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Old Posted Nov 27, 2014, 7:00 AM
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Mirrors could replace air conditioning by beaming heat into space

Mirrors could replace air conditioning by beaming heat into space


26 November 2014

By Ian Sample

Read More: http://www.theguardian.com/science/2...ing-heat-space

Quote:
A mirror that sends heat into the frigid expanse of space has been designed by scientists to replace air-conditioning units that keep buildings cool on Earth. Researchers believe the mirror could slash the amount of energy used to control air temperatures in business premises and shopping centres by doing away with power-hungry cooling systems.

- Around 15% of the energy used by buildings in the US goes on air conditioning, but the researchers’ calculations suggest that in some cases, the mirror could completely offset the need for extra cooling. --- In a rooftop comparison of the device in Stanford, California, scientists found that while a surface painted black reached 60C more than ambient temperature in sunlight, and bare aluminium reached 40C more, the mirror was up to 5C cooler than the surrounding air temperature.

- “If you cover significant parts of the roof with this mirror, you can see how much power it can save. You can significantly offset the electricity used for air conditioning,” said Shanhui Fan, an expert in photonics at Stanford University who led the development of the mirror. “In some situations the computations say you can completely offset the air conditioning.”

- The Stanford mirror was designed in such a way that it reflects 97% of the visible light that falls on it. But more importantly, it works as a thermal radiator. When the mirror is warmed up, it releases heat at a specific wavelength of infrared light that passes easily through the atmosphere and out into space.

- To make anything cool requires what engineers call a heat sink: somewhere to dump unwanted heat. The heat sink has to be cooler than the object that needs cooling or it will not do its job. For example, a bucket of ice will cool a bottle of wine because it becomes a sink for heat in the liquid. Use a bucket of hot coals and the result will the very different. The Stanford mirror relies on the ultimate heat sink: the universe itself.

- The mirror is built from several layers of wafer-thin materials. The first layer is reflective silver. On top of this are alternating layers of silicon dioxide and hafnium oxide. These layers improve the reflectivity, but also turn the mirror into a thermal radiator. When silicon dioxide heats up, it radiates the heat as infrared light at a wavelength of around 10 micrometres.

- Since there is very little in the atmosphere that absorbs at that wavelength, the heat passes straight out to space. The total thickness of the mirror is around two micrometres, or two thousandths of a millimetre. --- “The cold darkness of the universe can be used as a renewable thermodynamic resource, even during the hottest hours of the day,” the scientists write in Nature. In tests, the mirror had a cooling power of 40 watts per square metre at ambient temperature.

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When the mirror is warmed up, it releases heat at a specific wavelength of infrared light that passes easily through the atmosphere and out into space. Photograph: Fan Lab, Stanford Engineering


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Old Posted Mar 13, 2015, 12:24 AM
eleven=11 eleven=11 is offline
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what about solar electric panels with a mirror backing???
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Old Posted Mar 16, 2015, 12:39 PM
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However, there is a great deal which is not clearly explained within the article or even the paper itself. I did give the idea a bit more thought on the bus last night and think that I now follow the idea. However, when the paper announces that it is counter intuitive to expect something to cool when taken from the shade and placed in sunlight this it to the extent that real scrutiny of the idea is still needed.

Here is my latest understanding of how this is possible:

A perfect mirror would reflect 100% of EM in both directions.

Objects radiate heat based upon their temperature

If you placed this mirror atop a hot source such as building then the mirror is reflecting the same radiation back into the building which is then the same as if the outside temperature was the same as the building.

This is used for situations where you are trying to minimise heat exchange such as a thermos flask or a space blanket.

So if you have a situation where the outside temperature is hotter than the building then neither conduction, convection or radiation can possibly cool the building.

The sunlight will interact with the surfaces it illuminates and generate heat. The surface will continue to get hotter until the outgoing heat is in equilibrium with the incoming heat. If you block the incoming sunlight then you create a potentially lower equilibrium.

If the sunlight was very crudely represented as 2 blue and 1 red and this was matched with the radiation from the hot surface as 3 red lights. Then if you have a material which reflects the blue but lets the red through then you would now only have 1 incoming red light but 3 red going out. So the surface can cool until it is at level which only needs one red light.

So this mirror can allow a surface that was above the new equilibrium to cool. Because it is being tested where the building has been illuminate by morning light until 10am already this then means that the roof is hotter than the temperature that it would be if continuously covered by a mirror. This can then cool the roof before it eventually warms as the sun continues to heat.

The practical limits of the temperature at which this operates as cooling rather than retarding the warming is limited to the initial covering of the surface which has very limited practical applications. And in winter it too would continue to cool at certain temperatures so less effective for year round than a pure mirror.

Some applications such as perhaps bus-stops or cars might be useful but the temperature range is very limited and for fixed structures it may be to increase the efficiency of some devices which generate heat such as traditional air conditioners but most of the temperatures would be preventing an object hotter than the air cooling via convection.

The only real interesting applications might be for Geo-engineering arctic ice applications as the heat is dissipated to the atmosphere or a few specific chemical plants.

The roof scenario would almost certainly be better served by a stronger reflector and then using the light as either CSP or PV.
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Old Posted Mar 16, 2015, 6:26 PM
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Surely this isn't a very good idea when it comes to air travel / navigation.... not to mention environmental impact on fauna.
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