A method to send heat back into space, says S.Ananthanarayanan.
Goldfinger, the bad guy in Ian Fleming’s novel of the same name, had his victims painted in gold paint. But a strip along the backbone was left unpainted. This was because the gold paint did not allow the skin to breathe and a person painted all over would die.
The earth’s atmosphere is like gold paint over the earth. It traps the sun’s heat and acts like a blanket, to keep the earth from cooling by losing the trapped heat to space. So far, this has been a good thing, to give the earth a degree of uniform warmth, which enabled life to arise and flourish. But CO2 pollution has affected the balance and the earth is collecting more heat than it can handle.
Lyu Zhou, Haomin Song, Nan Zhang, Jacob Rada, Matthew Singer, Huafan Zhang, Boon S. Ooi, Zongfu Yu and Qiaoqiang Gan, from The State University of New York at Buffalo, University of Wisconsin and, King Abdullah University of Science and Technology, Saudi Arabia, report in the journal, Cell Reports Physical Science, a substantial improvement in technology to throw the heat that the earth accumulates right back into the chill of outer space.
The way things warm up is really the way things cool down. Nothing will get warm if left to itself. It warms because it receives heat from objects that are cooling, and the heat it receives is more than the heat it is radiating. The earth’s atmosphere is good at receiving heat, and it gets better when it contains more CO2. As the atmosphere is most dense at sea-level, it is nearest the earth that it is the warmest. And this heat stays there, because the layer immediately above is pretty warm too, and radiates back most of the heat that the lowest, warm layer radiates out.
Except, there is a part of the radiated heat that the upper layers do not absorb, to radiate back, but passes through the upper layer, and all the layers above that, and is lost to the earth for good. This is because the atmosphere is transparent, or allows free passage, to heat that is radiated in the form of infra-red light with wavelength in the window of 8 to 13 micrometers. What this implies is that a good way to cool the earth would be collect the heat we receive, turn as much as we can to IR light in the 8-13 µm range, and beam it out to space. We already know that whitewashing the roof can help reflect the heat away and keep a building somewhat cooler. But a lot of the reflected heat gets absorbed by the atmosphere and is radiated back. What would work is that we change the heat into that range of wavelengths that can pass through, before we send it out.
Normal objects, at usual temperatures of around 30˚C, radiate over a wide range of frequencies, as shown in Fig 1. As stated, most of this radiation would be absorbed by the atmosphere itself, with no reduction of the net heat content, except for emission in the 8µm to 13µm band. A first method to get emission to stay in this band was developed a few years ago, with a seven-layer stack of the materials, hafnium dioxide and silicon dioxide, mounted on a thin silver base. Hafnium oxide has large emission in 8-13µm, while silicon dioxide emits strongly at 9 µm. The thickness of the layers was regulated, for best efficiency of reflectivity and selective emissivity. The result was a device where sunlight that passed through the stack was reflected back, skywards, with very little radiation absorbed and strong emission at the special wavelength band. And an arrangement that was tried in Stanford University brought about a 4-5˚C drop in temperature. Another device by the same team had a wafer of silica, or silicon dioxide, laid over a crystal of silicon, the material of solar cells, with a backing of an aluminum mirror. The result was a solar cell with an arrangement to prevent heating. As the efficiency of solar cells falls steeply when the device warms, the arrangement promised a substantial improvement in output of solar cells.
While devices in use attain cooling of around 100 W/m2, the paper says that the best cooling, even in principle, that is possible for a sky facing device is 160W/m2. In comparison, the energy that comes in from the sun is 1,000 W/m2. A great part of the energy received is hence not made use of, the paper says.
A first feature that can be improved, the paper points out, is that the emission from one half of the emitter, in the sky facing systems developed, is directed to the ground and wasted. This is shown in the left side part of Fig 2. The authors remedy this by standing the emitter up, vertically, and deploying a pair of mirrors to capture the emissions from both faces of the emitter. This, they observe, “breaks the cooling power density limit (which is 160 W/m2) of the single-sided thermal emitter,” and lifts cooling performance to 273.3 W/m2.
A further feature is that the material of the reflectors is a cermet, or a composite of a ceramic and a metal, which is spectrally selective. This means it reflects light of certain wavelengths and allows light of other wavelengths to be absorbed. The optimised reflector that was used could absorb 90% of the solar radiation and reflect 90% of the radiation in the 8-13 µm band, the paper says.
The result was that along with emission of heat, out to space, of more than double what other devices could do, 90% of the remaining solar energy was absorbed. There was tremendous heating of the solar absorption plates, the paper says. The cooling, at the same time, was 14˚C in a laboratory trial and 12˚C in an outdoor trial, the paper says.
The important thing to note that radiative cooling, with simultaneous heat capture, is a passive process, that needs no electricity, compressors, etc. Cooling at the rate of 160W/m2 amounts to cooling like a 0.05 tonne conventional AC unit. A 20 m2passive emitter, which is a square with sides of 4.5 m, would thus cool like a 1 tonne AC unit – with no electricity. And it would also produce heat, to warm water, drive a generator, etc., all on solar energy!
An IEA report says air conditioning and electric fans for cooling accounts for 20% of energy used in buildings, and the demand for air conditioning is set to soar in the decades to come. Does it look like the end of the road for conventional cooling systems?
Net heat content
The first apparent disadvantage of conventional cooling systems is that they use power and pump CO2 into the atmosphere. Another disadvantage is that the heat extracted is dumped back into the environment, so that the net effect is that the earth’s heat content stays the same, in fact, it increases.The first apparent disadvantage of conventional cooling systems is that they use power and pump CO2 into the atmosphere. Another disadvantage is that the heat extracted is dumped back into the environment, so that the net effect is that the earth’s heat content stays the same, in fact, it increases.
Radiative cooling, on the other hand, apart from using no power, sends the heat out into space. The heat content of the universe stays the same, but the earth gets a little cooler!
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