This paper presents a new thermodynamic approach, based on the notion of power of separation, for the retrofit problem of finding the minimal energy requirement of an existing binary distillation column when coupling it in parallel with a membrane unit. A new geometric interpretation of this concept, which is supported by a rigorous mathematical proof, is introduced. From it results an efficient and accurate shortcut method to tackle the aforementioned problem. Numerical examples are considered for the energy intensive separation of olefins from paraffins, namely the retrofit of a C-3-splitter and the retrofit of a C-2-splitter through parallel hybridization with facilitated transport membranes. The results of the proposed shortcut method are compared to those obtained via the nonlinear programming optimization solvers GAMS-CONOPT and GAMS-CoinIpopt for a superstructure based problem formulation, which act as a reference. In both case studies, the shortcut method results in a significant reduction in problem size and in the number of solver iteration, while yielding only a small error on the minimal energy requirement of the column within the hybrid and on the hybrid architecture (i.e., the position of side-streams along the column). It could therefore be used to carry out a rapid screening of alternatives (e.g., different membrane technologies) in order to evaluate potential energy improvements. The proposed approach could also provide mathematical programming algorithms with a good initial guess of the solution.