“Conserve energy and save money!” is the ubiquitous message in an age of climate change, rising energy costs and possible future resource scarcity. Carry out an energy audit of your home, say environmentalists and consumer advisers, and see where savings can be made that could significantly reduce household bills.
But what these mass-media messages fail to show is exactly how energy use is distributed in modern society. However, there are plenty of statistics that show the proportion of electricity used by businesses, and a comparison of residential, commercial and industrial sectors is illuminating. Regarding the latter, the US Department of Energy has reported that large institutional cooling systems account for 13% of the energy consumed in US buildings, and around 9% of overall demand for electricity. Here we are talking about industrial-scale chillers; the figures do not account for office air conditioning and smaller-scale refrigeration.
Given the huge amounts of energy used in chillers, improving the efficiency of these machines might be seen as an environmental and economic imperative. This could soon happen, as new research shows that nanotechnology may be used to dramatically improve the lubricants and refrigerants used in cooling systems, and thus reduce the energy they consume.
Improving heat transfer in chiller systems
Mechanical engineer Mark Kedzierski at the National Institute of Standards and Technology (NIST) in Gaithersburg, US, and Maoqiong Gong at the Chinese Academy of Sciences in Beijing, have found that dispersing copper oxide nanoparticles in a polyester lubricant, and combining this with tetrafluoroethane – a common haloalkane refrigerant known as R134a – can improve heat transfer by between 50% and 275%. “We were astounded,” says Kedzierski, whose research results will soon be published in the Journal of Heat Transfer.
“As far as heat transfer goes, lubricants in chillers are a necessary evil,” says Kedzierski. “They are needed to lubricate the compressor, but most of the time the lubricant makes its way into the evaporator and causes a degradation in heat transfer as compared to pure refrigerant boiling. Much of my research has been focused on developing an understanding of how lubricants affect refrigerant boiling performance in order to mitigate the degradation.”
As part of this work, Kedzierski devised a fluorescence technique for measuring the mass of lubricant that accumulates on a boiling surface. From his studies, Kedzierski realised the importance of lubricant attaching itself to the boiling surface in determining the size and population of bubbles. “From that insight, it wasn’t too difficult to see the possibility of delivering highly conductive nanoparticles to the lubricant on the wall, like nanoparticles to cancer cells, in the hopes of improving boiling,” he says.
The work by Kedzierski and Gong builds on previous research which found that a more than 40% increase in the thermal conductivity of a liquid could be achieved by adding nanoparticles to a volume fraction of around 0.4%. There have to date been a number of studies carried out with various water-based nanofluids, but the results are inconsistent.
Kedzierski and Gong’s work focuses on the heat transfer properties of three R134a/nanolubricant mixtures on a roughened horizontal copper surface. A commercial synthetic oil known as RL68H was used as the base lubricant, and this was mixed with 30-nm diameter copper oxide nanoparticles (CuO-II). This metal oxide has many commercial applications, one of which is as a glass-polishing agent.
Particle size and dispersion were verified by a light scattering technique, and the particles were found to be well dispersed with little agglomeration. A proprietary surfactant with a mass of between 5% and 15% of the mass of the copper oxide was employed to disperse the particles in solution. In the tests of the nanolubricant mixture, heat fluxes ranging from 10 kWm–2 to 120 kWm–2 were used to simulate typical operating conditions in large chillers.
The proportion of nanoparticles is crucial when it comes to heat transfer, the researchers found. Mixtures with a 4% volume fraction showed boiling heat fluxes on average 140% larger than for mixtures with 2% by volume of CuO. And, for most heat fluxes, the 2% nanolubricant actually resulted in a heat transfer degradation.
Improvements in the thermal conductivity of a nanolubricant appear to be of secondary importance when it comes to boiling enhancement. More significant is the interaction of nanoparticles with bubbles in the boiling liquid, and the nanoparticle fraction sensitivity suggests that a critical volume fraction is necessary for boiling enhancement. It may be necessary for a certain threshold to be exceeded before there are sufficient nanoparticles to influence bubble growth and formation.
