ABSTRACT
Can we convert heat into work? The answer is “yes,” but with clear limitations which can be derived from the second law of thermodynamics. More precisely, if we have available heat at temperature TH and cold at a lower temperature TC , then the maximum fraction of heat that can be converted into work using a heat engine is 1− TC/TH (the Carnot factor). The reverse processes of obtaining cold from heat (refrigeration) and obtaining heat from cold (heat pump), which require work, are also limited by the Carnot factor. In practice, the efficiency is lower because of irreversibility that results in “lost work” or exergy (availability) losses. A thermodynamic (exergy) analysis can help to identify the main losses in the process. It can also be used to evaluate the overall efficiency of a technology.
