«CSAAR (7: 2010: Amman) Sustainable Architecture and Urban Development \ Edited by Steffen Lehmann, Husam Al Waer, Jamal AI-Qawasmi. Amman: The Center ...»
The world is alight with lowering energy consumption and the consequent carbon reduction but buildings which do not satisfy human needs are not sustainable. Another major consideration is that buildings need to be cared for. A major issue is that most of the variations between energy consumptions of similar buildings in similar locations is occupancy behaviour. Low energy architecture has to consider these matters and their interaction with other aspects such as water, waste and pollution. This session is a starting point.
Derek Clements-Croome The University of Reading, UK Sustainable Architecture and Urban Development 171 Ground Cooling and Heating Potential in a Mediterranean Climate: An Evaluation of Temperature Extremes in Malta Vincent Buhagiar, Cornelia Tabone, Tonio Sant University ofMalta Abstract In consideration of the ever-increasing energy demand for cooling in Malta, a typical Mediterranean Island, this paper evaluates the potential for ground cooling as part of a strategie passive design scheme towards low energy architecture. It also extrapolates the potential for heating in winter, given that inverse seasonal temperature differences occur over a six month time lag. The potential lies with the indigenous rock, globigerina limestone, having a high thermal mass and moisture content.
In principle this uses an air-to-air heat exchange process, where two sets of 200mm diameter pipes were laid vertically and horizontally in excavated trenches at 3.0m below ground level. Ambient air was extracted through the pipes into a purposely built test cel!. Environmental conditions were monitored throughout. Results indicated favourable ground conditions for cooling, more than for heating, given time lag response of globigerina limestone. The vertical pipe exceeded the horizontal pipe in its performance results can be exploited to reduce indoor spring and summertime temperatures, thus reducing the energy demand for cooling.
Keywords: energy, ground cooling, earth tubes, natural ventilation, passive cooling, carbonfootprint.
172 Vincent Buhagiar, Cornelia Tabone & Tonio Sant 1 Introduction In today's age of sustainability, architects and engineers strive towards a better overall perfonnance of buildings with a concerted effort to curtaü energy demand yet sustain thennal comfort and lifestyle.
In an effort to diminish the energy demand for cooling in wann to hot climates, this study investigates the potential for ground cooling in a Mediterranean Island, such as Malta. Given that winter conditions are considered relatively 'mild', this study extrapolates the potential for heating in winter, given that inverse seasonal temperature differences occur over a long time lag, particularly over spring and autumn (Francis, 1981).
1.1 Background Theory
A building exchanges heat with the environment by conduction, convection, radiation and evaporation or condensation. Although for any isolated building convection, followed by radiation are the more conspicuous modes of heat transfer, however for earth structures with greater ground contact, conduction is perhaps the most significant of these four modes of heat transfer with the underlying terrain. The concept of ground cooling is based on the heat-Ioss mechanism with tbe ground, which typically has a temperature lower than ambient conditions.
Various scientific studies have shown that soil temperatures at a depth in excess of circa 3.0 metres are relatively stable, with no diurnal fluctuation and experiencing only a small seasonal or annual variation. This stability is due to the fact that temperate beat waves are dampened as they penetrate through layers of soi!.
Sharan (2004) studied deep soil temperatures over a year in Ahmedabad, in western [ndia at an elevation of 53m above MSL. These studies lead to the
fonnation ofthe following equation:
This equation demonstrates that diurnal fluctuations in temperature are dampened and diminish within less than a metre depth of soi!. However, in consideration that annual waves have a much smaller amplitude of fluctuation, they have the ability of penetrating to greater depths of terrain. The equation also indicates that waves at deeper levels are out of phase, thus resulting in the occurrence oftemperature lag (Sharan, et al., (2004).
Sustainable Architecture and Urban 173 Lechner (2001) graphically demonstrates the variation of soil temperature with depth and time of year, where it can be c1early seen that the greater the depth, the smaller the variation in soil temperature over time (Figure I).
Therefore, large masses of soi! at a stable, quasi-constant temperature render it capable ofserving both as a sink and a source ofheat.
Despite receiving some attention in the 1970s and 1980s, earth tube systems were not appreciated by a wide audience. This was due to a number of perfonuance issues and limitations, namely associated with their moisture content and costs [i].
1.2 Literature Review
The open loop configuration entails the drawing of air by a fan into the tubes.
This air is delivered directly to the building interior and thus provides both ventilation and cooling of the internal environment. It is this system that has been adopted for this study.
Vincent Buhagiar, Cornelia Tabone & Tonio Sant 2 Aim of the Study The principal aim of this study was to monitor two options for ground cooling, namely horizontal and vertical earth-to-air heat exchangers, primarily designed in cooling mode. For this purpose earth pipes were set up in both vertical and horizontal configurations. A comparison and evaluation of their thermal efficiency was the subject ofthis stlldy.
Typical1y Mediterranean, the Maltese Islands have a warm to hot summer with a mild to cool winter, without sub-zero temperatures. Summer days are predictably elear with a solar insolation level of 8kW1m2 Diurnal variations range from 10 15°C, particularly in summer. The average RH of 76% is understandably high for a predominantly marine environment. It is typically windy al1 year round with prevailing wind directions being north-west to westerly; only 13% of the year has no measurable air movement (Leehner,200 1).
