Elsevier

Solar Energy

Volume 70, Issue 3, 2001, Pages 201-216
Solar Energy

On the impact of urban climate on the energy consumption of buildings

https://doi.org/10.1016/S0038-092X(00)00095-5Get rights and content

Abstract

Climatic measurements from almost 30 urban and suburban stations as well as specific measurements performed in 10 urban canyons in Athens, Greece, have been used to assess the impact of the urban climate on the energy consumption of buildings. It is found that for the city of Athens, where the mean heat island intensity exceeds 10°C, the cooling load of urban buildings may be doubled, the peak electricity load for cooling purposes may be tripled especially for higher set point temperatures, while the minimum COP value of air conditioners may be decreased up to 25% because of the higher ambient temperatures. During the winter period, the heating load of centrally located urban buildings is found to be reduced up to 30%. Regarding the potential of natural ventilation techniques when applied to buildings located in urban canyons, it is found that, mainly during the day, this is seriously reduced because of the important decrease of the wind speed inside the canyon. Air flow reduction may be up to 10 times the flow that corresponds to undisturbed ambient wind conditions.

Introduction

Cities are increasingly expanding their boundaries and populations and as stated ‘from the climatological point of view, human history is defined as the history of urbanization’. Increased industrialization and urbanization in recent years have affected dramatically the number of urban buildings with major effects on the energy consumption of this sector. It is expected that 700 million people will move to urban areas during the last decade of this century. The number of urban dwellers has risen from 600 million in 1920 to 2 billion in 1986 and if this growth continues, more than one-half of the world’s population will live in cities by the end of this century. One hundred years ago, only 14% lived in cities and in 1950, less than 30% of the world population was urban. Today, at least 170 cities support more than one million inhabitants each. As estimated, in the United States, 90% of the population is expected to be living in, or around, urban areas by the year 2000. Estimations show that urban populations will occupy 80% of the total world population in 2100.

Almost similar situations are found in Europe. It is reported that during the 1970s, urban tertiary buildings in Greece represented about 14% of these buildings in the country. Due to the dramatic urbanization, the corresponding percentage of new urban tertiary constructions increased up to 55% during the 1980s.

The situation will be even more dramatic in developing countries. Already, 23 of the 34 cities with more than 5 million inhabitants are in developing countries. Current projections estimate that 11 of those cities will have populations of between 20 and 30 million by the year 2000.

It is clear that urban areas without a high climatic quality use more energy for air conditioning in summer and even more electricity for lighting. Moreover, discomfort and inconvenience to the urban population due to high temperatures, wind tunnel effects in streets and unusual wind turbulence due to wrongly designed high rise buildings is very common (Bitan, 1992).

Data on the energy and specific electricity consumption of major European cities are given by Eurostat, 1995. Data on the electricity consumption in European cities range from 60 GWh/year for Valetta to 26,452 GWh/year for London. According to the same data, the average electricity consumption calculated on the basis of available data for cities with more than 1,000,000 inhabitants is around 4500 GWh per year. However, these data cannot be used to draw any conclusions.

Other, statistical data (Stanners and Bourdeau, 1995) show that the amount of energy consumed by cities for heating and cooling of offices and residential buildings in western and southern Europe has increased significantly in the last two decades. A recent analysis, (Jones, 1992), showed that a 1% increase in the per capita GNP leads to an almost equal (1.03), increase in energy consumption. However, as reported, an increase of the urban population by 1% increases the energy consumption by 2.2%, i.e. the rate of change in energy use is twice the rate of change in urbanization. These data show clearly the impact that urbanization may have on energy use.

Thus, it becomes increasingly important to study urban climatic environments and to apply this knowledge to improve people’s environment and decrease the energy consumption in cities.

