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Old May 18th, 2005, 08:51 AM   #1
Dubai-Lover
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How to cool a hot city

How to cool a hot city

By Mildred Fernandes, Special to Gulf News

International research on urban heat islands provides solutions for the UAE that can drop city temperatures by several degrees




Buildings, roads, parking lots and pavements are the foundations of every city.

They are also responsible for raising summer city air temperatures, sometimes by as much as 4 degrees Celsius (between 5 and 7 degrees Fahrenheit) more than surrounding areas, creating an urban heat island.

This phenomenon affects a city, its government and its residents in a number of ways, including public health.

Poor air quality is an important side effect of urban heat islands, as hotter city air boosts the creation of smog, leaving more pollutants in the air.

Urban heat islands also prolong and intensify heat waves in cities, making residents and workers uncomfortable and putting them at increased risk for heat exhaustion and heat stroke.

Hotter cities are also more expensive, with both government and residents paying the energy costs of additional air conditioning.

The increased electricity use also takes its toll on the planet, by producing carbon dioxide and contributing to global warming.

Scientific institutes around the world have long documented urban heat islands and researched ways to mitigate their effects.

The Lawrence Berkeley National Laboratory (LBNL) in California, the oldest of the US Department of Energy’s national laboratories and a leader in science and engineering research, is pre-eminent among them.

Dr Hashem Akbari, leader of the heat island group at LBNL, was in Dubai earlier this month to address the first International Conference on Green Buildings in the UAE.

He spoke to Gulf News about the Heat Island Group’s findings, and outlined some of the measures cities in this region can take to cool down.

“Measures to cool heat islands are simple and have been known to human beings for a long time,” said Dr Akbari.

“We just need to remind ourselves, and to put them back in practice.”

Passive solar orientation

In hot climates, one of the most basic and effective ways to build a green city, Dr Akbari explained, is to orient it in the right direction, towards prevailing winds and away from solar heat gain.

For this region, buildings would be ideally positioned in a north-south orientation, with the majority of windows and other openings located in these directions.

The more limited the opportunity for direct sunlight on the east and west surfaces of a building, the better.

The greatest amount of heat gain is produced on the west side.

“Most streets in the Middle East were typically designed in a north-south orientation, with the side streets at an angle,” said Dr Akbari.

“The understanding behind traditional building techniques and design still holds true today, but is often ignored. Some of the ideas like narrower streets may not be viable today due to population pressures, but the basic idea of minimising direct heat is still valid.”

Taking advantage of wind patterns means not putting any obstructions in the way to ensure natural cooling goes as far as it can, he added.

Buildings that incorporate plazas and other features that allow wind to pass through take this concept one step further.

Obstructions like buildings can come in handy, however, to block hot winds coming in from the desert.

Cool roofs

One of the major components of the Heat Island Group’s research involves cool roofs and cool pavements.

The underlying concept is a simple one: dark materials absorb more heat from the sun than lighter, reflective materials.

If those dark surfaces are roofs, some of that heat is transferred indoors.

In the sun, black surfaces can become up to 40 degrees Celsius (70 degrees Fahrenheit) hotter than the most reflective white surfaces.

Staying comfortable under a dark roof requires more air conditioning, meaning higher electricity bills.

Dark roofs also heat the air around them, contributing to the heat island effect.

Cool, reflective roofs can reduce this effect, save energy and make interior spaces cleaner and healthier.

Research in California and Florida showed that buildings with lightly coloured, more reflective roofs used up to 40 per cent less energy for cooling than buildings with darker roofs.

Cool roofs, therefore, are both financially and environmentally beneficial.

And while the finer details of the solar reflectivity of roofing materials are best left to architects and contractors, it is important to know that cool roofs are a function of both the materials used and their colour.

Research into cool roofing materials is ongoing at LBNL, with scientists trying to find the ideal roofing materials for different types of buildings in different climate zones and seasons, with different roof insulation levels, angles and orientations.

Cool pavements

Around the world, roads and parking lots are generally paved with black asphalt concrete and other dark materials that absorb most of the sunlight that falls on them.

Pavements get hot and heat the air around them, again adding to the heat island effect.

Researchers at the Heat Island Group have tried to determine exactly how black pavements are, how hot they get, the effect this heat gain has on city air and how to reverse these actions.

Their studies show that by a change to the asphalt coating can cause pavements and roads to reflect more heat, last longer and reduce city temperatures.

In one important experiment conducted last September in Berkeley, California, researchers measured the reflectivity of three types of asphalt - one newly laid, one aged and one with a prototype coating developed with the road construction industry - in the same area and at the same time.

The fresh asphalt recorded a temperature of 123 degrees Fahrenheit (50 degrees Celsius) with an albedo (reflectivity) level of 0.05; the aged asphalt 115 degrees Fahrenheit (46 degrees C) with an albedo of 0.15; and the coated prototype a temperature of 88 degrees Fahrenheit (31 degrees C) with an albedo level of 0.51.

For every minor increase in reflectivity, pavement temperature dropped several degrees.

Citywide simulations using cool pavement technologies showed an average decrease of about 3 degrees C (5.4 degrees F), and an estimated cost savings of $15 million (Dh55 million) annually.