3.1 Tbe Maltese Geology
The principle rock strata of the Maltese islands are composed of globigeria limestone, appearing in various types and layers, its density varying with depth.
The construction industry in Malta has always been heavily dependent on this local indigenous quarried material. It has its own unique properties perhaps giving it its almost unbeatable popularity among local folk as a standard building material used ror over 5,000 years. Hs struetural integrity, durability, weathering and thermal properties as well as its ease and low cost of quarrying combined with today's techno]ogy for eutting, have made it ideal for earlier dwellings to have their own cellar, cool, spacious and robust, hewn out of the limestone (Trump, 2004).
In consideration of the climatic elements as weH as the limestone'~ properties, environmental and physical eharacteristics point towards exploiting the potential for ground cooling by natural ventilation. This incllldes ail entrainment into underground pipes. Maltese folk have demonstrated their awareness ofthis in cellars in their early farmhouses.
4.1 Experimental Setup Two earth-to-air heat exchangers, a vertical and horizontal twm ~elS l)( PVC pipes, both 200mmO, 5mm thick and 15m total length. These we;.: m:,laikd ;ti il depth of 3.0m below the ground, conmlered idt:ai as Hldi,~alell r'mm pr.:V10li studies outlined above[ 1,2,3,4]. The whok' experimental set-li') was located lTI i' sub-urban, semi-rural area in an open lIeld in Nadur. (iozo, Mah.' ';;·.tc·,'l.; • •, Sustainable Architecture and Urban 175 The geological strata consisted of a layer of top soil followed by a layer of crushed limestone (a.k.a. 'torba '), followed by a bedrock of blue clay simulating an indoor space. The set up is illustrated in figure 2
In order to achieve the original aim of the study, the principal parameters monitored included, air temperature. relative humidity and air movement. All pipes and instrumentation were connected to a plenum underground chamber and into a monitoring room above. This simulated an indoor space, having double glazed aluminium windows and built in HeB (hollow core blockwork) to a height of 3.0m, as typical for Maltese dwellings.
Figure 3:The Monitoring Room set-up
Temperature and RH thermocouples were set up along the pipes at 3.0m intervals. Similar thermistors were used to monitor outdoor ambient conditions as weil as soil temperatures outside and away from the pipes. The horizontal pipe was laid to a nominal camber of 2° in order to drain any internal condensation.
Air was injected into hoth systems through a [an at each of the open ends. The Vincent Buhagiar, Cornelia Tabone & Tonio Sant velocity of the air circulated through the pipes was monitored through a single air velocity transducer located at one end of each pipe.
The Full Study With lessons leamt from a pilot study, the full study was carried out over periods ofbetween two to four days in April-May 2008, representing spring. Through the vertical pipe ambient air was extracted at a range ofvelocities between 0.17ms· 1, through 0.51 to 0.05ms· 1• Separately, the horizontal pipe was subjected to a mean air intake ofO.52ms· l.
5.1 Temperature Response (Vertical Pipe)
Air temperatures dropped steadily through the pipe, as recorded across thermocouples (numbered 6-10 with 6 elosest to air ingress and 10 furthest, just under monitoring mom). Conversely during the night, temperatures rose steadily as the air approached the monitoring room. Both effects indicate a potential for heat exchange with the ground's thermal inertia and its thermal lag. Fig.4 refers.
The notable difference in temperatures is attributed to the fact that thermocouple 6 was located at the entrance of the pipe circa I.2m above ground, meaning that inlet air was away from the intluence of subterranean conditions.
During the day, as the air is channelled deeper into the pipe, readings from thermocoupies 7, 8 and 9 showed a continuous decrease in air temperature, whereas the outlet temperatures recorded by thermocouple 10 increased. A logical explanation for such behaviour of air temperature within the vertical pipe is that as the air travels deeper into the buried pipe, its temperature would Sustainable Architecture and Urban Development decrease. However, given that the outlet of the pipe was at a shallower depth of circa 1.0m soil-embedded, as the air travelled upwards to thermocouple 10, its temperature experienced a slight increase.
An analysis of the same situation but during the night hours displays a reverse effect on air temperature, however with a sIight difference: As air travelled deeper in to the pipe, thermocouple readings marked a gradual increase in temperature. However, as the air was channelled upwards towards the outIet, the readings obtained from thermocouple 10 marked a further increase in temperature, despite it being located at a shallower depth of soi!. This may be attributed to the effect of solar radiation on shaIIow depths of soil, circa LOm below the surface.
The readings obtained from thermocouples 6, 7, 8, 9 and 10 demonstrate that the channelling of air along the length of the pipe has a damping effect on the air temperature. Figure 4 demonstrates how the outIet temperatures recorded from thermocouple 10 fluctuate much less than the inlet temperatures recorded from thermocouple 6. This can be interpreted as the damping effect that ground conditions have on air passed through it. However it is worth noting that the air flow rate is very critical. This depends on pipe diameter and air velocity.
Simulations with varying air flow were tested retaining the same pipe layout but varying only the fan speed, thus the air intake speed. This results in a different heat exchange rate, given ground conditions remain constant.
Within the same testing period, air velocity was deliberately varied through both pipes in order to assess its effect on temperature. When the velocity was increased to 0.51 ms· 1 an average increase in temperature of L5°e was recorded across a11 thermocouples.