When the heat is on, we need plans to keep cities cool

When the heat is on, we need plans to keep cities cool

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Climate change means Australian cities are facing more heat waves, but not all strategies to keep us cool are created equal.

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The Conversation

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The recent spate of heatwaves through eastern Australia has reminded us we’re in an Australian summer. On top of another record hot year globally, and as heatwaves become more frequent and intense, our cities are making us even hotter.

This is the urban heat island, where city temperatures can be significantly warmer than the surrounding rural regions.

The question, then, is what we can do to keep our cities cooler.

Why are cities hotter?

The temperature difference is caused by a range of factors, including dense building materials absorbing more of the sun’s energy, fewer trees to provide shade, and less soil to cool by evaporation.

Buildings can also act like the hairs on a husky, reducing wind speeds and blocking thermal radiation up to the night sky. On top of that, waste heat from car engines, air-conditioners and other energy use adds to overall air temperatures.

Why does this matter? Even a small increase in air temperature pushes up overall energy demand, and about 25% of our energy bills are for only 40 hours per year when the grid is most heavily used.

The most extreme heat events can buckle train lines, cause rolling blackouts and cost billions in lost productivity. And it’s not just bad for our wallets.

Heat stress can damage organs or exacerbate existing illnesses. Since 1900, extreme heat events have killed more Australians than bushfires, cyclones, earthquakes, floods and severe storms combined.

So, what can we do?

There are a number of things individuals can do to reduce the impact of heat in their homes, such as installing light coloured roofing material, insulation or an air-conditioner.

But it gets more complicated when considering the city as a whole, and how these small actions interact with each other and with the climate.

Air-conditioners

In heatwaves, air-conditioners save lives, allowing stressed bodies time to cool. But our homes can only be made cooler by blowing heat outside, along with the extra energy to run the system.

As well as increasing outside air temperatures in the short term, the fossil fuels burned add to global warming. A world cooled by air-conditioning probably isn’t the answer.

Trees and parks

Trees provide shade, but also cool the air, because evaporating water from leaves takes energy, reducing peak temperatures by 1-5° C.

Most city planners agree on the broad benefits of urban vegetation, with some metropolitan councils developing urban greening strategies.

However, urban trees can be a vexed issue for some councils; they use water, can be costly to maintain, can damage utilities and property, and can worsen air quality instead of improving it. Larger cities are often made up of dozens of councils; getting them to agree is a major challenge.

White roofs

We know that black surfaces get hotter in the sun, but demand for dark roof tiles still far outweighs demand for light colours. More reflective roofs can reduce a household’s energy bill, as well as the overall temperature of a city.

White roofs are most effective in warmer climates, because in cold climates, the cost savings in summer must be balanced with additional heating costs in winter.

Green roofs and walls

Green roofs and walls are building structures with integrated vegetation. They provide cooling benefits by shading buildings and through evaporation from leaves. They generally show less cooling benefit than white roofs, cost more to install and maintain, and use additional water and energy.

But they do look nice, improve biodiversity and make people happier.

Pavement watering

Prior to an extreme heatwave, it may be possible to reduce temperatures by wetting down building and road surfaces. It’s a traditional practice in Japan, and is now being considered in major cities like Paris.

But temperature and humidity are important factors in heat stress, so pavement watering should only be undertaken if the extra humidity does not increase heat stress.

Large scale rooftop solar

Solar panels convert energy from the sun into electricity, so less energy is required from the network overall. If enough roofs were covered with solar panels, could that lower air temperatures?

Probably a little. Other benefits include a reduction in the energy required for cooling (because the roofs are shaded by panels), and a stable, lower cost, decentralised renewable energy system.

Building density

A building with lots of thermal mass (think sturdy, double-brick home) can be an effective way to keep inside temperatures more stable. Heat is absorbed during the day and released at night. The same idea can work for an entire city.

An urban cool island can form in high-density cities like Hong Kong because tall buildings provide extra heat capacity and shade.

For similar reasons, the tight street layout of traditional Arabian and Mediterranean cities keep those streets cooler.

Shading structures

Installing light shading structures over streets, pavements and roofs can reduce the surface temperature of materials, and reduce the heat absorbed and radiated back into streets. Shading structures need to be designed so that they do not limit airflow, trapping heat and air pollution in streets.

Which is best?