Why should nanoparticles in lubricants improve heat transfer?
Precisely how adding nanoparticles to lubricants improves heat transfer in refrigerant/lubricant mixtures is not thoroughly understood, but in his forthcoming journal paper Kedzierski speculates on a number of factors that may be influencing the observed behaviour.
Nanoparticles of materials with a high thermal conductivity will improve heat transfer rates for the system. That is an obvious inference based on simple and well-understood physics.
But there are another two mechanisms that could be at work here. First, there is the interaction between nanoparticles and bubbles referred to above. Second, there is a loss of nanoscale nucleation sites due to nanoparticle filling of cavities in the roughened metal surface. The nanoparticles stimulate secondary bubbles that form on bubbles created at the boiling site. These carry heat away from the surface, and, if the presence of nanoparticles means that bubbles are being formed more efficiently, heat is transferred more readily.
Implications for industry
Kedzierski and Gong’s experimental results speak for themselves, but the researchers stress that several factors are likely to account for nanoparticle-enabled improvements in heat-transfer. “It’s hard to say because there are levels of understanding,” says Kedzierski. “I hope to have a good working knowledge of the basics in a few years.”
The results of this latest work could provide a stimulus to industry to design and employ better performing refrigerants and lubricants in their cooling systems. Asked if there has been interest from industry in the latest research developments, Kedzierski replies: “Cautious interest. Yes. Their business is reliability first with energy efficiency a very close second. The deal breaker would be if the nanoparticles harmed the compressor. But I feel confident that we can find the right particle to improve things for both the compressor and the evaporator, given that nanolubricants were first devised to improve lubricating properties via the ‘ball-bearing effect.’”
James Bogart, an expert in brazed-plate heat exchangers, and global R&D manager with GEA PHE Systems, is an industrialist with a keen interest in this field. “The work of Kedzierski and Gong adds significant credence to the theory of how nanoparticles can degrade surface performance, while also providing reasonable physical arguments as to how relatively higher particle concentrations can actually result in increased performance over the control condition,” he says. “As pointed out by the researchers, this is a first step into the investigation. The results are encouraging and certainly set the stage for effective and productive follow-on studies.”
And that is exactly what Kedzierski plans to do. Future research will look at the influence of particle material, shape, size, distribution and concentration on refrigerant performance. With further investigation, the observed effects may lead to a theory which can be used to develop nanolubricants that improve heat transfer in industrial-scale chillers and air-conditioning systems.
Success in optimising recipes for refrigerants, lubricants and nanoparticle additives would, says NIST, pay immediate and long-term dividends. If nanoparticle additives do not harm other aspects of system performance, nanolubricant/refrigerant mixtures could be incorporated into existing chillers, resulting in significant and immediate energy savings. Added to this, improved energy efficiency would result in smaller chillers that require fewer raw materials in their manufacture.
Given our reliance on power-hungry cooling systems, this technology could have a huge impact on industrial energy use if it can be implemented on a large scale. Kedzierski concurs, adding: “It would be a good feeling if I could play a small role in helping to make that happen.”
Mark Kedzierski is a mechanical engineer based in the Building and Fire Research Laboratory of the National Institute of Standards and Technology in Gaithersburg, Maryland. He received his PhD from Penn State University in 1987, and in that same year joined the Thermal Machinery Group (now the HVAC&R Equipment Performance Group) at the National Bureau of Standards. Since then he has investigated two-phase heat transfer of alternative and multi-component refrigerants. Kedzierski works closely with DuPont, Trane, Wolverine, UOP, ICI and other companies to ensure that his research is of value to industry.
Adding copper oxide nanoparticles to lubricants commonly combined with refrigerants used in chillers may encourage secondary nucleation – bubbles formed on top of bubbles. The double-bubble effect shown here enhances boiling heat transfer, and this could help boost the energy efficiency of industrial-scale cooling systems.
This is a revised version of an article first published in Nanomaterials World.