可用性是一种热力学性质，它衡量一个系统在to和Po受到不可避免的环境限制时的做功潜力。与能源不同，可用性是不守恒的，任何因摩擦或通过温差进行的热传递而导致的退化都会导致可用性的破坏。可用性的破坏被称为不可逆性。本文给出了R12的性能图（图1），它可以方便地分析蒸汽压缩制冷循环的可用性。该图以图形方式显示了循环内发生的不可逆性，同时也说明了进入或离开系统的可用性流。引文:巴哈马拿骚ASHRAE Transactions第78卷第2部分

Availability is the thermodynamic property which measures the potential of a system to do work when restricted by the inevitable surroundings at Toand Po. Unlike energy, availability is not conserved and any degradation by friction, or by heat transfers through a temperature difference, results in destruction of availability. The destruction of availability is called irreversibility.This paper presents a property diagram (Fig. 1), for R12, which allows convenient availability analyses of vapor-compression refrigeration cycles. The diagram graphically displays the irreversibilities occurring within the cycle while also illustrating the availability flow into or out of the system.