Sydney scientists claim ‘cool roof’ breakthrough could keep cities cooler, greener

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UTS team develops breakthrough ‘cool roof’ solution which could help reduce urban heat island effect, and peak power demand.

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As India emerges from what is shaping up to be the world’s fifth deadliest heatwave, scientists from Sydney’s University of Technology (UTS) claim to have made a technological breakthrough that achieves the “elusive target” of keeping a roof cooler than the air around it – even in hot Australian summers.

The development of the “coated polymer stack” cool roof material is the result of a study by Dr Angus Gentle and Emeritus Professor Geoff Smith, published in the latest edition of the journal Advanced Science.

roof1-Angus-Gentle
Dr Angus Gentle holding a piece of the special material over an existing cool roof used in testing. Picture supplied

And it is believed to have major implications for reducing both the heat island effect in urban areas, as well as peak power demand from air-conditioning – and greenhouse gas emissions.

In tests conducted by the UTS team on the roof of the university’s science building, the newly developed surface – a combination of specially chosen polyesters on a silver layer – stayed 11 degrees or more colder than an existing state-of-the-art white roof nearby, absorbing only 3 per cent of incident sunlight while simultaneously strongly radiating heat at infrared wavelengths that are not absorbed by the atmosphere.

“We demonstrate for the first time how to make a roof colder than the air temperature around it, even under the most intense summer conditions,” UTS Professor Smith said.

“Roofs heat up by absorbing sunlight, so darker roofs can get very hot. Even white roofs still absorb enough sunlight to warm up by 9 degrees Celsius to 12 degrees Celsius.

Smith says the plastic materials used for the demonstration are available commercially and potentially suited to use on basic roofing.

“Cool roofing reduces the severity of the urban heat island problem in towns and cities and helps eliminate peak power demand problems from the operation of many air conditioners,” he said.

“The added feedback benefits from cool roofs are not yet widely appreciated, but recent reports have shown they are substantial. Examples include ventilation with cooler air and higher performance of rooftop air-conditioning installations.”

Professor Smith also notes that the surface maintained its high performance in all conditions.

“Extensive dew formation is inevitable for a super cool roof and dew drops precipitate dirt. This roof site being 25 metres above a busy city transit road was a stern test. Results show that excellent thermal performance can be maintained.

“Much of the world’s population lives in warm climates. Keeping a roof cool saves energy and makes building interiors comfortable in summer. If enough roofs in a precinct are kept cool then the local climate can also be beneficially influenced,” he said.

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27 Comments
  1. Rob G 3 years ago

    Another great Aussie invention that can have a massive manufacturing future, but I doubt an Abbott government would back it.

    • Coley 3 years ago

      No prizes for guessing who they will have to turn to for funding to develop the idea.

  2. lin 3 years ago

    Looks interesting. Dew precipitation may also enable significant water collection in some locations.

  3. properly managed livestock 3 years ago

    Grasslands have almost 50% more reflectivity than bare ground. If we use properly managed livestock to restore our planets grasslands not only do we sequester carbon, boost biodiversity, enhance rainfall retention, produce healthy and truly sustainable food, we also put a planet cooling “green roof” back onto the soil surface

    • Alastair Leith 3 years ago

      Oh please. Enteric fermentation from ruminant livestock exceeds all the savings you are claiming. BZE Land Use Report found that 55% of Australian GHG emissions come from land use sector using 20yr GWP. Of that 90% are associated with livestock on northern ‘grasslands’/ranges.

  4. Jo 3 years ago

    A few things are weired here:

    “We demonstrate for the first time how to make a roof colder than the air temperature around it,…” —> So if the air temperature in shade is say 40 degrees, the roof will be cooler than 40? How does that work? If it worked, they could build a fridge out of the roof!

    “absorbing only 3 per cent of incident sunlight while simultaneously strongly radiating heat at infrared wavelengths that are not absorbed by the atmosphere….”

