E. N. Correa1*, C. de Rosa1 and G. lesino2

1 INCIHUSA — LAHV. Instituto Ciencias Humanas Sociales y Ambientales — Laboratorio de Ambiente
Humano y Vivienda. (CONICET — CCT-Mendoza). C. C.131 C. P. 5500 — Mendoza. Argentina

2 INENCO — Instituto de Investigaciones en Energias No Convencionales — U. N.Sa.- CONICET.
Universidad Nacional de Salta. Avda. Bolivia 5150 — CP 4400 — Salta Capital — Argentina.

ecorrea@lab. cricyt. edu. ar

Abstract

This paper presents the geographical distribution of heating and cooling degree-days in Mendoza’s Metropolitan Area (MMA) taking into account the influence of urban heat island’s intensity over the heating and cooling energy requirements in the city and quantifies the green house gases emissions derived from that impact.

The value of HDD and CDD has been calculated from temperature data recorded at 16 fixed weather stations installed within MMA’s, measuring temperature and humidity in the urban canyons during a full yearly cycle. The calculation is performed using the Erbs’s method and the interpolated data for the considered metropolitan area are mapped using GIS software.

The results obtained have been compared with those obtained from the meteorological standards data computations, indicating that there is an under-estimation of CDD for the city’s center of approximately 20% respect to the value obtained from the meteorological station values, and in the case of HDD there is an over-estimation close to 50%.

Keywords: HDD and CDD; Urban Heat Island; Mendoza’s metropolitan area; energy consumption, climate change.

1. Introduction

The influence of weather on energy consumption, particularly on fossil fuel demand, has been widely reported in the past [1]. Although the energy used in the construction is a function of the climate and of the final use or destination of buildings (public offices, scholar, residential; etc), it is also of the city’s architectural features, which modify the local climate as well, generating a microclimate. Particularly the differences between de air temperature in cities respect to de rural or edge area is known like “heat island effect”. Particularly, MMA, presents a heat island effect whose maximums reach the 10 °C, in winter as well in summer, with an average value of 6°C throughout the yearly cycle [2].

Studies performed during the last decade which correlate energy consumption with the heat island effect, have demonstrated that for cities of more than 100,000 inhabitants, the energy consumption during pick hours is raised by 1.5 to 2.0 % for each degree in the city’s temperature’s increase [3]. Particularly in Argentina, the residential demand of electric energy represents more than 40% of the distributor’s total demand and shows growth’s rates that increase every year since 2002. At the same time, the installation of air conditioned equipment in the country is growing steadily; and is known that the increased use of household electric appliances is the cause greater energy demand.

According to the INDEC (National Institute for Statistics and Census), the production of air conditioned equipment grew a 250% between 2000 and 2005

Besides, it contributes to the increase of environmental pollution in two ways: directly, given that higher urban temperatures operate as a catalyst of the chemical reactions of the combustion gases present in the atmosphere, generating larger quantities of smog; the smog production increases by 5 % for each 0.5 °C of the maximum temperature increased above 20 °С; and in an indirect way, since the increase of the energy consumption required for additional cooling, causes that the generating plants emit larger quantities of combustion gases (CO2, CO, NOx, SOx, water vapour and methane); all responsible for global warming or green-house effect and the acid rain, among the best known environmental effects.

Often the discussion is about how hot or cold is the city, in terms of thermal comfort; in this case it is simple enough to measure the air temperature. But in some cases, it is important to find a way of measuring the impact of the temperature increase over the energy consumed and the air quality of the city. In this sense the degree-day method is a well-known and the simple method used in the heating, ventilating and air-conditioning industry to estimate heating and cooling energy requirements in buildings. The severity of a climate can be characterized concisely in terms of degree-days. But in general the degree days calculations are performed taking into consideration the meteorological data provided by standard meteorological stations which are situated generally in the city’s surroundings.

For this reasons this paper explore the geographical distribution of heating and cooling degree-days in MMA taking into account the influence of urban heat island’s intensity over the heating and cooling energy requirements in the city and quantifies the green house gases emissions derived from this impact.

On other hand degree days affect urban dwellers not only in terms of energy consumed, also the growth rate of many organisms is controlled by temperature. Diverse concepts related to degree — days are used to connect plant growth, development, and maturity to air temperature. In addition degree-days are linked to air quality and health problems. The modification of the urban ecological balance plus the worsening of life quality affect the city’s sustainable condition.

