There have been some interesting developments at the farther reaches of research lately that could have an interesting bearing on the future of the refrigeration sector.
You may recall that last month I reported on the potential for magnetic refrigeration techniques to challenge the conventions of vapor compression. Although not at the same level of commercial readiness, an alternative technique is showing interesting progress towards achieving similar goals.
In Nanyang Technological University, Singapore, researchers have achieved successful results using a laser to cool the semiconductor material cadmium sulphate from a temperature of 20 degrees C down to minus-20 C.
In simple terms, laser cooling takes place because more photons leave each atom of the material than are fired at it by the laser, thus heat energy is lost from the material. Semiconductors offer good properties for heat transfer thanks to their particular behavior under laser irradiation.
Cadmium sulphate is such a semiconductor — an inorganic material commonly used in pigments to form the color yellow. It is also a used as a thin-film layer in solar cells and electronics. Significantly, it is a far more readily available material than the rare-earth materials previously used in optical cooling.
The Nanyang team believes that "optical cooling" if commercialized could offer a range of benefits over conventional vapor compression. It would replace the traditional compressor-and-refrigerant configuration with a microlaser and semiconductor — thereby reducing the noise, vibration, energy consumption and risk of leakage associated with the conventional systems.
Having a refrigerator with a microlaser system would also open up the prospect of allowing much smaller physical "boxes" — and wouldn't necessarily even require a "box" as we know it.
This also opens up another intriguing development — the potential for "self-cooling" chips in computers, where external fans are not required.
According to the researchers, the reduced physical size offers the tantalizing potential for reducing heat — and thus prolonging battery life — in high-energy-draw devices from night-vision goggles to smartphones.
Professor Xiong Qihua said: “If we are able to harness the power of laser cooling, it would mean that medical devices that require extreme cooling, such as MRI, which uses liquid helium, could do away with bulky refrigerant systems, with just an optical refrigeration device in its place.”
The research team is now looking to bring their laser cooling technique down to the liquid helium temperature of minus-269 C, so as to open up the possibilities of cryogenic applications. This is something that researchers at Copenhagen University in Sweden managed to achieve last year with their own laser cooling method, using a very thin nanomembrane, comprising the material gallium arsenide.
Another breakthrough was achieved in September by Massachusetts Institute of Technology, whose researchers created a hydrophobic polymer, suitable for coating condensers, cooling towers and the like, which is one thousand times thinner than current commercially available coatings.
The advantage of the extremely thin coating is that thermal or electrical conductivity are virtually unaffected. Other advantages are the fact that the particular polymer does not degrade in the presence of steam, making it well suited to power stations.
MIT Doherty Associate Professor of Mechanical Engineering Kripa Varanasi says: “Over the last several decades, people have always searched for a durable surface treatment to make condensers hydrophobic. With the discovery of a way to make highly durable polymer coatings on metal surfaces, the potential impact this can have has now become real.”
MIT says the covalent-bonding process the team developed is significantly more stable than previous coatings, even under harsh conditions.
This polymer prevents liquid forming a film on the surface, which can lead to degradation or reduction in performance, even at 100 C.
According to MIT, the new coating can easily be retrofitted to conventional condenser materials — typically titanium, steel, copper or aluminum — in existing facilities, using a process called initiated chemical vapor deposition (iCVD).
Sumanta Acharya, director of the National Science Foundation's Thermal Transport Processes Programme (who was not involved in this research), is impressed, saying: "In my opinion, this work represents a major breakthrough in condenser technology. It offers the potential for significantly higher heat-transfer coefficients, high vapor-condensation rates and rapid removal of the condensate."
