In ancient times, buildings were massive in the absence of sophisticated structural analysis and design methods, as well as mechanical and electrical systems. They were designed using materials with a large amount of thermal mass and appropriate building orientation that considered the direction and movement of the Sun and wind, provision of natural ventilation, etc. In other words, the design was primitive yet climate-responsive that builders developed from their ingenuity, empirical observations, trial and error, and experience. Following the industrial revolution, as modern technologies emerged, engineers devised Heating, Ventilation, and Air-Conditioning (HVAC) systems, and architects designed buildings drawing upon primitive notions but with more sophisticationerudite. The energy crisis of the 1970s was a revelation for energy consumers and building designers to appreciate the importance of fuel efficiency, not only for tall buildings but also across the board for the entire building enterprise. It functioned as a strong impetus to do something about energy conservation.
2. Sources of Energy
The primary sources of energy generation can be broadly categorized as non-renewable and renewable. The non-renewable sources are natural fossil fuels such as coal, crude oil, and natural gas, formed underground in the remote geological past from the remains of living organisms. These are depleting due to their non-stop consumption, and the depletion is exacerbated by the growing world population, raising the concern for energy deficiency. Unlike fossil fuels, renewable energy is free from depletion. This type of energy is also called green or clean energy. While these last two terms have the same meaning, they have some minor distinctions.
2.1. Non-Renewable Energy
Natural fossil fuel contains hydrocarbon formed from the remains of dead plants and animals and is extracted and burned as fuels. Pollution is a significant problem of fossil fuels as they give off carbon dioxide when burned, which causes a greenhouse effect, leading to global warming. Coal is the worst of the three fossil fuels (i.e., coal, crude oil, and natural gas) as it produces more carbon dioxide and gives off sulfur oxide, creating acid rain. The mining of coal destroys vast areas of land. Capturing the carbon and diverting it to the green forest, the greenery, plants, vegetation, and crops that could absorb it will go a long way toward reducing global warming. The byproduct of this might be to strengthen the trees, plants, and crops. The crops will produce better foods and positively contribute to the food chain
[5][9][10][11][5,11,14,15].
Oil causes pollution and poses environmental hazards such as oil spills in oceans and seas from oil tankers and releases toxic chemicals when combusted, causing air pollution. Using natural gas can cause unpleasant odors during transportation and accidents due to explosions. Methane is a potent greenhouse gas emitted from oil and gas infrastructure. When non-renewable energy is spent less in buildings, it naturally leads to less carbon emission into the atmosphere and, hence, less global warming
[9][12][13][14][11,16,17,18].
2.2. Renewable Energy
Unlimited renewable energy sources are derived from natural sources such as solar and wind that do not run out. Furthermore, hydro, biomass, i.e., plant and waste material, and geothermal energy are other sources. These do not result in carbon emissions but may impact the environment. Nuclear energy is clean, but if a meltdown occurs due to human error, it will be disastrous. This discourages the use of atomic energy for power generation. Biomass causes deforestation, and hydropower creates land use problems and affects marine life. Green energy is a renewable energy causing no carbon emissions and has a minimal environmental impact. It includes solar, wind, low-impact hydro, and limited types of biomasses. Likewise, clean energy has zero carbon emissions, but many biogases from organic matter, household wastes, and manure are clean but not completely renewable. The capture of biofuel and landfill gas can produce clean energy
[15][16][17][19,20,21].
The most innovative new building designs are those that double as energy generators. Many adjustments are being made to the way electricity is distributed. Coal-fired power facilities and large hydroelectric projects are used to account for most of the electricity generation in the US, for example, and distribution to customers throughout the country has resulted in significant losses. “Distributed resources” such as rooftop solar PVs are increasingly being used to generate electricity, rather than traditional centralized power facilities. In the past, buildings have been passive users of the electric grid, even though they account for 70% of all electricity use in the US. With distributed energy systems at the helm, buildings are increasingly taking on a more proactive role in the energy network, acting as generators as well as consumers. In some areas, renewable energy is already mandated for brand new buildings. “Solar-ready” building codes are becoming increasingly mandatory in the new construction industry. As climate change becomes a more pressing concern, it stands to reason that stringent regulations such as these will always be necessary
[8][18][8,9].
