Risks of summertime extreme thermal conditions in buildings as a result of climate change and exacerbation of urban heat islands
Introduction
One of the fundamental purposes of buildings is to serve as protection from the ambient environment. Buildings provide shelter from wind and precipitation, but also act as buffers against heat in summer and cold in winter. Building energy codes and standards help to ensure that the building thermal envelope and the installed Heating, Ventilation, and Air-Conditioning (HVAC) systems are able to maintain the building's interior environment within reasonable bounds. Such comfort boundaries are typically defined based on temperature and humidity limits (e.g., as specified in ANSI/ASHRAE Standard 55 [1]). Building designers and engineers employ complex whole-building energy simulation software that assists them in sizing and selecting HVAC equipment. These simulation models integrate information regarding building geometry, construction materials, and anticipated building use patterns (e.g., occupancy, lighting, and plug loads) with typical meteorological year (TMY) weather data to estimate building performance under typical conditions (based on 30-years of historical weather data for the nearest airport weather station).
A reasonable question is whether buildings designed and constructed to operate under climatic conditions of the past 30 years will be resilient to weather conditions experienced during the lifetime of the building and its installed equipment. Prompted by concerns of a warming climate, this manuscript addresses two questions: (1) to what extent is building thermal performance compromised when the building is exposed to significantly warmer conditions than it was designed for? and (2) how is this compromised performance further impacted when a heat wave is coincident with a major loss of power/HVAC equipment failure?
Cities tend to be warmer than their natural (unbuilt) surroundings. This urban heat island (UHI) phenomenon is a result of a number of factors including the prevalence of thermally massive and low reflectivity surfaces, the general lack of surface moisture, and waste heat emissions from energy-consuming activities [2]. Urban heat islands are temporally and spatially complex. One can define a UHI based on differences in surface temperatures or air temperatures. Furthermore, air temperature heat islands can be defined at a range of vertical heights above the surface.
It is the urban canopy air-temperature UHI that is most relevant with respect to direct effects on building occupants. For buildings located in or near the centre of a large city, the summertime urban canopy UHI tends to be largest in the early morning hours [2], [3]. In fact, numerous studies have found remarkably similar results regarding summer differences in UHI magnitudes from day to night. For example, in an observationally-validated modelling study of the London heat island, Bohnenstengel et al. [4] found locations within the city centre to be 4–5 °C warmer than rural locations in the early morning hours. The same study found that the UHI in early afternoon was no more than 1 °C. In a similar study of London, Kolokotroni and Giridharan [5] found that summer daytime UHI magnitude was relatively small (<1 °C) from 10 am to 6 pm, while throughout most of the night the UHI magnitude remained relatively constant at 2–3 °C. Chan [6] found similar summertime results for Hong Kong: the nocturnal UHI was between 2 and 3 °C while during the day it was consistently between 0.5 and 1.0 °C. In a long-term analysis of 32 years of observational data for Buenos Aires, Camilloni and Barracand [7] found that night time UHI magnitudes were typically about 2 °C while the daytime UHI was negligible. Likewise, in a study of summer (July 2006 and 2007) UHI in Bucharest, Cheval et al. [8] found daytime UHI in the range of −1 to +1 °C and night time UHI on the order of 2.5–3 °C. So, as a general conclusion it is reasonable to state that near surface air temperature heat island magnitudes in summer are typically less than 1 °C, while at night the UHI magnitude may approach 2–5 °C. Actual UHI magnitudes depend on many factors including synoptic weather conditions (e.g., heat islands are typically greater during calm conditions).
Global climate change is likely to add to the UHI and to be magnified in cities in summer due to feedback mechanisms involving air conditioning of buildings [9], [10], [11]. Specifically, as the global climate warms energy use for air conditioning will increase and urban residents are likely to spend even more time indoors. These effects will interact with other risk factors related to building construction and insulation levels [12]. For example, Riberon [13] demonstrated that in the case of the 2003 heat wave in France individuals living on the top floor of uninsulated buildings had mortality risk that was roughly four times that of the general population. Further exacerbating these conditions is the continuing densification of urban populations. These trends will lead to increased waste heat emissions associated with air conditioning and will further increase summertime outdoor air temperatures.
Diurnal variation of warming under climate scenarios is perhaps more important than the annual or even daily averages; although, it is far less studied. Most future climate assessment efforts focus on seasonal or annual increases in air temperature. Even the most detailed analyses resulting from downscaling of climate model simulations generally present only daily maximum and minimum temperatures. Results from such studies consistently suggest that minimum temperatures are expected to increase more than maximum temperatures, resulting in a decrease in the diurnal temperature range [14], [15]. Nevertheless, it is possible that for some locations and some seasons a different trend may emerge. In any case, climate model predictions for changes in maximum and minimum temperatures can be used to construct hourly profiles of air temperatures under climate change scenarios [16].
Urban warming associated with concurrent global warming and urban growth will take place amid a backdrop of increasingly stressed electric utility grids and will, in some areas, result in increased frequency of utility system failures. Such events will have significant consequences for the health and comfort of building occupants [17].
This study explores the role of global and local warming on the indoor thermal environments of representative apartment buildings in two distinctly different warm climate cities. These scenarios are studied both in the context of typical operations and under the scenario of power outages and equipment failures during heat waves.
Section snippets
Methods
Climate change scenarios are used in this study to construct whole-building model simulations of representative apartment buildings. In each case, the building design and sizing of cooling equipment are based on current building codes and Typical Meteorological Year-TMY weather data from local airports. Simulations are conducted under (a) current climate (CC) conditions; (b) conditions that include a global warming effect (2050); and (c) conditions that include global warming with a concurrent
Results
Results are presented for both the driving climate scenarios constructed for building simulations as well as for indoor air temperature and several thermal comfort metrics for simulations in which building HVAC equipment functioned normally. Additional simulation results are presented for cases in which air conditioning failed during the hottest part of the summer, or there was a complete power outage to the building.
Discussion and conclusions
This study demonstrates the importance of designing buildings to account for future ambient conditions, and highlights the risks to building occupants associated with heat waves that may occur simultaneously with loss of power or air conditioning system failures.
The results clearly show that in some instances, apartment buildings designed to meet comfort needs based on historical climate conditions will be resilient to climate change. Specifically, the modelled apartment building in Chicago was
Acknowledgements
The author wishes to acknowledge the World Climate Research Programme's Working Group on Coupled Modelling, which is responsible for CMIP, and thanks the climate modelling groups (listed in Table 1 of this paper) for producing and making available their model output. For CMIP the U.S. Department of Energy's Program for Climate Model Diagnosis and Intercomparison provides coordinating support and led development of software infrastructure in partnership with the Global Organization for Earth
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