To figure out what works best, we need to be able to model the physics of different strategies, in different types of cities and in different climates. We can then assess the economic and health impacts and decide on appropriate and plans that give us the biggest bang for our buck.

Here we have focused on heat in cities, but there are other important concerns like air quality or flooding.

In colder cities, an urban heat island could actually be a good thing. Each city is different; each requires a tailored and integrated plan developed over the entire metropolitan region, and then implemented locally by councils, businesses and households.

Source: The Conversation. Reproduced with permission.

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13 Comments
  1. MaxG 4 years ago

    “Since 1900, extreme heat events have killed more Australians than bushfires, cyclones, earthquakes, floods and severe storms combined.”
    I am still being laughed at for having catered for more extreme wind speeds, provided extreme insulation, and other measures, (requiring only 2.5kW on the worst day for heating and cooling the whole house of 180m2,) dealing with the future weather being expected. Let’s see who is laughing last 🙂

  2. George Darroch 4 years ago

    So, trees and rooftop solar then?

  3. solarguy 4 years ago

    As I write this post in my air conditioned office, I’m pleased to say my whole house is cool inside thanks to all my Inverter split a/c’s, which are all powered by PV. It’s 40 outside.The hot water is being taken care of by an E.T SHW system. It reached top out temperature of 80 degrees at 10.30am and perhaps this year I will only need to use grid power to boost it 8 days like I did last year and used no more than 57.6kwhrs.

    I’m achieving this and still exporting power to the grid. I can’t wait until the battery system is up and running, then these hot nights won’t impact my already meagre power bills. With the export helping to pay for the SAC charges, I’ll be unlucky to pay any more than $50/quarter.

    • Greg Hudson 4 years ago

      $50 a quarter is very good. My total for 2016 was $414 mostly fixed daily fees by the distributor (i.e. not the retailer).

  4. wmh 4 years ago

    Might I put in a recommendation for the kind of city street shade structure that you find for instance in the south of Spain. Here shade cloth is stretched between 5 to 6 storey buildings. It is does not cover the entire gap but is positioned to cover the central part of the street to allow fresh air to penetrate. An instant reduction in street level temperature.

  5. wmh 4 years ago

    Combine solar with storage and the household energy paradigm changes from central generator/ distribution network/ individual consumer, a system which has been in force for the whole of civilisation, to one where energy generation and consumption is entirely local to the consumer. Solar power will eventually fulfill the old 1950’s nuclear power promise of “energy, too cheap to meter”. Solar panels are cheap but the consumer is still faced with high cost storage as batteries or ultra low cost storage in the form of house thermal mass (useful in both summer and winter) or hot water storage (useful for domestic hot water provision and winter home heating).

    • solarguy 4 years ago

      Ah, but what will you use to cool the house in summer?

      • wmh 4 years ago

        Solarguy, as you do in your house, during the day solar power runs the
        A/C cooling and at night you have an amount of house thermal mass to keep things cool, with the advantage less heat is gained at night
        because the night time outside temperature is cooler. Cold water energy storage for summer is possible but not as practicable as hot water storage for winter because the minimum attainable temperature, even with a (for instance) brine mixture, is limited.

        • solarguy 4 years ago

          Minus 21c is very good, but at what cost for winter heat and summer chilling?

          • wmh 4 years ago

            I’m not a refrigeration engineer so Google is my source of information about NaCl brine. Apparently CaCl brine will reach -51 C. For some fluids It seems that corrosion is the big problem. However every commercial fishing trawler has a refrigeration system that uses some kind of low freezing point working fluid so it must be possible to build a domestic version, although combining the hot and cold systems may not be practicable.
            See http://www.cool-info.co.uk/brines_steam/secondary_refrigerants/sec_refrig01.html

          • solarguy 4 years ago

            May not be practicable……. on a cost standpoint is exactly my point. Even using Evacuated tubes (free energy) the cost of heating or cooling comes in to dear for residential use, unless you do it yourself and have light loads. I wish it wasn’t the case but it is for now. Maybe one day!

  6. Rod 4 years ago

    I think, for the suburbs anyway, bang for buck has to be light coloured roofing.
    Good for the individual households and the surrounding area.
    Councils could mandate for new builds and renovations and education might convince those upgrading or painting their roof to consider light colours.
    Households could also reduce hard surfaces to encourage water retention.

  7. Ren Stimpy 4 years ago

    Why are cities hotter????? Because of climate change you idiots!

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