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Smart dust / The power of powderHot stuffMagic dustA sprinkle of nanoA pinch of powder(New Scientist Via Acquire Media NewsEdge) IMAGINE you could cut your electricity bill simply by adding a pinch of special powder to the coolant in your fridge. What if the same stuff could also reduce your car's fuel consumption, improve your home's central heating and make electricity that little bit cheaper? It might sound fanciful, yet it turns out that there is more to this dream than fairy dust. Back in the early 1990s, engineers made a remarkable discovery: adding nanometre-sized particles to a liquid makes it far more effective at carrying away heat than anyone expected. This was exciting news. Keeping machinery cool is a hugely expensive business, in the power industry, in motor vehicles and in electronics of all kinds. Improvements of just a few per cent could potentially save billions. It seemed that there were fortunes to be made, and saved, thanks to nanopowders. Conflicting results soon left experts scratching their heads, and excitement gave way to confusion. Now 20 years after the effect was uncovered, the first commercial uses are finally in sight. So why the delay? And can nanoparticle coolants live up to their promise? The story began in 1993 when Hidetoshi Matsuda and his team at Tohoku University in Sendai published a curious result in a Japanese journal called Netsu Bussei. They had found that water's thermal conductivity – its ability to transfer heat – could be increased by adding nanometre-sized particles of aluminium and titanium oxides. Intriguingly, the more of the particles they mixed in, the more the liquid's heat-carrying capacity went up. This wasn't simply of academic interest. Devices that transfer heat from one place to another are vital in all kinds of technologies – from radiator systems in cars and refrigeration circuits in air conditioning to the tiny metal fins that cool processor chips inside every computer. Any improvement, however tiny, could cut costs significantly. On the other side of the world, Stephen Choi at the Argonne National Laboratory near Chicago, who could read Japanese, came across Matsuda's paper. Choi and his colleague Jeff Eastman had also seen evidence of this strange effect. They had lost out to Matsuda in the race to publish first, but they now set out to explore the benefits the materials could offer. Choi even coined the term "nanofluids" to describe these liquids. Engineers had already tested metallic powders as additives in liquid coolants. Metals like copper have a thermal conductivity hundreds of times higher than water, so it makes sense that these additives should help shift more heat. Yet experiments with micrometre-sized particles were a failure – the grains tended to sink unless the fluid was mixed vigorously. Worse, they damaged the cooling system as they circulated, abrading pipes and wearing out pumps and bearings. High hopes This is where nanoparticles have the advantage. They are small enough to remain in suspension, and because they are only about the size of a large molecule, they don't wreck pumping machinery. Couple this with their intriguing chemical properties and enormous surface areas and perhaps nanoparticles could offer a new way to keep things cool. Sure enough, when Choi and Eastman added a tiny amount of copper nanoparticles to ethylene glycol – a liquid coolant commonly used in vehicle engines – they measured a 40 per cent enhancement in the liquid's heat transfer properties. Copper oxide nanoparticles in water performed even better, but the most impressive result came when Choi added carbon nanotubes to silicone oil, boosting the liquid's thermal conductivity by 160 per cent. It was remarkable. "We even got calls from Formula-1 racing teams" wanting to know if nanofluids could give their cars an edge over their competitors by cooling their engines more efficiently, Eastman recalls. "It got a lot of people excited." Exhilaration soon turned to consternation. While some researchers found nanoparticles enhanced heat transfer, others reported no effect. A few even suggested that the stuff made heat transfer worse. "They weren't able to duplicate the data," says Thomas McKrell, a nanomaterials specialist at the Massachusetts Institute of Technology. So researchers began to devise new physical mechanisms to explain the effect. A century-old theory, developed by Scottish physicist James Clerk Maxwell, already explains how well a liquid will transfer heat if it has particles suspended in it. The model predicts that heat transfer increases in a simple linear way, taking into account both materials' individual ability to conduct heat, and is not affected by the particles' size or the temperature. Yet in many cases the results using nanomaterials in fluid didn't fit that model. There was a tantalising prospect that some exotic behaviour was occurring on the nanoscale. But what? Perhaps, suggested some people, random fluctuations of Brownian motion were making the nanoparticles bash into one another, so they could pass on their heat energy. Because nanoparticles are so small, they might have more collisions. Others wondered whether the nanoparticles clumped together and so the heat would travel faster through these large clusters. Or maybe the liquid formed an ordered layer around each particle, and these structures helped to pass on the heat? Arguments continued until 2007, when engineer Jacopo Buongiorno and his colleagues at MIT decided enough was enough. They would solve the mystery by getting teams around the globe to repeat identical nanofluid experiments. Thirty-four research institutes, from Tokyo to Texas, got involved in these benchmarking experiments and tested the same samples using the same protocols, and then gathered in Beverley Hills, California, in 2009 to compare notes. The picture that emerged seemed a blow for those who thought that nano was a byword for magic. "The consensus was that thermal conductivity can be described by conventional theory," says Buongiorno – Maxwell's model seemed to hold true, even on these tiny scales. So why the confusion? It turned out that not only did every researcher have their own favourite nanomaterials – from titanium and aluminium oxide, to silicon carbide, carbon nanotubes and copper – some also added dispersing agents such as surfactants to keep the nanoparticles in solution. Others avoided these agents all together. Even something as simple as the way a fluid was stirred might affect things, says Wenhua Yu from Argonne. It seemed the case was finally closed. Not so fast, says Yulong Ding who studies nanofluids at Leeds University, UK. "This story isn't over." He says that tests on flowing samples continue to show impressive cooling performance, an effect that can't be explained wholly by Maxwell's model. One 2008 study by a team at