Refrigeration as well as air conditioning operates on the principle of the compression and expansion of refrigerants. When compressed, the refrigerant changes into a liquid state. When it expands, it evaporates again. To evaporate it needs heat which is taken from whatever it touches and cools it down.
But now, research being done at the University of Cambridge in England is focusing on what is called “magnetic cooling,” based on the straining of materials. The results have been published in the journal Nature Materials and quoted at the www.basqueresearch.com website.