Can one have a roof with variable albedo?

[Garald]

Is there a realistic, non-science-fictional way to have one's roof be (much) more reflective in summer than in winter? (The relevance to energy efficiency is obvious.)

[Conor]

Interesting, no idea and no data to add, but,

 

Paint whitewash on in early summer? it'll wash off again with the winter rains.

 

Tho, overall, a permanently white roof would be better from an access / cost / safety point of view. The cooling effect of a white roof in the summer would be much grater than the gain of a dark roof in winter. 

 

And of course depends on the ratio of roof to building volume. A skinny, tall building with a small roof would have marginal benefits. 

[SteamyTea]

Plants are meant to do this, but when I did an experiment on it, the variation was tiny as the grass grew.

 

There are paints that change colour with temperature.

 

In practice, as radiation is proportional to the cube of the absolute temperature difference, the only times that a meaningful difference happens is during summer, so a reflective coating is all you need, in winter the differences are too small most of the time (maybe 10 days in the UK).

 

City in Sierra Leone covers buildings in mirrors to fight extreme heat

People in Freetown, Sierra Leone, are increasingly exposed to extreme heat due to climate change and the urban heat island effect, but covering homes in a reflective film significantly cut indoor temperatures

By James Dinneen

3 June 2023

 

 

 

 

Reflective film installed on the roofs of two buildings in Freetown, Sierra Leone

MEER

 

Tens of thousands of people in Freetown, Sierra Leone, live in informal settlements dense with metal and concrete buildings that trap heat and make heatwaves more dangerous. An experiment has shown that covering such buildings with a cheap mirrored film can substantially reduce the temperature inside.

Since becoming the chief heat officer of Freetown in Sierra Leone, Eugenia Kargbo has experimented with all kinds of ways to protect the city from heat. When a group of researchers approached her with a plan to cover the city’s buildings in mirrors to cool them off, she thought it was worth a try. “I said, ‘why not?’”

The proposal came from a US non-profit called MEER, an acronym for Mirrors for Earth’s Energy Rebalancing. Founder Ye Tao was in search of a place to test the cooling effects of a reflective film the organisation had developed out of recycled PET plastic and aluminium. In theory, a building covered in the film would absorb less of the sun’s radiation and be cooler than one roofed with metal or tar.

Tao had heard about Kargbo’s efforts to mitigate heat in Freetown, which have included planting hundreds of thousands of trees across the city and installing shading structures made of reflective plastic in a crowded marketplace. So, he asked her about testing the film.

Temperatures in Freetown regularly spike above 40°C (104°F) during the dry season from December to April, and remain hot even at night. The hottest days are projected to become more frequent with climate change.

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The heat is also exacerbated by the urban heat island effect – a phenomenon in which urban areas get hotter than nearby rural areas due to less vegetation, less air flow and more heat-absorbing material on buildings and roads. Kargbo was interested in anything that might provide cooling effects indoors. The city’s buildings are hot and getting hotter.

“The heat data shows everywhere in Freetown is getting hotter, but there are also communities that stay hotter throughout the day,” says Kargbo.

This was particularly true of the numerous informal settlements built along the coast and the deforested hills around the city. Most buildings in the impoverished settlements are made from uninsulated concrete walls and corrugated zinc roofing that absorb and trap heat.

Kargbo says the heat and humidity, along with high levels of air pollution in the settlements, make for a “toxic combination” for residents’ health and well-being.

In consultation with residents, Kargbo and other city officials along with Tao and his colleagues decided to test their reflective film in a settlement called Kroo Bay. The settlement is one of Freetown’s largest with more than 10,000 people living in roughly 1 square kilometre.

 

Read more:

El Niño climate events cost tropical countries trillions of dollars

 

Jalahan Sesay, a recent graduate from the University of Sierra Leone who surveyed residents as part of the MEER project, says most people in Kroo Bay sleep outside during the hottest time of year, because staying indoors is intolerable. Most buildings lack a ceiling to separate the living space from the roof. “It’s like having a radiator on top of people’s heads,” says Tao.

Working with local carpenters, Tao and his team installed mirrored film on the roofs of two residences. To compare its effectiveness against other cooling strategies, they also painted the roof of one residence white and added a new metal roof to another. All four buildings were similarly constructed and had around 180 square metres of roof area.

