According to the UN, in 2007, when the world's population was approaching 6.7 billion, the number of people living in urban areas was approximately equal to the number living elsewhere. However, by 2050, when the global population is predicted to have reached 9.8 billion, twice as many people will live in urban areas than elsewhere. If you're wondering where an additional 3 billion-plus people will live and work: look skyward.
Advances in construction methods and materials, alongside unmanageable traffic congestion and a resurgence in city centre living have seen a growth in the construction of tall buildings. Figures from the Council on Tall Buildings and Urban Habitat (CTBUH) show that the number of buildings over 150m high has increased from 2,376 to 4,991 in the past 10 years. Among buildings taller than 300m numbers have increased from 542 to 1,616.
"Taller buildings need more heating in winter and more cooling in summer" Prefessor Philip Steadman, The Bartlett School of Architecture
But the ever-increasing number of people living and working in cities, and the construction of the buildings that will facilitate it, could be very bad news for the planet, as early research indicates that tall buildings are proportionately more energy intensive than short ones.
A 2017 study of 600 British office buildings by a team at University College London (UCL) showed that as a building's height increases from five to 20 storeys, carbon emissions from energy use, per unit of floor area, double.
Taller buildings are subject to forces that smaller buildings aren't. "[They] rise above their neighbours and are exposed to stronger winds, lower temperatures, and more direct sunshine," explains Philip Steadman Emeritus Professor of Urban Studies and Built Form Studies at the Bartlett School of Architecture, who led the research. "[So] they need more heating in winter and more cooling in summer."
There is a scarcity of studies about the carbon impact of running tall buildings. One of the reasons for that is operators are notoriously guarded about releasing the necessary data, for fear they might give away their competitive edge. The data from private building operators that was used in the UCL study, for example was provided on strict condition that no individual building data would be published.
"Currently transparent solar cells and high on embodied carbon" Simon Sturgis, Targeting Zero
Another reason is the current primacy of computer modelling to determine buildings' energy consumption, which according to Steadman has consistently suggested that tall buildings are as, or more, efficient to run – results that are unsurprisingly favoured by those with financial interests in building tall buildings.
Steadman claims that a powerful lobby for computer simulation models is still reluctant to accept the results of his research. "I have given a number of talks [about the research] that are well attended by planners, but not developers. And where I encounter them [developers] they are much harder to convince," he says.
Despite this reluctance to accept Steadmans results, comparable evidence has been available for a while. As long ago as 2004, a study of 20 government office buildings in Hong Kong measured each building's height and the energy used from functions such as air-conditioning, lighting and lift operations. However, despite gathering all the necessary information, the study's analysis was focused elsewhere. "There was enough data to draw comparable conclusions to our research but the paper did not," says Steadman.
At the heart of tall buildings' energy inefficiency is their use of glass, whose high conductivity heats the building in summer and cools it in winter. Steadman's research found that the proportion of a building's exterior that was glazed "increases systematically with [a building's] height, increasing heat loss and heat gain."
Most of the world's tallest buildings are fully or nearly fully glazed, according to Simon Sturgis founder of Targeting Zero, a sustainability consultancy in London. And the problem with glass isn't limited to its thermal inefficiency – maintenance of glass buildings is relatively wasteful, too. To reduce the amount of air-conditioning needed to cool the interior of a mostly glass building, they are typically built with a triple-glazed facade with a large gap between the outer pane and double- glazed inner panes, in which electronically operated blinds fit. There is also the laminate required for strengthening, and then a heavy aluminium framing system to support the five sheets of glass.
Furthermore, these glazing units don't last as long as the traditional stone-and- steel facades of older skyscrapers. Sturgis estimates that they must be replaced every 30 to 40 years compared with every 100 years for a building with exterior glazing of 40% –a figure that reflects that of Empire State Building-era skyscrapers – and one that he suggests should be a target for future buildings.
A further issue with fully glazed facades is that they come in large units, meaning that a fault in one section often requires a wholesale replacement. "And the material is hard to recycle, so there is a huge resource impact," adds Sturgis.
It seems clear that if the fast-urbanising world cannot curb its appetite for tall buildings, it will need to start building them with less glass. Transparent solar cells coating large skyscrapers theoretically provide a future power bounty, given the uninterrupted access to sunlight enjoyed by taller structures. "Currently, though these are expensive and high on embodied carbon [the carbon produced in their manufacture and installation]," says Sturgis.
When it comes to how to build at scale more sustainably, the principles of Passive House could prove to be a valuable resource. The stringent design standard was developed to reduce the energy required to heat or cool a building by harnessing the power of the sun and internal energy sources, while conserving heat with airtight construction methods.
