US scientists have developed a cooling circuit that does not use refrigerants that are harmful to the climate and the environment. To do this, they use an effect comparable to that of road salt, which is used when roads are slippery: the salt lowers the dew point of the ice, so that it melts even at slightly below zero. The researchers use ethylene carbonate as the coolant and sodium iodide as the salt, Drew Lilley and Ravi Prasher of Lawrence Berkeley National Laboratory in Berkeley, California, write in the journal Science.
Chlorofluorocarbon (CFC) refrigerants, which deplete the protective ozone layer in the upper atmosphere, have long been banned. However, the hydrofluorocarbons (HFCs) used today as refrigerants are not harmless either: their global warming potential is around 2000 times greater than that of carbon dioxide (CO₂). While HFCs do not normally escape from the refrigeration circuit, they can become airborne if leaked or improperly disposed of. As the need for cooling increases in a warming world, the amount of HFCs released is also likely to increase in the future.
“The development of high-efficiency refrigeration with safe, low-GWP refrigerants is a major challenge in combating climate change,” the researchers write. They found that the dew point of ethylene carbonate drops by 28 degrees Celsius when sprinkled with sodium iodide and isolated from its surroundings.
The salt lowers the melting point so that the solid ethylene carbonate becomes a liquid. This process consumes energy and if the ethylene carbonate cannot obtain energy from its surroundings because of the insulation, it uses the energy of its own heat – this cools it down.
When cooled, the mixture of ethylene carbonate and sodium iodide can be used as a refrigerant. However, in order to create a cycle, the two substances must be separated from each other again. To do this, the scientists used electrodialysis: chambers containing the liquid are separated by membranes and an electrical voltage is applied. The membranes only allow certain ions to pass through, so that both the positively and negatively charged salt ions escape from one chamber. Without the salt, the melting point of ethylene carbonate rises again and it solidifies.
The concentrated ethylene carbonate-sodium iodide mixture from the electrodialysis is sent in a separate line to the mixer where it is added to the solid ethylene carbonate to melt it. This closes the cooling circuit. The authors of the study write that the efficiency of the process could be increased further if more suitable membranes for electrodialysis were developed.
Because the processes are still relatively slow at the moment. Emmanuel Defay from the Luxembourg Institute of Science and Technology in Belvaux also points this out in a “Science” comment: “A cycle can last between five minutes and several hours,” writes the materials researcher. Nevertheless, like the study authors, he is convinced that the circulatory process can be significantly improved. His conclusion: “This is a serious contender for the future of cooling.”
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