A ventilated courtyard as a passive cooling strategy in the warm humid tropics
The parameters considered for evaluating thermal comfort are air temperature and relative humidity. Potential of the courtyard to act as a passive cooling strategy is a function of the indoor airflow pattern. Better indoor thermal modification is seen when the courtyard acts as an air funnel discharging indoor air into the sky, rather than as a suction zone inducing air from its sky opening. The thermal comfort of the courtyard is measured with Givoni’s bio-climatic chart. (H. Nagai, I. Rajapaksha, M. Okumiya, 2002)
Thermal performance analysis of courtyards in a hot humid climate using CFD method
The study is undertaken with the Computational Fluid Dynamics (CFD) method for the estimation of thermal performance of the building. The parameters measured are air temperature (Cº) and air velocity (m/s). Thermal comfort of the courtyard is evaluated by the Predicted Mean Vote (PMV) index. The building under study is a low-rise building. The study established that a U-shape courtyard with aspect ratio of 1:2 performs better than the aspect ratio of 1:1. Increasing the shading area using cantilevered roof provides better thermal comfort. PMV model - treats all occupants the same, disregards location and adaptation to thermal environment. (Abdulbasit Almhafdy, Josmin Yahya, Norhati Ibrahim, Sabarinah S H Ahmad, 2014)
Effect of courtyard shape factor on heating and cooling energy loads in hot-dry climate
For the estimation of the thermal performance of the building, shape factor (W/L) and form factor (V/A) of the courtyard are measured. Annual heating and cooling loads of the courtyard of the respective shape factors are calculated. Geometry of the courtyard form affects the shadows produced on the building envelope. This consequently affects the received solar radiation and the cooling and heating loads of the building. Buildings with lower shape factor are found to have lower heating loads. (G. Koçlar Orala, G. Manio Lua, 2015)
Adaptive Comfort Model
The model establishes that the comfort temperature is a result of interaction between the occupant and the environment - Variable Temperature Approach. It accounts for the following:
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Change of thermal environment can be through – clothing, posture, windows, blinds, doors, change of position, mechanical controls
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Change of human physical paramaters can be through - Activity level (M), Heat loss – H (from the body).
ACS model – gives freedom to the occupants, considers location and adaptation to thermal environment.
Methodology
The entire methodology of the study is as charted below:
Note:
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The combined effects of air temperature and air velocity are studied by the plotting of a derivative comfort parameter – CORRECTED EFFECTIVE TEMPERATURE.
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For plotting the isotherm, EXCEL is used.
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The values for analyzing thermal comfort with the acquired temperature values are established with the help of the software – CLIMATE CONSULTANT (CC)
Field Survey
Stage 1: Documentation of micro-climatic factors
The elements of the building and its surroundings that affect the wind speed are to be documented. The elements include:
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Building orientation
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Adjacent buildings
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Vegetation
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Cross ventilation
Stage 2: Recording real-time air temperature & velocity values
Courtyards work on the principle of convective cooling. It primarily involves the movement of air currents due to difference in pressure induced by difference in temperature of the ground surface and the atmosphere. This effect is enhanced by the external air velocities. Therefore, to evaluate the thermal performance of the mid-rise building, the following thermal comfort parameters are measured:
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Dry bulb temperature
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Wet bulb temperature
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Air velocity
at various points along the horizontal and vertical planes of the courtyard enclosure
Data Handling
Stage 1: Manual – CET values plotted with the nomogram
CET – Corrected Effective Temperature is a thermal comfort index that integrates the effects of air temperature and air velocity. All the DBT and WBT values will be plotted on the nomogram, to get the corresponding CET values.
Stage 2: Software – Isotherms are plotted
With the acquired CET values, isotherms are plotted along the vertical plane for all the four surfaces of enclosure. The variation of temperature with respect to height can be interpreted from the plotted CET isotherms.
Data Interpretation
Stage 1: Software – Real-time thermal comfort analyzed with Climate Consultant data
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WBT max and WBT min are derived from Mean max temp, RH max and Mean min temp, RH min values
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CET max and CET min are derived from the respective WBT values
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CET values for entire year are derived from the hourly temperature calculator
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CET isopleth is plotted and the comfort zone is mapped for still air conditions
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Thermal comfort in different floors for various months and the comfort percentage for the year are inferred based on the above observations
Stage 2: Manual - Relation between building dimensions and respective isotherms
Shape factor = Width of courtyard / Length of courtyard
Form factor = Surface area of the building / Volume of the building
The percentage of space lost on ground, in terms of the shape factor is compared with the percentage of thermal comfort provided and inferences are made.
