Cool as carbon dioxide

Earlier this month, the Union Cabinet approved the ratification of the ‘Kigali Amendment to the Montreal Protocol on Substances that Deplete the Ozone Layer’. This means phasing out the hydrofluorocarbons (HFCs) that are used in our refrigerators.

Actually, HFCs do not deplete the ozone layer, which protects life on earth from harmful ultraviolet rays, but they have a high ‘global warming potential’, or GWP, of 12,000-14,000. One GWP represents one molecule of carbon dioxide.

So, no HFCs anymore. Fine, but what do you replace them with?

There is a global call to move away from ‘synthetic refrigerants’ (like r-134a, r-152a) towards ‘natural refrigerants’ like ammonia, hydrocarbons such as propane and butane, and carbon dioxide. Hydrocarbons are the best, but they can catch fire. Overall, the best natural refrigerant is carbon dioxide.

Carbon dioxide, however, has a problem. It becomes ‘supercritical’ at 31 degrees C — namely, it exists in a state where there is no distinction between gaseous and liquid states — it will not liquefy, no matter how much pressure you apply. A refrigerant must be liquefiable.

Refrigerators work by vaporising the refrigerants in them, which leads to cooling. But to keep the machine running the gas is converted back to liquid by compressing it. More pressure means higher temperature. We need the pressure but not the temperature, so we ‘reject’ the heat in a condenser. When the gas cools inside the condenser while still under high pressure, it becomes liquid — and ready for a new cycle.

In hot climes like India, the ambient temperature is already high; the temperature of the refrigerant from the compressor needs to be higher than the ambient temperature — a temperature gradient is needed to ‘reject’ the heat. But beyond 31 degrees C, carbon dioxide refuses to liquefy.

Indian researchers have solved this problem by “a clever arrangement of heat exchangers, so that the ‘cold’ available within the system” is used to liquefy carbon dioxide. The liquid that has vaporised is very cold; the trick is to tap the low temperature to cool the compressed gas into a liquid, explains Prof Satyanarayanan Seshadri of the Department of Applied Mechanics, IIT-Madras, who has been working on a carbon dioxide-based refrigeration project.

“The system works quite well, because carbon dioxide is 15-20 per cent more efficient than conventional refrigerants,” Seshadri tells Quantum . Synthetic refrigerants with low global warming potential, such as r-34, are less efficient, calling for more electricity. In India, every kWhr of electricity produced leads to an emission of 0.83 kg of carbon dioxide (emission factor); the more electricity you consume, the more carbon dioxide you let out.

A carbon dioxide system today costs thrice as conventional refrigeration, but, Seshadri points out, costs will come down as you manufacture more — the costs are not inherent in the physics.

INDEE+

Seshadri’s work is part of an Indo-Norwegian programme, called INDEE+, “an umbrella project supporting the Indian refrigeration and air conditioning sector in the transition towards more environmentally friendly technology”.

The project is supported by Danfoss, the Danish multinational which has operations in India. Five Indian partners are involved — IIT-Madras, IISc, BITS, Central Institute of Fisheries Technology, and the Council for Energy, Environment and Water.

Now that the demo plant in IIT-Madras is working well, INDEE+ plans to set up three commercial-scale plants — in a community kitchen (Akshaya Patra), a supermarket, and a fishing vessel. “The future is carbon dioxide,” says Seshadri.

Magnetic refrigeration

The future also holds room for other technologies, including one in which Seshadri too is involved. Gas-free, magnet-based refrigeration, a joint project of the International Advanced Research Centre for Powder Metallurgy and New Materials (ARCI) and IIT-Madras, has seen some success, even if it might take a decade to hit the market.

At its heart are special materials that become hot when put inside a magnetic field. While this ‘magneto-caloric effect’ was discovered 130 years ago, global research is currently fabricating better alloys, which heat up more in a magnetic field. ARCI scientists R Gopalan and Srikanti Kavita have made “Heusler” alloys using manganese and nickel.

Seshadri has designed a prototype, which uses commercially procured lanthanum-iron-silicon, a good but expensive alloy that is also being tried out by others such as the Fraunhofer Institute, Germany.

The results will be the benchmarks for ARCI’s Heusler alloys.

In the contrivance, a tube containing the magneto-caloric materials is pushed into, and taken out of a powerful magnetic field. When inside the field, it becomes hot; when pulled out, it chills.

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