In the fast-expanding city of Gaobeidian, in China's central Hebei province, 1.2m m2 (13m ft2) of living space has just been Passive House-certified. But the application of these standards to commercial buildings can provide particularly big dividends.
In 2015, a full Passive House retrofit of a university building in Innsbruck, Austria, reduced heating demand from 180 kWh/m² a year, to just 21 kWh/m² a year reports the Passive House Institute. In 2018, Klinikum Frankfurt Hoechst, the world's first certified Passive House hospital opened. Typically, electricity consumption in a hospital is between three and four times higher per m2 than that of an equivalent- sized residential building, the institute estimates. According to its figures 40-60% of a hospital's energy use can be saved using a Passive House construction.
Dr Wolfgang Feist, director of the Passive House Institute, says that sourcing power entirely from locally generated renewables is a far more useful measure of a self-sustaining building than the popular "annual net zero" balance where electricity fed into the grid is offset against energy consumed. One reason is that large-scale electricity grids waste a proportion of the power that runs through them, and this is not taken into account in calculations. In 2014 8.3% of all electricity generated across the world was lost in transmission, according to the World Bank.
The Innsbruck building combines renewable generation, including a groundwater heat pump, a solar thermal system and solar panels, with traditional Passive House building methods to reduce heat loss. "The combination of energy efficiency and renewables is the futureproof solution for all buildings, including multi-storey projects... [In the Innsbruck building] the actual amount of regionally and seasonally available renewable energy is taken into account so that a completely sustainable supply system becomes possible," says Feist.
Since battery technology doesn't yet allow a building's self-generated power to be stored onsite – which, if it were possible, would allow heat or power generated in the summer to be kept for use during the winter – the best solution is creating buildings that need less energy in the first place, and can generate all they need.
Until buildings generate their own energy, smart technology may localise electricity grids, reducing transmission waste. Shrinking the grid to a very local level is almost as effective as removing it altogether, since the further electricity must travel, the greater the waste. (Ironically, the growth of renewables has in some cases increased the distances across which electricity must be transmitted.)
Wide-scale adoption of solar panels and the growing number of local wind farms raises the prospect of a future where local energy demands are met by locally generated electricity, which would vastly reduce waste from transmission. For example, a business could need energy for air-conditioning on a hot summer day, and use power generated by solar panels on nearby houses, while residents are at work.
But local power generation is only one factor. To support a local network you need a model for local consumption, which means building a marketplace where consumers and generators can trade.
David Shipworth, Professor of Energy and the Built Environment at the Bartlett, believes that blockchain can play a crucial role in facilitating peer-to-peer energy trading, which could hold the key to the future of sustainable energy generation.
"Think of blockchain simply as a way of exchanging data that ensures an accurate record of that exchange between generators and consumers," says Shipworth. Say, for example, that you buy a unit of energy that I have generated from solar panels on my roof, next door to you. My smart meter registers one more unit of energy exported to the grid; yours registers one more used. The blockchain register would work as an electronic ledger: a unit is added to my "produced" column and one is added to your "consumed" column. Everyone has a real-time copy of the ledger. So, you and I don't even need to trust each other: transactions are recorded on everyones ledger meaning that transactions are virtually impossible to forge. It is the perfect mechanism to support a market.
A local power network supported by blockchain could also support socially responsible investment. For example, a householder might support her daughter's local primary school by buying energy produced by the school.
"The next obstacle is regulatory," says Shipworth. "Currently the tax and regulatory model for energy markets is partly a function of the huge costs of maintaining the grid. Today, the same amount is spent on keeping the UK's National Grid running –strengthening wires, replacing transformers and pylons and so forth – as is spent on all renewable energy generation", says Shipworth.
Reducing the size of wasteful national electricity grids would release money that could be used to fund sustainable power generation.
By then, perhaps the world's skyscrapers will finally be paying their way in environmental economics.
At 88m (289ft) tall, the Bolueta high-rise in Bilbao, Spain, is the tallest Passive House-certified in the world.
Completed in 2018, Bolueta – named after the district in which it is located –comprises 171 apartments. A second Passive House-certified tower of 21 storeys is planned next to the first tower.
A Passive House typically consumes approximately 90% less energy than an existing building and 75% less energy than an average new construction.
The projects architect Germain Velazquez said, at the time of its completion: "Now that Bolueta is complete, there are no excuses any more: it is possible to realise such a project and it is just as possible to realise one almost anywhere out there."
PHOTOS BY PASSIVE HOUSE INSTITUTE; LONGFOR