4.2 Simulation in Ghardaia, Algeria
The main points:
• Daytime overheating can be eliminated by internal storage mass
• Reflective outer surface color reduces the indoor temperature.
• The cooling effect of continous ventilation depends on the structure (heavy or light) and the outdoor temperature.
• A reduction of the ventilation rate during the day is only advantageous in the case of a heavy structure (with high thermal capacity).
• Thermal insulation decreases the indoor temperature in the hot season and increases it in the cold season. This is especially the case for surface temperature.
• The influence of the window size is higher in the case of a light structure than in the case of a heavy structure.
• The influence of double glazed window panes is negligible.
Source: Parametric study by Lund University, (LCHS) and CNERIB, Algeria, carried out by Hans Rosenlund.[ 156 ]
4.2.1 The simulation configuration
The geographical location and climatic characteristics have already been described in chapter 4.1.1
In this example a model building has been simulated by a computer program, where the influence of various parameters on the indoor temperatures were examined. The project was carried out within the framework of the research cooperation “Building in Hot and Arid Climates” between Lund University, Sweden (LCHS) and Center National d’Etudes et de Recherches IntegrÄes du Batiment (CNERIB), Algeria. The principal researcher was Mr. Hans Rosenlund. The simulation program used was JULOTTA (Kèllblad 1986).
The building model chosen for this study is a two story row house. The performance of the upper floor of the middle section of the house is calculated, with a width of 7.2 m and a depth of 10.2 m. The main facade can be varied with respect to window size and the depth of the horizontal projecting roof slab above the windows.
The basic types
Two basic types of structures are examined: a heavy one and a light one.
The light structure consists of outer walls and roof of light, insulated timber and mineral wool 54 mm thick (U-value = 0.83 W/m²K), with negligible internal thermal storage capacity. The window covers an area of 5.23 m².
The heavy structure, which acts as a thermal storage with a capacity equal to 100 m² of 200 mm concrete, consists of heavy outer walls and roof of 200 mm concrete (U-value = 3.0-W/m²K). The window measures 3.34 m².
In both cases, no roof overhang above the windows, and an air change of 1.1 per hour is calculated.
The absorptivity of the outside of the walls and roof is 80% (a = 0.8) for the case in Chapter 4.2.2; for the cases in Chapter 4.2.4 - 4.2.7 the absorptivity is 20% (a = 0.2).
The internal heat load assumed is about 400 W at nighttime between 6 pm and 8 am, and 200 W during daytime.
The variable parameters
Based on the calculation of the thermal performance of the basic types, a number of variables have been introduced and their influence on the thermal performance calculated:
a) Influence of internal storage capacity
Restrictions and purpose of the study
The main results of this parametric study which are summarized hereafter, give a somewhat theoretical picture. In reality the indoor climate is always influenced by a combination of these factors as well as other factors. Simply adding the various influences of the parameters is not possible. Also, the human factor is not considered, i.e. the unreliability of controlled ventilation, which depends on the active participation of the inhabitants. Furthermore, the JULOTTA-program does not sufficiently incorporate the influence of radiation to the night sky, thus giving slightly too high indoor temperatures. The results therefore do not represent real indoor temperature figures.
It also has to be kept in mind that the building chosen represents a “general model” which is not adapted to the climatic conditions of Ghardaia at all.
However, the results can be used to judge the importance of the different parameters and to estimate their effects, positive or negative, strong or weak.
4.2.2 Influence of internal thermal storage capacity
Results of 4.2.2
As Fig 4/4 a) and b) show, on a hot day the maximum temperature can be significantly reduced with the help of the thermal mass of the whole structure. The internal thermal storage mass is cooled down during the night by an increased ventilation (10 air change per hour). Thus, during the day, when the ventilation rate is lower (1.1 air change per hour), the internal mass maintains a lower indoor temperature. The increased internal mass, however, means a slightly higher night temperature.
An interesting comparison between the heavy and the light structure house is that, in the former, the time lag of the heavy outer walls causes the temperature to rise to a maximum of 43.5°C as late as at 6 pm. In the light structure house, with internal mass, the maximum temperature is lower, 40.9°C, and occurs earlier, at 3 pm. The high altitude of the sun makes the difference in window size less important, while the effect of roof insulation in the light structure house is obvious.
The negative effect of the non-insulated concrete roof in the heavy structure is clearly seen in the case without internal heat storage mass, with a temperature rising up to 49°C towards evening and only dropping then, due to the increased ventilation that starts at 9 pm.
Fig 4/4 c) and d) show that on a cold day an internal mass decreases the variation in temperature while the average temperature remains the same. This phenomenon is more pronounced in the light structure house, being more affected by the solar radiation through the bigger windows.
The relatively high average indoor temperatures are a consequence of the assumed internal load, the low air change rate and the solar heat radiation gain. Moreover, the result is slightly falsified because of the radiation losses to the night sky that are not sufficiently considered in the calculation program.
As a consequence, the internal thermal storage capacity of the building, not being exposed to solar radiation, is an important means of increasing winter night temperatures and decreasing maximum temperatures during the hot season.