2.2.1. Harnessing Solar and Wind Energy
There are two types of solar energy: active and passive. Active solar energy is implemented through technological installations such as solar collectors and photovoltaic (PV) panels. Researchers are working hard and advancing PV to make it a practical solution for the sustainable energy supply in buildings. PV cells convert light into electrical energy. Commercial PV cell performance has been steadily improving depending upon the type of cell and density to permit the transmission of sunlight. The application of PV technology for high-rise buildings can be significant as these structures offer an opportunity for direct sunlight if neighboring buildings do not over-tower them. The disadvantage of PV technology is the massive power required to produce them, the source of which is fossil-based fuels. Moreover, strict management and recycling assessments are needed to produce toxic and flammable gases containing phosphate and cadmium. Extensive research is continuing to overcome these difficulties. Whereas active solar energy is applied via technological installations, the passive solar energy concept is applied in practice as a design strategy to realize space heating, daylighting, etc.
[19][20][21][22][22,23,24,25].
To exploit wind energy, wind turbines can be installed on tall buildings to generate electricity. At higher heights of tall buildings where wind speed is particularly strong, wind can be used as a source of energy. Tall buildings can be shaped to funnel wind into a zone containing wind turbines without negatively affecting the structure, the surroundings, and the occupants. In fact, by such form-giving of the building, wind speed can be magnified to enhance energy production. A disadvantage of wind turbines is that, together with other mechanical components, they can cause vibration in slender buildings. It is, for this reason, that the initial employment of a windmill at the top of the 542 m (1778 ft) tall One World Center in New York was subsequently dropped. Wind turbines, however, do not impact tall concrete buildings much, unlike steel buildings, as these buildings have considerable mass and damping characteristics
[23][24][25][26][12,26,27,28].
2.2.2. Geothermal Energy
Tall buildings are often built deep into the ground; hence, this form of plentiful thermal energy from the ground source can be exploited to support the HVAC system of the buildings. The common surface manifestation of this type of energy is the hot water from springs. Geothermal heat pumps can exploit the high temperature of the upper layers of the planet’s crust
[27][28][29][29,30,31].
The rate of temperature increase in the ground, or the “geothermal gradient”, averages 2.5 to 3 °C (36.5 to 37.5 °F) every 100 m (330 ft) of depth
[30][31][32][13,32,33]. Modern drilling methods can reach depths of up to 9.5 km (6 mi). Hot water from springs is the most frequent surface manifestation of geothermal energy. Since the 19th century, natural hot water has been utilized in industrial settings. Built in 1913, the first geothermal power plant generated 250 J/s (250 kW) of power
[10][33][34][35][14,34,35,36]. The heated, dry rock with a high temperature is another source. It is necessary to bring geothermal heat to the surface. The method of heating a place by pumping water via boreholes and then returning it to the surface is referred to as borehole heat exchange. Geothermal energy has the advantage of being unaffected by seasonal variations and climatic changes
[36][37].
A significant area of innovation is the pairing of geothermal energy with heat pump technology. This technology has incrementally been upgraded, especially in the US. During the last few years, the number of geothermal ground-source heat pumps has grown significantly, with most of the development in the US and Europe. As the foundation of tall buildings necessitates deep excavation, its application to these buildings could prove more relevant than any other building type. The technology needed to tap into this energy warrants considerable advancement, refinement, and expertise. More research on this technology is necessary to make geothermal energy economically competitive with conventional energy sources
[37][38].
2.2.3. Biomass Energy
Another renewable energy source is biomass energy derived from biomass fuel, i.e., living and once-living things or organic materials, such as wastepaper, which is available in abundance in office buildings. Biomass is the sum of all the Earth’s living matter within the biosphere. More specifically, it refers to the concept of growing plants as a source of energy. When biomass is converted to fuel as a source of chemical energy, the process is carbon-neutral. This energy can be used for generating electricity. Humans have historically made fires from wood for cooking and staying warm in cold climates. Now, biomass is utilized to fuel electric generators and machinery. Common biomass materials are from plant sources that can be burned to generate heat and transformed into electricity. Biomass is a clean, renewable energy source. Its initial energy comes from the Sun that plants need to grow. Trees, crops such as corn and soybean, and municipal solid wastes are generally available and can be managed sustainably
[38][39].
Substantial amounts of biomass are abundant in tall office buildings in the form of paper, most of which is used only briefly and then trashed. Biomass fuel, such as wastepaper in office buildings, can be used for generating electricity and steam for tall buildings. The Illinois Institute of Technology investigated a 73-story Chicago multi-use high-rise project in this research area
[11][15]. Based on the investigative data, the study assumed a wastepaper production of 0.110 kg/m
2 (0.022 lb/ft
2) per day for offices and commercial space. The study concluded that a biomass-integrated gasifier/steam-injected gas (BIG/STIG) turbine would be the most efficient system for using biomass fuel. The Princeton Center for Energy and Environmental Studies researched using gas turbines with biomass fuels
[12][39][40][16,40,41]. Bioenergy is renewable due to the continuously growing botanical sources, and its generation does not contribute to global warming
, as it is essentially carbon-neutral.
