It is known that one of the worst places to be on a hot summer day is about leaving the outside of an air conditioner. This is because in order to cool the interior of buildings, these devices extract heat from the interior to spew into the atmosphere. However, not being ideal machines, the amount of heat discharged into the atmosphere is more removed from inside the buildings (some up to 30% more). In a way, the price paid to maintain pleasant interior temperature is a further warming of the urban atmosphere. In recent years, the number of buildings with air conditioning has been increasing rapidly in many Spanish cities, particularly in commercial areas, and most buildings already have such facilities. The question then arises: does the heat generated by air conditioners is large enough to modify the urban climate?

A study by Francisco Salamanca, CIEMAT researcher, public research body under the Ministry of Science and Innovation, funded by the Ministry of Environment, Rural and Marine Affairs, suggests that air temperature in a city like Madrid could rise to 1, 5-2 degrees centigrade due to air conditioning in summer. This analysis forms part of the doctoral thesis presented at the Universidad Complutense de Madrid (UCM), co-directed by Alberto Martilli (CIEMAT researcher) and Carlos Yagüe (Professor at the UCM) and the researchers who participated also Polytechnic School Federal de Lausanne (Switzerland) and the National Center for Atmospheric Research (Boulder, Colorado, USA). This research was conducted using an atmospheric model (Weather Research and Forecast Model, WRF), which has built a simple model of building energy that considers the heat generation in the buildings occupants and equipment, trade with the outside by ventilation, diffusion of heat through the walls, and radiation through the windows (Salamanca et al. 2010a). From these data, the model estimates the amount of heat that needs to be removed from buildings and it ends up heating the atmosphere. The novelty of this approach is that pioneered by coupling the model building with the atmospheric model, we can consider all the interactions between buildings and atmosphere. This model has been used to simulate the behavior of the atmosphere over the region of Madrid for two days of summer 2008 (June 30 and July 1). Once validated the results with experimental data DESIREX campaign (Sobrino et al., 2010) which took place in those days was considered a scenario where the air conditioners do not emit heat to the atmosphere. As mentioned above, from these simulations has been estimated in 1,5-2 degrees increased air temperature due to the use of air conditioners in some areas of the city (Salamanca, 2010). However, this increase does not occur during the hottest hours of the day, but during the evening. This is particularly significant because it is known that one of the main causes of increased mortality during heat waves is connected to a prolonged heat stress (eg.: Temperatures do not drop enough at night to allow rest population, Clarke, 1972).

Another advantage of the methodology presented in this study is the possibility of estimating the energy consumption due to air conditioning units on the entire city. This amount, as well as meteorological parameters, depends on the morphological parameters of the city as the total volume of air contained in the buildings, the area of the buildings themselves that exchanges heat with the atmosphere, the size of the streets and height buildings, which determine the shadows and the amount of energy lost by radiative cooling. A previous study (Salamanca et al., 2010b) on the city of Houston (Texas, USA) has shown that if you have detailed spatial information of these morphological parameters, the model is able to estimate with good approximation consumption values energy obtained with other techniques that require much more contrasting prior (Heiple and Sailor, 2008). For the city of Madrid do not have this detailed morphological information. For this reason, urban settings are set based on specific information about some neighborhoods considered representative of the urban typology of Madrid. Considering these limitations and to assess the potential of the tool, they have studied several possible scenarios representative of simple strategies for reducing energy consumption, as increased albedo (ability to reflect solar radiation) from the rooftops, increase thickness layer of insulation on the roof, or the use of air conditioning systems that do not emit heat to the atmosphere. Each separate strategy may represent a saving of between 3 and 5% of total energy consumption due to air conditioners, while if taken together, would save 10%. It is important to consider that the energy consumption for cooling or heating the building represents a significant share of total domestic energy consumption (for example, in the U.S. at the national level this type of consumption represents 50% of annual domestic consumption, Heiple and Sailor, 2008). If one considers, moreover, that three-quarters of global energy consumption occurs in cities, and the percentage of the world’s urban population is growing continuously (for the first time in history is more than 50%) can understand how cities are a key place where action to control global energy consumption, a necessary action to limit the impact of climate change.

Tools such as the one developed in this study may allow not only to evaluate different control strategies in energy consumption and improving the urban environment, but also to plan the future development of cities in a sustainable sense. In addition, to assess also the impact on air pollution from urban development scenarios. Clearly, not always the choice of an urban development scenario will be clear (what to do if a modification of the urban structure can improve the weather, but air quality worse?), And the final political decision will also depend other factors (epidemiological, social, economic, etc.). However, it is important that you start to have numerical tools to quantify the complex interactions between urban structure and atmosphere, so that decisions can be based on the most current scientific knowledge.

Source: CIEMAT