4.2.3 Influence of outer color of walls and roof
A lower absorptivity - or a higher reflectivity - results in generally significantly lower indoor temperature, both in the heavy and light structure houses. The heavy structure house also has clearly lower night temperatures due to decreased heat storage of solar radiation in the outer walls and roof. For the same reason the variation between day and night temperature’s is smaller.
4.2.4 Influence of continuous ventilation
Conclusion for 4.2.4
Fig 4/6 a) and b) show that an increased continuous ventilation causes the indoor temperature to approach the outer one. This means - in the heavy structure house - a decreased night temperature, while the maximum temperature, being lower than the outdoor one, increases with the increased ventilation rate. The light structure house, with low thermal storage capacity, generally has lower indoor temperatures with the increased ventilation rate, due to the fact that the indoor temperature exceeds the outdoor one at all hours.
During winter, the indoor temperature can be increased by a decrease in ventilation rate.
4.2.5 Influence of reduced ventilation during the daytime
Conclusion for 4.2.5
Fig 4/7 displays the difference between a permanent ventilation of 10 air changes per hour and a ventilation reduced to 1.1 ach during the daytime. In the heavy structure house, the day temperature decreases with reduced day ventilation. In the light structure the opposite is the case.
As a consequence, reduced day ventilation is only advantageous in heavy structures. In other words, the advantage of storage mass is only fully exploited if combined with reduced ventilation in daytime.
The case of a very cold day with the same variation in ventilation rates is not relevant.
4.2.6 Influence of thermal insulation on air temperature
Conclusion for 4.2.6
Fig 4/8 a) and b) show the importance of roof insulation especially during the hot period when the solar radiation effect is at its greatest. Both structures - the heavy and the light one - perform better with increased insulation, but the light structure house has higher temperatures and a wider range. The heavy house has lower night temperatures with increased insulation due to “internalization” of mass which makes it more efficient in combination with night ventilation.
In the cold season, the heavy structure house has generally higher temperatures due to increased insulation, while the light one has lower maximum, but higher minimum temperatures.
4.2.7 Influence of thermal insulation on ceiling surface temperature
Conclusion for 4.2.7
During the hot season, the ceiling temperatures are equally decreased by roof insulation. The heavy roof has an almost constant temperature around 36°C. Without insulation the surface temperature of the heavy roof rises up to 42°C. This is a rather modest value, due to an assumed high reflectivity (a = 0.2). A normal grey concrete surface exposed to solar radiation would result in an inner surface temperature far above 50°C. The light structure case shows much higher maximum values due to the absence of mass.
During the cold season, the ceiling surface of the heavy structure house generally has higher and more even temperatures with increased insulation, while the light house has higher minimum, but lower maximum temperatures.
4.2.8 Influence of size of south facing windows
Comparing the cases of windows measuring 3.2 m² and 10.4 m² respectively, the indoor temperatures differ during the cold season. In the case of the heavy structure, a larger window results in an increased air temperature of 2 - 4°C. In the case of the light structure the increase is up to 6°C.
During the hot season the differences are negligible. The reason for this is that the direct radiation hardly reaches the window area and for small angles of incidence, most of the radiation is reflected. Furthermore, the portion of diffuse radiation in this climate is very low in July.
4.2.9 Influence of number of window panes
If the effect of double glazing is also calculated; a double glass sealed with 12 mm air space between the panes is studied. In both cases of the heavy as well as the light structure house, the influence of the double glazing is less than 0.4°C and can be neglected. This applies to both winter and summer. The only remarkable difference lies in the inner surface temperature of the window.
Where indoor and outdoor temperatures are similar, the main heat transfer through windows is by radiation which is only marginally affected by a second pane.
4.2.10 Concluding recommendations
• A high internal thermal storage capacity is essential to decrease temperature variations and to profit from an increased night ventilation.
• A white outer surface or a heavily ventilated double roof construction is necessary to prevent the solar radiation penetrating the building structure, especially the roof, during the summer when the angle of the sun is high.
• Southern windows of moderate size receive a great deal of the solar radiation during the cold period, while during summer they are rarely exposed to direct radiation. However, during spring and fall, when temperatures are still high and the angle of the sun is less, the quantity of solar radiation through windows can be considerably higher and needs to be considered, e.g. additional shading devices would be needed.
• The ventilation rate should be kept as low as possible during the winter period. However, due to hygienic reasons, the ventilation rate must not be too low. Another problem which could occur is condensation, especially in the case of structures with a relatively low internal surface temperature in combination with rooms with a high rate of added vapour such as kitchens.
• During summer nights the ventilation should be increased as much as possible by catching the wind or using stack effects. In the daytime, when the outdoor temperature exceeds the indoor one, the ventilation rate should be kept to a minimum. The internal mass thus retains the cool of the night until daytime.
• A good roof insulation is preferable to protect the building from the intense direct solar radiation during summer. The effect of wall insulation is, however, negligible in the hot period, as long as the house has a significant internal storage capacity. Insulation of east and west wall can be considered.
• In the winter an external insulation is very efficient in raising indoor temperatures to acceptable levels. The outer walls could even be of a lightweight and well insulated construction if the building has a considerable mass.
• However, insulation is normally scarcely available and/or expensive. Efforts should be made to develop local insulating building materials in desert regions.
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