During the day, the interior temperature of the building with the new zinc roof was on average 1 to 2°C cooler than before, and the building with white paint was around 3°C cooler. Inside the two buildings covered in film, it was 6°C cooler, says Tao. The temperature of the roof on the two mirrored buildings was 15°C cooler than without the film, on average.

 

 

A heat map (left) shows the temperatures of the four roofs (pictured right), with darker areas signifying lower temperatures. While the new metal roof (bottom left) appears coolest, temperature measurements revealed the two roofs covered in reflective film (right) were actually cooler.

MEER

 

Sedie Sowa, another MEER intern who surveyed residents in Kroo Bay, says families who live in the mirrored buildings are pleased with it. “They say they sleep comfortably,” he says.

David Sailor at Arizona State University says assessing the amount of cooling depends a lot on measurement conditions – cooling effects from reflecting sunlight will be greatest during days with the most solar radiation, for instance. But he says a 6°C reduction is substantial. “There’s a lot of potential there not just to improve comfort, but to save lives.”

Tests are ongoing, but Kargbo says she is encouraged by the results, and aims to roof many more buildings in Freetown with reflective film. If an entire neighbourhood or more were to be covered, Tao says the cooling effects could accumulate as air flows across roofs, lowering air temperature across whole areas and not just within individual buildings.

Infrared-reflecting paint can cool buildings even when it is black

By Layal Liverpool

24 April 2020

 

 

 

A new paint reflects infrared with a layer of polymer, which helps keep objects cool

Jyotirmoy Mandal

 

A two-layered paint that reflects infrared light while maintaining its colour could help keep buildings and vehicles cool under hot sun. This could help reduce energy used in cooling, such as by air conditioners.

This coating was developed by Yuan Yang at Columbia University in New York and his colleagues. It consists of a top layer of commercial paint, which provides the colour, and a bottom layer made of a polymer similar to Teflon, which reflects infrared light.

Sunlight contains both visible and infrared light but the infrared accounts for the majority of the solar energy, says Yang.

When the sun shines on an object coated with this paint, the top layer absorbs certain wavelengths of light, depending on the paint’s colour, while the bottom layer reflects infrared light, preventing the object from heating up.

A similar cooling effect can be achieved using white paint or metal mirrors, but Yang says the advantage of this new paint is that it can be any colour desired.

Read more:Earthquake-resistant paint could help brick buildings survive tremors

Usually black paint absorbs heat, but painting an object with a black version of this new coating kept it about 16°C cooler than when an object painted with commercial black paint was exposed to the same amount of sunlight.

In another test, the new paint coating was found to be able to maintain its colour despite being placed in an oven at 60°C for 30 days.

Yang says this paint could help save electricity and reduce carbon dioxide emissions.

“Solar reflective and thermally emissive surfaces offer a sustainable way to cool objects under sunlight,” says Mingqing Wang at University College London, who was not involved with the work. This could be useful in tropical locations to help keep buildings cool and reduce electricity consumption from air conditioning, as well as to prevent cars, buses and trains from getting too hot, she says.

An intriguing next step would be to try and add more functionality to the coating, for instance to enable the energy from the reflected infrared light to be harvested to generate electricity, says Wang.

 

Science Advances DOI: 10.1126/sciadv.aaz5413

 

 

Superwhite paint can cool buildings even in hot sunlight

By Adam Vaughan

21 October 2020

 

 

 

Some buildings are already painted white to help with cooling

Johner RF/Getty Images

 

A new superwhite paint is so reflective that it can cool a surface to below the surrounding air temperature, even under sunlight. It could help reduce the use of energy-intensive air conditioning in hot countries.

With global energy use expected to grow 90 per cent by 2050, ways to passively keep cool without using energy will be vital in coming decades. While “cool roofs” painted white are a common sight in hot climates, materials experts think they can do one better.

Xiulin Ruan at Purdue University and his colleagues developed a white paint that was so reflective and good at radiating heat that it cooled a surface to 1.7°C below the surrounding noon air temperature during tests in Indiana. Compared with existing, commercial heat-reflective paints that reflect about 80-90 per cent of solar energy, the new one managed 95.5 per cent.

Although it sounds counter-intuitive, the surface can be cooled below the surrounding temperature because it emits enough heat through radiative sky cooling, the natural process of a body under the sky – such as a roof – radiating heat out to space. Light-coloured surfaces regularly do this on cloudless nights, but it wasn’t until 2014 that we found a material that managed the feat in daylight, when our cooling needs are greatest.