    —> if the roof is reflecting 97% of light, how can it strongly radiate at infrared wavelengths? Reflected light is light, And the solar spectrum has only a small part of infrared radiation.
    And what does “not absorbed by the atmosphere” mean? Do they want to say that this roof does not cause climate change?

    Something is wrong here. It smells like snake oil.

    • Pedro 3 years ago

      It appears that the roof was 11 degrees cooler than the white roof next to it. So it is not ambient temperature in the shade.

      I take it that certain gases absorb only certain parts of the EM spectrum. So if the roof radiates IR that the atmosphere does not absorb it should travel out to space.

      • Jo 3 years ago

        This is what the article said: “… breakthrough that achieves the “elusive target” of keeping a roof cooler than the air around it …”

        If a surface is radiating more infrared radiation than it receives, it must first adsorb other radiation (in this case light) and convert it to a lower wavelength (= heating up) which makes no sense in the context of this invention as the roof would be hotter, not cooler.

        • WR 3 years ago

          The roof is not in thermal equilibrium with the air above it. The roof would only be in thermal equilibrium, and therefore at the same temperature as the air, if this was a closed, isolated system. This isn’t the case, so the roof can be cooler.

          The air is receiving energy from a number of sources and reradiating that energy in the infrared part of the spectrum. The roof will be reflecting most of the incident IR radiation from the air, along with the incident visible light. So there is no reason why it should have the same temperature as the air above it.

        • Pedro 3 years ago

          I see your point. Wording in the article is ambiguous and confusing. Of course the roofing material has to heat up, but I think they meant by far less than other roofing materials. I think the laws of thermodynamics are still intact.

    • WR 3 years ago

      You can’t build a fridge out of it because it does not make the air cooler. It simply stops the air under the roof from warming as much as it normally would. Its basically acting like insulation.

      Everything with temperature above absolute zero radiates energy. The wavelengths of this radiant energy will depend on the material and temperature of the surface of the object. For the typical temperatures of the roof, the material from which it is made must radiate at infrared wavelengths that are not absorbed by greenhouse gases in the atmosphere.

      • Jo 3 years ago

        How can the material “… strongly radiating heat at infrared wavelengths …” if it is “… absorbing only 3 per cent of incident sunlight …”?
        Where does the energy come from?
        If the material was converting visible light into infrared radiation it would be heating up not cooling down.

        • WR 3 years ago

          The energy comes from the visible light and IR radiation that it will be absorbing, and it will also be getting some energy via conduction from the surrounding air.
          It reradiates this energy as IR radiation to STOP from heating up. If it WASN’T radiating energy, it would keep getting hotter.

          • Jo 3 years ago

            All I am saying is that it does not make sense to describe a material as “…absorbing only 3 per cent of incident sunlight …” and at the same time as “… strongly radiating heat at infrared wavelengths …”.

            Of course every body radiates heat. I have also no problem accepting that a ‘mirror’ material is heating up less in sunlight than a white material.
            But absorbing 3% of radiation and ’emitting strongly’ does not makes sense. In my mental dictionary ‘strongly’ is certainly more than 3%.

            I think the author simply did not understand and got it wrong. I will leave it with that.

        • G3980 3 years ago

          If you look at figure 1 in the paper, You can see that the reflectance is high in the solar wavelengths (300nm-2500nm) while at the longer wavelengths (greater than 5000nm) the surface emits well, which coincides with the peak of planck spectrum [the radiation given off] by the surface at room temperature (300K). Strongly emits is referring to the thermal infrared wavelengths > 5000nm, rather than the solar near-infrared (700-2500nm).

  5. john 3 years ago

    Before using an insulating material on a iron roof the temperature under the roof was 50 to 60 degrees C after treatment this dropped by 15 degrees on average now this treatment is simular and perhaps of greater benefit.

  6. JohnRD 3 years ago

    Keep in mind that net radiation between two objects depends on the temperature of the objects, not the temperature of what is in between. That is why the sun can radiate heat to earth despite all that cold space in between.

