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5.2 Temperature Response (Horizontal Pipe)
When extracting air through the horizontal pipe the maximum and minimum temperatures noted during daytime and night time respectively were across thermocouple 1, the closest to the air intake point. This was as expected, similar to the vertical pipe.
During the day, when outdoor temperatures were as high as 23.0o e, (typical spring-time in Malta), indoor air temperature was reduced by 2.5°e to 2I.5°C.
However during the night hours, when extemal temperatures were around 16°e, indoor temperatures were still warmer by 2°e standing constantly stable around 18°e. Figure 5 demonstrates the shallower pitch of the sinusoidal curve between temperature extremes outdoors as reflected indoors. It is also worth noting that when outdoor temperature shot up to 27°e indoor temperatures remained fairly stable, only responding by about 1.8°e. This further highlights the potential of the ground to moderate outdoor conditions, particularly in April-May (during field tests).
Vincent Buhagiar, ComeJia Tabone & Tonio Sant 178 Onee more a quick simulation of changes in velocity revealed that at higher air velocities, the temperature differenee between the inlet and outlet decreases.
In this case air speed was increased from O.l7ms-l to 1.02ms- 1• It was also noted that the difference between inlet and outtet air temperatures dropped by I.2°C from an earlier 2.5°C. Naturally this is attributed 0 the higher convective losses through the pipe.
5.3 Relative Humidity Profile (Vertical Pipe) Figure 6 refers. This demonstrates how the RH profile varies as air is extraeted through the vertical pipe at O.52ms-l, showing a drier air ingress by 6% (99 96%) into the indoor spaee, although admittedly still high by thermal comfort standards, (Saberi's psychrometric chart).
98.00 ~.oo ~4.00 ~ i ~92.00.]1 ~90.00 88.00 84.00
When the air velocity was decreased to O.05ms- l, it was noticed that the outtet relative humidity increased by 2%. On the other hand, when air velocity was Sustainable Architecture and Urban Development 179 increased to 0.46ms·\ the outlet saw a 3.3% drop in RH at the pipe outlet, indoors.
5.4 Relative Humidity Profile (Horizontal Pipe) When the air velocity was increased to 1.02m/s, the relative humidity at the pipe outlet experieneed a drop from almost 2% (73.5-71.2%). Onee the velocity was redueed to O.52m1s RH gradually inereased to 72.5%. Fig. 7 refers. These results further confirm that witb an increase in air flow rate through such a 200mm diameter pipe (both vertical and horizontal), RH is diminished, albeit even if marginaJly.
74.00 72.00 70.00
6 Conclusions6.] Temperature Variations with Air Velocity Changes The above readings were obtained when air velocity through the vertical and horizontal pipes was O.18ms· 1 and O.05ms· ' respectively. Higber air velocities imply higher indoor air temperatures (c1oser to outdoors). However, in spite of the higher air velocity in the vertical pipe (almost quadrupled), it still managed to reduce the air temperature inside it by double the amount the horizontal pipe did (both had equallengtbs of 15m each).
This gives a c1ear indication of the superiority of the vertieal pipe, over the horizontal one (for a given length and diameter), even with higher air velocities.
Perhaps a mathematical relationship can be derived with parameters comprising outdoor and indoor temperatures, pipe diameter, air speed and the latent heat of the ground or its diffusivity (quotient of conductivity over the product of specific Vineent Buhagiar, Cornelia Tabone & Tonio Sant heat capacity and density) (Santamouris, Asimakopoulos, 2001) However this was eonsidered beyond the seope of this study.
6.2 Relative Humidity Variations with Air Velocity Changes An assessment of thennal eomfort parameters was made with the monitored values of indoor air temperature and RH by inserting these along the eomfort envelope on the psyehrometrie ehart, figure 8. This is the result of an overall relationship of a1l relevant parameters, partieularly air temperature, relative humidity and air speed (Leehner, 2001).
When eomparing values of indoor temperatures and RH, it was observed that for the vertical pipe, for air velocities wh ich ranged from 0.16 0.20 ms· l, the outlet temperature and relative humidity obtained ranged from 20.1 - 20.3°C and 53.2 67.8% respeetively. On the other hand, for the horizontal pipe, with air velocities between 0.05 - O.lOms· l, indoor temperatures and RH ranged from 20
- 21.5°C and 47- 73% respeetively.
Therefore, results show that in order to aehieve outlet air temperature and relative humidity levels which fall within the comfort zone in the psyehrometric ehart, air had to be ehanneled at a velocity wh ich ranged between 0.16 - 0.20 mls through the vertical pipe and 0.05 0.10 mls through the horizontal pipe.
The results obtained from the on-site tests earried out prove the effectiveness of both horizontal and vertieal earth pipes. The stable globigerina limestone eonditions resulted in the eooling of entrained wann outdoor daytime air, while during night time, it resulted in the marginal heating of cooler outdoor air. This is particularly significant in spring in a Mediterranean climate such as Malta, where temperatures around 20-27°C, may attraet spaee eooling as early as April-May.
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Hence ground cooling does prove to be an effective passive cooling regime, particularly with Malta's staple thermal inertia attributed to be one salient characteristic of its indigenous porous sedimentary rock.
Experiment Critique In an ideal situation, tests on earth-to-air heat exchangers would be carried out in the summer months, when temperaturcs escalate to relatively high levels.
