Both passive and low energy cooling strategies employ natural phenomena to exchange heat with the surrounding environment using architectural elements and natural heat sinks (Voss et al. 2007, Santamouris 2007). The effectiveness of these cooling strategies is therefore mainly dependent on climate and some building fabric components. Since the building is not well defined in the early stages of the design process, the selection of effective cooling strategies depends mainly on climate.
Five main passive cooling strategies are commonly adopted; natural ventilation, nocturnal/night ventilation, direct evaporative cooling, indirect evaporative cooling and ground cooling (Givoni 1994). Night ventilation and direct evaporative cooling are suggested for hot-dry climates, while natural ventilation and indirect evaporative cooling may be strategies suitable for hot-humid climates (Szokolay 2003). Passive designs are very energy efficient and are expected to consume the least energy of the many different cooling strategies. However, the cooling potential of these strategies is sometimes insufficient to satisfy the cooling requirements of all the building zones especially at extreme weather conditions. Moreover, the performance of some passive strategies –evaporative and ground cooling– could hardly be controlled.
Low energy cooling strategies integrate some active cooling components in order to improve the cooling potential, control the cooling performance, and satisfy the majority of cooling needs of the building. Examples of these applications include slab radiant cooling with embedded chilled pipes, evaporative coolers and geothermal borehole heat exchangers (Florides et al. 2002, Liddament 2000, Tassou 1998, Santamouris 2007). Low energy cooling strategies are expected to provide more controllable cooling potential than passive designs but with higher associated energy consumption. This energy consumption is expected to be lower than conventional active systems since these strategies exploit natural energy sources; solar energy, geothermal energy, wetbulb depression and material properties (Santamouris 2007).
High thermal mass is usually used in conjunction with these passive and low energy cooling strategies to act as a heat sink that controls the heat absorption and discharge heat transfer mechanisms. At the same time, thermal mass reduces the fluctuation of the indoor environment and so protects it from the severe dry outdoor climate and the large temperature swings (Abanomi et al. 2005, Zhou et al. 2006, Antinucci et al. 1992). The effectiveness of the thermal mass depends on the exposed surface, material properties and the diurnal dry-bulb temperature variation (Givoni 1998).
Since passive strategies should satisfy part of the cooling demands and consume the least energy consumption, the research focuses more on the
five different passive strategies (below) together with possible improvements using low energy cooling technologies.
3.4.1 Natural Ventilation
Houses and office buildings were designed to enhance natural cooling, and people spent summer days and evenings on porches or fire escapes. They cooled off by getting wet--opening up fire hydrants, going to the beach, or diving into swimming holes. Before air conditioning, American life followed seasonal cycles determined by weather. Workers' productivity declined in direct proportion to the heat and humidity outside-on the hottest days employees left work early and businesses shut their doors. Stores and theatres also closed down, unable to comfortably accommodate large groups of people in stifling interiors. Cities emptied in summers as people fled the city for mountain and seaside resorts.
3.4.2 Night Ventilation (Nocturnal Ventilation)
Night or nocturnal ventilation makes use of the low night temperature at non-occupied periods to flush the accumulated daytime internal heat such that the structure is cooled down at night while the thermal mass provides a cold discharge mechanism during the following daytime (La Roche 2001). The effect of night ventilation with an exposed thermal mass could reduce the cooling loads, the size of cooling equipments, and the overall energy consumption (Santamouris 2007). Night ventilation has been suggested for dry climates with large diurnal range of 15ºC – 20ºC and night temperature below 20ºC (Santamouris et al.1996). Accordingly, comfort and nocturnal ventilation depend on day and night temperatures, humidity, wind speed and direction.
3.4.3 Direct Evaporative Cooling
As air flows across a wet surface or a mist, direct evaporative cooling occurs through water evaporation. This increases the moisture content of the air and reduces its drybulb temperature (Antinucci et al. 1992); in this process, sensible heat is converted into latent heat at a constant wet-bulb temperature (Szokolay 2003). The dry-bulb temperature can be reduced by about 70% - 80% of the wet bulb depression (Givoni 1991) and this is defined as the difference between dry-bulb temperature (DBT) and the bulb temperature (WBT) (Rosenlund 2000, Santamouris 2007). A larger wet-bulb depression promotes greater reduction in dry-wet-bulb temperature. This strategy has been suggested for dry climates with noon relative humidity below 40% (Smith 2005), maximum WBT 22ºC – 24ºC and maximum DBT 42ºC – 44ºC (Givoni 1994).
Direct evaporative cooling can be enhanced as a passive strategy by implementing indoor fountains, waterfalls and vegetation, by designing a
lake at the windward side, or by supplementing moisture pads or sprinklers in integrated wind scoops (Antinucci et al. 1992). The latter strategy is known as Passive Downdraught Evaporative Cooling (PDEC) (Santamouris 2007).
The same cooling mechanism can be enhanced as a low-energy cooling system by integrating fans and fibrous wet pads known as direct evaporative coolers (Florides et al. 2002). In this system, the fan drives airflow across the wet pads and into the space. The moisture content of the air increases associated whilst there is significant dry-bulb temperature reduction. This low-energy cooling system permits more temperature and humidity control of the supplied air than passive applications and its performance could be simulated using EnergyPlus (DOE 2011).
