A REVIEW OF PASSIVE COOLING FOR HIGH-RISE & HIGH-DENSITY RESIDENTIAL BUILDINGS IN HONG KONG Key words: Hong Kong, hot and humid climate, passive cooling, high-rise & high-density residence ZHAOYIYUN_1155020213_SENV7700
Outline Residential Energy Consumption in Hong Kong HK Background Problems Brought about by the HK Climate and the Key Point of Energy-saving Design The Definition of Passive Cooling Historical Development Passive Cooling Benefits Principles of Heat Transfer Reasons of Indoor Overheating and Passive Cooling Strategies Passive Cooling of High-rise & High-density Residence in Hot & Humid Region The definition of High-rise & High-density Residence Passive Cooling of High-rise & High-density Residence —Case Study (HK, Singapore) What Passive Cooling Strategies are Suitable • for High rise & High-density Residential Buildings in HK ? Discussion & Conclusion Limitations
HK Background Residential Energy Consumption in Hong Kong Fig. 1 Hong Kong residential development Fig. 2 residential energy consumption composition Passive design is more energy saving with no pollution to environment, more conducive to economic development and won’t increase the burden to national electric energy system. Therefore, with the energy crisis and environmental problem facing today, we should pay more attention to the use of passive approach to heating and cooling.
HK Background Problems Brought about by the HK Climate and the Key Point of Energy-saving Design Fig. 4 Hong Kong energy-saving design measures Fig.3 Hong Kong climate summary Hong Kong is located at east longitude 114°15' and north latitude 22°15', in the south coast of China. It is the subtropical climate, having higher annual temperatures, with an annual average temperature of 22.8℃. The summers are hot and humid. The temperature is about 26 ~ 30℃. Winter, cool and dry, but rarely drops below 5℃. It is always rainy from May to September and sometimes has typhoons from July to September. Hong Kong's average annual rainfall is 2,214.3 mm and the wettest month is August. The dominant wind is from east.
Passive Cooling • The Definition • Historical Development Passive cooling can be defined like using natural methods to remove heat from buildings by convection, radiation and conduction, making the buildings cool. simple intuitive applications of natural cooling mechanical cooling systems passive cooling revitalized • Benefits The energy required for heating and cooling of buildings is approximately 6.7% of the total world energy consumption. By proper environmental design, at least 2.35% of the world energy output can be saved . In hot climate countries, energy needs for cooling can amount to two or three times for heating, on an annual basis. Fig. 5 traditional homes in hot and humid climates
Passive Cooling Principles of Heat Transfer Heat transfer is the phenomena of transfer of heat inside the object, or between the objects. Any temperature difference existing between the various parts of the object or the objects themselves, there is bound to the delivery of heat transfer. Depending on the heat transfer mechanism, the basic method of heat transfer is divided into three kinds, conduction, radiation and convection. Fig. 6 conduction Fig. 7 ground cooling and heating Fig. 8 roof reflection Fig. 9 windows Fig. 10 convection in glazing
Passive Cooling Reasons of Indoor Overheating 1Outdoor high temperature transfers heat into the room through the indoor and outdoor air convection. In naturally ventilated buildings, indoor air temperature changes by means of the exchange of indoor and outdoor air, causing indoor overheating. 2The solar radiant heatgoes directly into the room through the sunny window. 3The reflective heat near the buildings, grounds, road and the long-wave radiation heat cause indoor overheating. 4Some heat is from the maintaining structure. Under the joint action of the intense solar radiation and high temperatures, the roofs and walls surface temperatures are rising, transferring the heat into the room. So the inner surface with temperatures rise often causes indoor overheating. 5Some heat is generated by the indoor production, living and equipment. Fig. 11 reasons of overheating
Passive Cooling Passive Cooling Strategies Three levels, prevention of heat gains in the building, modulation of heat gains, rejection of heat from the interior of the building to heat sinks modulation of heat gains can be classified as four strategies, rediative cooling, cooling with ventilation, earth cooling and evaporative cooling[M.SANTAMOURIS AND D.ASIMAKOPOULOS ]. 1 Minimizing outdoor and indoor heat gains 2 Window glass and shading Radiative cooling 3 The structure of buildings 4 Cooling with ventilation 5 Earth cooling 6 Evaporative cooling Fig. 12 passive cooling strategies
Passive Cooling Passive Cooling Strategies 1 Minimizing outdoor and indoor heat gains Fig. 13 Fig. 14 First of all, choose the correct orientation and layout of the buildings, and strive to avoid the main use of space and transparent shielding space exposed to the east and west sun. Green the surroundings, in order to reduce the environmental radiation and air temperature.