  7. John Silvester 3 years ago

    There seems to be some confusion about how a material can be cooler than the temperature of the air around it.

    If at a given temperature, the material is able to radiate more energy than is being absorbed from the air around it, the material will cool relative to the surrounding air. This cooling effect is limited by the fact that as the material cools the amount of energy it is able radiate diminishes and the temperature difference between the air and material increases allowing more energy to flow into the material from the air. The cooling effect will cease when the amount of energy being emitted is equal to the amount of energy absorbed by the temperature difference.

    This article is claiming they have created a material that is able to reflect 97% of incoming visible light and radiate more energy as infrared (IR) than is being absorbed from the 3% of visible light not being reflected.

  8. Alastair Leith 3 years ago

    Interesting material(s) here. Does the roof also stay cool in winter when gaining heat from sun radiation and atmosphere (conduction) is desirable?

    In Melbourne the gains of white roofs are offset (to whatever extent I’m not sure) by the loses in winter. Since heating is a bigger household energy cost in Melbourne (and other temperate locations) than cooling is it worth it. A roof that could switch from this in summer to heat gain and minimised losses in winter would be science fiction, I guess. Ventilating roof cavities in summer but not in winter is one example of ‘switching’ that I’m talking about.

    • Chris Fraser 3 years ago

      I think windows and thermal mass in the walls are the best solar heat gain. I’ll be plugging up the colorbond with batts to reduce heat loss in winter.

  9. Phound 3 years ago

    Is it more reflective than a mirrored surface on the roof?

    • Mike Dill 3 years ago

      The surface reflects 97% of the sunlight, and also emits energy in the infrared from the background heat. So, it is not more reflective than a mirror, but has the ability to remove heat from the planet.

      • Gordon 3 years ago

        If it reflects 97% of sunlight, then it is more reflective than a common household mirror, which would be more like 90% reflective. You would only see over 97% on telescope mirrors with enhanced coatings (and minimal dust) – which is very expensive.

  10. Ian 3 years ago

    As I see it , this coating has a silver base layer that reflects most light, visible and infrared. It then has a semi- opaque top layer which absorbs/ emits a certain infrared spectrum. The sun’s spectral irradiance in W/m2/nm is highest in the visible spectrum and then drops off in intensity in the infrared part of the spectrum. Absorption and emission of electromagnetic radiation is usually characteristic for a substance. The researchers must have chosen a substance that emits infrared light at about room temperature. As it heats up by conduction from the warm air and the small quantity, (3%), of sunlight it starts to radiate infrared rays. As said before, the sun’s infrared irradiance is relatively weak at the surface of the earth and so this material appears ‘hotter’ than the sun at those frequencies and the material emits more radiant heat than it absorbs at that frequency. The intense irradiation of the sunlight in the visible spectrum goes right through this material and strikes the mirrored backing and is reflected unchanged out the material again. There you have it, a reflective surface and semi-opaque surface giving the best of both worlds. Hence the roof is cooler than the surrounding air. Borosilicate glass transmits light nicely up to 3500nm and then becomes very opaque. A glass mirror would be nearly as good!

  11. Stephen Young 3 years ago

    What is the cost per square meter? When likely to be commercially available?

  12. David Rossiter 3 years ago

    Another approach to getting a ‘cool roof’ is to build what used to be called a tropical roof – that is shade the roof with another roof above the existing roof and leave an air gap in between.
    Even better put solar cells on the roof covering part or all of the roof with what becomes effectively another form of tropical roof. I recently had some data on my roof on a summers day – where there was no solar cells the uninsulated roof temperature (mostly ridge capping) was in excess of 60 degrees C, whereas those bits that were covered by solar cells were close to ambient air temperature.
    This is a rarely noted benefit of rooftop PV in hot weather.

  13. Richard M 3 years ago

    This was proposed in America as well. It was pooped on by the progressive establishment even after it was proved by scientific models. Just doesn’t allow the progressives enough ways to control everyone else’s livfe.

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