Biomass, however, has some disadvantages. For example, if biomass feedstocks are not restocked as rapidly as they are used, they can be converted to non-renewable energy. Biomass generally requires arable land to grow. This means that the land used for biofuel crops are unavailable for other uses. In addition, burning biomass releases carbon, nitrogen oxides, and other pollutants and particulates. If these pollutants are not captured and reprocessed, they can produce smog. Moreover, the amount of wastepaper generated by office buildings has decreased due to the digitization of documents and their internet transmission
[41][42].
2.2.4. Fuel Cells
Another source of renewable energy is the fuel cell. A fuel cell is an electromagnetic device that generates electricity like batteries and that can be considered an electrochemical internal combustion engine. It is a reactor that combines hydrogen and oxygen to produce electricity, heat, and water. It is used for spacecraft, airplanes, and other mechanical transportation systems. An example is 4 Times Square in New York City, which employs two 200 J/s (200 kW) fuel cells utilizing natural gas to generate power. The cells provide 100% of the nighttime electric demand without combustion, and hot water and carbon dioxide are the only byproducts. Similarly, the One World Trade Center Tower is partly powered by 12 hydrogen fuel cells, which produce 4.8 MW of power. Notably, the waste heat output from the fuel cell system is used for hot water and heating, amounting to 73,899 kJ (70,000 BTU
) of high-grade heat and 527,550 kJ (500,000 BTU) of low-grade heat. At present, its cost is high, but with future mass production, it is bound to go down
[23][12]. Soon, fuel cells will provide heat and electricity for many offices and residences. More research is needed to make fuel cells economically competitive and to improve their performance to broaden their application on a larger scale
[42][43].
3. Energy-Saving Mechanisms
3.1. Passive Design
3.1.1. Façades, Daylighting, and Electric Lighting
Daylighting is a crucial aspect of façade design for sustainable tall buildings. A façade acts as a building’s “skin”. The energy loss or gain of a tall building depends much upon the materiality and technology employed in the façade treatment. Glazed façades were considered a weak link in tall buildings for energy performance because of their insignificant insulating capability other than preventing the inflow of outside air and altering the inside temperature. These have now become sophisticated with the application of innovative technologies. The total-building concept, including the systems of HVAC, electricity, structure, and the façade, promotes the notion of integrated design in which all the systems are interdependent
[24][26]. A high-performance façade engineered in the early stages of design development has become critical as the energy efficiency to achieve sustainability has become an indispensable performance criterion for buildings
[25][27].
In the la
tter half of the 20th century, the usage of double-skin façades—two glass layers separated by an air space—and occasionally triple-skin façades with a natural ventilation system gained popularity
[26][28]. A double-skin façade lessens the heat input in the summer and heat loss in the winter; this type of glazing serves as a barrier between internal and external conditions. Likewise, double-glazing with argon-filled cavities, triple-glazing, and glass coatings can increase U-values and screen ultraviolet rays of light
[27][29]. The internal temperature is maintained via passive thermal processes, which include heating and cooling without needing electricity, in conjunction with ventilation of the space between the skins. The extra cost incurred for materials with a higher embodied energy to improve the thermal insulating capabilities of façades is typically recovered through lower energy usage over the course of the building’s life.
Using energy-efficient lighting is vital in tall buildings, in which optimal solutions can result in substantial savings in energy consumption. Lights in a tall building have a relatively low embodied energy cost and, thus, these savings are real. Energy-efficient lamps and lighting control systems can be integrated with daylight to provide reductions in total consumption. Lighting in tall buildings can make up 10 to 25% of the total electrical load, depending upon the power requirements and the amount of heat load that is transferred back into the cooling load
[21][24]. When daylighting is combined with electric illumination produced from renewable and non-renewable sources, it can significantly reduce the amount of energy needed for indoor lighting. According to estimates, daylighting combined with continuous dimming of electric lighting can reduce the energy required for interior illumination by 25 to 40%
[22][25]. Automatic management of electric lighting in response to ambient daylight levels offers the highest benefit.