It is well well-known the energetic and environmental crisis that affects our planet as a result of the irresponsible way of using natural resources during the last century. The energy performance of buildings has taken on a greater significance with the setting of domestic and international targets for the reduction of greenhouse gas emissions. Energy use is the largest contributor to these emissions. It is expected that the results presented in this paper and their availability fill a gap in information needed by building designers and engineers for simplified energy calculations and construction of buildings with rational energy use and minimal emissions in the city of Mendoza.

2. Methodology

The studies performed by Oke [4] offer a valuable help to collect meteorological data in urban areas. It is possible to obtain results of acceptable quality over the heterogeneity of the urban areas, but this requires paying careful attention to the principles and concepts specific to urban areas. In this work, the guidelines suggested by the WMO on “Instruments and Observing Methods”, report Nr. 81 entitled “Guidance to Obtain Representative Meteorological Observations at Urban Sites”

[4] are applied.

With the purpose of quantifying of Urban Heat Island intensity; the thermal behaviour of the city was monitored in a continuous fashion, during a complete yearly cycle, starting from January 2005, 16 fixed points, equipped with automatic stations, measuring temperature and humidity in the urban canyons every 15 minutes, were installed. The stations installed are of the type: H08- 003-02, two channel logger with internal temperature and user-replaceable RH sensors, temperature measurement range: -20 to 70 °С, temperature accuracy: +/- 0.7° at 21°C, RH measurement range: 25 to 95 % RH (user replaceable RH sensor), RH accuracy: +/- 5% RH. The sensors were placed at a height of 2.5 m from the street level floor [4], within perforated PVC white boxes, in order to avoid irradiation and assure an adequate air circulation. Figure 1 presents the locations of fixed measurements within MMA and their installation features.

The severity of a climate can be characterized concisely in terms of degree-days, but in general, the degree-day’s calculations are performed taking into consideration the meteorological data provided by standard meteorological stations which are generally located in the city’s surroundings. Particularly in Mendoza city the three stations corresponding to the National Meteorological Service of Argentina are placed on opposite sides of the city’s edge and the third one in a central position of a densely forested large urban park. See red stars figurel.

The degree-day method estimations are accurate if the internal temperature, thermal gains and building properties are relatively constant. In the study, the calculation of heating and cooling degrees days has been carried-out from the registered data, applying the method developed by Erbs, to each series of data, as it is described in Al-Homud [5]. The monthly degrees-day values were calculated according to equation 2.1. In the equations, Ta is the monthly average temperature; D m is the amount of days of the month; aDy is the standard deviation of the monthly average temperature respect to the annual average temperature and aDm is the standard deviation of the daily average temperature respect to the monthly average temperature.

The calculations have been made using 18 °C bases for the computations, this temperature has been selected from the distribution graph of the energy consumptions versus the registered average temperatures [1], the tendency curve of this distribution shows a point of inflexion around 18°C. The degree-days obtained have been compared with those corresponding to the weather station airport (- 32,85°, 68,78° longitude and 700 m. a.s. l. of altitude) located in the NE sector of the city’s outskirts.

Although the zoning obtained from the collected data during 2005, does not have meteorological rigor, because greater extensions time series are required, its value is to point-out the range of the urbanization impact on the HDD-CDD parameters calculation. Table 1 shows the comparison between the data registered by the airport’s station during 2005 versus the data registered in the 90’s. It is observed that the monitored year does not display atypical behaviour that underestimates the conclusions derived from this study.

The heating and cooling degree-day calculated from the data registered by each fixed station within the urban grid have been interpolated with the purpose of zoning their distribution within Mendoza’s Metropolitan Area (MMA). The interpolations have been done using the IDW method (distance’s inverse), which, compared to the Krigging Universal and the Spline methods, has demonstrated to be the most accurate, since it minimizes the quadratic error. In order to ease the calculations, the number and distance to neighboring points to be taken into consideration, should avoid those too distant and restrict them to a determined number. A variable radius, with a maximum limit of 2,000 m and a number of 12 points has been considered for the analysis. Their cartographic representation and digitalization in (GIS) was made using Arc. View 3.6.

To predict the impact over the global warming emissions that produce the heat island effect, the calculations were made taking into consideration de percentage variation between the urban HDD and CDD values and those calculated from the data registered at the airport. The factor emission values derived from the electricity generation and natural gas production in Argentina are expressed in kg of CO2 and were taken from the information provided by National Direction of Energetic Prospective. With the scope of compare the annual emissions, this values was expressed by MJ, considering: Natural Gas PCI=8400 kcal/m3 and 1KWh =3.6 MJ.

3. Results