the US National Institute of Standards and Technology in Gaithersburg, for example, showed that adding copper oxide nanoparticles to a refrigerant almost tripled its ability to transfer heat. Secret in the flow The explanation could lie in the international benchmarking experiments. They measured thermal conductivity using a small sample of nanofluid held in sealed cells. But heat transfer also depends on the way energy is exchanged as the liquid moves and flows. And here nanofluids are rather unusual – unlike most liquids, their viscosity can vary non-linearly, depending on how fast they flow. This behaviour could be particularly important in the narrow pipes found inside heat exchangers. Where a flowing fluid meets the wall of a pipe, friction slows it to create a layer of sluggish or static liquid. If this layer is relatively thick it should act like an insulator and reduce heat transfer, says Buongiorno, but he suspects that nanoparticles are somehow making this layer thinner, so helping to boost heat flow into and out of the pipes. It is just a theory, he stresses, but it might explain the anomalous results. Buongiorno's theory could be good news for those hoping to put nanofluids to work. Take the prototype heat exchanger built by Thermacore of Lancaster, Pennsylvania, in partnership with a European project called NanoHex. In tests this unit halved cooling energy costs. To create a nanofluid for the device, David Mullen of Thermacore, is now working with ITN Nanovation of Saarbrücken, Germany. Ultimately, the commercial product will be used in data centres, which are rapidly becoming one of the planet's biggest carbon emitters – a 2008 study by management consulting firm McKinsey predicts that carbon emissions from data centres will surpass emissions from the aviation industry by 2020. According to Ding, there are still some major challenges to solve. It isn't yet clear whether the nanoparticles in nanofluids will start to stick to the inside of pipes with long term use. But more importantly, adding nanoparticles makes a fluid more viscous. That means any heat transfer benefits could be outweighed by the extra energy needed to pump the gloopier liquid around the cooling circuit. Engineers are now trying to figure out whether this is a showstopper. So far, signs are good: in 2010, for instance, a Chinese research team led by Shengshan Bi at Xi'an Jiaotong University modified a domestic fridge to run on a conventional refrigerant containing titanium dioxide nanoparticles. Despite the extra viscosity, their fridge used about 10 per cent less energy. Similar results have been found in other studies. According to Geoff Smyth, head of technology and delivery at the UK's Carbon Trust, this kind of performance could prove significant. In the UK alone people spend £1.2 billion on electricity every year to cool and freeze food and drinks. Making the nation's fridges and freezers 10 per cent more efficient would save £195 million and 710,000 tonnes of carbon dioxide emissions each year, he says. What's more, experiments with air conditioning units suggest that nanofluids could bring energy savings of up to a third. Improve chiller efficiency by just 1 per cent and it would save 320 billion killowatt-hours of electricity annually in the US alone, reducing the nation's energy bill by more than $3 billon and cutting about 150 million tonnes of CO2 emissions annually. As well as saving electricity, nanofluids could also make generating it more efficient. Most electricity plants use heat to convert water into steam, but if the surface of the heating element becomes too hot, the fluid around it starts to boil. Bubbles act as insulators and when they become too large, or the bubble layer becomes thick, the heating process slows. At some point, called the critical heat flux, the heater surface temperature rises rapidly because there is nowhere for the heat to go, says McKrell. At this point the energy could be released explosively, risking serious damage to the equipment. It turns out that nanoparticles can help keep the heat flowing. Studies show they seem to be deposited onto the heater surface, and somehow this alters the interactions between the surface and the surrounding liquid. The exact mechanism is still a mystery but the result is clear: nanofluids raise the critical heat flux. This could prove especially useful in nuclear reactors. When the critical heat flux of a coolant is exceeded, the reactor goes into meltdown and to help understand how nanofluids might prevent this, Buongiorno is working with French nuclear power company Areva. Should an accident occur, he suggests that pressurised tanks could flood the reactor with nanoparticles and cool it rapidly. He also suggests that nanofluids could be added to conventional reactor coolants, allowing a reactor to run safely at slightly elevated power levels. "We've found that nanofluids increase the critical heat flux by about 9 per cent," says McKrell – although some researchers put the figure as high as 200 per cent. At Argonne, engineers are also revisiting an idea first investigated by Choi and Eastman. A few years ago, Jules Routbort demonstrated an alternative to the liquid coolant used in vehicle radiators. It was a nanofluid containing graphite particles that enhanced thermal conductivity by an impressive 50 per cent. Routbort, who died last year, claimed that this eliminated the need for a second heat exchanger which is often used in vehicles to cool electronic components. This would translate into a lighter engine, making the vehicle cheaper to run. Engineer Calvin Li at Villanova University in Pennsylvania calculates that improving cooling efficiency by just 15 per cent would save 2.9 billion tonnes of petrol annually in the US alone. Energy company Chevron has begun to test these nanofluids in vehicle engines, says Li. This stuff may not hit the road for a while, however. Replacing your car's coolant with nanofluids would cost hundreds of dollars (see diagram, left). Then there are possible environmental effects to consider. Leaky car radiators spill thousands of tonnes of coolant onto the highway each year in the UK alone, yet the impact of nanoparticles on human health is still not fully understood. The nanofluid dream may not yet have come true, admits Buongiorno, but there are good reasons to believe that two decades of hard graft have not been wasted. These materials have a healthy future, he says. "The years have maybe taken away a bit of the glamour," he adds, "but now that's out of the way, the real work can begin." n Katharine Sanderson is a freelance writer based in the UK Fluid thinking Nano magic: the power of powder to save energy The strange tale of how a simple ingredient could slash fuel bills With nanofluid coolants, nuclear reactors could operate with greater safety margins (c) 2013 Reed Business Information - UK. All Rights Reserved. |