Reduction of energy consumption in buildings can be achieved by simple methods and techniques using an appropriate building design and energy-efficient system and technology, such as passive cooling system. The passive air cooling system provides cooling through the use of passive process without using customary mechanical units such as fan, compressor, etc. in regions where cooling is a dominant problem. Passive cooling strategy maximises the efficiency of the building envelope by minimising heat gain from the external sources and assisting heat loss to the natural sources of cooling such as air movement, cooling breezes, evaporation and earth coupling. The principle of passive cooling is to prevent heat from getting into a building during a warm day and bringing in cool air from outside when the temperature drops. Buildings consume a lot of energy, and the building sector is responsible for almost 40% of the total energy consumption on a national level, where the Australian buildings use up to one-third of their electricity on air conditioning (Cooling demand management 2014). The average energy consumption per person increased 10% from 1990 to 2008, where the world population increased 27% (Pérez-Lombard et al. 2008). During this period, the use of regional energy increased in the Middle East by 170%, in China by 146%, in India by 91%, in Africa by 70%, in Latin America by 66%, in the USA by 20%, in the EU-27 block by 7% and in the world overall by 39%. World energy consumption will grow by 56% between 2010 and 2040. Total world energy use rises from 524 quadrillion British thermal units (Btu) in 2010 to 630 quadrillion Btu in 2020 and to 820 quadrillion Btu in 2040 (IEO, 2013). The net energy consumption of Australia increased at an average annual rate of 1.8% over 10 years from 1999-2000 to 2009-2010 (Cooling demand management 2014). The net energy consumption has increased by 1.1% (5,945 petajoules (PJ)) compared to the previous 10 years. According to the Department of Climate Change and Energy Efficiency (Australia), the Australian residential sector energy consumption in 1990 was about 299 PJ (electricity, gas, LPG and wood) and that by 2008 had grown to about 402 PJ and is projected to increase to 467 PJ by 2020 under the current trends. This represents a 56% increase of energy consumption in residential sector over the period 1990 to 2020.
To reduce energy consumption, passive air cooling strategy is seen as a suitable option for all subtropical zones with both high humid summer and warm winter and subtropical zones with warm humid summer and mild winter. Two case studies for residential apartment buildings of Tehran, Iran and Swansea, UK, were discussed, and the cooling strategies were explored which could be adopted to reduce energy usage and the associated greenhouse gas emissions (Nooraei et al. 2013). A bioclimatic chart was developed for passive cooling strategies namely natural ventilation, evaporative cooling, high thermal mass and high thermal mass with night ventilation (DeKay and Brown 2013). The bioclimatic chart suggests different passive cooling strategies for different months in a year. All these passive cooling strategies depend on the daily changes in temperature and relative humidity. Among these strategies, the influence of thermal mass with night ventilation on the maximum indoor temperature was calculated for summer of hot humid climate of Israel (Shaviv et al. 2001).
Passive cooling strategies of evaporative cooling, natural ventilation, ground cooling and radiant cooling were discussed by Santamouris (2007). Three passive cooling strategies were tested for shading and non-shading system to determine their applicability in warm climates (La Roche and Milne 2004). Thermal performance of a passive cooling system of earth pipe cooling was investigated experimentally and numerically for a subtropical climate in Queensland, Australia (Ahmed et al. 2013, 2014a, b). To reduce energy consumption, an air-cooled chiller system in an office building was analysed using passive cooling (Chowdhury et al. 2009). The use of advanced passive cooling was explored in the context of complex non-domestic buildings (Rajapaksha and Hyde 2012). This study evaluated the present barriers and opportunities associated with the challenges of passive cooling through monitoring of an innovative case study of a building in southeast Queensland in Australia. The feasibility of passive cooling in newly built office buildings in the temperate climate of Belgium was assessed using standardised adaptive comfort criteria (Parys et al. 2012). In this research, two passive cooling schemes were studied: diurnal manual window operation and the combination of diurnal manual window operation and passive night ventilation.