3.4.4 Indirect Evaporative Cooling
Indirect evaporative cooling uses the same evaporative cooling phenomenon to reduce the zone dry-bulb temperature but without increasing its moisture content. This is achieved by the introduction of a heat exchanger with a completely separated air stream, which is cooled by direct-evaporative cooling (La Roche 2001). This can be adopted as a passive strategy by integrating roof sprays, moving water film over the surface and roof ponds together with a high conductivity roof slab (Antinucci et al. 1992, Givoni 1992). In these passive applications, the building structure is cooled by water evaporation from the exterior surface which acts as a heat exchanger to cool the adjacent spaces without raising their moisture content (Antinucci et al. 1992, Givoni 1992). This passive strategy might be applicable with higher maximum WBT of 25ºC and maximum DBT of 46ºC and could suit more humid climates with low wet-bulb temperatures (Santamouris et al.
1996). The passive cooling efficiency is improved with high insolation level and high wind speed which make it appropriate for dry climates (Nahar et al.
2003, Verma et al. 1986).
Indirect evaporative cooling can also be adopted as a low energy cooling strategy in either air systems or radiant systems. For air systems, the outdoor air is cooled by an evaporative cooler in a completely separate circuit and passes through a heat exchanger to cool the supplied air to the space (Florides et al. 2002). Although the supplied air is cooled without any increase in its moisture content, the cooling potential is expected to be lower than with direct evaporative cooling (Liddament 2000). For radiant systems, the building structure with embedded water pipes that is used to reject excessive heat from the indoor space is firstly cooled though water evaporation within cooling towers (Tian et al. 2009b, Strand 2001). This radiant system applies the same concept of passive indirect evaporative cooling (Costelloe et al. 2003) which is considered within bioclimatic analysis methods (Givoni 1992, Brown et al. 2001) but with more system control and wider range of application.
3.4.5 Ground Cooling
The ground can act as a heat sink for the building to absorb heat by conduction through the ground and by convection within a circulating fluid (Givoni 1991). The ground temperature changes at a slower rate than the air and is highly affected by the soil materials; conductivity, heat capacity and density (Santamouris 2007). Ground cooling effectiveness depends on the difference between ambient air and ground temperatures (Antinucci et al.
1992) that was suggested to be about 14K – 16K for dry and 10K – 12K for humid climates (Givoni 1994). The highest ground cooling potential is expected to take place during the summer time where maximum temperature difference between the ambient air and the ground exists.
Semi-buried buildings (known as contact cooling) or passing fresh outdoor air through earth tubes could exploit the cooling potential of the ground (Antinucci et al. 1992). These passive applications deal mainly with the surface ground temperature which follows the ambient conditions at a slower rate. For hot climates, the ground may require to be cooled either by shading, planting or irrigation.
Since the temperature of the deep ground is almost stable, the difference between maximum ambient air and ground temperature increases and higher cooling potential could then be achieved using vertical geothermal Borehole Heat Exchangers (BHE). This low-energy cooling strategy can be applied by circulating a fluid within buried closed loop pipe system coupled to heat pumps or directly to radiant cooling devices (Santamouris 2007).
Recent research findings show that the long term operation of this system causes a gradual increase in the deep ground temperature which has to be taken into consideration when designing such system (Fisher et al. 2005).
3.4.6 Mixed-Mode/Hybrid Cooling
“Mixed mode is a term used to describe servicing strategies that combine natural ventilation with mechanical ventilation and/or cooling in the most effective manner” (CIBSE 2000).
The mixed-mode cooling concept –sometimes named hybrid ventilation–
dates back to late 1980s when research began to address issues such as carbon emissions, building related health problems, productivity and occupant satisfaction. In the 1990s, several research projects studied exploitation of the best of passive and active cooling strategies in mixed-mode/hybrid schemes. Mixed-mixed-mode/hybrid strategies seek to maximise the use of passive methods but incorporate supplementary mechanical systems for use in the most extreme conditions (Brager 2006). The main objective is to maintain satisfactory indoor air quality IAQ and thermal comfort during occupied hours while minimizing energy use (Gids 2001). Mixed-mode/hybrid strategies are expected to consume more energy than passive
strategies and less than mechanical ones (Lomas et al. 2007, Charvat et al.
2005). Regarding the integration of both natural and mechanical systems, the components needed are therefore some combination of components and features such as low pressure ductworks, variable speed fans and heat recovery systems (Wouters et al. 1999).
Analysis methods, control algorithms and appropriate prediction tools help designers to evaluate and optimize mixed-mode systems (Li 2001). Since mixed-mode strategies are concerned with both IAQ and thermal comfort, coupled thermal multi-zone airflow tools are useful for the detailed analysis of the performance of these strategies (Heiselberg 2002). Due to the high level of uncertainty at the early design stages, and inexperienced users, preliminary analysis tools should be simpler and easy to use (Li and Heiselberg 2003). Various simplified tools have been developed to predict the potential of mixed-mode ventilation at early design stages (Axley et al.
2002, Fracastoro et al. 2001, Luo et al. 2007) but they don’t integrate low energy cooling strategies.
The design challenge of mixed-mode strategies is to overcome existing barriers. Heiselberg et al. (2001) summarized the list of barriers identified in AIOLOS (Allard 1998), NatVent (http://projects.bre.co.uk/natvent/) and Annex 35 projects (IEA 2002) and highlighted on fire, smoke and noise regulations as major barriers. Roth et al. (2006) and Kossik (2001) also suggested unfamiliarity with those strategies, climatic limitations (as most of research and implementation was done in Europe), insufficient guidance within codes and standards as special challenges.
The research conducted to date has mostly been concerned with applications in temperate climates such as that of northern Europe. The application of Mixed-Mode ventilation in severe hot climates and its integration with other passive and low energy cooling strategies is very challenging, has not been systematically studied and so this work presents a distinction with respect to previous work.