Bioclimatic Environment of San Luis Potosí

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5 Design of an Earth-to-Air Heat Exchanger

As it was stated on Chapter 3.2, the usage of natural ventilation in architecture has many benefits. It is a useful strategy to achieve adequate indoor air quality levels to create healthy conditions for the users of a building, as well as it can be used to achieve passive acclimatization of the interior spaces, reaching comfort temperatures and acceptable humidity levels. As García Chávez & Fuentes Freixanet (1995) state, a good control of the wind allows cooling a building through the airflow of natural ventilation, heating it through thermo-convective processes, enhance humidification through evaporative cooling or dehumidification through condensation. Nevertheless, the random behavior of wind creates difficulties to properly implement natural ventilation in an architectural project (Mermet, 2005).

On the other hand, under some climate conditions, the implementation of natural ventilation in a building can increase its energy consumption, due to the necessity of being heated, cooled or just circulated (Autodesk, 2013). The advantage of integrating a passive strategy such as an EAHE is that the energy transfer of the air and the soil not only helps pre-cooling the air in hot seasons, but it can also pre-heat it in cold seasons. Brown &

DeKeys (2001), state that the massive properties of the ground allow its temperature peak to lag between 10 to 13 weeks behind the seasonal changes, making it feasible to be used as a heat sink (Larson, 2014). Therefore, “air may be passed through buried ducts and thus used to cool or heat incoming fresh ventilation air” (Brown & DeKay, 2001).

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77 Table 11. Bioclimatic environment of the city of SLP

Bioclimatic Environment San Luis Potosí

Latitude: 22.3° N Longitude: 100.9° W Altitude: 1887 MASL Mean Annual Temperature……….…….………. 18.2° C

Mean Annual Thermal Oscillation ……….….………. 15.0° C Mean Annual Relative Humidity……….……….. 52 % Mean Annual Precipitation………..………. 315 mm Mean Annual Solar Radiation………..……… 6.3 kWh/m² day Wind……….………... 4.8 m/s SW, 4.1 m/s NE

Comfort Temperature San Luis Potosí

Comfort T° Min Comfort T° Max Comfort T°

23°C 19°C 27°C

Source: (Aguillón Robles, 2007)

Seasonal Climate

Over the year, the different proportion of climate elements in the bioclimatic environment causes different characteristics, creating four specific microclimates in the city of San Luis Potosí: Cold semi-dry; Warm-dry; Temperate semi-dry and Temperate-dry (See Table 12).

Table 12. Seasonal climate of the city of SLP

Source: Self-construction based on Aguillón Robles (2007)

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78 According to the previous table, Aguillón (2007) describes four primary acclimatization objectives along the year:

1. To control and optimize exterior heat gain: With the strategy of solar control, it is possible to let the solar radiation into the spaces or block it when necessary.

2. To avoid nocturnal heat dissipation: High thermal mass materials release energy more slowly over the night. Well sealed windows and elements as fabric curtains reduce heat dissipation outside the building.

3. To control and take advantage of exterior, natural ventilation: Natural ventilation must be avoided in cold seasons to prevent heat losses (See Table 13). In hot seasons, natural ventilation must be stimulated to cool the internal spaces.

4. To enhance humidification: In hot seasons, natural ventilation can be enhanced with humidification, letting the air pass through wet elements or even water fountains.

Tables 13 to 16 show the characteristics of each seasonal-climate found in the city of San Luis Potosí, as well as their specific acclimatization requirements. Since this research project focuses on an acclimatization/ventilation system for social housing, natural ventilation and humidification requirements are highlighted in every table.

Table 13. Cold semi-dry seasonal weather

COLD SEMI-DRY (January, February, December)

Temperature Maximum: 22.3°C Minimum: 6.7°C Average: 14.5°C

Relative Humidity Maximum: 98% Minimum: 11% Average: 50%

Comfort Temperature 22.1°C

Wind Direction: SW Speed (m/s): 4.6

Solar Exposure 9 hr/day Sun’s maximum height: 47°

Solar Radiation 7.248 kW/m²/ day

Precipitation Total: 5mm Maximum: 48mm Maximum in 24hrs: 17mm Seasonal acclimatization requirements:

• Take advantage of internal heat production

• Take advantage of thermal gain, optimizing solar radiation, if possible, all day

• Block exterior ventilation

• Humidification control between 11h -17h

• Foster the usage of high thermal mass materials

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79 Table 14. Warm-dry seasonal climate

WARM-DRY (March, April)

Temperature Maximum: 28.7°C Minimum: 10.9°C Average: 19.8°C

Relative Humidity Maximum: 97% Minimum: 6% Average: 38%

Comfort Temperature 23.7°C

Wind Direction: E Speed (m/s): 4.3

Solar Exposure 10 hr/day Sun’s maximum height: 78°

Solar Radiation 7.624 kW/m²/ day

Precipitation Total: 8mm Maximum: 32mm Maximum in 24hrs: 13mm Seasonal acclimatization requirements:

• Control internal heat production

• Block solar radiation to avoid heat gains between 11h and 18h

• Incite humidification between 11h and 16h

• Control natural ventilation

• High thermal mass materials

Table 15. Temperate semi-dry seasonal climate

TEMPERATE SEMI-DRY (May, June, July, August, September)

Temperature Maximum: 27.4°C Minimum: 10.9°C Average: 20.4 °C

Relative Humidity Maximum: 97% Minimum: 19% Average: 57%

Comfort Temperature 23.9°C

Wind Direction: NEE Speed (m/s): 4.1

Solar Exposure 11 hr/day Sun’s maximum height: 98°

Solar Radiation 10.851 kW/m²/ day

Precipitation Total: 49mm Maximum: 128mm Maximum in 24hrs: 51mm Seasonal acclimatization requirements:

• Control internal heat production

• Block solar radiation to avoid heat gains from 9h

• Optimize humidification

• Control rain infiltration

• High thermal mass materials

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80 Table 16. Temperate-dry seasonal climate

Source: (Aguillón Robles, 2007)

These tables give an overview on the specific time of every season where the EAHE must be turned on or stay off, or even enhanced with a humidification device to fulfill the acclimatization requirements, as well as the wind direction and speed registered along the year, which can help deduce the behavior of the wind on the housing unit. It is important to mention that, depending on the weather conditions, the 230W fan can be turned on or off as well. If the sky is clear, solar radiation will induce the air suction by the solar chimney, avoiding the usage of the fan. On the other hand, if the cloudiness doesn’t allow the proper function of the solar chimney, the fan must be turned on to supply the air on the required spaces of the house.

Temperate-Dry (October, November)

Temperature Maximum: 23.5°C Minimum: 9.4°C Average: 16.5°C

Relative Humidity Maximum: 98% Minimum: 16% Average: 60%

Comfort Temperature 22.7°C

Wind Direction: E Speed (m/s): 3.0

Solar Exposure 10 hr/day Sun’s maximum height: 43°

Solar Radiation 7.624 kW/m²/ day

Precipitation Total: 18mm Maximum: 55mm Maximum in 24hrs: 34mm Seasonal acclimatization requirements:

• Control and optimize internal heat production

• Control solar radiation, inciting heat gain from 11h to 16h

• Incite humidification from 11h to 16h

• Stimulate and control natural ventilation

• High thermal mass materials

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