electricity: CO2 produced in power station is a function of the season and the hour of the day.
Assumptions:
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heating system: natural gas ≫ 0.198 kg CO2/kWh;
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cooling system: electricity ≫ 0.342 kg CO2/kWh;
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mechanical ventilation: electricity ≫ 0.342 kg CO2/kWh.
During the analysis of the CO2 emission impact, two cases were studied:
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it is possible to naturally
Conclusions
This study mainly consists in evaluating the impact of a double-skin on energy consumptions according to the orientation of the building and the double-skin, for three various levels of insulation. The presence or not of the double-skin has an impact on the energy consumptions. However, as we see, this one is weak compared to the induced impact:
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by the use of the strategies (primarily natural cooling strategies);
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by the insulation level;
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by the level of internal gains.
Fig. 11 illustrates, for a
Acknowledgements
The authors wish to thank the Walloon Regional Government of Belgium for its support in funding this research. We would also thank José Flémal (draftsman) who helped us to illustrate the article.
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