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Although an unambiguous definition of heat is available for closed systems, the question of how best to define heat in open systems is not yet settled. After introducing a set of physical requirements for the definition of heat, this article reviews the non-equivalent definitions of heat for open systems used by Callen, Casas-Vazquez, DeGroot, Fox, Haase, Jou, Kondepudi, Lebon, Mazur, Misner, Prigogine, Smith, Thorne, and Wheeler, emphasizing which physical requirements are not met.  A subsequent section deals with the main objective of this article and introduces a new definition of heat that avoids the difficulties of the existent definitions, providing (i) a complete distinction between open and closed systems, (ii) non-redundancy, (iii) natural variables for the thermodynamic potentials, and (iv) a sound, complete, and intuitive generalization of classical thermodynamic expressions.

Non-redundant and natural variables definition of heat valid for open systems