The present paper presents the results of an urban study carried out in Athens aiming, among other things, to investigate the impact of the urban climate on the energy consumption of urban buildings. An extended network of measuring stations has been installed and climatic data collected for a 3-year period. Specific air flow and temperature distribution experiments have been also carried out in 10 urban canyons. The data have been used to evaluate the impact of increased ambient temperatures on the heating and cooling performance of buildings. Also, air flow and temperature distribution data in urban canyons have been used to evaluate the impact of canyon geometry and characteristics on the potential of natural ventilation techniques to provide passive cooling to urban buildings.

Section snippets

URBAN CLIMATE AND ENERGY CONSUMPTION OF BUILDINGS

A very comprehensive description of the urban climate is presented by Landsberg (1981). It is important to describe the mean features by which the urban climate differs from the climatic conditions of the surrounding rural areas.

According to Oke (1977), the air space above a city can be divided into the so-called urban air ‘canopy’, and the boundary layer over the city space called ‘the urban air dome’. The urban air canopy is the space bounded by the urban buildings up to their roofs. The

EXPERIMENTAL PROCEDURE

In the frame of the urban climate experiment carried out in Athens, 20 automatic temperature and humidity stations have been installed in the major Athens area during spring 1996. At a later phase the number of stations has been increased to 30. The instrumentation used was selected to satisfy several criteria like acceptable cost, in order to cover as many locations as possible, satisfactory performance according to the international meteorological standards, low maintenance, internal power

TEMPERATURE AND DEGREE DAYS DISTRIBUTION

The collected data have been analyzed in detail in order to assess the heat island intensity in the city of Athens as well as the specific distribution of the ambient temperature in the city. Specific and detailed statistical and climatological analyses have been performed, however, presentation of this analysis is out of the scope of the present paper.

During the summer period much higher temperatures have been recorded in the central Athens area especially during the daytime. Fig. 2 plots the

CALCULATION METHODOLOGY

The collected data have been used to calculate the distribution of the cooling and heating needs of a representative office building for all locations where climatic data were available.

The considered building is constructed in seven different levels, and has a total surface of 500 m2. It is used by 25 people and is a low energy building involving many energy conservation features to decrease its heating and cooling needs, Fig. 10. A full description of the building is given in Allard (1998).

IMPACT ON THE SUMMER PERFORMANCE OF URBAN BUILDINGS

The calculated spatial variation of the cooling load of the reference building for a set point of 27°C and for August 1996 is given in Fig. 11. Values are in kWh per square metre and month. Very local phenomena and conditions are not shown in the maps unless a measuring station was placed locally. Fig. 11 gives the iso-cooling load lines, (in kWh per square metre and month), indicating the spatial variation of the cooling load for the whole region of Athens. As shown, the cooling load at the

IMPACT ON THE WINTER PERFORMANCE OF URBAN BUILDINGS

Increased urban temperatures may have a serious impact on the heating load of urban buildings. Calculations of the heating load of the same reference building have been performed for all stations and the whole monitoring period. All stations have been grouped in three cluster stations located in the very central Athens area, suburban stations and urban parks and green areas. The corresponding heating load for each station is given in Fig. 14. The heating load in the central Athens region was

IMPACT ON THE NATURAL VENTILATION POTENTIAL

Natural ventilation of buildings located in urban canyons is seriously reduced because of the important decrease of the wind velocity inside the canyons. Air flow phenomena associated with urban canyons are extensively discussed in Santamouris (1999). Experiments in 10 deep canyons during the summer 1997, have shown that mean wind speed inside the canyon rarely exceeds 1 m/s, independent of the free wind speed above the buildings. Fig. 15 shows, as an example, the variation of the air speed

CONCLUSIONS

The present paper discusses the energy impact of higher ambient temperatures in the city centers. Data from the Athens urban climate have been used to evaluate the increase of the cooling load of urban buildings. It is found that for a representative building, the cooling load almost doubled in the central Athens area, while peak electricity load may be tripled for higher set point temperatures. In parallel, the minimum COP value of the air conditioners in the central Athens area is reduced by

Acknowledgements

The present research is partly financed by the European Commission, Directorate General for Science, Research and Technology under the contract JOR3-CT95-0024. The contribution of the Commission is gratefully acknowledged.

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