‘Tiny tubes could bring big savings on fuel bills’ screamed a headline in The Guardian last year (2006). Researchers at the University of Leeds, the paper reported, have developed nanofluids that they claim could significantly improve the heat transfer of central heating systems. The basic principle – as several laboratories have observed – is that dispersions of nanoparticles in ordinary liquids, such as water or ethylene glycol, show thermal conductivities far superior to that of the liquid alone.
But would it make sense to put a complex fluid into our radiators when plain water is doing a terrific job and the overall efficiency is dictated by the insulation of the whole house, not by the heat transfer rate within it? Detlev Belder, a nanofluidics expert at the University of Regensburg, Germany, is unconvinced. ‘The idea sounds a bit crazy,’ he said. If optimising the heat transfer rate were the key issue in domestic heating, we would be filling our radiators with mercury or gallium, he suggests.
Many researchers, meanwhile, believe that more promising applications for the resulting nanofluids will be in cooling rather than heating. ‘I would anticipate that the first commercial use of nanofluids would be in industries that can improve their productivity by increasing cooling rates, such as the rubber or paper industry,’ says Jules Routbort, who studies nanofluids at the Argonne National Laboratory near Chicago, Illinois.
Whatever their end application, the suspended nanoparticles used to produce nanofluids can be of many different kinds. Richard Williams and Yulong Ding at the University of Leeds use carbon nanotubes (CNTs) to create their nanofluids. ‘We are not limited to CNTs but CNT nanofluids give the best performance,’ says Yulong Ding. Even though pure and well-defined carbon nanotubes for many nanotech and electronic applications can be prohibitively expensive, Ding says ‘CNTs are not necessarily expensive as purity is not a problem for the formulation’.
Using these CNTs in domestic radiators can lead to energy savings, he insists: ‘If you want to increase heat transfer by a factor of two using your current system, you would need to increase the pumping power by a factor of about 12. If you have a better heat transfer fluid, which transfers heat quickly without much increase in pumping power, you would save lots of energy.’
Argonne’s Routbort is sceptical. The increased viscosity of the nanofluid might eat any energy saving away,1 while clogging and corrosion could make a heating system based on carbon nanotube nanofluids impractical, he argues. Instead, researchers at Argonne, supported by the Office of Vehicle Technologies of the US Department of Energy, are working to develop nanofluids for cooling radiators for transport applications.
The nanofluid option is most attractive in cases where there are tight limits on the size or weight of a cooling system, such as in vehicles, airplanes, or computers, Routbort explains. ‘Computer simulation has shown that we can reduce the size of truck radiators by 5% if we use nanofluids as coolants. At highway speeds, this would result in 2.5% fuel savings.’ Other applications that Routbort and colleagues have listed in a recent review of the field include defence, space travel, and biomedicine.
Apart from trucks, the fluids may also end up cooling computers. With the ongoing miniaturisation of semiconductor components, the waste heat they produce is more and more concentrated, making it more difficult to dissipate. Researchers at the National Taiwan University at Taipei have already started to develop nanofluid cooling systems specifically for desktop and laptop computers. Circulating in tiny heat pipes, nanofluids would help to carry heat away from the processors and towards the periphery of the computer, where it can escape to the outside.2
Researchers at the Massachusetts Institute of Technology (MIT) in Cambridge, US, meanwhile, are also investigating the possibility of using nanofluids for heat transfer in nuclear power stations. This application, however, does not rely on the ‘anomalous’ heat conductivity of the particles. Rather, their tendency to aggregate at the hot surface of the fuel rods, from which the heat needs to be carried away, would avoid the formation of an insulating layer of steam, which in current procedures limits the efficiency of heat transfer above a certain temperature.
Jacopo Buongiorno and his team at MIT have demonstrated this improvement in bench experiments and also in preliminary trials using MIT’s nuclear research reactor. They hope that their technology could make all pressurised water reactors – around two thirds of the nuclear power plants running in the US – more efficient, with an extra 20% of power to be gained.
