A single phase perturbation model has been developed for the characterization of the behavior of packedbed thermocline thermal energy storage tanks, derived from the one-dimensional two-phase energy equations. The non-dimensional parameters governing the problem have been identified and separated into two groups, those related to the fluid and solid-filler material thermo-physical properties (beta,gamma), and those relating to the process flow (Peclet and Blot numbers). A series expansion solution for the perturbation model is derived. The new perturbation model is an improvement over the current one-phase models as it more accurately captures the effect of the diffusion term, and allows for a direct comparison with the two-phase model. (C) 2014 Elsevier Ltd. All rights reserved.