Night ventilation: an effective strategy to reduce the use of air-conditioning in office buildings
The use of air conditioning in the building sector is increasing rapidly. In almost the 46% of the houses in the Organisation for Economic Co-operation and Development (OECD) it has been rising by 7% each year. It is reasonable to think that in the case of office buildings this figure is even higher.
Since 1990, the energy consumption in office buildings has increased by 300%. Office buildings are responsible for the 50% of the energy consumption in the building the sector and 8.6% of the total energy consumption in Spain (IDAE 2011). The 26.2% of the energy consumption in office buildings is due to air conditioning and it is expected to increase considering the proliferation of squanderer glass buildings in the last decade, since concepts related to modernity, technology and transparency are playing a predominant role in their design.
The environmental impact associated to the intensive use of air conditioning in terms of CO2 emissions will have achieved the value of 18.1 Mt in Europe by 2020, far from the figure 0.516 Mt registered in 1990 (Adnot 1999). This disproportioned increase in CO2 levels does not reflect the international compromise adopted with the Kyoto protocol that established a reduction of 5% in CO2 and CFC emissions, habitual in air condition systems.
Intensive use of air conditioning is the result of many processes (Santamouris 2007), in particular:
- adoption of an universal style of buildings that does not consider climatic issues and results in increasing energy demands during the summer period;
- increase of ambient temperature, particularly in the urban environment, owing the heat island phenomenon, which exacerbates cooling demand in buildings;
- changes in comfort culture, consumer behaviour and expectations;
- improving of living standards and increased affluence of consumers;
- increase in buildings’ internal loads.
A significant number of studies are focusing on the energy efficiency and applicability of night ventilation cooling in office buildings under various climatic conditions. In (Blondeau et al 1997) is shown a reduction of diurnal variation from 1.5 to 2°C, resulting in a significant comfort improvement for the occupants. In addition, (Givoni 1991, 1996) argues that the night ventilation techniques are efficient particularly for arid regions where day time ventilation is insufficient to ensure thermal comfort. Moreover, a study conducted in the hot humid climate of Israel argues that it is possible to achieve a reduction of 3–6ºC in a heavy structured building without operating an air conditioning unit (Shaviv et al 2001).
The role of intensive night cooling in the warm Mediterranean climates is also analysed in (Becker et al 2002) (Artmann et al 2006). In buildings with large internal loads, intensive night ventilation enables the lowest internal mass temperature and the lowest power loads throughout the whole working day. The peak power load is reduced by 13%.
Because of all these reasons, counteracting the environmental and energy impact of air conditioning is one of the main objectives in the near future. Passive cooling is presented as an effective strategy to achieve the Kyoto Agreement, reducing the energy demand of office buildings and providing an adequate thermal comfort (Santamouris et al 2010) (Santamouris et al 2013).