How does it work?
The nanofluids pursued for many of these applications, experts say, are no different from colloids, some of which have been known for over a century. Danziger Goldwasser, an alcoholic beverage containing a suspension of colloidal gold, for example, was produced as far back as the 16th century. But while every chemist must have come across colloidal suspensions at some point, not many appear to have wondered about their thermal properties. It was only in 2001 that Jeff Eastman and Steve Choi, with their colleagues at Argonne,3 reported observing ‘anomalously increased effective thermal conductivities’ in suspensions of copper nanoparticles in the cooling fluid ethylene glycol.
Since then, the flow of papers in this field has increased steadily. In 2004, Seok Pil Yang and Steve Choi at Argonne proposed a theoretical model of the phenomenon. They suggested that Brownian motion is key to the surprising conductivity.
In contrast to traditional models of two-phase systems, which assume essentially a static distribution of particles in the liquid, Yang and Choi have taken into account that nanoparticles will be kicked about quite violently by the thermal motions of the smaller molecules around them. These interactions also contribute to the exchange of energy between the components of the system. The 2004 model predicts that the increase in conductivity is higher when the nanoparticles get smaller, which agreed with previous experimental observations.4
Since then, however, researchers have obtained conflicting results. Argonne’s Jules Routbort thinks that the phenomenon is still far from fully understood. ‘It is clear that the current theories lack some essential physics,’ he says.
Things get more complicated when different shapes are considered, such as elongated nanotubes versus round(ish) nanoparticles. ‘One can imagine that there would be an enhancement for non-spherical nanoparticles, or nanotubes, if a percolation process exists,’ Routbort explains. Percolation would involve nanotubes lining up to pass on the heat from one to the other. While there are theories covering this type of scenario, they don’t explain the enhancement observed in highly diluted suspensions of spherical particles.
Introducing a new, all optical method to measure thermal conductivity and diffusion at the same time, the group of Roberto Piazza at the CSGI Polytechnic at Milan, Italy, caution that, at least in the spherical colloids they studied, there is no anomalous behaviour at all.5 Thus, more research is needed to clear up the fundamental issues.
Road to commercialisation
Still, the empirical observations have already inspired commercial ventures. Leeds researchers Richard Williams and Yulong Ding have already set up a spin-out company, Dispersia, supported by investment from UK technology transfer organisation IP Group. Dispersia aims to develop recipes for heat transfer fluids for specific applications.
‘The use of nanofluids in heat transfer could radically change the design of many industrial systems,’ says Rob Rule from IP Group’s Leeds subsidiary, Techtran, the university’s former technology transfer office. He says that Dispersia is already engaged with several industrial partners, although he cannot divulge their names.
Cooling, industrial heat transfer and automotive applications are the key areas the company is aiming at.
The company currently consists of the two founders, a non-executive director, and one researcher working from a dedicated lab within Leeds University. Massive growth is neither planned nor required, Rule explains, as the company will keep focusing on developing the fluids, for which industrial partners will then develop applications. Rule hopes that within 12 months, at least one of the fluids will be ready to be passed on to the industrial partners.
The biggest commercial prize in the field is the car industry. In the US, car manufacturers GM and Ford are both said to be running their own research programmes on nanofluid applications. Whoever designs the nanofluids that will cool the cars of the future is bound to make it big. ‘That’s what we are all aiming for,’ Rule admits.
Thus, it appears that nanofluids may end up making your car or computer more efficient, even if they may not reduce your heating bills.
1. R Prasher et al. Appl. Phys. Lett. 2006, 89
2. CY Tsai et al. Mater. Lett. 2004, 58, 1461.
3. JA Eastman et al. Appl. Phys. Lett. 2001, 78, 718.
4. SP Jang et al. Appl. Phys. Lett. 2004, 84, 4316.
5. R Rusconi et al. Appl. Phys. Lett. 2006, 89