Application of two passive cooling systems in a hot and humid climate in Rome was discovered to sustain the room temperature of the test cell to be lower than the ambient air by 2.0°C to 6.2°C and lower than the controlled cell by 1.4°C to 3.0°C (Calderaro and Agnoli 2007). A study was investigated how air temperature in rooms of a residential building is affected by some natural ventilation strategies in a hot dry climate (Idowu 2011). This study found that the variation in wind direction and the location of spaces have significant effect on cooling. Recommendations on the selection of appropriate ventilation strategies in relation to the prevailing external conditions were derived and the appropriateness of the control methods was discussed (Kolokotroni et al. 2001). Specific passive cooling strategies were identified over a part of the Sahara desert of Libya (Agrawal 1992). The potential of passive cooling techniques was discussed for Malaysian modern houses with the aim of reducing air-conditioning usage. A full-scale field experiment was carried out in this study to reveal the detailed indoor thermal environment for various ventilation strategies (Kubota and Chyee 2010). Persson and Westermark (2011) studied energy-efficient specific cooling strategies for a comfortable indoor climate in summer for Swedish climate.
The performance of different mixed-mode cooling strategies for a single-zone office space in four main arid cities representing diverse arid climates was evaluated, and the most effective strategies were considered for each city (Ezzeldin et al. 2009). The new concept of incorporating phase change materials (PCM) inside the building material for enhancing the room air quality and reducing the energy consumption consumed by the air conditionings in the buildings was examined (Madhumathi 2012). The experimental results showed an improvement of thermal comfort and reduction of energy consumption of the building containing PCM without substantial increase in the weight of the construction materials. A research was conducted to evaluate the performance of the existing materials integrated with PCM and to propose a passive design strategy that would improve the system (Isa et al. 2010). This research suggested copper foam as a medium to be integrated with microencapsulated PCM. The relationship between the building design and the natural ventilation was examined by Kleiven (2003). A concept to take ventilation air into the building from the top and to draw it down into the spaces below was examined using the stack effect associated with the difference in temperature between the internal and external environments. Methods of occasionally cooling the vertical intake ducts of passively ventilated buildings, adopting the top-down system both to boost airflows were also discussed (Gage et al. 2001). The effect of most important parameters affecting night ventilation performance such as building construction, heat gains, air change rates, heat transfer coefficients and climatic conditions was evaluated including annual variations on the number of overheating degree hours (operative room temperature >26°C) (Artmann et al. 2008). A bioclimatic chart was developed for Muscat, Oman which suggested some passive cooling strategies for different months in a year (Al-Azri et al. 2013). Most of these studies concentrated on more than one passive cooling strategies selection for different months in a year for hot and humid subtropical climate, which are not cost effective and difficult to implement. Therefore, it is necessary to find a procedure to select a suitable passive cooling strategy for a particular hot and humid climatic location which will assist to install only a specific passive cooling method for the entire life of the building. In view of these, the main aim of this study is to find a procedure to identify that cooling strategy for any hot and humid subtropical climate.
Passive cooling strategies
Passive cooling involves designing buildings for cooling load avoidance (Natural ventilation: cross ventilation 2014). Design strategies that minimise the need for mechanical cooling systems include proper window selection and orientation and day lighting design, selection of appropriate varnishing for windows and skylights, proper shading of glass when heat gains are not desired, use of light-coloured materials for the building envelope and roof, careful sitting and orientation decisions and good landscaping design. Buildings should be designed in relation to specific climatic conditions, the changed function or the time of use or occupancy levels of internal and external spaces, and in relation to how these results will impact the parts that remain unchanged. A passive cooling system is capable of transferring heat from a building to various natural heat sinks (Givoni 1994).
Passive air cooling is the least expensive means of cooling a room with the lowest environmental impact. The system uses elements of the building to store and distribute energy and when prevailing conditions are favourable to discharge heat to the cooler parts of the environment like the sky, atmosphere and ground. The passive cooling strategies eliminate mechanical air conditioning requirements such as fan, compressor, etc. in the modern buildings where cooling is a dominant requirement. Thus, the passive cooling is considered an alternative to mechanical cooling that requires complicated refrigeration systems. Four major common strategies are discussed below.
