This is a potential new cooling system that consumes no energy for buildings and that could save massive amounts of energy on air conditioner systems normally used to keep buildings cool inside throughout the world! At first when I read that it uses "no energy", I immediately though "how that could be given the laws of physics?" But then I read on and found that this can conform perfectly to the laws of physics for is explains:
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Objects can cool themselves without consuming energy by radiating energy in the form of infrared light. This process is not normally very efficient because objects can also be warmed by convective air currents and by absorbing radiation emitted by other objects and by the air. However, air absorbs and emits very little infrared radiation at wavelengths of around 8–13 μm. It is through this "window" that the Earth lowers its temperature at night – especially when the sky is clear – by sending radiation out into space.
To make practical use of this effect during the day, the surface of an object must emit radiation within this window, while at the same time reflecting sunlight to minimize the amount of heat it absorbs. The problem is that no known naturally occurring material can do both of these things.
Layered material
Now, physicist Shanhui Fan and colleagues at Stanford University have made a device that fits the bill. The team designed a structure of seven alternating layers of silicon dioxide – essentially glass – and hafnium dioxide. Both materials are transparent to visible light but emit radiation strongly at wavelengths of around 10 μm. These layers are stacked on a layer of silver to create a mirror that reflects visible light. Fan and his team used a computer simulation to choose thicknesses for the different layers that would maximize both how much sunlight the combined device reflects and how much infrared radiation it emits.
The researchers then mounted the device, which was just under 2 μm thick, onto a 20 cm-diameter circular silicon wafer, added a plastic sheet to block convective air flows, and placed the apparatus on the rooftop of a building at Stanford. They found that on a sunny day, the device cooled to between 4 and 5 degrees below the surrounding air temperature. The device therefore appears to be the first object known to achieve such cooling under direct sunlight without consuming energy.
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Originally posted by humy At first when I read that it uses "no energy", I immediately though "how that could be given the laws of physics?"
In Livingstone, Zambia, we know all about cooling and energy costs. The best ways to achieve cooling are trees, high ceilings and insulation under the roof.
We have often laughed at people who buy a house, chop down all the trees (because they are too lazy to clean up the leaves that trees drop) and then 6 months later they install air conditioners.
My sister says that by planting trees around her house she has brought down the average temperature by about 4 degrees.
They found that on a sunny day, the device cooled to between 4 and 5 degrees below the surrounding air temperature. The device therefore appears to be the first object known to achieve such cooling under direct sunlight without consuming energy. OK that does seem to violate the laws of physics. Is the device radiating downwards, or in some other way interacting with its lower surface?
Do we call this the 'reverse green house effect'?
Originally posted by twhitehead In Livingstone, Zambia, we know all about cooling and energy costs. The best ways to achieve cooling are trees, high ceilings and insulation under the roof.
We have often laughed at people who buy a house, chop down all the trees (because they are too lazy to clean up the leaves that trees drop) and then 6 months later they install air conditioners.
My ...[text shortened]... other way interacting with its lower surface?
Do we call this the 'reverse green house effect'?
OK that does seem to violate the laws of physics. Is the device radiating downwards, or in some other way interacting with its lower surface?
no; the cooling effect of the device works by radiating some of its heat to outer space i.e. it radiates it upwards. Remember, the material's surface is made highly reflective of the wavelengths that the sun mainly radiates and, take away direct sunlight, the effective 'temperature' of space is extremely low (just a few kelvin ) . So there is in effect a kind of 'temperature difference' between the surfaces of the Earth and outer space and the material's surface is designed to merely efficiently radiate roughly around 10 micron wavelengths, which both the sun and the surrounding air radiates relatively little of, to outer space and thus tends to slightly head towards equalizing its temperature with the freezing cold ( 'freezing cold' at around the 10 micron wavelengths as opposed to solar radiation wavelengths ) of outer space. This cooling effect is consistent with the laws of thermodynamics as I understand them.
It actually works in a slightly similar way to how "frost pockets" work (I tried and failed to find a decent link to properly explaining the physics of frost pockets. Anyone? ) ; when I used to cycle (on push bike ) to work in cold mid winter early mornings after a clear cloudless night and even before sunrise, I often find that the temperature seems to feel a lot colder when I cycle downwards to a dip in the land surrounded by hills and then it feels a lot less cold when I cycle up to the top of a hill; the eccentric temperature difference is partly explained by the top of the clouds of freezing fog in the dips of the land radiating heat to outer space more efficiently than the surfaces of the surrounding hills thus creating what is called a "frost pocket". I assume you might be not be familiar with the term "frost pocket" if you live in a part of the world where there is no frost pockets? I am all too familiar with the term because I used to be a farmer and farmers just hate frost pockets because, if your farm is on a natural frost pocket, you would be massively disadvantaged by it as it in affect make the winter last longer on your farm and thus make you early crops less early which tends to greatly reduce their profitability. Farmers that have the misfortune of having their farms on frost pockets tend to be poor farmers.
Originally posted by humy This cooling effect is consistent with the laws of thermodynamics as I understand them.
All very interesting. So the device will only work when exposed to the open sky, and not inside a closed room. So we have a very cold sky above our heads and we don't typically realize it because the air and sunlight keep warming us up.
Originally posted by twhitehead All very interesting. So the device will only work when exposed to the open sky, and not inside a closed room. So we have a very cold sky above our heads and we don't typically realize it because the air and sunlight keep warming us up.
If it can generate a difference in temperature, can that be used to generate energy, actually go MORE than 100% free cooling?
You could have solar panels as the bottom layer, instead of a mirror. If you can use this to pump heat out of buildings then it's acting as a heat pump and I'd like to know what the energy input is that's driving the heat pump.
Originally posted by twhitehead All very interesting. So the device will only work when exposed to the open sky, and not inside a closed room. So we have a very cold sky above our heads and we don't typically realize it because the air and sunlight keep warming us up.
Originally posted by sonhouse If it can generate a difference in temperature, can that be used to generate energy, actually go MORE than 100% free cooling?
I think I heard of research on this but don't remember where. If I remember correctly, you can exploit that temperature difference to generate energy (electrical ) but so far no cost effective way has been found and you are not talking here about an impressive theoretical maximum average power density ( in terms of per unit area ) compared with the theoretical maximum for more traditional solar power. But, that said, traditional solar power doesn't cool; so perhaps that is missing the point?
Originally posted by sonhouse If it can generate a difference in temperature, can that be used to generate energy, actually go MORE than 100% free cooling?
The temperature difference is insignificant when compared to the heat supplied by the sun, or the difference between daytime air temperature and underground, so I it would make more sense to use those for the power generation - which is certainly possible. It probably wouldn't typically be as good as wind or solar.
One system I know of is using daytime heat to heat a nearly empty water tank, then you close it at night and a vacuum forms and it sucks up water - essentially a passive water pump driven by the suns heat.
The use of glass and the greenhouse effect would increase the temperature difference even more.
Originally posted by twhitehead In Livingstone, Zambia, we know all about cooling and energy costs. The best ways to achieve cooling are trees, high ceilings and insulation under the roof.
We have often laughed at people who buy a house, chop down all the trees (because they are too lazy to clean up the leaves that trees drop) and then 6 months later they install air conditioners.
My ...[text shortened]... planting trees around her house she has brought down the average temperature by about 4 degrees.
This is true even here in the Netherlands. I have a beech tree in my garden, and as the garden is small enough to be more or less shaded in its entirety by its canopy, my garden is always nice and cool in summer, and so is my sitting room.
cooling system that consumes no energy!
28 Nov '14 09:20
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