3.1.2. Natural Ventilation
An analysis of the site conditions is vital to natural building ventilation, especially concerning sufficient air circulation in sheltered areas (e.g., atria, sky gardens, and sky lobbies)
[13][17]. A basic assessment of wind directions and intensities can be made using wind roses, which display the predominant winds throughout the seasons. Through vector analyses of virtual models, a more thorough study can be accomplished. Engineers and architects can now comprehend how wind affects structures and optimize their designs by utilizing a range of computerized programs. Wind tunnel studies using physical models fitted with sensors that provide accurate measurements of the effects of the wind on the structure and urban setting can result in a more thorough and complex experimental investigation.
Tall buildings must be constructed in a way that makes use of both positive and negative pressures acting on their exteriors to enhance natural ventilation and prevent internal building issues such as wind pressure on doors and windows. When outdoor temperatures and air velocity are above 29 km/h (18 mi/h) during the transitional seasons (autumn and spring) in temperate climates, high-rises can benefit from natural ventilation. When the outside temperature is above 22 °C (72 °F), additional mechanical ventilation is needed, especially in the summer. When the outdoor temperature drops below 5 °C (40 °F), mechanical ventilation and heat recovery devices are also advised
[14][18].
3.2. Active Design
3.2.1. HVAC Systems
Manufacturers of air-conditioning appliances have developed high-performance, energy-efficient appliances for saving energy. These have EPA-certified ENERGY STAR labels on them. For any HVAC, the airflow distribution system is of vital importance. When the air distribution is well designed, it will result in energy efficiency and occupant comfort. The use of an underfloor air distribution system (UFAD) is one of the most efficient HVAC systems. It has several advantages over the traditional overhead system, such as raised-access floors for environmental control and improved access to building services. It also improves the ventilation system, causing high-quality airflow. Its layout reduces frictional losses in the system and causes a consistent temperature throughout the building. Furthermore, the UFAD permits complete flexibility for office areas and improved individual temperature control. Using highly efficient system components and an ice storage system, the building’s central cooling and heating plant lowers the power demand. A LEED-certified design is something that architects are increasingly looking for.
3.2.2. Combined Heat and Power
Combined Heat and Power (CHP) (or a cogeneration system) is a very efficient energy-saving technology. In addition, its application is suitable for tri-generation systems. CHP allows concurrent production of heat, power, and, infrequently, chilled water for air-conditioning. An advantage of it is that, unlike conventional systems, it averts transmission losses as electricity is produced near the point of use. The simultaneous generation of power and heat enables overall thermal efficiencies, reducing a significant amount of fuel. Efficiency and operational time are enhanced further by adding a boiler system or another heat storage medium. The generated electricity can be used by a facility or transferred into a public power grid. The heat energy can be utilized for heating water and producing steam. Consequently, this system reduces fuel consumption, costs, and CO
2 emission. The CHP system is increasingly popular in several European cities for servicing commercial and institutional buildings. Stockholm, Helsinki, Copenhagen, etc., provide much of their electricity and heating from CHP systems
[10][14]. Thus, CHP employment is an attractive option as
form
ofost energy generat
ioned is valuable. It is a flexible system that can be adapted to low-to-zero-carbon applications.
3.2.3. Vertical Transportation
Elevators and escalators represent vertical mobility, which affects how efficiently energy is consumed and how effectively people move around. Vertical transportation makes up 5 to 10% of the building’s energy use. Elevator technology is a persistent area of research and development as a result of the recent construction of numerous supertalls and some megatall buildings. This mode of transportation is currently more efficient due to technological advancements. In a “regenerative elevator”, for instance, energy from descending vehicles is absorbed and used to power ascending ones. Additionally, design choices can increase the effectiveness of the net-to-gross-floor-area ratio. For instance, to increase energy efficiency, the double-decker elevators at the Petronas Towers in Kuala Lumpur have doubled the ridership of single-decker elevators. Another way to cut energy use by up to 50% is to use variable-speed gearless elevators with sophisticated programming and energy recovery
[20][23].
Destination-oriented elevators can optimize the trip schedule and further save energy. In traditional systems, users wait after pressing an up-or-down call button. They then rush to choose their location and board the first available car, stopping at each floor they have chosen. The building’s residents and visitors enter their destination floor at the central lobby booth using the destination-oriented system, which groups people traveling to the same floor together and cuts down on travel time. By reducing the number of elevators and conserving energy, this configuration creates more floor space that may be rented out or used for other purposes. Energy-efficient machine-room-free elevators that use permanent-magnet synchronous motor technology are also a significant breakthrough.