Las razones de Cuba traspasan sus fronteras

In 2001, a study of the urban heat island of Rio Gallegos in the south of Argentina was undertaken by students of the Postgraduate course organised by the National University of Southern Patagonia. This city, located at latitude 51° S, is the most important city of Southern Patagonia with a population of 100,000 inhabitants.

When this study was first proposed as a practical exercise to search for evidence of the impact of the built-up area to the environment, it was considered doubtful that a significant heat island would be detected, due to the following series of unfavourable conditions for the detection of this phenomenon:

• Persistent and very high wind velocities, typical of the Patagonian Region.

• Open urban tissue with wide streets.

• Predominantly low buildings, many with low thermal mass, and scarce vegetation.

• Very limited traffic at the hour of the measurements.

• Insignificant heat generation by industry.

However, when the measurements were made, the variation of temperature detected was even higher than that measured in the previous experiments in the dense, temperate and busy city of Buenos Aires, with a total population one hundred times larger than the city of Rio Gallegos.

It is indeed relevant for the purpose of the exercise to note that this was detected in spite of the unfavourable climate, the lower population, the lower building density, the lack of vegetation and the lower emissions of heat from transport and industry.

10.5.1. Location and climate.

The city of Rio Gallegos, typical of Patagonian settlements, is situated on the southern side of the estuary, with a relatively flat topography with little natural vegetation. The majority of the buildings are one or two storeys high, with a few higher buildings in the centre and in social housing projects, though these rarely exceed 4 storeys.

This city is located in the Bioclimatic Zone 6, Very Cold, according to the classification in the IRAM Standard 11.603 (1996) and the heating degree days exceed 3100 with a base temperature of 18°C, three times the value found in Buenos Aires.

In spite of this difference and the low design temperatures, the typical values of thermal transmittance of walls and roofs of heated buildings are very high, resulting in high heat losses.

10.5.2. Results.

The resulting isotherms, plotted on the aerial photo of the city, seen in Figure 10.10., show the influence of the following factors:

• The isotherms based on the temperatures measured by the mobile stations clearly show the formation of a heat island, with higher temperatures related to the central area of the city.

• The tidal estuary of the Ría, which forms the northern boundary of the city, shows it’s influence on the temperature distribution.

• The sectors with little urban development to the south and south-west, occupied by army bases, have lower temperatures than the adjacent urban areas.

• The prevailing winds from the west and south-west, with a velocity between 14-18 km / hour (4.5 m/sec), measured at a height of 1.50 metres in the urban area, produce a displacement of the warmed air towards the east. In the airport, an average wind speed of 44 km/hr was recorded at a height of 10 metres with a westerly orientation, and gusts of up to 66 km/hr at the time of the experiment.

• The form of the isotherms and the variation in temperatures measured exceed the values that could be generated by geographical factors, such as the river estuary to the north, the Atlantic ocean several kilometres to the west, or the limited differences of altitude.

The minimum temperature of -2,7° C was registered in open country on the road to the airport, situated some 4 kilometres to the west of the city. The temperature measurements, registered at the airport met station, indicate a temperature of 1°-2° C at the time of the survey.

The shorter distances covered during the heat island experiment, compared with those of Buenos Aires, reduce the time to complete the circuits and minimise possible errors due to temperature variations. The total daily temperature variation was only 7°C, with high winds and predominately cloudy skies.

10.5.3. Discussion.

The significant intensity of the urban heat island measured in the city of Rio Gallegos is surprising, considering the very limited vehicular circulation at that hour, the open urban tissue and wide streets, high wind velocities and overcast skies.

The majority of the buildings, especially those of traditional buildings, are of lightweight construction with timber structure, corrugated iron roofs and, in many cases, walls of metal sheets or timber siding. More recently, housing was constructed of concrete block or hollow clay blocks, with lightweight roofs. The typical construction, therefore, has little thermal capacity to store the limited heat of the day to night-time.

The maximum temperature difference between open countryside and the city centre exceeds 5° C. In this case, the contribution of transport, industrial activities and the thermal capacity of buildings to store heat from the day to the evening hours is very limited. Therefore, it is deduced that the principal cause of the heat island detected is the high heat loss from buildings.

The thermal performance of typical buildings and the contribution of the special energy tariff structure, heavily subsidised by the central government with special consideration in this remote and cold region of the country, are considered to be the main contributary factors.