However, due to the fact that such dissertation projects are typically undertaken between Oct-lune, such tests had to be carried out between April and May.
One shortcoming was that a plenum chamber had to be excavated under the monitoring room to interconnect all pipework feeding ingress air and for wiring thermistors to the data logger. This may have influenced the indoor temperatures, as it may have had a buffering effect on the simulated indoor space.
The monitoring room was (2.4 x 2.4 x 3.0m high). This was considered fairly small compared to a standard habitable room in a dwel1ing. Hence internal convective air flow may have influenced indoor air temperatures and RH.
However this room had double glazed window and door, constructed of single cavity hollow core concrete blockwork.
Although ideally a proper dwelling should have been tested, this was not possible due to constraints of land availability in an urban area. Hence a purposely built room was constructed, serving to interconnect and house all monitoring equipment.
8 Scope for Further Research
Therefore with hindsight, if the full study had to be repeated the following
refinements will be necessary for further and deeper research:
In a more extensive pilot study, different pipe sizes would be tested first (say 100,150, 200, 250, 300mm). However these would need to be tested with different air speeds, as it is a known fact that for a smaller pipe diameter there is a lower convective heat exchange (smaller surtace area of pipe contact with terrain), hence this needs to be compensated for with a slower air speed.
Conversely for a larger pipe diameter (e.g. 200mm i, as for this study) a higher air velocity can be afforded.
In practice, for large-scale experiments, this is particularly relevant for human-scale indoor environments, typically a living room or a whole dwelling.
Air change rate is therefore equally important in order to cool a larger indoor space.
Another refinement would be testing out for a longer period, say 8-10 days, also testing for winter conditions, possibly utilising areal habitable space, such as a living room in an inhabited house. In this way a more realistic, live testing would ensue, with the lO-day period spanning a whole working week and two weekends. Such a monitoring exercise could easily be complemented with a Vineent Cornelia Tabone & Tonio Sant questionnaire survey, for subjeetive assessment of spaee thermal eomfort of oeeupants.
The tests earried out were on pipes designed as an open loop configuration with air as the eireulating medium. These mayaIso be tested on ditferent eireulating media, such as liquids/eompressed gas, tested on closed loop pipe eonfigurations.
Due to the nature of the typical Maltese plot of land, being narrow and long, and the eurrent seareity of land available, the implementation of vertieal earth-to-air heat exehangers would be an ideal remedy for maintaining eomfortable internal environments, partieularly throughout the hot summer months. This aetive means of eooling requires a minimal amount of eleetrieal energy to funetion and thus ean contribute to Malta's eurrent need to reduee eonsumption of fossil-driven energy. Moreover it effeetively reduees a building's carbon footprint.
10 Acknowledgements This paper was the outeome of a 2008 final year B.E.&A.(Hons.), dissertation projeet by Cornelia Tabone,(2008) supervised by the lead author. Apart from continuous moral support, her family also provided both the financial support to buy her own equipment, as weil as the land resouree, made available for the experimental set-up. She is indebted to them forever.
Sineere thanks are also due to all the aeademie and supporting staff at the University of Malta as weH as third parties involved who made -these patience trying experiments possible.
References Leehner, N. (2001). Heating, Cooling and Lighting: Design Methods fm Arehiteets. 2nd Edition. USA: lohn Wiley and Sons, Ine.
Givoni.B. (1984) Passive and Low Energy Cooling of Buildings. lohn Wiley and Sons. Ine., USA Meteorologieal Data provided by the Department of Civil Aviation, Malta International Airport, Luqa, Malta.
Trump, D.H. (2004). Malta Prehistory and Temples. Midsea Books Ltd. Saberi, O. Et al., n.d. Thermal Comfort in Arehitecture.
Santamouris.M., Asimakopoulos, D., (2001) Passive Cooling ofBuildings.
lames & James Seienee Publishers., p.p. 367-371.
Tabone, C. (2008), The Potential of Ground Cooling in Malta Using Earth Tubes, B.E.&A.(Hons.) Dissertation, Faeulty of Arehitecture & Civil Engineering, University ofMalta.
Sustainable Architecture and Urban Development 183 Energy Efficient Design Features for Residential Buildings in Tropical Climates: The Context of Dhaka, Bangladesh Tahmina Ahsan & Orjan Svane Royal Institute ofTechnology, Sweden Abstract This study aimed at identifying passive design features through literature study that can be incorporated in residential buildings of Dhaka to make them energy efficient. The study also aimed at identifying changes in the design process that can affect energy efficiency in residential buildings. It has analyzed the present eJectric energy use for cooling and lighting typical residential buildings of upper middle income households in Dhaka through a case study conducted in Dhaka. It has also calculated the possible energy savings by adopting certain energy efficient features in the case study building.
The findings from this study indicate that doubling the thickness of externat walls on east and west of the building, use of hollow clay tiles instead of weathering course for roofs and use of appropriate horizontal overhang ratios for all four orientations can reduce the cooling load of the case study building by 64% and thus reduce the total energy use of the building by 26%. Finally, it can be concluded that the process of designing energy efficient residential buildings is not a 'one-man's show'. Architects, developers, interior designers and clients are the other actors who can bring a change in the design practice.