A thin resurfacing with the prototype coating is all that is needed. Cities, say Group researchers, would have cooler summers at no extra cost if these materials were used to pave new roads and resurface old ones.

Leafy air conditioners

Vegetation is crucial in cooling cities, first by providing shade for buildings, cars and pedestrians, and secondly by reducing urban temperatures through a process known as evapotranspiration.

Trees placed strategically next to offices, homes, parking places and even paved areas, for example, minimise heat buildup in those areas and make air conditioning more effective by blocking direct sunlight and heat gain.

Researchers at LBNL estimate that properly planted trees can reduce cooling costs by an average of 10 to 20 per cent.

Over their lives, it is estimated that trees can be much less expensive than air conditioners and the energy needed to run them.

Plants also sweat or transpire water through pores in their leaves.

This water draws heat as it evaporates, cooling the air and lowering ambient temperature in the process.

Researchers estimate that a single mature, properly watered tree with a canopy of 30 feet can evapotranspire up to 40 gallons of water in a day.

Vegetation also brings other benefits, including providing habitat for wildlife, stabilising soil and preventing erosion, reducing wind speed and filtering pollutants from the air.

Planting the wrong trees in the wrong areas, however, can negate many of the benefits of green spaces.

As with the solar orientation, it is best to locate shade trees in the east and west of windows and other building surfaces to provide shade from morning and afternoon sun, which adds the most heat to buildings.

Dr Akbari also stressed the need for trees with big, dense canopies to provide as much relief as possible for buildings and pedestrians, and the use of salt-tolerant trees and shrubs to minimise fresh water consumption.

“Fountains and artificial ponds and lakes can also encourage evaporation,” he added.

“It seems many of these ideas are here already, but they need careful planning.”

In general, he added, trees should be planted within 20 feet of windows and grow to a mature size of at least 10 feet higher than the window.

And in the same way that evergreen trees are used to shelter homes in cold climates from harsh winter winds, desert bushes and trees can be used to block sandstorms and hot desert winds.

Trees grouped together not only protect each other from the sun and wind, but also make for refreshing green belts in the city and cooler neighbourhoods.

Time to act

A growing city like Dubai, Dr Akbari said, needs to integrate cool technologies as soon as possible, while the construction boom is still underway and with new residential and business communities being created.

“Retrofit measures can be taken in some cases, but some things need to be in place from the beginning,” he added.

“The irony is that cities like Dubai have mushroomed in the last 20 years, but there is no think tank to ask all the questions that need to be asked. But once they have developed, then they will want to know how they can retrofit."

“Architectural competition for the largest landmark throws energy efficiency out the window, but these energy costs will always be there for the government, businesses and residents to face,” he said.

Cooling a city requires a combination of traditional and contemporary knowledge and technology, and the UAE has access to both.

The solar orientation, recessed windows and ventilation patterns of decades past can be combined with the smart systems, breathing walls and vegetation research of the modern world.

The hottest cities of the world, temperature and business-wise, can become cool, healthy and energy-friendly communities.

For more information on Urban Heat Islands and the Lawrence Berkeley National Laboratory, visit: HeatIsland.LBL.gov.

The conference

The 1st International Conference on Green Buildings: The Future in the UAE, held on May 2 and 3 at the Dubai Chamber of Commerce and Industry, featured 12 international experts.

Twelve UAE-based industry professionals also spoke on a variety of subjects related to green buildings.

The topics included innovations in technology to daylighting, efficient glass, indoor air quality, photo-voltaics and recycled materials.

Conference co-chairman Dr Mohsen Aboulnaga, dean of the College of Design and Applied Arts at Dubai University College, said the two-day event succeeded in creating awareness of the concept of green buildings for industry executives, government bodies and the public.

It also suceeded in taking definitive steps towards creating a local green building council and green rating guidelines for residential and commercial buildings in the emirates, Dr Aboulnaga added.

For more information on the conference, visit icbg05.duc.ac.ae
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Old May 18th, 2005, 09:11 AM   #2
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sounds interesting, but i am sure there might be a lot of possibilities to cool the city and some have already been esablished for sure.
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Old May 18th, 2005, 11:41 AM   #3
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sounds complicated, I dont see it happening any time in the near future.
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Old May 18th, 2005, 12:52 PM   #4
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too long of a read, so what are we suppose to do to get a cooler city ?
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Old May 18th, 2005, 03:01 PM   #5
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Quote:
Originally Posted by Qatar4Ever
too long of a read, so what are we suppose to do to get a cooler city ?

Haha I second that.
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Old May 18th, 2005, 06:36 PM   #6
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well according to the article , you get an ice cold blanket and cover the whole city with it , that sums it up pretty well
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Old May 19th, 2005, 09:30 AM   #7
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^ works for me , someone call up dubai holding and tell them about it ,
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Old May 19th, 2005, 02:44 PM   #8
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They should fill Dubai with palm trees, though they are expensive they really are effective in providing shading. Also, every building in Dubai should install AC vents on top that blow into the streets below, creating 20-25C temps
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Old February 1st, 2017, 10:16 PM   #9
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