Passive Cooling Passive Cooling Strategies 2 Window glass and shading Figs. 15 different windows Fig. 16 the AC energy consumption of glass Selecting the special glass for glass windows can also make an effect on heat-resistant shading. It is found that there are two types of main windows used as Hong Kong residential glass, white glass and colored glass. White glass is the most widely used, accounting for about 75%, with a good ability of translucent. As can be seen from the figure 16, the energy consumption of white glass is higher. While using blue glass, energy consumption relative to white glass decreased by 1.5% . window overhangs or operable sunshades (extend in summer, retract in winter) can reduce or eliminate heat. Fig. 17 window overhangs or operable sunshades
Passive Cooling Passive Cooling Strategies 3 The structure of buildings (1) ROOF Fig.19 Fig.20 ventilated insulated roof Fig.21 heat transfer of ventilated insulated roof Fig. 18 light colored roof Fig.22 water insulated roof The roof is the most effective radiator for radiant cooling. Use light colored building materials and cool roofs (with high emissivity) to minimize conducted heat gain and cool. There is a ventilated insulated roof originated in the South China’s folk tile roof. Such roofs use inside ventilation under the surface layer to take away heat, achieving the purpose of passive cooling.
Passive Cooling Passive Cooling Strategies 3 The structure of buildings (1) WALL Fig.24 Fig.25 Fig.26 Fig.23 Storing the coolness in a specialized storage mass or in the structure mass of the building is a way of indirect radiative cooling, which is obtained by cooling a fluid through radiation to the sky. The cool air can also be flushed directly into the building, preferably next to the storage mass. With the deepening of the wall reform, a number of new wall materials appeared, such as brick, reinforced concrete panels, reinforced concrete hollow slab and composite large plate. In the passive cooling design of walls, the traditional architecture of the LINGNAN region used oyster shells affixed to the outer walls. The oyster shells on the surface of the wall itself form the shading of buildings. Inside the oyster shells, air plays a role of insulation. In addition, planting green plants the outer wall can generate a very good heat insulation effect.
Passive Cooling 4 Cooling with ventilation Passive Cooling Strategies Fig.27 Fig.28 In areas of hot climatic and high-humidity conditions, ventilation is important. Air movements through buildings result from the difference in pressure indoors and outdoors which can be achieved by natural forces and mechanical forces (pressure difference induced mechanically, like a fan). One of the reason of natural ventilation is different air pressure, another is stack effect-pressure difference induced by temperature gradients between the inside and outside of the building.
Passive Cooling 4 Cooling with ventilation Passive Cooling Strategies Fig.32 Fig.33 Fig.29 Fig.30 Fig.31 The overall layout of the residential area and single building design should be conducive to natural ventilation. Locate door and window openings on opposite sides of building to facilitate cross ventilation, with large areas facing up wind if possible. Use open plan interiors to promote natural cross ventilation, or use louvered doors, or instead use jump duct if privacy is required.
Passive Cooling 4 Cooling with ventilation Passive Cooling Strategies Ventilation of cooling building can be categorized into ‘daytime ventilated cooling’ and ‘nocturnal ventilated cooling’. (1) Daytime Ventilated Cooling (2) Nocturnal Ventilated Cooling Nocturnal ventilated cooling means lowering the indoor day-time temperature by ventilating the building at night. The effectiveness of the nocturnal ventilation is linked to 1 the ventilation rate 2 the storage area 3 the area that comes into contact with the flowing air 4 the heat capacity and thermal conductivity of the storage material.
