NANOTECHNOLOGY used in the form of a lubricant additive could significantly boost the energy efficiency of chillers, according to research being undertaken in the USA.
The government-run National Institute of Standards and Technology (NIST) has found that dispersing the right amount of copper oxide particles in a standard polyester refrigerant lubricant and combining it with R134a improves heat transfer by between 50% and 275%.

The particles, which are just 30 nanometres in diameter, are thought to encourage secondary nucleation - bubbles on top of bubbles. The double-bubble effect is said to enhance boiling heat transfer and, ultimately, could help to boost the energy efficiency of industrial-sized cooling systems.

Results of this work have been presented at recent conferences and will be reported in an upcoming issue of the ASME Journal of Heat Transfer.

Concentration

Just how nanomaterial additives to lubricants improve the dynamics of heat transfer in refrigerant/lubricant mixtures is not thoroughly understood but the right concentration of nanoparticles appears to be crucial.

'As with all good things, the process is far from foolproof,' commented NIST researcher Mark Kedzierski. 'In fact, an insufficient amount or the wrong type of particles might lead to degradation in performance.'

On the basis of work so far, it appears that nanoparticles of materials with high thermal conductivity improve heat transfer rates for the system.

Preliminary results of the NIST research also indicate that, in sufficient concentrations, nanomaterials enhance heat transfer by encouraging more vigorous boiling of the mixture. The tiny particles stimulate secondary bubbles that form atop bubbles initiated at the boiling site. Bubbles carry heat away from the surface, and the fact that they're being formed more efficiently because of the nanoparticles means the heat gets transferred more readily.

No research has yet been done into whether there may be any adverse long-term effect of these additives on the compressor but, as Mark Kedzierski points out, nanolubricants were first created with improved lubricity in mind.

While work so far has revolved around R134a chillers, the technology is expected to have a similar effect with other refrigerants.
It is expected to work on any system relying on shell-side boiling on the outside of tubes in a pool of liquid.

For this reason Mark Kedzierski says: 'I wouldn't expect to see performance improvements for direct expansion residential applications where the boiling occurs inside the tubes.'

Although no work has been done on this yet, the technology could also work with the increasing number of chillers which have adopted the Turbocor oil-less compressor.

'As long as there was some provision for allowing some lubricant to make its way to the evaporator, then it should work,' said Mark Kedzierski.

'This in fact may be the best application for the technology because you would be able to 'dial' or optimise the concentration of lubricant in the evaporator to optimise performance without regard to the lubricant requirements of the compressor.'

The team even expect improved performances with chillers using enhanced surface tubes.

'We have data at NIST that shows that additives and specially chosen lubricants can improve the performance of enhanced tubes. So I am confident that there is a place for nanolubricants for enhanced tube geometries,' confirmed Kedzierski.

'Also, the mechanism of the tube coatings is significantly different from how the nanolubricant works to improve heat transfer. The nanoparticles work together in a fluidic way with the lubricant in the lubricant excess layer while the coatings are a purely surface controlled phenomenon.'

Long-term benefits

Success in optimising recipes of refrigerants, lubricants and nanoparticle additives would pay immediate and long-term dividends, according to NIST researchers, and if they did not harm other aspects of equipment performance, high-performance mixtures could be swapped into existing chillers, resulting in immediate energy savings.

The improved energy efficiency, would allow next-generation equipment to be smaller, requiring fewer raw materials in their manufacture.