|Fig. 1: Guangzhou, China's "net zero energy building" Pearl River Tower which harnesses wind power and directs it into turbines. (Source: Wikimedia Commons)|
With population doubling every 65 years, there has been a continued flow of people from rural to urban areas. Since 2007, more than half the world's population is living in urban areas and so the need for high-rise buildings has never been greater. Indeed, high-rise construction allows cities to improve urban connectivity and functionality.  This comes in stark contrast with increasing concerns regarding the depletion of natural resources and pollution in densely populated areas. By definition, skyscrapers are not ecological. In fact, the construction, day-to-day running and eventual demolition of high-rise buildings require a huge input of energy and often carbon footprint-heavy raw materials. This lack of ecological efficiency can simply be attributed to their tallness. This tallness affects needs to withstand large forces of high wind speeds, greater energy demands to transport materials, and increased energy needs related to moving people up and down floors.  As the development of high-rise buildings in the 20th century intensified, the oil crisis in the 1970s prompted a new discussion around how to make skyscrapers more energy-savvy. 
Over the past several decades, salient green design approaches have surfaced. Here, we will survey only a few of these. Firstly, as skyscrapers reach newer heights, structural engineers have concentrated on aerodynamics and the effects of wind. For tall slender buildings, vortex shredding is the source of major wind-induced excitation. This phenomenon describes wind hitting a facade and travelling across adjacent faces in vortices, increasing wind speeds across the surface of a building. [2,4] Thus, buildings must adopt aerodynamic forms by adopting a tripod-type, rounded corners and a domed top.
Secondly, buildings can also take advantage of renewable energy sources in the form of wind energy. For example, tall buildings can be built to direct wind into turbines and generate significant energy.  Thirdly, a building's facade has become more than just an encasing: facades can adapt to changing environmental conditions. High-tech skins, such as a lining of photovoltaic skins, can provide natural ventilation, a shaded interior and reduced wind pressure.  With growing global populations, both food and housing needs are inevitably increasing. Thus, the third development in many of today's buildings is a heavy uses of greeneries, from green roofs to sky gardens vertical landscaping. Such characteristics not only infuse green into urban life, but also reduce energy consumption through better ventilation, provide space for water collection and for growing produce in vertical farms.  Finally, green buildings have started to investigate biosmic designs which mimic nature. For example, the Cactus Building in Doha, Qatar, mimics the behavior of a cactus in heat through smart shades that open and close automatically to control internal heat. [2,5]
These approaches to building skyscrapers can be encapsulated within Ken Yeang's bioclimatic methodology, which advocates buildings that respond to climatic conditions in the surrounding. These bioclimatic principles, then, incorporate key features of the local region and thus result in low-energy buildings with unique designs. This is a trend that will continue to gain momentum in the 21st century. [1,6]
Even if a skyscraper were to apply all the key features discussed, a building that is poorly integrated into its environment would not necessarily generate less greenhouse gases than a building without these features. This highlights a great need to invest time and research not only into tomorrow's green technologies, but also better urban planning. Therefore, a sustainable approach to green skyscrapers should consider other issues including social, infrastructure, construction, operation and a building's functional adaptability to future market changes.  Indeed, a skyscraper loses its green title if it is placed away from important services such as public transit, schools and stores.
Despite the world's acceptance of climate change, the realization of a need for more sustainable approaches to life in general, and huge advances in the development of green skyscrapers, there remains some real concerns regarding the development of these buildings. More precisely, there is a growing concern that sustainable building in developing countries around the world imports Western models of aesthetic appearance - and so the green skyscraper is in fact grey with steel. With time, urban centers around the world have become increasingly homogenous and monotonous in their aesthetic, because of the "one size fits all" skyscraper. Thus, there is a great need to incorporate local culture in new high-rise buildings. 
Designed by Adrian Smith and SOM, this 300m and 71-storeyed tower was the first "zero energy" building. The tower's innovative design produces as much energy as it consumes, in the true spirit of a green skyscraper. Its semielliptical form is adapted to wind as it guides it to two openings which push turbines that generate energy. These openings also act as structural relief by allowing wind to pass through the building instead of pressing against it. [2,7] The building also makes use of photovoltaic cells, raised floors for ventilation and radiant heating.
© Quentin Perrot. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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