Nanostructured coated copper tube boiling refrigerant R-134a. Image credit: the University of Pretoria
By Dr Bradley Bock, senior lecturer, Department of Engineering, Built Environment and Information Technology at the University of Pretoria
Nanotechnology may provide the breakthrough to push the HVAC&R industry towards the next level of performance and efficiency, and a team at the University of Pretoria’s Department of Mechanical and Aeronautical Engineering, in collaboration with colleagues from the Massachusetts Institute of Technology (MIT) and Imperial College London, are making progress on understanding this complex field and the possible applications to the HVAC&R industry.
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The HVAC&R industry is under ever increasing pressure due to global warming, as the industry is estimated to be the cause of approximately 10% of our global warming emissions[1], either directly through the leaking of the refrigerants used in refrigeration equipment, or indirectly through the emissions at the power plants used to supply the electricity that operates these machines. Greater efficiencies are therefore essential to allow for lower electricity consumption as well as reduced equipment size so that less refrigerant is needed.
This is particularly true for Africa, which is expected to be the largest growth market for the worldwide HVAC&R industry[2],[3] in the near future, driven by urbanisation and industrialisation[4],[5]. This growth unlocks great potential for the African continent, particularly in the field of agriculture, where cold storage infrastructure will allow for the modernisation of Africa’s still often rural farming sector, reducing wastage and spoilage and expanding export opportunities. However, this growth will bring with it an even greater burden on global warming emissions, meaning Africa will have to be part of the charge to reduce the global warming impact of HVAC&R.
Nanotechnology may be part of the solution. Nanotechnology is any technology that operates on the nano scale, between 1 to 100 nm (that’s between 0.000001 to 0.0001 mm).
A wide range of technologies have been investigated, with advances in fields as diverse as solar panels, wastewater cleaning and cancer treatment[6].
The team at the University of Pretoria are investigating nanostructures, nanometre-sized structures applied to surfaces to change their properties. These structures have properties that make them attractive for heat transfer, allowing for the possibility of more efficient and smaller evaporators and condensers.
The team at the University of Pretoria, consisting of Dr Bradley Bock and Prof Josua Meyer (now at the University of Stellenbosch), collaborated with researchers at MIT and Imperial College London to apply nanostructures developed originally for the nuclear industry to the outside of copper tubes used in refrigeration evaporators.
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- Silica nanoparticle structures. Image credit: the University of Pretoria
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- Forest of tiny nanospikes. Image credit: the University of Pretoria
Two types of nanostructures were investigated. The first consisted of applying silica nanoparticles to the outside of a copper tube. These silica nanoparticles are made up of the same material as beach sand but are only 20 nm wide (so 0.00002 mm). By applying these small balls to the copper surface, intricate cavities and voids can be created on the surface, which traditionally has been found to improve heat transfer under certain conditions.
The second nanostructure type was applied using a special chemical formulation, which after encountering the copper surface, causes a chemical reaction that forms a forest of tiny nanospikes. These nanospikes change the surface properties of the copper, giving the surface the ability to wick the refrigerant, in effect absorbing refrigerant in the same way paper towels can absorb water when cleaning up a spill.
The research[7],[8] so far has led to several innovative findings. The nanospikes increased boiling heat transfer by up to three times that of a plain copper tube. But the silica nanoparticles decreased heat transfer. This showed that the simple fact that the nanostructures create voids and cavities is not enough to guarantee improved heat transfer, and careful selection and understanding of the nanostructures and their geometries is needed.
The nanostructures also changed the way the heat transfer from the copper tubes to the refrigerant occurred, with new heat transfer mechanisms noted that had not been seen before. “Using high-speed camera footage, we were able to see that the refrigerant liquid was wicked into the surface, and it appeared to allow refrigerant to actually creep under the bubbles created during boiling, allowing the bubbles to grow larger and quicker,” suggests Dr. Bock. “But more work is needed to really figure out more precisely what is going on.”
Despite these promising initial findings, more work will need to be done if nanostructures are to compete with existing technologies. “These nanostructures don’t currently compete with commercially enhanced tubes,” Dr Bock points out, “But with further research we can optimise these nanostructures and hopefully unlock their full potential.”
“3D printed heat exchangers are a particularly promising application of these nanostructures,” Dr Bock also noted. Nanostructures can be applied to the complex geometries that can be produced with 3D printing by simply pumping the chemicals through the heat exchangers, increasing the capacity of these already enhanced heat transfer devices.
The team at the University of Pretoria are continuing their research on this topic, not only in the hope of making the next big breakthrough in the HVAC&R industry, but to ensure Africa is at the forefront of the new technologies that will be built over the next decades on its shores.
[1] https://www.birmingham.ac.uk/Documents/college-eps/energy/Publications/Clean-Cold-and-the-Global-Goals.pdf
[2] https://www.designingbuildings.co.uk/wiki/Africa_tops_world_AC_growth_forecasts
[3] https://www.goldsteinresearch.com/report/africa-air-conditioner-market-trends-analysis
[4] https://www.cbn.co.za/industry-news/food-dairy-processing-manufacturing/cold-storage-infrastructure-and-refrigeration-tech-are-key-for-growth-in-sa-food-
production/
[5] https://www.newyorker.com/magazine/2022/08/22/africas-cold-rush-and-the-promise-of-refrigeration
[6] https://www.nano.gov/about-nanotechnology/applications-nanotechnology
[7] https://doi.org/10.1016/j.ijheatmasstransfer.2020.120452
[8] https://doi.org/10.1016/j.ijheatmasstransfer.2020.120387
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