Natural ventilation
There are two major techniques in natural ventilation systems: cross ventilation and single-sided ventilation. Cross ventilation is attained when rooms with a double orientation with at least two walls face externally in opposite directions as shown in Figure 1a, and single ventilation is achieved when there is only one external facade as shown in Figure 1b. In the cross ventilation system, the action of any wind will then generate pressure differences between those openings and so will promote a robust airflow through an internal space. But in the single ventilation system, wind-driven ventilation flow is dominated by the turbulence of the wind, as caused by temporal changes in wind speed and direction. Hence, the cross ventilation is the design type of choice.
The benefits of using natural ventilation are 40% lower energy cost than the air-conditioned equivalents, capital costs savings in the region of 10% to 15%, increased fresh air supply to a space which may result in higher thermal comfort levels and increased productivity and so on. Furthermore, all the typical cost indicators such as installation cost, capital and maintenance are low.
Evaporative cooling
A method of converting hot air into a cool breeze using the process of evaporating water is the evaporative cooling. By evaporating water, energy is taken from the air and the temperature is reduced. The natural process of water evaporation along with an air-moving system is utilised by evaporative coolers to generate effective cooling. Two temperatures are vital when dealing with evaporative cooling systems such as dry bulb temperature and wet bulb temperature. If the dry bulb temperature and wet bulb temperature are 35°C and 15°C, respectively, the maximum drop in temperature due to evaporative cooling would theoretically be 20°C. The cooling effect due to perspiration on the human skin is an example of evaporative cooling. In hot and humid climates, the cooling effect is less because of the high moisture content of the surrounding air. The four major factors that affect the rate of evaporation are relative humidity, air temperatures, air movement and surface area. There are two general methods of evaporative cooling: direct and indirect.
Direct evaporative cooling involves the movement of air past a water spray (air washer/water spraying chamber) or other wetted medium (evaporative pads, rigid media or evaporating wheel) as shown in Figure 2a. The energy performances of direct evaporative cooler are 250 W per 3,600 m3/h for typical electric fan power and 60 to 100 W for electricity consumption for the pump (Patel 2011). Investment costs are a bit higher than those for standard (vapour compression) air conditioning systems: A direct evaporative cooler costs about one third (1/3), a two-stage evaporative cooler about two thirds (2/3) more than comparable mechanical cooling equipment. Indirect evaporative cooling systems attempt to solve the problem of the high level of humidity that is produced by direct evaporative cooling with the help of a secondary heat exchanger as shown in Figure 2. Indirect evaporative coolers can operate only if the indoor wet bulb temperature is lower than the outdoor dry bulb temperature. In extremely dry climates, evaporative cooling of air has the added benefit of conditioning the air with more moisture for the comfort of building occupants. Lower energy consumption and lower CO2 emission and indoor air quality may be improved due to higher outside air which are the main benefits of using evaporative cooling. Evaporative cooling can save up to 80% of the energy used by a refrigerated air conditioner. The typical cost indicators such as installation cost, capital and maintenance are low in evaporative cooling.
High thermal mass
A high thermal mass structure has the ability to absorb and store heat during the day as shown in Figure 3a and save it for night as shown in Figure 3b. When thermal mass is exposed to the interior, it absorbs heat from internal sources and dampens the amplitude of the interior temperature swing. All matter has thermal mass; however when used in reference to a building, thermal mass generally means materials capable of absorbing, holding and gradually releasing heat. Heavy, dense building materials with high specific heat like concrete, brick and other masonry have high thermal mass. Thermally massive materials absorb heat and slowly release it when there is a temperature difference between the mass and the surrounding space.
By this process, internal temperatures can be held significantly below external ambient temperatures during the summer. Equally, in winter, mass can absorb heat gained which builds up during the day and releases it into the space at night. This can potentially reduce heating demand. Appreciable reduction of the indoor temperature can be achieved in such buildings, with indoor air maxima about 3°C below outdoor air maxima having been observed in some cases (Givoni 1994). The main advantages of using high thermal mass are reduced annual energy use, reduced peak demand, a more stable internal environment, increased acoustic insulation of assemblies and improved fire ratings of assemblies. The installation cost, capital and maintenance cost are low to install this passive cooling system.