Tools for Data Collection
CET Nomogram – The nomogram is used to graphically measure the corrected effective temperature with the recorded values of DBT, WBT and air velocity. It also has the comfort range marked, which helps in examining the thermal comfort of the respective readings
Psychrometric Chart – The chart is used to graphically measure WBT with the recorded values of DBT and Relative Humidity
Hourly Temperature Calculator – The graphical calculator helps to extrapolate the calculated CET values from the CET nomogram for CET values for the entire year
Tools for Data Analysis
Isotherm – Isotherm is a graphical representation that consists of curves that join the points on a surface that have the same value of CET
Isopleth – Isopleth is a graphical representation that consists of curves that join the points with the same value of CET, for values throughout the year
Experimental Setup
The site chosen for the study of the thermal efficiency of the courtyard in mid-rise buildings is the Mega Ladies Hostel in National Institute of Technology, Calicut. The building is of seven floors and consists of a central courtyard open to sky. It is enclosed by windowed corridors in the north-western and the south-eastern sides and windowed rooms in the north-eastern and the south-western sides of the building. The dimension of the courtyard is 42 metres by 24 metres (length x width) with the height of the enclosure as 21 metres.
The psychrometer is used to measure the DBT and WBT values of all the grid points in the grid as shown below.
The anemometer is used to record the wind velocity values in all the floor levels of the building. All the values are measured at a height of 80cm from the floor level of each floor of the building.
FACTORS INFLUENCING THE MICRO CLIMATE
The micro-climate of an area is more relevant to the building’s environment than the macro-climatic conditions of the area. Some of the factors that influence the micro-climate of the building under study are:
Building Orientation
The length of the building is oriented along the North-west – South-east axis. Owing to the sun-path, the enclosure with the highest exposure is the one along the south-west orientation.
Adjacent Buildings
The building under study is bounded by low-rise buildings with a maximum of four stories on three sides with the North-western side open to vegetation. The buildings are aligned perpendicular to the building under study and so channel the prevalent major winds towards the building.
Vegetation
The dense vegetation along the North-western side of the building funnels the wind up to four stories height in the building. The remaining floors experience airflow without any obstruction.
Cross Ventilation
The building enclosure is composed of rooms with fenestrations on two sides and corridors with fenestrations on the other two sides enabling cross ventilation throughout the building.
Analysis
ISOTHERM
The isotherms were plotted for all the four vertical enclosures of the courtyard. These graphs give the trend of change in corrected effective temperature with height.
Corridor-side enclosures
In the south-eastern enclosure, the CET values vary from 26.8⁰C to 28.8⁰C with a range of 2⁰C, while in the north-western enclosure, the CET values range from 26.5⁰C to 28.9⁰C with a range of 2.4⁰C. The CET is observed to decrease with height in both the cases, but the change is more gradual in the south-eastern enclosure. There are abnormalities in grid points 5 and 4 observed in the north-western enclosure where there is a sudden change in CET, within the same level. North-western enclosure is comparatively more comfortable.
Room-side enclosures
In the south-western enclosure, the CET values vary from 28.3⁰C to 29.2⁰C with a range of 0.9⁰C, while in the north-eastern enclosure, the CET values range from 28.3⁰C to 29.4⁰C with a range of 1.1⁰C. The CET is observed to increase with height in both cases, gradually. Comparatively, the South-western enclosure is more comfortable, across levels. In general, the rooms close to centre of the enclosures are more comfortable than the ones close to the edges.
In comparison with the corridor-side enclosures, the CET values for room-side enclosures are recorded in still-air condition. The range of variation in CET in the room-side enclosure is comparatively smaller, compared to its counterpart. The trend in change is more gradual in room-side enclosures. Comparatively, the CET values are lower in the corridor-side enclosures, thereby having better thermal comfort than the room-side enclosures. CET values increase with height in room-side enclosures, while in the counterpart, it is the opposite.
Isopleth
The CET isopleth is plotted based on the secondary data obtained as discussed in section, with values for 24 hours for 12 months, thereby giving a clear picture of the heating conditions throughout the year. The isopleth is plotted for still air conditions, considering the most vulnerable condition for discomfort.More than half a year is comfortable thermally, with reference to the isopleths. The range for thermal comfort is considered as 22⁰C to 27⁰C based on the CET Nomogram for 1 clo.
Thermal Discomfort
As per figure, all the regions outside the range 22⁰C to 27⁰C represent thermal discomfort. The discomfort zones can be further divided as:
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Above the comfort zone
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Below the comfort zone
Above Comfort Zone
The region above the comfort zone causes discomfort due to higher CET values, which can be brought within the comfort zone by inducing more air movement and increasing the exposure to wind of the space under study. This kind of discomfort is experienced from the end of February to mid-June.
For the situation under study, the maximum CET value acquired, considering both the room-side enclosures is 29.4⁰C. With that marked on the isopleths, vertical projection from the point gives the level of comfort throughout the day and horizontal projection gives the level of comfort across months for the same time.
Daily Extrapolation - For the maximum condition under study, discomfort ranges from 10am to 7 pm, for the day the study was conducted – as per the isopleth.
Yearly Extrapolation – For the maximum condition under study, discomfort ranges from the mid-March to mid-June.
Below Comfort Zone
The region below the comfort zone causes discomfort due to lower CET values, which can be brought within the comfort range by increasing the exposure to solar radiation of the space under study. This kind of discomfort is experienced from the month of September to April.