Read more:Infrared-reflecting paint can cool buildings even when it is black

Compared with that breakthrough, Ruan says his team’s paint is thinner, cheaper and could be easily scaled up. The acrylic paint is made with calcium carbonate, and partly achieves its qualities by containing particles of many different sizes, which help to scatter different wavelengths in the solar spectrum. Ruan estimates a typical US home of 200 square metres would save about $50 per month on cooling costs, compared with using an existing heat-resistant paint.

“This is a very nice result,” says Aaswath Raman at the University of California, Los Angeles. “It demonstrates a paintable solution that employs materials commonly used by the paint industry, and gets reasonably good cooling performance. One potential limitation could be its use of organic solvents, but that could be addressed in the future.”

 

Cell Reports Physical Science DOI: 10.1016/j.xcrp.2020.100221

[Garald]

1 hour ago, Conor said:

Interesting, no idea and no data to add, but,

 

Paint whitewash on in early summer? it'll wash off again with the winter rains.

 

Tho, overall, a permanently white roof would be better from an access / cost / safety point of view. The cooling effect of a white roof in the summer would be much grater than the gain of a dark roof in winter. 

 

And of course depends on the ratio of roof to building volume. A skinny, tall building with a small roof would have marginal benefits. 

 

I have (most of) a broad, short rowhouse with a clay tile roof. The ground-floor (well, the bit of it that I have) is pleasantly cool in summer, the first floor is quite all right, but the attic gets hot. At some point, I may raise the attic roof and in the process redo the roof insulation entirely. I'd obviously want to do as good a job as possible - that was one of the motivations for the question. Of course I don't know whether townhall would give me permission to have something other than a clay tile roof (though every neighbor seems to have a different kind of roof).

[Garald]

16 minutes ago, SteamyTea said:

Plants are meant to do this, but when I did an experiment on it, the variation was tiny as the grass grew.

 

There are paints that change colour with temperature.

 

In practice, as radiation is proportional to the cube of the absolute temperature difference, the only times that a meaningful difference happens is during summer, so a reflective coating is all you need, in winter the differences are too small most of the time (maybe 10 days in the UK).

I thought radiation was proportional to the cube of *absolute temperature*? Wouldn't we be talking of the temperature of the sun, at any rate?

[Garald]

Actually, wouldn't the ideal thing be something like a Trombe wall *on the roof*, to be covered in foil during the warmer half of the year?

[SteamyTea]

2 minutes ago, Garald said:

I thought radiation was proportional to the cube of *absolute temperature*? Wouldn't we be talking of the temperature of the sun, at any rate

https://en.wikipedia.org/wiki/Stefan–Boltzmann_law

[SteamyTea]

I am actually being a bit dumb this morning, just fit PV, that will take out 20% of the IR on the sunniest days, and nothing on the coldest night.

[Garald]

37 minutes ago, SteamyTea said:

https://en.wikipedia.org/wiki/Stefan–Boltzmann_law

*Fourth* power, right. Oops.

[SteamyTea]

1 minute ago, Garald said:

*Fourth* power, right. Oops.

Yes, just realised that, was a long day driving yesterday and I think I may go back to bed to sleep it off.

[saveasteading]

2 hours ago, Garald said:

something other than a clay tile

Clay works well. It absorbs a great deal of heat. Then there is an air gap underneath as a thermal break. Depending on the tile type, there is probably air movement dispersing heat, as well as radiating and wind heat loss.

It's nog enough and you still need something underneath.

 

I have poked around with Mediterranean houses that are cool indoors. 25C in when it it is 35C out.

The tiles are scorching. Then there is a mass concrete roof 100 thick which gets quite hot but slowly. Then a suspended ceiling. No insulation.

At night the roof cools to the ambient temperature.

Shiny or light colours would reflect some heat of course.

[Garald]

What about this - does it really work?

 

https://www.3m.com/3M/en_US/energy-conservation-us/applications/passive-radiative-cooling/?fbclid=IwY2xjawFksOlleHRuA2FlbQIxMAABHUIw6wTdqv5HwAj5NW5qUxQY8ieuzPDxidYH4a-5zm2WiJLgE0U8Y2G6jg_aem_n1OSWXTEyGv0_g2AfBHrig