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I was reading that a new type of refrigerator might reach a coefficient of performance (COP) of 10. This seems quite the achievement and the authors state that their approach might achieve a Carnot efficiency of 75%. (I could not get hold of the original source as it is a part of an expensive book series "Zimm C, Jastrab A, Sternberg A, Pecharsky V K, Gschneidner K Jr, Osborne M and Anderson I 1998 Adv. Cryog. Eng. 43 1759".)

Now I am a bit confused. Wikipedia states that the COP should be the inverse of the Carnot efficiency but than these numbers do not fit together or am I missing something?

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do you know the temperatures at which those figures where achieved? Does Carnot efficiency of 75% mean effiency = 75%, or 75% of the Carnot efficiency at the relevant temepratures? high COP implies low $\delta T$, the other way round for a heat engine. So I agree, the numbers don't fit. –  mart Aug 9 '12 at 14:02
The exact temperatures are not stated, they achieved a $\Delta T$ of 10 Kelvin in the room temperature range. It just says "a Carnot efficiency approaching 75%" in a review article by the same authors ("Recent developments in magnetocaloric materials" by Gschneidner et al.) –  Alexander Aug 9 '12 at 14:09
well, 10K and a cOP of 10 would mean the upper temperature is 100K ... there must be a mistake somwhere –  mart Aug 9 '12 at 14:44

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