High thermal mass with night ventilation
Night ventilation is the use of the cold night air to cool down the structure of a building so that it can absorb heat gains during the daytime reducing the daytime temperature rise. Night ventilation is an overheating prevention strategy which uses little or no fossil energy. The success of this strategy is however highly dependent on large diurnal temperature differences. Heat is normally absorbed and stored during the day by building structure elements, and it is released back into spaces in the second half of the day. Warm air would build up by heat gain from interior surfaces during the night and hence increases air temperatures the next day. As a result, night ventilation is needed to flush the heat out. To optimise the daytime cooling capacity of thermal mass, the mass should be ventilated at night to allow relatively cool night air to remove heat absorbed in the mass during the day as shown in Figure 4. A reduction in the indoor temperature of about 3°C to 6°C below the exterior air may be achievable, depending on the local climate, the amount of mass, its distribution and the ventilation details. Night ventilation in this case can utilise the fluctuation in air temperatures to cool the building envelope and bring fresh air into building spaces.
If the windows are open during the night and early hours of the morning, the building would be cooled and the heat would be eliminated. Night ventilation may be used from 00:00 to 9:00 a.m. during the overheated periods of the year. It may not, however, be without initial costs, since the requirement for ducts and controls may represent an additional cost.
Thermal comfort analysis
Thermal comfort is the condition of mind which expresses satisfaction with the thermal environment (Fanger 1970). In other words, an individual who is experiencing thermal comfort is the one who is satisfied and feels thermally comfortable with his surrounding environment. Four indoor environmental factors such as air temperature, air humidity, air velocity and surface temperatures affect the thermal comfort. Each factor affects thermal comfort differently. The factors most commonly addressed in the conventional design process - air temperature and air humidity - in fact affect only 6% and 18% of our perception of thermal comfort, respectively. To take a more effective comfort-focused approach, temperature of surrounding surfaces and air velocity must be considered, which account for 50% and 26% of thermal comfort perception, respectively. There are three main reasons behind the study of thermal comfort, which is to achieve user satisfaction, efficient energy consumption and to set a standard with a range of thermal comfort temperature for a particular environment. In 2004, ASHRAE Standard 55 has a lower margin of relative humidity that expands the comfort temperature. When the relative humidity is as low as 10%, the suggested comfort range is from 25°C to 28°C, while when the relative humidity is 55%, the suggested comfort range is from 24°C to 27°C (Standard 2004). Various researches have been carried out in search of the correct thermal comfort range for hot and humid climate such as the Australian climate. These researches investigated on occupants living in naturally ventilated, air-conditioned or mixed both naturally ventilated and air-conditioned buildings.
From the previous researches of the indoor thermal comfort range and neutral temperature under hot and humid climate, it has seen that the researches for naturally ventilated buildings were carried out from 1952 to 2009 and for air-conditioned or mixed both naturally ventilated and air-conditioned buildings were carried out from 1990 to 2009. The findings on the occupants living in naturally ventilated buildings have shown that the neutral temperature, Tn ranges from 26.1°C to 28.9°C and the average Tn is 28.1°C. Meanwhile, the thermal comfort temperature ranges from 22.7°C to 33.0°C and the average upper limit of the thermal comfort temperature is 30.3°C. The findings on the occupants living in air-conditioned or mixed both naturally ventilated and air-conditioned buildings have shown that the neutral temperature, Tn ranges from 24.2°C to 27.5°C and the average Tn is 25.9°C. Meanwhile, the thermal comfort temperature ranges from 20.8°C to 29.5°C and the average upper limit of the thermal comfort temperature is 28.3°C. The overall findings for naturally ventilated, air-conditioned or mixed both naturally ventilated and air-conditioned buildings have shown that the occupants living in air-conditioned building have less tolerance to high dry bulb temperature as compared to occupants living in naturally ventilated building. This is the reason for lower neutral temperature, Tn, and thermal comfort temperature range in fully and partially air-conditioned building.
