# Heat pumps and efficiency

by unchained1978
Tags: efficiency, heat, pumps
 P: 99 In my thermodynamics class we're talking about heat pumps (i.e. refrigerators and such). When talking about a typical heat engine (carnot or not) it makes sense to me to talk about the efficiency of the engine $η=1-\frac{Q_{l}}{Q_{h}}$, because it clearly represents how much bang you get for you buck (to be colloquial). But when you discuss heat pumps, it's possible to have an efficiency greater than 1, and I have a difficult time understanding this. It's clear from the definition in the case of a refrigerator for example, because you have $η=\frac{Q_{l}}{W_{in}}$. In Blundell & Blundell's Concepts in Thermal Physics p.133, they reason this is because efficiency is essentially "what you want to achieve" divided by "what you have to do." Can someone clarify this idea of efficiency in the context of heat pumps or explain why it makes sense to talk about an efficiency $\geq 1$?
 Quote by unchained1978 In my thermodynamics class we're talking about heat pumps (i.e. refrigerators and such). When talking about a typical heat engine (carnot or not) it makes sense to me to talk about the efficiency of the engine $η=1-\frac{Q_{l}}{Q_{h}}$, because it clearly represents how much bang you get for you buck (to be colloquial). But when you discuss heat pumps, it's possible to have an efficiency greater than 1, and I have a difficult time understanding this. It's clear from the definition in the case of a refrigerator for example, because you have $η=\frac{Q_{l}}{W_{in}}$. In Blundell & Blundell's Concepts in Thermal Physics p.133, they reason this is because efficiency is essentially "what you want to achieve" divided by "what you have to do." Can someone clarify this idea of efficiency in the context of heat pumps or explain why it makes sense to talk about an efficiency $\geq 1$?