Mitigating Heat in Cities
Regardless of where you are in the world, we can all agree that this summer has been a hot one. More than that, it has been a scorcher. With high temperature records being broken all over the world.
If you live or work in a city, chances are you will have felt the heat even more keenly than your rural counterparts. Cities not only absorb heat and hold onto it almost greedily in asphalt and concrete surfaces, radiating it away again over a long period of time, but actually generate their own too. Vehicles stuck in traffic release heat and air conditioners, which are so crucial for Summers in cities, vent waste heat into the streets. The buildings themselves, ever taller and larger, create ‘urban canyons’ trapping heat and making it hard to diffuse quickly.
All of these factors add together to form what is known as an urban heat island, essentially meaning large cities tend to be about 10◦c warmer than the surrounding areas. In a summer like the one we are in the midst of, that can be a terrifying temperature to reach and endanger the lives of those who dwell within.
The question is, how do we in the construction sector, plan for a warming world? Are we really headed for a world in which wealth is denoted not by the value of your property or the worth of your belongings, but instead on how cool you can keep your home? And of course, all that expensive air conditioning actively warms the environment for the rest of us, so are we willing to heat everything so a few areas are kept cool?
Before air-conditioning was so broadly used, there were many building techniques employed throughout history in very warm or even desert regions to mitigate the sun’s rays. In many ways, our reliance on air conditioning has allowed us to drop these often more time or space consuming methods and create buildings which would be unbearable to be in without it. Can you imagine trying to work in a glass fronted tower with windows that don’t open and almost no ventilation if the AC simply stopped functioning?
It is plain that, regardless of where in the world you happen to be, planning for hotter weather is going to be key to mitigating the worst of its effects. For those in the construction industry, this planning needs to involve the buildings of tomorrow. Offices which no longer require power hungry AC units to stay cool all summer, homes which are naturally cool and allow air to flow easily when required and so on. We need to start looking at ways to achieve these things.
The following are some power-light innovations either already in use or soon to be. Inspired both by new and old technologies.
Vegetation can be a surprisingly effective tool in fighting heat. It provides shade which is helpful but it also encourages a process called evapotranspiration, water evaporating from plants’ leaves which in turn reduces the temperature of adjacent air.
Many cities worldwide take advantage of the cooling provided by parks and trees as well as the positive effect plants and green spaces have on the psychological wellbeing of citizens. However, none so far have utilised greenery to the extent of Singapore.
In 1967 Singapore started its ‘garden city’ plan. This initially involved intensive tree planting and the creation of many public parks. However, as the population of the city state grew and its buildings grew ever taller and more densely packed, focus shifted to include greenery on high rise buildings. Things like vertical planting on the facades of buildings and green roofs or ‘skygardens’.
Singapore today has over 240 acres of skyrise greenery, with the plan to try to double this amount by 2030. The reason for this focus on green space within the city is thanks to building regulations such as the Lush (Landscaping for Urban Spaces and High-rises) policy, which essentially requires any new developments within the city to include areas of greenery the size of the site itself. This greenery can be placed at ground level, on rooftops, balconies or even growing vertically on the sides of buildings. Doing so can help temperatures drop by between 2◦C and 3◦C and many developers go even further than required, draping their whole buildings in greenery!
To make cities coolers we need to start looking at the materials that they’re built from and be prepared to change things. Most urban centres in the world are primarily made up of hard, dark materials (paving, concrete, tarmac etc.) the majority of which absorb solar radiation. Over the course of hours, these surfaces then release their absorbed heat, keeping urban areas stiflingly warm even as the day cools elsewhere. On hot days, studies have shown that standard paving can reach temperatures up to 67◦C and roofs between 50-90◦C. These temperatures have a negative effect on the health of a city’s population, especially for those unfortunate enough to live on the top floors of building with dark roofs whose lives could be at risk during heatwaves.
A great way to overcome the issues caused by so many dark, heat absorbing surfaces is to use cool coatings. These are usually lighter pigments of asphalt or white coatings applied to roofs, roads and facades, allowing surfaces to reflect more heat away from the city.
Research by Nasa showed that a white roof could be up to 5.5◦C cooler than a dark one on the hottest day of summer in New York.
Water, the original Coolant
In the time before air-conditioning, water was a commonly used tool for cooling structures and urban centres in hot areas. The 14th century palace of the Alhambra had courtyards with pools and fountains, this water evaporated into the air, cooling it naturally.
Similar techniques could be used in cities today to help cool the outdoor spaces. If you were to pair the creation of bodies of water along with our earlier look at creating green spaces full of vegetation, this would perhaps be the most impactful.
Cities in the warmest parts of China have started to use large water misters in public spaces like bus stops to spray clouds of cooled water easing the temperature in the air and of course providing a welcome relief to waiting passengers.
Studies have suggested that adding water features and cool coatings to an area could help reduce the cooling requirements in that area by over 29%. This would mean less need for air-conditioning which in turn would mean less waste heat being pumped from buildings and into the environment.
Throwing Shade in useful Directions
A key challenge when attempting to keep the built environment cool is the wide-spread use of fully-glazed facades. Lots of windows in a building allow lots of natural light which is desirable, they also grant those within the building views of the surrounding area. However, windows also trap unwanted heat in the summer and let it escape too easily in the winter making such buildings expensive to climate control appropriately.
Shade systems are fairly rudimentary methods of keeping the sun off and its fairly straight forward to design one for a building. A static shade will only have limited effectiveness however, so a more dynamic shade which moves in tune with local weather and the path of the sun would be much more impactful.
An example of a dynamic shading system would be in Ab Dhabi, a city in which temperatures rise as high as 48◦C and structures need sheltering from an intense desert sun. The Al Bahr Towers have a shading system which takes its inspiration for the traditional middle eastern shading device known as a mashrabiya. These were wooden screens patterned with Islamic geometry, that allowed filtered light into a room along with views of surrounds all while protecting inhabitants from the harsh sun.
The modern mashrabiya on the Al Bahr towers create a dynamic façade capable of adapting to the position of the sun and creating shade against it to keep the building cool (also reducing CO2 emissions). The buildings management system operated 1059 hexagonal shades which open and close like flowers. Their movement are programmed to follow the sun, shading the parts of the buildings in direct sun but retracting to let in natural light in areas where the sun is less intense.