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Figure 10.8. Temperature at the airport on the day of the heat island experiment: the red line shows the temperature range measured in the city during the experiment: 1^st June, 2001, at 9 pm.

In most cases, the thermal performance of walls and roofs are designed to comply with the minimum levels established in the IRAM Standards (IRAM 11.605, 1998), scarcely sufficient to avoid surface condensation, though even this level of insulation is not compulsory. Housing construction using hollow concrete blocks, frequently used in the low cost private sector, does not even reach this minimum standard, while the earthquake resistant structures of concrete frames introduce important cold bridges, increasing heat losses and the incidence of internal surface condensation.

From direct experience of the indoor temperatures in housing and conversations with local residents, it was confirmed that indoor air temperatures frequently exceed 23° C and may reach 25° C, values well in excess of those required to ensure comfort.

However, these temperatures may be adopted to compensate for the low surface temperatures of walls and roofs without adequate thermal insulation. This increase in the indoor temperature may produce additional heat losses in the order of 30 to 40 %.

An additional factor that contributes to high heat losses in buildings is the subsidy included in the provision of natural gas in Patagonia, intended as a social benefit for the inhabitants of urban areas in the southern region, many of whom are government employees with relatively low salaries. This allows families to heat the poorly insulated public housing during the extended cold season. However, this benefit does not extend to rural areas without access to the natural gas network which only serves urban areas.

The subsidy is also intended as a compensation for the inhabitants of provinces which produce natural gas and petrol.

It should be noted that the internal domestic natural gas tariff in urban areas is well below the industrial tariff, which in turn is below the cost in most other countries. The additional subsidy of 50 % in Patagonia is now equivalent to over 80 %.

The gas in canisters used in rural areas and marginal suburban areas has no subsidy, leading to prices 5 to 6 times the urban tariff, resulting in social inequality. The result is a total lack of incentives to save energy or improve the thermal characteristics of the building envelope. Studies by ENERGAS (2006), the organization responsible for controlling the privatised gas production and distribution industry, found that gas consumption in areas with this additional subsidy was twice as those found in areas without subsidy, for the same difference between outdoor and indoor temperature (set at 20° C).

An important result of this study is the evidence of large heat losses from buildings, especially residential ones, for it is argued that these are the main cause of the Rio Gallegos heat island measured and included in this study. These heat losses are not only a result of the poor thermal transmittance of walls and roofs, but also the form and grouping of buildings. A large proportion of residential buildings in Rio Gallegos are single storey and free standing.

Therefore, a 5° C increase in the air temperature implies a significant environmental impact due to the use of fossil fuels, directly related to the architectural design and construction systems used. Although the small increase in the outdoor temperature could be considered favourable, both for users of outdoor spaces and for the reduction in the heating demand, this is achieved as a result of excessive heat losses and uncontrolled energy use in buildings.

There is an apparent difference between the temperatures measured in the airport in a Stevens Screen, obtained from Wunderground (2006) and in the mobile stations measured at the same time during the heat island experiment. Despite the strong winds, the Stevens Screen appears to produce a slight delay in the temperature readings, in addition to the 45 minute difference between solar and civil time.

Following the heat island experiment, the Airport reported a continuing drop in temperature to -1° at midnight. It should be added that small ponds in the peripheral urban area remained frozen during the day of the heat island experiment, showing the prevailing temperatures.

Figure 10.8. Rio Gallegos winter urban heat island, measured at 9 pm, 1st June, 2001.

Reference: Red circle: starting and finishing point of the six simultaneous circuits.

Grey zone: central area of the city.

Source: Aerial photograph obtained from Google Earth (2006).

The estimated temperature range in the city centre is 6°, from +1 to + 7, with an average temperature of +4°C, while the temperature to the west of the city ranges from -2° to +4.5°C, with a swing of 6,5 degrees and an average temperature of 1,5°C. This difference, leads to a 16 % increase in the heating load of houses in the peripheral areas to the west, assuming an average daily indoor temperature of 18°C. This corresponds to a temperature of 20°C in living rooms during the day and 18°C in bedrooms, dropping to 18° and 16° respectively during the night.

Although the rise in the urban temperature can be considered favourable, it is only obtained at the expense of excessive heat losses from the building fabric. The saving of 16 % of the heating load is small, compared with the 60 % saving that could be obtained by improved insulation of buildings.

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