The fact a controversial cornucopia of towers is coming London’s way is well known – and some who support this development claim that towers are “greener” than low-rise living.
But are they? My new research project at UCL’s Energy Institute (conducted in association with Create Streets) will try to discover if high-rise buildings are more energy-intensive than equivalent low-rise buildings. At present the evidence is uncertain, but there are suggestive hints. We intend to test them.
Many variables can affect tall buildings’ energy use – and only some of them relate to height. There is the use of energy in lifts. There is the use, or not, of air conditioning. There can be important differences due to building construction, especially between glass curtain walls and solid walls, since rates of heat loss are greater through the glass. Finally, there will be effects related to the local environments of buildings – such as orientation, overshadowing and exposure to sun and wind.
There is some evidence on the relationship between height and energy use. A 2003 study of 20 comparable Hong Kong office towers found that, as high increased, there was a steady increase in energy too. Each additional storey added on average 3 kilowatt hours per M2.
But the use per M2 for lighting and – perhaps surprisingly – lifts did not increase with height. The biggest increase was in the energy used for heating, ventilating and air conditioning. The Hong Kong climate is hot and steamy: we would expect heavy loads for air conditioning. But these were greater, per unit of floor area, the taller the building.
The energy element of the English Housing Survey provides evidence of the relationship between energy use and height in tall residential buildings. A preliminary analysis of data on both low- and high-rise purpose-built flats shows that the latter use on average more than twice the amount of electricity annually (although the sample is small).
That said, there have been studies that have shown little or no difference in energy use with height. Aedas Architects analysed theoretical designs for tall office buildings, using simulation models, and found only small increases in energy intensity with height.
Researchers at the LSE carried out a study of energy use for heating in residential buildings of many different types. They found that energy use decreased with height. However, they too were working with simulations. Furthermore, their sample only included buildings up to 11 storeys.
Why might we expect energy use to increase with height? The obvious characteristic of tall buildings is that they stick up above their neighbours: they are exposed to higher winds. Data from the Chartered Institution of Building Services Engineers shows that wind speeds do indeed rise with height
And higher winds can contribute to increased energy consumption in several ways. They remove heat from the surfaces of buildings. They increase drafts, meaning that the air inside requires more heating or cooling. And they can increase the rates of conduction of heat through the building’s envelope, especially through glazing. Tall buildings tend also to be more exposed to the heat of the sun, and are less likely than low-rise to be overshadowed by trees or other buildings. This can lead to increased energy requirements for cooling in summer.
It seems at least possible that such effects are insufficiently represented in the simulation models used to estimate energy consumption during the design of tall buildings. This could lead to underestimates of predicted energy consumption in practice.
It’s sometimes suggested that a concentration of tall buildings around public transport hubs can help shift travellers from cars to buses or trains, and so reduce energy consumption in transport. This may be true. However, it is an argument for higher densities rather than skyscrapers, as such.
Some researchers have looked at the potential for “green” retrofitting of existing tall buildings. Valuable as this work is, it seems possible that most, if not all, of the conservation measures and renewable technologies employed in “green” or low-energy skyscrapers could equally be applied – perhaps with greater effectiveness – in low-rise buildings. One could also imagine that the potential for adaptation and refurbishment would be greater in low buildings than in tall ones. Might the most sustainable skyscraper not be a skyscraper at all?
Philip Steadman is professor of urban and built form studies at University College London. This article is adapted from an essay he wrote for Create Streets.
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