Passive Cooling 5 Earth cooling Passive Cooling Strategies Fig. 34 underground dwellings Fig.35 earth contact cooling system Cool the soil for use the cooled soil as a cooling source for a building. The earth provides a useful heat sink since although daytime temperature may be excessively high, the mean annual temperatures may be well with the comfort zone. The building may be coupled by direct conduction, and underground building, or by other devices such as air pipes. 6 Evaporative cooling Evaporation is the phase change of water from liquid to vapor. This is accompanied by release of high amounts of heat from the air that comes in contact with the wet surface or from the surface where evaporation takes place. It is especially effective in arid and dry climates.
Passive Cooling of High-rise & High-density Residence in Hot & Humid Region The definition of High-rise & High-density Residence The definition of high-rise and high-density residence in our country is the residential buildings of not less than 100 sets each hectares and more than 24 meters high and more than 10 layers. mountainous with larger gap less land and more people the global warming with rising ocean area makes reclamation costs increase HK the local land price is very high upward trend in population density well-developed economy higher-income residents To stabilize land prices, increase housing supply, the Hong Kong government develops building technology greatly and higher and denser residential construction has become the first choice of the local government.
Passive Cooling of High-rise & High-density Residence in Hot & Humid Region Passive Cooling of High-rise and High-density Residence—Case Study 1 Hong Kong ‘XIN YI Garden’ Natural ventilation: Building layout focuses on the use of wind around to cool. Use wind tunnel experiments to optimize each household natural ventilation. A combination of high-building and low-building reduces the unfavorable impact of wind on the pedestrian area. Shading and heat insulation: Use exterior shading and roof shading, insulation layer to control the building heat absorption. Use computer simulation to optimize the design of the shade, sunshine facilities. Passive utilization of solar energy: Encourage the use of good sunshine, ventilation conditions to laundry space. Set the “ventilation tower “ to take advantage of solar thermal energy to promote air circulation. Fig.36
Passive Cooling of High-rise & High-density Residence in Hot & Humid Region Passive Cooling of High-rise and High-density Residence—Case Study 2 The Met residence in Singapore In hot and humid areas, residential buildings take full advantage of the local climate, the breeze, reasonable organizing the balcony, garden and outdoor space layout to achieve the good results of cross-ventilation. Fig.40 north façade Fig.37 the Met Fig.38 the Met section Fig.39 planting on the wall Fig.41 the gap-structure between the towers generating a chimney effect also plays a role to increase ventilation. Shading systems effectively block the strong tropical light directly into the interior.
Discussion & Conclusion By summarize and review all the mentioned passive cooling strategies, considering the climate condition and types of residential buildings, measures suitable in Hong Kong can be discussed as follow. 1 Minimizing outdoor heat gains through choosing the correct orientation and layout of the buildings, striving to avoid the main use of space and transparent shielding space exposed to the east and west sun. Green the surroundings, in order to reduce the environmental radiation and air temperature. 2 Using blue glasses and shading to reduce the energy demand for cooling substantially. 3 In terms of walls, phase change material used for walls in HK with hot and humid climatic condition is good. Don’t add the thickness of the insulation layer on the thin wall. Planting green plants on the outer wall can generate a very good heat insulation effect. What’s more, storing the coolness in a specialized storage mass or in the structure mass of the building is a way of indirect radiative cooling can be applied in HK. 4 Use renewable natural resources, wind energy to improve building ventilation effect for passive cooling. The overall layout of the residential area and single building design should be conducive to natural ventilation, letting the natural wind round about between the buildings. What’s more, reasonable organizing the balcony, garden can achieve the good results of cross-ventilation. In high-mass, well insulated residential buildings, shaded and closed during the day, a drop in the internal air temperature, relative to the outdoor temperature, is available through nocturnal ventilation.
Discussion & Conclusion limitations The applicability of passive cooling strategies could be limited by climate and microclimate, air pollution and noise level, site topography and building regulations, lack of national regulations and evaluation tools. What’s more, for the assessment of indoor thermal environment, users’ expectations are not the same under different design strategies. The end